Networking Working Group                                  T. Winter, Ed.
Internet-Draft
Intended status: Standards Track                         P. Thubert, Ed.
Expires: April 7, 29, 2010                                    Cisco Systems
                                                        ROLL Design Team
                                                            IETF ROLL WG
                                                        October 4, 26, 2009

      RPL: IPv6 Routing Protocol for Low Power power and Lossy Networks
                         draft-ietf-roll-rpl-03
                         draft-ietf-roll-rpl-04

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Abstract

   Low Power power and Lossy Networks (LLNs) are made largely a class of constrained
   nodes (with limited network in which
   both the routers and their interconnect are constrained: LLN routers
   typically operate with constraints on (any subset of) processing
   power, memory, memory and sometimes energy
   when they are battery operated).  These routers (battery), and their interconnects are interconnected
   characterized by
   lossy links, most of the time supporting only (any subset of) high loss rates, low data rates, that rates and
   instability.  LLNs are usually fairly unstable with relatively low packet delivery
   rates.  Another characteristic comprised 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 anything from a large number of nodes, few dozen and up
   to several dozens or
   hundreds or more nodes in thousands of LLN routers, and support point-to- point traffic
   (between devices inside the network.  These characteristics offer
   unique challenges LLN), point-to-multipoint traffic (from a
   central control point to a routing solution: subset of devices inside the IETF ROLL Working Group
   has defined application-specific routing requirements for a Low Power LLN) and Lossy Network (LLN) routing protocol.
   multipoint-to- point traffic (from devices inside the LLN towards a
   central control point).  This document specifies the IPv6 Routing
   Protocol for Low Power and Lossy Networks (RPL).

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted LLNs (RPL), which provides a mechanism whereby
   multipoint-to-point traffic from devices inside the LLN towards a
   central control point, as described in RFC 2119 [RFC2119]. 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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.1.  Design Principles  . . . . . . . . . . . . . . . . . . . .  6
     1.2.  Expectations of Link Layer Behavior Type  . . . . . . . . . . . . .  7
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Protocol Model . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Protocol Properties  Overview . . . . . . . . . . . . . . .  9
       3.1.1.  IPv6 Architecture  . . . . . . . . . . .  9
       3.1.1.  Topology Instance and Objectives . . . . . . .  9
       3.1.2.  Typical LLN Traffic Patterns . . . .  9
       3.1.2.  Multipoint-to-Point Traffic Flows and DAGs . . . . . . 11
       3.1.3.  Point-to-Multipoint Traffic Flows  . . . . . . 10
       3.1.3.  Constraint Based Routing . . . . 11
       3.1.4.  Point-to-Point Traffic Flows . . . . . . . . . . . . 10 . 12
     3.2.  Protocol Operation . . . . . . . . . . . . . . . . . . . . 10 12
       3.2.1.  DAG Construction . . . . . . . . . . . . . . . . . . . 12
       3.2.2.  Destination Advertisement  . . . . . . . . . . . . . . 19 15
     3.3.  Loop Avoidance and Stability . . . . . . . . . . . . . . . 21 17
       3.3.1.  Greediness and Rank-based Instabilities  . . . . . . . 22 17
       3.3.2.  Merging DAGs . . . . . . . . . . . . . . . . . . . . . 22
       3.3.3.  DAG Loops  . . . . . . . . . . . . . . . . . . . . . . 23
       3.3.4. 18
       3.3.3.  DAO Loops  . . . . . . . . . . . . . . . . . . . . . . 23
       3.3.5. 18
       3.3.4.  Sibling Loops  . . . . . . . . . . . . . . . . . . . . 23
     3.4.  Local and Temporary Routing Decision . . . . . . . . . . . 24
     3.5.  Maintenance of 18
   4.  Routing Adjacency . . . . . . . . Metrics and Constraints Used By RPL  . . . . . 25
   4.  Constraint Based Routing in LLNs . . . . 18
   5.  RPL Protocol Specification . . . . . . . . . . . 25
     4.1.  Routing Metrics . . . . . . . 19
     5.1.  RPL Messages . . . . . . . . . . . . . . 25
     4.2.  Routing Constraints . . . . . . . . . 19
       5.1.1.  ICMPv6 RPL Control Message . . . . . . . . . . 26
     4.3.  Constraint Based Routing . . . . 19
       5.1.2.  DAG Information Solicitation (DIS) . . . . . . . . . . 20
       5.1.3.  DAG Information Object (DIO) . . . 26
   5.  RPL Protocol Specification . . . . . . . . . . 20
       5.1.4.  Destination Advertisement Object (DAO) . . . . . . . . 27
     5.1.  DAG Information Option
     5.2.  Conceptual Data Structures . . . . . . . . . . . . . . . . 28
       5.2.1.  Candidate Neighbors Data Structure . . 27
       5.1.1.  DAG Information Option (DIO) base option . . . . . . . 27
     5.2.  Conceptual Data Structures . 28
       5.2.2.  Directed Acyclic Graphs (DAGs) Data Structure  . . . . 29
     5.3.  DAG Rank . . . . . . . . . . . 34
       5.2.1.  Candidate Neighbors Data Structure . . . . . . . . . . 34
       5.2.2.  Directed Acyclic Graphs (DAGs) Data Structure . . . . 35
     5.3. 30
     5.4.  DAG Discovery and Maintenance  . . . . . . . . . . . . . . 36
       5.3.1. 31
       5.4.1.  DAG Discovery Rules  . . . . . . . . . . . . . . . . . 37
       5.3.2. 32
       5.4.2.  Reception and Processing of RA-DIO DIO messages . . . . . 39
       5.3.3.  RA-DIO . . 36
       5.4.3.  DIO Transmission . . . . . . . . . . . . . . . . . 41
       5.3.4. . . 38
       5.4.4.  Trickle Timer for RA DIO Transmission . . . . . . . . . . 42
     5.4. 39
     5.5.  DAG Heartbeat  . . . . . . . . Sequence Number Increment  . . . . . . . . . . . . . . 44
     5.5. 40
     5.6.  DAG Selection  . . . . . . . . . . . . . . . . . . . . . . 44
     5.6. 41
     5.7.  Administrative rank  . . . . . . . . . . . . . . . . . . . 45
     5.7.  Candidate DAG Parent States and Stability  . . 41
     5.8.  Collision  . . . . . . 45
       5.7.1.  Held-Up . . . . . . . . . . . . . . . . . . 42
     5.9.  Guidelines for Objective Functions . . . . . 45
       5.7.2.  Held-Down . . . . . . . 42
       5.9.1.  Objective Function . . . . . . . . . . . . . . . 46
       5.7.3.  Collision . . . 42
       5.9.2.  Objective Function 0 (OF0) . . . . . . . . . . . . . . 44
     5.10. Establishing Routing State Outward Along the DAG . . . . . 46
       5.7.4.  Instability  . . .
       5.10.1. Destination Advertisement Operation  . . . . . . . . . 47
     5.11. Loop Detection . . . . . . . . . 47
     5.8.  Guidelines for Objective Code Points . . . . . . . . . . . 48
       5.8.1.  Objective Function . . 54
       5.11.1. Host Basic Operation . . . . . . . . . . . . . . . . 48
       5.8.2.  Objective Code Point 0 (OCP 0) . 55
       5.11.2. Instance Forwarding  . . . . . . . . . . . 50

     5.9.  Establishing Routing State Outward Along the . . . . . . 55
       5.11.3. DAG Inconsistency Loop Detection . . . . . 52
       5.9.1.  Destination Advertisement Message Formats . . . . . . 53
       5.9.2.  Destination Advertisement Operation 56
       5.11.4. Sibling Loop Avoidance . . . . . . . . . 55
     5.10. . . . . . . . 56
       5.11.5. DAO Inconsistency Loop Detection and Recovery  . . . . 57
     5.12. Multicast Operation  . . . . . . . . . . . . . . . . . . . 62
     5.11. 57
     5.13. Maintenance of Routing Adjacency . . . . . . . . . . . . . 63
     5.12. 58
     5.14. Packet Forwarding  . . . . . . . . . . . . . . . . . . . . 64 59
   6.  RPL Constants and Variables  . . . . . . . . . . . . . . . . . . . . . . . . 65 60
   7.  Manageability Considerations . . . . . . . . . . . . . . . . . 66 61
     7.1.  Control of Function and Policy . . . . . . . . . . . . . . 66 61
       7.1.1.  Initialization Mode  . . . . . . . . . . . . . . . . . 66 61
       7.1.2.  DIO Base option  . . . . . . . . . . . . . . . . . . . 66 61
       7.1.3.  Trickle Timers . . . . . . . . . . . . . . . . . . . . 67 62
       7.1.4.  DAG Heartbeat  . . . . . . . . Sequence Number Increment  . . . . . . . . . . . . 68 63
       7.1.5.  The  Destination Advertisement Option Timers . . . . . . . . . 68 . . 63
       7.1.6.  Policy Control . . . . . . . . . . . . . . . . . . . . 68 63
       7.1.7.  Data Structures  . . . . . . . . . . . . . . . . . . . 68 63
     7.2.  Information and Data Models  . . . . . . . . . . . . . . . 69 64
     7.3.  Liveness Detection and Monitoring  . . . . . . . . . . . . 69 64
       7.3.1.  Candidate Neighbor Data Structure  . . . . . . . . . . 69 64
       7.3.2.  Directed Acyclic Graph (DAG) Table . . . . . . . . . . 69 64
       7.3.3.  Routing Table  . . . . . . . . . . . . . . . . . . . . 70 65
       7.3.4.  Other RPL Monitoring Parameters  . . . . . . . . . . . 70 65
       7.3.5.  RPL Trickle Timers . . . . . . . . . . . . . . . . . . 70 66
     7.4.  Verifying Correct Operation  . . . . . . . . . . . . . . . 71 66
     7.5.  Requirements on Other Protocols and Functional
           Components . . . . . . . . . . . . . . . . . . . . . . . . 71 66
     7.6.  Impact on Network Operation  . . . . . . . . . . . . . . . 71 66
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 71 66
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 71 66
     9.1.  DAG Information Option (DIO) Base Option  RPL Control Message  . . . . . . . . . 71 . . . . . . . . . . 66
     9.2.  New Registry for RPL Control Codes . . . . . . . . . . . . 67
     9.3.  New Registry for the Flag Control Field of the DIO Base
           Option . . 71
     9.3.  DAG Information Option (DIO) Suboption . . . . . . . . . . 72 . . . . . . . . . . . . . . 67
     9.4.  Destination Advertisement Option (DAO) Option  DAG Information Object (DIO) Suboption . . . . . . 72 . . . . 68
     9.5.  Objective Code Point for the Default Objective
           Function OF0 . . . . . . . . . . . . . . . . . . . 72 . . . . 68
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 73 68
   11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 73 69
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 74 70
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 74 70
     12.2. Informative References . . . . . . . . . . . . . . . . . . 74 71
   Appendix A.  Deferred  Requirements  . . . . . . . . . . . . . . . . 76
   Appendix B.  Examples  . . . . . . 72
     A.1.  Protocol Properties Overview . . . . . . . . . . . . . . . 72
       A.1.1.  IPv6 Architecture  . 77
     B.1.  Moving Down a DAG . . . . . . . . . . . . . . . . . . . . 78
     B.2.  Link Removed . . . . . . . . . . . . . . . . . . . . . . . 79
     B.3.  Link Added . 73
       A.1.2.  Typical LLN Traffic Patterns . . . . . . . . . . . . . 73
       A.1.3.  Constraint Based Routing . . . . . . . . . . 79
     B.4.  Node Removed . . . . . 73
     A.2.  Deferred Requirements  . . . . . . . . . . . . . . . . . . 80
     B.5.  New LBR Added 74
   Appendix B.  Examples  . . . . . . . . . . . . . . . . . . . . . . 80
     B.6. 74
     B.1.  Destination Advertisement  . . . . . . . . . . . . . . . . 81
     B.7. 76
     B.2.  Example: DAG Parent Selection  . . . . . . . . . . . . . . 82
     B.8. 77
     B.3.  Example: DAG Maintenance . . . . . . . . . . . . . . . . . 83
     B.9. 78
     B.4.  Example: Greedy Parent Selection and Instability . . . . . 84
     B.10. Example: DAG Merge . . . . . . . . . . . . . . . . . . . . 86 79
   Appendix C.  Additional Examples . . . . . . . . . . . . . . . . . 88
   Appendix D.  Outstanding Issues  . . . . . . . . . . . . . . . . . 92
     D.1. 81
     C.1.  Additional Support for P2P Routing . . . . . . . . . . . . 92
     D.2. 81
     C.2.  Loop Detection . . . . . . . . . . . . . . . . . . . . . . 92
     D.3. 81
     C.3.  Destination Advertisement / DAO Fan-out  . . . . . . . . . 92
     D.4. 81
     C.4.  Source Routing . . . . . . . . . . . . . . . . . . . . . . 92
     D.5. 82
     C.5.  Address / Header Compression . . . . . . . . . . . . . . . 93 82
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 93 82

1.  Introduction

   Low Power 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, most of the typically time supporting only low data rates, that are
   usually fairly 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 point-to-multipoint or multipoint-to-point. multipoint-
   to-point.  Furthermore such networks may potentially comprise a large number of nodes, up to several dozens or
   hundreds or more nodes in the network.
   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 power and Lossy
   Network (LLN) routing protocol, specified in
   [I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs],
   [I-D.ietf-roll-indus-routing-reqs], [RFC5673], and [RFC5548].  This
   document specifies the IPv6 Routing Protocol for Low Power 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],
   [I-D.ietf-roll-indus-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 instantiation implementation of the protocol to be optimized in terms of of,
   e.g., minimizing required code space. 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
   "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 Behavior 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) link layer 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 terminology used key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document is consistent with and
   incorporates that are to be interpreted as described in `Terminology 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].  The terminology is extended
   in this document as follows:

   Autonomous:  The ability of a routing protocol to independently
         function without relying on any external influence or guidance.
         Includes self-organization capabilities.

   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 a one or more root node nodes (a DAG root,
         or sink- typically a Low Power power and Lossy Network Border Router
         (LBR)).

   DAGID:  For the purpose of this document, the term DAG is
         often used to refer to a DAG Iteration as defined below.

   DAG Identifier. Instance:  A globally unique identifier for DAG Instance is a DAG.  All
         nodes who are part set of possibly multiple
         Destination Oriented DAGs.  A network may have more than one
         DAG Instance, and a given RPL router can participate to multiple DAG have knowledge
         instances.  Each DAG Instance operates independently of the DAGID. other
         DAG Instances.  This knowledge 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 used 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 identify peer nodes 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
         order 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 coordinate form a new Iteration of a DAG.  A DAG maintenance while avoiding loops. 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.  For
         each DAGID that a node is a member of, the node will maintain a
         set containing one or more DAG parents.  If a node is a member
         of multiple DAGs then it must conceptually maintain a set of
         DAG parents for each DAGID.

   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 (depth) within a DAG.  Note that siblings defined in this
         manner do not necessarily share a common parent.  For
         each DAG that a node is a member of, the node will maintain a
         set of DAG siblings.  If a node is a member of multiple DAGs
         then it must conceptually maintain a set of DAG siblings for
         each DAG.

   DAG root:  A DAG root is a sink node within the DAG.  All paths in the DAG
         terminate that has no outgoing
         edges.  Because the graph is acyclic, by definition all DAGs
         must have at a least one DAG root, root and all DAG edges contained in the paths terminating terminate at a
         DAG root are oriented toward the DAG root.  There must be at least one DAG root per DAG, and

   Sub-DAG  The sub-DAG of a node is the set of other nodes in some
         cases there may be more than one.  In many the DAG
         that might use cases, source-
         sink represents a dominant traffic flow, where path towards the sink is a DAG root or is located behind that contains the DAG root.  Maintaining routes
         towards DAG roots is therefore
         node.  Nodes in the sub-DAG of a prominent functionality for
         RPL. 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.

   P2P:  Point-to-point.  This refers to traffic exchanged between two
         nodes.

   P2MP: Point-to-Multipoint.  This refers

   OCP:  Objective Code Point.  The Objective Code Point is used to traffic between one node
         and
         indicate which Objective Function is in use in a set of nodes.  This DAG.  The
         Objective Code Point is similar to the P2MP concept further described in
         Multicast or MPLS Traffic Engineering ([RFC4461]
         [I-D.ietf-roll-routing-metrics].

   OF:   Objective Function.  The Objective Function (OF) defines which
         routing metrics, optimization objectives, and
         [RFC4875]).  A common RPL use case involves P2MP flows from or
         through a DAG root outward towards other nodes contained in the
         DAG.

   MP2P: Multipoint-to-Point; used to describe a particular traffic
         pattern.  A common RPL use case involves MP2P flows collecting
         information from many nodes in the DAG, flowing inwards towards
         DAG roots.  Note that a DAG root may not be the ultimate
         destination of the information, but it is a common transit
         node.

   OCP:  Objective Code Point.  In RPL, the Objective Code Point (OCP)
         indicates which routing metrics, optimization objectives, and
         related functions are in related functions
         are in use in a DAG.  Instances of the  The Objective Code Point are 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.  Protocol Properties  Overview

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

3.1.1.  IPv6 Architecture

   RPL is strictly compliant with layered IPv6 architecture.

   Further, RPL is designed with consideration to the practical support  Topology Instance and implementation Objectives

   A topology instance 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 exists over lossy
   links (usually low bandwidth with low packet delivery success rate).

3.1.2.  Typical LLN Traffic Patterns

   Multipoint-to-Point (MP2P) and Point-to-multipoint (P2MP) traffic
   flows from nodes within the scope of an 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 support
   of such flows, although such flows
   are not exclusively rooted at LBRs a particular application, or service, and is optimized according
   to a certain objective, as determined on by an application-
   specific basis.  In particular, several applications such Objective Function (OF),
   and may be characterized by certain destination prefixes as building well.  A
   topology instance, or home automation do require P2P (Point-to-Point) communication.

   As required by the aforementioned routing requirements documents, RPL
   supports the installation of 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 paths.  The 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
   paths include sending duplicated traffic along diverse paths, as well
   as to support advanced features such DAGs as Class of Service (CoS) based
   routing, or simple load balancing among a set of paths (which could
   be useful for the LLN means to spread traffic load dynamically and avoid fast energy
   depletion on some, e.g. battery powered, nodes).

3.1.3.  Constraint Based Routing

   The RPL design supports constraint based routing, based on
         autonomously partition the network.

   o  a set of
   routing metrics.  The routing metrics for links and nodes single Destination Oriented DAG with
   capabilities supported by RPL are specified multiple DAG roots
      coordinating over some backbone

      *  For example, multiple border routers operating with a reliable
         backbone, e.g. in support of a companion document 6LowPAN application, that are
         capable to act as logically equivalent sinks to this specification, [I-D.ietf-roll-routing-metrics].  RPL signals the metrics and related objective functions in use in same DAG.

   o  a particular
   implementation by means combination of an Objective Code Point (OCP).  Both the
   routing metrics and the OCP help determine the construction one of the
   Directed Acyclic Graphs (DAG) using a distributed path computation
   algorithm.

   RPL supports above as suited to some application
      scenario

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

   Traffic is bound to a set of application and implementation specific constraints, as guided DAG Instance by an OCP.  Traffic may subsequently
   be directed along the appropriate constrained path based on traffic a marking within in the flow
   label of the IPv6 header.  For more details on the approach
   towards constraint-based routing, see Section 4.

3.2.  Protocol Operation

   A LLN deployment will consist  Traffic originating in support of a number
   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 nodes and a number of
   edges (links) mapping
   between them, whose characteristics will depend on
   implementation an InstanceID and link layer (L2) specifics.  Due to the nature a type or service of application traffic is
   beyond the LLN environment the L2 links are expected scope of this specification.

   Conceptually a node running RPL may capable to demonstrate maintain a large
   degree membership
   in multiple DAG Instances in support of variance as to their availability, quality, different application
   services and/or optimization objectives.  For example, one instance
   may optimize for minimizing latency and other
   related parameters.  Certain links, demonstrating a viability above a
   confidence threshold separate orthogonal
   instance may optimize for particular node minimizing energy.  This scenario does
   introduce some additional considerations, for example loop avoidance
   and link metrics, as based
   on guidelines from [I-D.ietf-roll-routing-metrics], will be extracted
   from default routing behavior.  These considerations are beyond the L2 graph,
   scope of this specification and the resulting graph will are intended to be used as the basis elaborated on which to operate the routing protocol.  Note that as the
   characteristics of the L2 topology vary over time the set in a
   future revision of viable
   links is to be updated and the routing protocol thus continues to
   evaluate the LLN.  In RPL this process happens in or a distributed
   manner, and from companion specification.  As such, this
   specification will deal exclusively with the perspective of scenario where a single node running
   implements RPL this
   process results in a set support of candidate neighbors, with associated node a single DAG Instance.

3.1.2.  Multipoint-to-Point Traffic Flows and link metrics as well as confidence values. 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],
   [I-D.ietf-roll-indus-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 top to refer to the public
   Internet or a core private (non-LLN) IP network.  In support of this dominant flow
   RPL constructs Directed Acyclic Graphs (DAGs) on top of the viable
   LLN topology, selecting and orienting links among candidate neighbors
   toward DAG roots which root the MP2P flows.

   LLN nodes running RPL will construct Directed Acyclic Graphs (DAGs)
   rooted at DAG roots, which may be naturally designated nodes that generally have some application
   significance, such as providing connectivity to an external routed
   infrastructure.  The term "external" is used top refer according to the public
   Internet or a core private (non-LLN) IP network.
   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, as will be
   discussed further. 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.

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 who 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
   specification
   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

   RPL constructs one or more DAGs, over gradients defined by optimizing
   cost metrics along paths rooted at designated nodes.

   The DAG construction algorithm is distributed; each node running RPL
   invokes a set of DAG construction rules and objective functions when
   considering its role with respect to neighboring nodes such that the
   DAG structure emerges.

3.2.1.1.  IP Router Advertisement -  DAG Information Option (RA-DIO)

   The IPv6 Router Advertisement (RA) mechanism (as specified in
   [RFC4861]) Object (DIO)

   A DAG Information Object is defined and used by RPL in order to build
   and maintain a DAG.

   The IPv6 RA message is augmented with a DAG Information Option (DIO),
   forming  This document defines an RA-DIO message, ICMPv6 Message Type RPL
   Control Message, which is capable to convey carry the DIO.  The DIO message
   conveys information about the DAG 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.  This is not a metric, although its
      derivation is typically closely related to one or more metrics as
      specified by the OCP.  The rank information is used to support
      loop avoidance strategies and in support of ordering alternate
      successors when engaged in path maintenance.

   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 vector set of path metrics, 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 RA 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
   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 RA DIO messages,
   along with the triggered RA DIO messages in response to inconsistency,
   is one feature that enables RPL to operate in the presence of
   unreliable links.

3.2.1.2.  DAG Identification

   Each DAG is identified by a particular identifier (DAGID) as well as
   its supported optimization objectives  Grounded and available destination
   prefixes.  The optimization objectives are conveyed as Floating DAGs

   Certain LLN nodes may offer connectivity to an Objective
   Code Point (OCP) as described further below.  Available destination
   prefixes, which external routed
   infrastructure in support of an application scenario.  These nodes
   are designated `grounded', and may include destinations available beyond serve as the DAG
   root, multicast destinations, or IPv6 node addresses, roots of a
   grounded DAG.  DAGs that do not have a grounded DAG root are advertised
   outwards along floating
   DAGs.  In either case routes may be provisioned toward the DAG and recipient nodes may then provision routing
   tables with entries inwards towards root,
   although in the destinations.  The RPL
   implementation at each node 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 provisioned by the application associated with sufficient information to determine which objectives and
   destinations are required, each DAG root,
   and thus thereby each DAG, in order that some DAGs in the RPL implementation may
   determine which DAG to join.

   The decision for a node to join a DAG LLN may be optimized according to
   implementation specific policy functions on the node as indicated by
   one or more specific OCP values.
   preferred over other DAGs.  For example, a node DAG root that is sinking
   traffic in support of a data collection application may be configured for one goal
   by the application to optimize a bandwidth metric (OCP-1), and
   with be very preferred.  A transient DAG, e.g. a parallel goal to optimize for DAG
   that is only existing until a reliability metric (OCP-2).
   Thus two DAGs, with two unique DAGIDs, permanent DAG is found, may be constructed and
   maintained in the LLN: DAG-1 would be optimized according
   configured to OCP-1,
   whereas DAG-2 would be optimized according less preferred.  The administrative preference
   offers a way to OCP-2.  A node may then
   maintain independent sets engineer the formation of DAG parents and related data structures
   for each DAG.  Note that in such a case traffic may directed along the appropriate constrained DAG based on traffic marking within the
   IPv6 header.  This specification will focus on the case where in support of the
   node only joins one DAG; further elaboration on
   application.

3.2.1.4.  Objective Function (OF)

   The Objective Function (OF) conveys and controls the proper operation
   of RPL optimization
   objectives in use within the presence of multiple DAGs, including traffic marking DAG.  The Objective Function is
   indicated by an Objective Code Point (OCP), and related rules, are to be specified is further specified
   in future revisions [I-D.ietf-roll-routing-metrics].  Each instance of
   this or companion specifications.

3.2.1.3.  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.

3.2.1.4.  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 in support of DAG repair 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.

3.2.1.5.  Objective Code Point (OCP)

   The OCP serves to convey and control the optimization objectives in
   use within the DAG.  The OCP is further specified in
   [I-D.ietf-roll-routing-metrics].  Each instance of an allocated OCP
   indicates:

   o  The set allocated OF
   indicates:

   o  The set of metrics used within the DAG

   o  The objective functions method used for least cost path determination.

   o  The function method used to compute DAG Rank

   o  The functions methods used to accumulate prepare metrics for propagation within a
      RA-DIO DIO
      message

   For example, an objective code point might indicate that the DAG is
   using the Expected Number of Transmissions (ETX) as a metric, that
   the optimization goal is to minimize ETX, that DAG Rank is equivalent
   to ETX, and that RA-DIO propagation entails adding the advertised ETX
   of the most preferred parent to the ETX of the link to the most
   preferred parent.

   By using defined OCPs that are understood by all nodes in a
   particular implementation, and by conveying them in the RA-DIO DIO message,
   RPL nodes may work to build optimized LLN using a variety of
   application and implementation specific metrics and goals.

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

3.2.1.6. 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 based on guidelines in
      [I-D.ietf-roll-routing-metrics] determined by the implementation.  This
      data structure will also track DAG information as learned from and
      associated with each neighbor.

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

   o  Nodes who that receive RA-DIO DIO messages will take into consideration
      several criteria when processing the extracted DAG information.
      The node may discount either discard the RA-DIO according to loop avoidance rules DIO based
      on rank as described further below.  Nodes will consider the
      information in the RA-DIO in order to determine whether several criteria, including rank-based loop avoidance rules, or not
      that candidate neighbor offers a better attachment point
      process the DIO to maintain a position in an existing DAG
      (which the node may or may not be
      improve a member of) position as according to the
      implementation specific optimization goals, OCP, an Objective Function (OF) and
      current
      metrics. path cost.

   o  Nodes may join a new DAG or move within the current DAG, in
      response to the information contained in the RA-DIO message, and
      in accordance with loop avoidance rules described further in this
      specification.  For the successors within the DAG, a node manages manage a set of DAG Parents.  Joining, moving within, and leaving the DAG
      is accomplished through managing this set Parents according to the rules specified
      by RPL.

   o  As nodes join, move within, and leave DAGs they emit updated RA-
      DIOs which are received and acted on by neighboring nodes.  When
      inconsistencies (such as caused by movement or link loss) are
      detected within the  This set is also augmented to include DAG structure, RA-DIO siblings.

   o  DIO messages are may be emitted more frequently.  When the or less frequently as a function
      of DAG structure becomes consistent, RA-
      DIO messages taper off. consistency.

   o  As less preferred DAGs encounter more preferred DAGs that offer
      equivalent or better optimization objectives, 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.

   o  As the DAG construction operation proceeds, nodes accumulate onto
      the DAG in progressively outward tiers, centered around the DAG
      root.

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

3.2.1.7.  DAG Rank

   When

3.2.2.  Destination Advertisement

   As RPL constructs DAGs, nodes select DAG may provision routes toward
   destinations advertised through DIO messages through their selected
   parents, they will select the most preferred
   parent according and are thus able to their implementation specific objectives, using
   the cost metrics conveyed in the RA-DIO messages send traffic inward along the DAG in
   conjunction with the related objective functions as specified by the
   OCP.

   Based on
   forwarding to their selected parents.  However, this selection, the metrics conveyed by mechanism alone
   is not sufficient to support P2MP traffic flowing outward along the most preferred
   DAG parent, from the nodes own metrics and configuration, and a related
   function defined DAG root toward nodes.  A destination advertisement
   mechanism is employed by the OCP, a node will be able RPL to compute a value
   for its rank as a consequence build up routing state in support of selecting a most preferred DAG
   parent.
   these outward flows.  The rank value feeds back into the DAG parent selection according destination advertisement mechanism may not
   be supported in all implementations, as appropriate to
   a loop-avoidance strategy.  Once a DAG parent has been added, and a
   rank value for the node within the
   application requirements.  A DAG has been computed, root that supports using the nodes
   further options with regard
   destination advertisement mechanism to DAG parent selection and movement
   within build up routing state will
   indicate such in the DIO message.  A DAG are restricted in favor root that supports using the
   destination advertisement mechanism must be capable of loop avoidance.

   It is important allocating
   enough state to note that store the DAG Rank is not itself a metric,
   although its value is derived routing state received from the LLN.

3.2.2.1.  Destination Advertisement Object (DAO)

   A Destination Advertisement Object is defined and influenced used by the use of
   metrics RPL in
   order to select convey the destination information inward along the DAG parents and take up a position
   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 DAG.  In
   other words, routing metrics DAO message includes the following:

   o  A lifetime and OCP (not rank directly) are used sequence counter to determine the DAG structure and consequently freshness of the path cost.  The only
   aim
      destination advertisement.

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

   o  Prefix information to inform loop avoidance as explained hereafter.
   The computation of identify the DAG Rank MUST destination, which may be done in such a way so as
      prefix, an individual host, or multicast listeners

   o  Reverse Route information to
   maintain record the following properties for any nodes M 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 N who maintained, nodes are
   neighbors in the LLN:

      For capable to emit
   DAO messages to a subset of their DAG parents.

3.2.2.2.1.  `One-Hop' Neighbors

   As a special case, a node N, and may periodically emit a link-local
   multicast IPv6 DAO message advertising its most preferred parent M, DAGRank(N) >
      DAGRank(M) must hold.  Further, all parents in locally available
   destination prefixes.  This mechanism allows for the DAG parent set
      must be one-hop
   neighbors of a 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 to learn explicitly of its parents.

      If DAGRank(M) < DAGRank(N), then M the prefixes on the node,
   and in some application specific scenarios this is probably located desirable in
   support of provisioning a more
      preferred position than N in trivial `one-hop' route.  In this case,
   nodes that receive the DAG with respect multicast destination advertisement may use it
   to provision the metrics one-hop route only, and optimizations defined by the objective code point.  In not engage in any
      fashion, Node M may safely be additional
   processing (so as not to engage the mechanisms used by a DAG parent for Node N without risk parent).

3.2.2.2.2.  Operation in Support of creating Stateful Nodes

   When a loop.

         For example, (unicast) DAO message reaches a Node M node capable of rank 3 is likely located in a more
         optimum position than storing
   routing state, the node extracts information from the DAO message and
   updates its local database with a Node N record of rank 5.  A packet directed
         inwards the DAO message and forwarded from Node N to Node M will always make
         forward progress with respect to the DAG organization on
   neighbor that
         link; there is no risk of Node M at rank 3 forwarding it was received from.  When the
         packet back into Node N's sub-DAG at rank of 5 or greater
         (which would be a sufficient condition node later propagates
   DAO messages, it selects the best (least depth) information for a loop to occur).

      If DAGRank(M) == DAGRank(N), then M each
   destination and N are located positions of
      relatively the same optimality within conveys this information again in the DAG.  In some cases,
      Node M may be used as a successor by Node N, but with related
      chance form of creating DAO
   messages to a loop that must be detected and broken by some
      other means.

         If Node M is at rank 3 and subset of its own DAG parents.  At this time the node N
   may perform route aggregation if it is at rank 3, then they are
         siblings; by definition Node M and N cannot be able, thus reducing the
   overall number of DAO messages.

3.2.2.2.3.  Operation in each others
         sub-DAG.  They may then forward 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 each other failing
         serviceable parents, making `sideways' progress (but not
         reverse progress).  If another sibling or more gets involved
         there may a Reverse Route Stack
   carried within the DAO message.  The node then be some chance for 3 passes the DAO message
   on to one or more way loops, which is
         the risk of sibling forwarding.

      If DAGRank(M) > DAGRank(N), then its DAG parents without storing any additional
   state.

   When a node M that is located in capable of storing routing state encounters a less
      preferred position than N in the DAG
   (unicast) DAO message with respect 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 metrics Reverse Route State and optimizations defined associate it with the
   destination described by the objective code point.  Further,
      Node (M) DAO message.  Subsequently the node may in fact be in Node (N)'s sub-DAG.  There is no
      advantage
   use this information to Node (N) selecting Node (M) as a DAG parent, and such
      a selection may create construct a loop.

         For example, if Node M is source route in order to bridge
   the region of rank 3 and Node N is nodes that are unable to support Hop-By-Hop routing to
   reach the destination.

3.2.2.2.4.  Additional Considerations

   Further aggregations of rank 5,
         then DAO messages prefix reachability information
   by definition Node N is destinations are possible in order to support additional
   scalability.

   A special case of an DAO message, termed a less optimum position than
         Node N. Further, Node N at rank 5 `no-DAO', may in fact be used to
   tear down the routing state that has been established by the
   destination advertisement mechanism in Node M's
         own sub-DAG, and forwarding a packet directed inwards towards case of, e.g., unreachability
   or other events that affect the DAG root from M to N will result outward routing state.

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

3.3.  Loop Avoidance and
         possibly Stability

   The goal of a loop.

   As guaranteed consistent and loop free global routing
   solution for an example, the DAG Rank could LLN may not be computed in such practically achieved given the real
   behavior and volatility of the underlying metrics.  The trade offs to
   achieve a way so as stable approximation of global convergence may be too
   restrictive with respect to
   closely track ETX when the objective function is to minimize ETX, or
   latency when need of the objective function is LLN to minimize latency, or react quickly in a
   more complicated way as appropriate
   response to the objective code point being
   used within lossy environment.  Globally the DAG.

   The DAG rank is subsequently used LLN may be able to restrict the options
   achieve a node has
   for movement within the DAG and weak convergence, in particular as link changes are able to coordinate movements
   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
   avoid reduce control overhead, in
   particular the creation of loops.

3.2.1.8.  Sub-DAG

   The sub-DAG expense of a node is mechanisms such as count-to-infinity, RPL
   does try to avoid the set of other nodes creation of greater rank in loops when undergoing topology
   changes.

   RPL includes rank-based mechanisms for detecting loops to ensure that
   packets make forward progress within the DAG, DAG and thus might use a path towards the trigger DAG root that contains
   this node.  This is an important property that is leveraged for loop
   avoidance- repair
   if necessary.

3.3.1.  Greediness and Rank-based Instabilities

   Once a node has lesser rank then it is not joined a DAG, RPL disallows certain behaviors,
   including greediness, in order to prevent resulting instabilities in
   the sub-DAG.
   (An arbitrary DAG.

   If a node with greater rank may or may not is allowed to be contained greedy and attempts to move deeper in the sub-DAG).  Paths through siblings are not contained
   DAG, beyond its most preferred parent, in this set.

   As a further illustration, consider order to increase the DAG examples in Appendix B.
   Consider Node (24) in size
   of the DAG Example depicted parent set, then an instability can result.  This is
   illustrated in Figure 9.  In this
   example, the sub-DAG of Node (24) 14.

   Suppose a node is comprised of Nodes (34), (44), willing to receive and (45).

   A frozen sub-DAG is process a subset of nodes DIO messages from
   a node in the sub-DAG of its own sub-DAG, and in general a node who
   have been informed of deeper than it.  In
   such cases a change chance exists to the node, and choose create a feedback loop, wherein two or
   more nodes continue to follow the
   node try and move in a manner consistent with the change, for example DAG in
   preparation for a coordinated move.  Nodes order to optimize
   against each other.  In some cases this will result in the sub-DAG who hear of an
   instability.  It is for this reason that RPL mandates that a change and have other options than to follow the node do not have
   to become part of the frozen sub-DAG, for example such
   never receive and process DIO messages from deeper nodes.  This rule
   creates an `event horizon', whereby a node may cannot be
   able to remain attached to influenced into
   an instability by the original DAG through a different DAG
   parent. 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.8.

3.2.1.9.  Moving up in a B.4

3.3.2.  DAG Loops

   A node DAG loop may safely move `up' in occur when a node detaches from the DAG, causing its DAG rank to
   decrease and moving closer reattaches
   to a device in its prior sub-DAG.  This may happen in particular when
   DIO messages are missed.  Strict use of the DAG root without risking the
   formation sequence number can
   eliminate this type of a loop.

3.2.1.10.  Moving down in a DAG

3.3.3.  DAO Loops

   A node DAO loop may not consider to move `down' occur when the DAG, causing its DAG rank
   to increase and moving further from the DAG root.  In the case where
   a node looses connectivity to the DAG, it must first leave the DAG
   before it may then rejoin at parent has a deeper point.  This allows for the
   node to coordinate moving down, freezing its own sub-DAG and
   poisoning stale routes to the DAG, route installed upon
   receiving and minimizing processing a DAO message from a child, but the chances of re-
   attaching to its own sub-DAG thinking that it child
   has found subsequently cleaned up the original
   DAG again.  If state.  This loop happens when a node where allowed to re-attach into its own sub-DAG no-
   DAO was missed till a heartbeat cleans up all states.  RPL includes
   loop would most certainly occur, and detection mechanisms that may not be broken until a
   count-to-infinity process elapses.  The procedure mitigate the impact of detaching before
   moving down eliminates DAO loops
   and trigger their repair.

   In the need to count-to-infinity.

3.2.1.11.  DAG Jumps

   A jump from one DAG to another DAG case where stateless DAO operation is attaching to 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 new DAGID, in group of siblings kept choosing
   amongst themselves as successors such that a way packet does not make
   forward progress.  This specification limits the number of times that an old DAGID is replaced
   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 new DAGID.  In
   particular, when an old DAGID is left, all associated parents are no
   longer feasible, shortest
   paths.  Interior Gateway Protocols (IGPs) such as IS-IS ([RFC5120])
   and a new DAGID is joined.

   When a node in a DAG follows a DAG parent, it means that the DAG
   parent has changed its DAGID (e.g. by joining a new DAG) and that the
   node updates its own DAGID in order to keep OSPF ([RFC4915]) use static link metrics.  Such link metrics can
   simply reflect the DAG parent.

3.2.1.12.  Floating DAGs for DAG Repair

   A DAG may bandwidth or can also be floating.  Floating DAGs may be encountered, for
   example, during coordinated reconfigurations of the network topology
   wherein a node and its sub-DAG breaks off the DAG, temporarily
   becomes a floating DAG, and reattaches computed according to a grounded DAG.  (Such
   coordination endeavors to avoid the construction
   polynomial function of transient loops several metrics defining different link
   characteristics; in all cases they are static metrics.  Some routing
   protocols support more than one metric: in the LLN).

   A DAG, or a sub-DAG temporarily promoted to vast majority of the
   cases, one metric is used per (sub)topology.  Less often, a DAG, second
   metric may also become
   floating because of be used as a network element failure.  If tie-breaker in the DAG parent set presence of the node becomes completely depleted, the node will have detached
   from the DAG, and may, if so configured, become the root Equal Cost
   Multiple Paths (ECMP).  The optimization of its own
   transient floating DAG with a less desirable administrative
   preference (thus beginning multiple metrics is known
   as an NP complete problem and is sometimes supported by some
   centralized path computation engine.

   In contrast, LLNs do require the process support of establishing the frozen
   sub-DAG), both static and then may reattach to dynamic
   metrics.  Furthermore, both link and node metrics are required.  In
   the original DAG at a lower point
   if case of RPL, it is able (after hearing RA-DIO messages from alternate
   attachment points).

3.2.2.  Destination Advertisement

   As RPL constructs DAGs, nodes may provision routes toward
   destinations advertised through RA-DIO messages through their
   selected parents, and are thus able virtually impossible to send traffic inward along the
   DAG by forwarding 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 their selected parents.  However, this mechanism
   alone is link and nodes.  If a link or a node does not sufficient to support P2MP traffic flowing outward along
   the DAG
   satisfy a required constraint, it is `pruned' from the DAG root toward nodes.  A destination advertisement
   mechanism is employed by RPL candidate list
   thus leading to build up routing state in support of
   these outward flows. a constrained shortest path.

   The destination advertisement mechanism may not
   be set of supported link/node constraints and metrics is specified
   in all implementations, as appropriate to the
   application requirements.  A DAG root that supports using [I-D.ietf-roll-routing-metrics].

   The role of the
   destination advertisement mechanism Objective Function is to build up advertise routing state will
   indicate such metrics
   and constraints in the RA-DIO message.  A DAG root that supports using
   the destination advertisement mechanism must be capable of allocating
   enough state addition to store the routing state received from the LLN.

3.2.2.1.  IPv6 Neighbor Advertisement - Destination Advertisement Option
          (NA-DAO)

   An IPv6 Neighbor Advertisement Message with Destination Advertisement
   Options (NA-DAO) is objectives used to convey the destination information inward
   along the DAG toward compute the DAG root.

   The information conveyed in
   (constrained) shortest path.

   Example 1: Shortest path: path offering the NA-DAO message includes shortest end-to-end delay

   Example 2: Constrained shortest path: the
   following:

   o  A lifetime path that does traverse any
              battery-operated node and sequence counter to determine the freshness of that optimizes the
      destination advertisement.

   o  Depth information used by nodes to determine how far away path
              reliability

5.  RPL Protocol Specification

5.1.  RPL Messages

5.1.1.  ICMPv6 RPL Control Message

   This document defines the
      destination (the source of RPL Control Message, a new ICMPv6 message.
   The RPL Control Message has the destination advertisement) 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

   The RPL Control message is

   o  Prefix an ICMPv6 information message with a
   requested Type of 155.

   The Code will be used to identify the destination, which may be a
      prefix, an individual host, or multicast listeners RPL Control Messages as follows:

   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.  0x01: DAG Information Solicitation (Section 5.1.2)

   o  0x02: DAG Information Object (Section 5.1.3)

   o  0x04: Destination Advertisement Operation

   As the Object (Section 5.1.4)

5.1.2.  DAG is constructed and maintained, nodes are capable to emit
   NA-DAO messages Information Solicitation (DIS)

   The DAG Information Solicitation (DIS) message may be used to solicit
   a subset, or all, of their DAG parents.  The
   selection of this subset Information Object from a RPL node.  Its use is according analogous to an implementation specific
   policy.

   As
   that of a special case, Router Solicitation; a node may periodically emit a link-local
   multicast IPv6 NA-DAO message advertising use DIS to probe its locally available
   destination prefixes.  This mechanism allows
   neighborhood 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 who 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 nearby DAGs.  The DAG parent).

   When a (unicast) NA-DAO Information Solicitation
   carries no additional message reaches body.

5.1.3.  DAG Information Object (DIO)

   The DAG Information Object carries a node capable number of storing
   routing state, the node extracts information from the NA-DAO message metrics and updates its local database with other
   information that allows a record of the NA-DAO message
   and who it was received from.  When the node later propagates NA-DAO
   messages, it selects the best (least depth) information for each
   destination and conveys this information again in the form of NA-DAO
   messages to discover a subset of DAG, select its own DAG parents.  At this time the node
   may perform route aggregation if it
   parents, and identify its siblings while employing loop avoidance
   strategies.

5.1.3.1.  DIO Base Option

   The DIO Base Option is able, thus reducing the
   overall number of NA-DAO messages.

   When a (unicast) NA-DAO message reaches a node incapable of storing
   additional state, the node must append the next-hop address (from container option, which neighbor the NA-DAO message was received) to is always present,
   and might contain a Reverse Route
   Stack carried within the NA-DAO message. number of suboptions.  The node then passes base option regroups
   the
   NA-DAO message on to one or more of its DAG parents without storing
   any additional state.

   When a node minimum information set that is capable of storing routing state encounters a
   (unicast) NA-DAO message with a Reverse Route Stack that has been
   populated, the node knows that the NA-DAO message has traversed a
   region of nodes that did not record any routing state. mandatory in all cases.

        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 node DAG Control Field is
   able to detach and store the Reverse Route State and associate it
   with currently allocated as
         follows:

         Grounded (G):  The Grounded (G) flag is set when the destination described by DAG root
               is offering connectivity to an external routed
               infrastructure such as the NA-DAO message.  Subsequently Internet.

         Destination Advertisement Trigger (D):  The Destination
               Advertisement Trigger (D) flag is set when the DAG root
               or another 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 successor chain decides to reach the destination.

   In this way trigger
               the sending of destination advertisement mechanism is able advertisements in order to
   provision
               update routing state in support of P2MP traffic flows for the outward direction along the
               DAG, and as according to the available resources further detailed in Section 5.10.  Note that the
   network.

   Further aggregations
               use and semantics of NA-DAO messages prefix reachability
   information by destinations this flag are possible in order to support
   additional scalability.

   A further example of 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 is available in Appendix B.6

3.3.  Loop Avoidance within the DAG.

         DAGPreference (Prf):  3-bit unsigned integer set by the DAG
               root to its preference and Stability unchanged at propagation.
               DAGPreference ranges from 0x00 (least preferred) to 0x07
               (most preferred).  The goal of a guaranteed consistent and loop free global routing
   solution for default is 0 (least preferred).
               The DAG preference provides an LLN may not be practically achieved given administrative mechanism
               to engineer the real
   behavior and volatility self-organization of the underlying metrics.  The trade offs LLN, for example
               indicating the most preferred LBR.  If a node has the
               option to
   achieve join a stable approximation of global convergence may be too
   restrictive with respect more preferred DAG while still meeting
               other optimization objectives, then the node will
               generally seek to join the need more preferred DAG as
               determined by the OF.

         Unassigned bits of the LLN Control Field are considered as
         reserved.  They MUST be set to react quickly in
   response 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 lossy environment.  Globally DAG root,
         and propagated with no change outwards along the LLN may be able to
   achieve DAG.  Each
         increment SHOULD have a weak convergence, in particular as link changes are able to
   be handled locally value of 1 and result in minimal changes to global topology.

   RPL does not aim may cause a wrap back to guarantee loop free path selection, or strong
   global convergence.  In order to reduce control overhead, in
   particular
         zero.

   InstanceID:  8-bit field indicating the expense of mechanisms such topology instance associated
         with the DAG, as count-to-infinity, RPL
   does try to avoid provisioned at the creation of loops when undergoing topology
   changes.  Further mechanisms to mitigate DAG root.

   DAGRank:  8-bit unsigned integer indicating the impact DAG rank of loops, such as
   loop detection when forwarding, are under investigation.

3.3.1.  Greediness and Rank-based Instabilities

   If a node is greedy and attempts to move deeper in the DAG, beyond
   its most preferred parent, in order to increase node
         sending the size DIO message.  The DAGRank of the DAG
   parent set, then an instability can result.  This root is illustrated in
   Figure 11.

   Suppose a node
         ROOT_RANK.  DAGRank is willing to receive and process a RA-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 further described in an
   instability.  It is for this reason that RPL mandates that Section 5.4.

   DAGID:  128-bit unsigned integer which uniquely identify a node
   never receive and process RA-DIO messages from deeper nodes. DAG.  This
   rule creates an `event horizon', whereby a node cannot be influenced
   into an instability
         value is set by the action DAG root.  The global IPv6 address of nodes that may the
         DAG root can be in its own
   sub-DAG.

   A further example of used, however. the consequences of greedy operation, and
   instability related to processing RA-DIO messages from nodes of
   greater rank, may DAGID MUST be found in Appendix B.9

3.3.2.  Merging DAGs

   The merging of DAGs is coordinated in a way such as to try and merge
   two DAGs cleanly, preserving as much unique per DAG structure as possible, and
   in
         within the process effecting a clean merge with minimal likelihood scope of
   forming transient DAG loops.  The coordinated merge is also intended
   to minimize the related control cost.

   When a node, and perhaps a related frozen sub-DAG, jumps to a
   different DAG, LLN.  In the move is coordinated by case where a set of timers (DAG Hop
   timers).  The DAG Hop timers allow the nodes who will attach closer
   to the sink of root is
         rooting multiple DAGs the new DAGID MUST be unique for each DAG to `jump' first, and then drag dependent
   nodes behind them, thus endeavoring to efficiently coordinate
         rooted at a specific DAG root.

   The following values MUST NOT change during the
   attachment propagation of DIO
   messages outwards along the frozen sub-DAG into the new DAG.

   A further example DAG:
      Grounded (G)
      Destination Advertisement Supported (A)
      DAGPreference (Prf)
      Sequence
      InstanceID
      DAGID
   All other fields of a DAG Merge operation the DIO message may be found in
   Appendix B.10

3.3.3.  DAG Loops

   A DAG loop may occur when a node detaches from updated at each hop of the DAG and reattaches
   propagation.

5.1.3.1.1.  DIO Suboptions

   In addition to a device in its prior sub-DAG that has missed the whole detachment
   sequence and kept advertising the original DAG.  This may happen minimum options presented in
   particular when RA-DIO messages are missed.  Use of the DAG sequence
   number can eliminate this type of loop.  If the DAG sequence number
   is not in use, base option,
   several suboptions are defined for the protection is limited (it depends on propagation
   of RA-DIO messages during DAG hop timer), and temporary loops might
   occur.  RPL will move to eliminate such a loop as soon as DIO message:

5.1.3.1.1.1.  Format
        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 RA-DIO DIO message is received from containing a parent that appears to be going down, as suboption for which the child has to detach from it immediately.  (The alternate choice
   of staying attached and following
         Suboption Type value is not recognized by the parent in its fall would have
   counted to infinity and led receiver, the
         receiver MUST silently ignore the unrecognized option, continue
         to detach as well).

   Consider node (24) process the following suboption, correctly handling any
         remaining options in the DAG Example depicted message.

   Suboption Length:  16-bit unsigned integer, representing the length
         in Figure 9, and its
   sub-DAG nodes (34), (44), and (45).  An example octets of a DAG loop would
   be if node (24) were to detach from the DAG rooted at (LBR), and
   nodes (34) suboption, not including the suboption Type
         and (45) were Length fields.

   Suboption Data:  A variable length field that contains data specific
         to miss the detachment sequence.
   Subsequently, if option.

   The following subsections specify the link (24)--(45) were to become viable and node
   (24) heard node (45) advertising DIO message suboptions which
   are currently defined for use in the DAG rooted at (LBR), a DAG loop
   (45->34->24->45) may form if node (24) attaches to node (45).

3.3.4.  DAO Loops

   A DAO loop may occur when the parent has a route installed upon
   receiving and processing a NA-DAO Information Object.

   Implementations MUST silently ignore any DIO 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.  The DAO loop
   is suboptions
   options that they do not explicitly handled by understand.

   DIO message suboptions may have alignment requirements.  Following
   the current specification.  Split
   horizon, not forwarding convention in IPv6, these options are aligned in a packet back to such
   that multi-octet values within the node it came from, may
   mitigate 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 DAO loop in some cases, but start of the header, for
   n = 1, 2, 4, or 8).

5.1.3.1.1.2.  Pad1

   The Pad1 suboption does not eliminate it.

   Consider node (24) in the DAG Example depicted in have any alignment requirements.  Its
   format is as follows:

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

                              Figure 9.  Suppose
   node (24) has received a DA from node (34) advertising a destination
   at node (45).  Subsequently, if node (34) tears down 4: Pad 1

   NOTE! the routing
   state for format of the destination and node (24) did not hear Pad1 option is a no-DAO message special case - it has
   neither Option Length nor Option Data fields.

   The Pad1 option is used to clean up insert one or two octets of padding in the routing state, a DAO loop may exist. node (24) will
   forward traffic destined for node (45)
   DIO message to node (34), who may then
   naively return it into a loop (if split horizon is not in place).  A enable suboptions alignment.  If more complicated DAO loop may result if node (34) instead passes the
   traffic to it's sibling, node (33), potentially resulting in a
   (24->34->33->23->13->24) loop.

3.3.5.  Sibling Loops

   Sibling loops occur when a group than two octets
   of siblings keep choosing amongst
   themselves as successors such that a packet 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 make forward
   progress. 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 current draft limits those loops to some degree by
   split horizon (do not send back PadN option is used to insert three or more octets of padding in
   the same sibling) and parent
   preference (always prefer parents vs. siblings).

   Consider DIO message to enable suboptions alignment.  For N (N > 2) octets
   of padding, the DAG Example depicted in Figure 9.  Suppose that Node
   (32) and (34) are reliable neighbors, and thus are siblings.  Then,
   in Option Length field contains the case where Nodes (22), (23), and (24) are transiently
   unavailable, value N-3, and with no other guiding strategy, a sibling loop may
   exist, e.g. (33->34->32->33) as the siblings keep choosing amongst
   each other in an uncoordinated manner.

3.4.  Local and Temporary Routing Decision

   Although implementation specific, it is worth noting that a node
   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
   decide be aligned as necessary to implement some local routing decision based on some
   metrics,
   support its contents.  Its format is as observed locally or reported in the RA-DIO message.  For
   example, 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 routing DAG.  The DAG Metric Container may reflect contain a set number of successors (next-hop),
   along with various aggregated metrics used to load balance the
   traffic according to some local policy.  Such decisions are local
   discrete node, link, and
   implementation specific.

   Routing stability is crucial in a LLN: in aggregate path metrics as chosen by the presence of unstable
   links,
   implementer.  The Container Length field contains the first option consists length in
   octets of removing the link from the DAG Metric Data.  The order, content, and triggering a DAG recomputation across all coding of the nodes affected
   by the removed link.  Such a naive approach could unavoidably lead to
   frequent
   DAG Metric Container data is as specified in

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

   The processing and undesirable changes propagation of the DAG, routing instability, and
   high-energy consumption.  The alternative approach adopted DAG Metric Container is
   governed by RPL
   relies on the ability to temporarily not use a link toward a
   successor marked 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 valid, with no change on the 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 structure.  If
   the link Destination Prefix

   The Destination Prefix suboption is perceived as non-usable for some period of time (locally
   configurable), this triggers a used when the DAG recomputation, through root, or
   another node located inwards along the DAG
   discovery mechanism further detailed in Section 5.3, after reporting on the link failure.  Note path to the DAG
   root, needs to indicate that this concept it offers connectivity to destination
   prefixes other than the default.  This may be extended useful in cases where
   more than one LBR is operating within the LLN and offering
   connectivity to take
   into account other link characteristics: for different administrative domains, e.g. a home network
   and a utility network.  In such cases, upon observing the sake of
   illustration, Destination
   Prefixes offered by a particular DAG, a node may MAY decide to send a fixed number join
   multiple DAGs in support of packets to a particular successor (because of limited buffering capability application.

   The Length is coded as the length of the successor) before starting to send traffic to another successor.

   According to suboption in octets,
   excluding the local policy function, it Type and Length fields.

   Prf is possible for the node
   to order the DAG parent Route Preference as in [RFC4191].  The reserved fields
   MUST be set from `most preferred' to `least
   preferred'.  By constructing such an ordered set, zero on transmission and by appending MUST be ignored on receipt.

   The Prefix Lifetime is a 32-bit unsigned integer representing the set with siblings,
   length of time in seconds (relative to the node time the packet is able to construct an ordered list sent)
   that the Destination Prefix is valid for route determination.  A
   value of preferred next hops to assist in local and temporary routing
   decisions.  The use all one bits (0xFFFFFFFF) represents infinity.  A value of the ordered list by
   all zero bits (0x00000000) indicates a forwarding engine loss of reachability.

   The Prefix Length is
   loosely constrained, and may take into account an 16-bit unsigned integer that indicates the dynamics
   number of leading bits in the
   LLN.  Further, a forwarding engine implementation may decide to
   perform load balancing functions using hash-based mechanisms to avoid
   packet re-ordering.  Note however, that specific details destination prefix.

   The Destination Prefix contains Prefix Length significant bits of a
   forwarding engine implementation are beyond the scope
   destination prefix.  The remaining bits of this
   document.

   These decisions may be local and/or temporary with the objective Destination Prefix, as
   required to
   maintain complete the DAG shape while preserving routing stability.

3.5.  Maintenance of Routing Adjacency

   In order trailing octet, are set to relieve the LLN of 0.

   In the overhead of periodic keepalives,
   RPL event that a DIO message may employ an as-needed mechanism of NS/NA in order need to verify
   routing adjacencies just prior specify connectivity to forwarding data.  Pending the
   outcome of verifying the routing adjacency,
   more than one destination, the packet may either be
   forwarded or an alternate next-hop Destination Prefix suboption may be selected.

4.  Constraint Based Routing in LLNs

   This aim of this section is to make a clear distinction between
   routing metrics and constraints and define the term constraint based
   routing
   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 document.

4.1.  Routing Metrics

   Routing metrics are used by suboption is generally static
   and unchanging within the routing protocol DAG, therefore it is not necessary to compute
   include in every DIO.  This suboption MAY be included periodically by
   the
   shortest path according to one of more defined metrics.  IGPs such as
   IS-IS ([RFC5120]) DAG Root, and OSPF ([RFC4915]) compute the shortest path
   according SHOULD be included in response to a Link State Data Base (LSDB) using link metrics
   configured by 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 network administrator.  Such metrics can represent
   DAG root and used to configure the link bandwidth (in which case trickle timer governing when DIO
   message should be sent within the metric DAG.  DIOIntervalDoublings is usually inversely
   proportional to the bandwidth), delay, etc.  Note
   number of times that in some cases the metric is a polynomial function of several metrics defining
   different link characteristics.  The resulting shortest path cost DIOIntervalMin is
   equal allowed to be doubled
   during the sum (or multiplication) of the link metrics along trickle timer operation.

   DIOIntervalMin is an 8-bit unsigned integer.  Configured on the
   path: such metrics are said DAG
   root and used to configure the trickle timer governing when DIO
   message should be additive or multiplicative metrics.

   Some routing protocols support more than one metric: in sent within the vast
   majority of DAG.  The minimum configured
   interval for the cases, one metric is used per (sub)topology.  Less
   often, a second metric may be used as a tie breaker DIO trickle timer in the presence units of ECMP (Equal Cost Multiple Paths).  The optimization ms is
   2^DIOIntervalMin.  For example, a DIOIntervalMin value of multiple
   metrics 16ms is known
   expressed as an NP complete problem and is sometimes supported
   by some centralized path computation engine.

   In the case of RPL, it 4.

5.1.4.  Destination Advertisement Object (DAO)

   The Destination Advertisement Object (DAO) is virtually impossible used to define *the*
   metric, or even a composite, that will fit it all:

   o  Some information apply when determining routes, other propagate
   destination information
      may apply only when forwarding packets inwards along provisioned routes.

   o  Some values are aggregated hop-by-hop, others are triggers from
      L2.

   o  Some properties are very stable, others vary rapidly.

   o  Some data are useful in a given scenario and useless in another.

   o  Some arguments are scalar, others statistical.

   For that reason, the DAG.  The RPL protocol core is agnostic to the logic that
   handles metrics.  A node will be configured with some external logic
   to use and prioritize certain metrics for a specific scenario.  As
   new heterogeneous devices are installed to support the evolution of a
   network, or as networks form in a totally ad-hoc fashion, it will
   happen that the
   DAO allows the nodes that are programmed with antagonistic logics and
   conflicting or orthogonal priorities end up participating in the same
   network.  It is thus recommended DAG to use consistent parent selection
   policy, as per Objective Code Points (OCP), to ensure consistent
   optimized paths.

   RPL is designed to survive and still operate, though build up routing state for nodes
   contained in a somewhat
   degraded fashion, when confronted to such heterogeneity. 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 key
   design point is Destination Advertisement Object (DAO)

   DAO Sequence:  Incremented by the node that owns the prefix for each node is solely responsible
         new DAO message for setting the
   vector of metrics that it sources in prefix.

   InstanceID:  8-bit field indicating the topology instance associated
         with the DAG, derived in part as learned from the metrics sourced from its preferred parent.  As a result, DIO.

   DAO Rank:  Set by the DAG
   is not broken if another node makes that owns the prefix and first issues the
         DAO message to its decisions in as antagonistic
   fashion, though an end-to-end path might not fully achieve any rank.

   DAO Lifetime:  32-bit unsigned integer.  The length of time in
         seconds (relative to the
   optimizations time the packet is sent) that nodes along the way expect.  The default operation
   specified in OCP 0 clarifies this point.

4.2.  Routing Constraints

   A constraint
         prefix is valid for route determination.  A value of all one
         bits (0xFFFFFFFF) represents infinity.  A value of all zero
         bits (0x00000000) indicates a link or a node characteristic loss of reachability.

   Route Tag:  32-bit unsigned integer.  The Route Tag may be used to
         give a priority to prefixes that must should be
   satisfied by the computed path (using boolean values or lower/upper
   bounds) and is by definition neither additive nor multiplicative.
   Examples of links constraints stored.  This may be
         useful in cases where intermediate nodes are "available bandwidth",
   "administrative values (e.g. link coloring)", "protected versus non-
   protected links", "link quality" whereas capable of storing
         a node constraint can be the
   level limited amount of battery power, CPU processing power, etc.

4.3.  Constraint Based Routing routing state.  The notion further specification
         of constraint based routing consists this field and its use is under investigation.

   Prefix Length:  Number of finding valid leading bits in the
   shortest path according IPv6 Prefix.

   RRCount:  8-bit unsigned integer.  This counter is used to some metrics satisfying a set count the
         number of
   constraints. entries in the Reverse Route Stack.  A technique consists value of first filtering out all links
   and nodes `0'
         indicates that cannot satisfy the constraints (resulting in no Reverse Route Stack is present.

   Prefix:  Variable-length field containing an IPv6 address or a sub-
   topology) and then computing the shortest path.

      Example 1:
         Link Metric:     Bandwidth
         Link Constraint: Blue
         Node Constraint: Mains-powered node

      Objective function 1:
         "Find prefix
         of an IPv6 address.  The Prefix Length field contains the shortest path (path with lowest cost where
         number of valid leading bits in the path
         cost is prefix.  The bits in the sum of all link costs (Bandwidth)) along
         prefix after the path
         such that all links prefix length (if any) are colored `Blue' reserved and that only traverses
         Mains-powered nodes."

      Example 2:
         Link Metric:     Delay
         Link Constraint: Bandwidth

      Objective function 2:
         "Find the shortest path (path with lowest cost where the path
         cost is the sum of all link costs (Delay)) along the path such
         that all links provide at least X Bit/s of reservable
         bandwidth."

5.  RPL Protocol Specification

5.1.  DAG Information Option

   The DAG Information Option carries a number of metrics MUST
         be set to zero on transmission and other
   information that allows 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 discover a DAG, select its 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
   parents, discovery:

   o  A set of candidate neighbors

   o  For each DAG:

      *  A set of DAG parents and identify its siblings while employing loop avoidance
   strategies.

5.1.1.  DAG Information Option (DIO) base option

5.2.1.  Candidate Neighbors Data Structure

   The DAG Information Option set of candidate neighbors is a container option carried within an
   IPv6 Router Advertisement message to be populated by neighbors that
   are discovered by the neighbor discovery mechanism and further
   qualified as defined statistically stable as per the mechanisms discussed in [RFC4861], which
   might contain
   [I-D.ietf-roll-routing-metrics].  The candidate neighbors, and
   related metrics, should demonstrate stability/reliability beyond a number
   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 suboptions.  The base option regroups the
   minimum information set that is mandatory
   candidate neighbor set, in all cases.

        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      |    Length     |G|D|A|  00000  |   Sequence    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | DAGPreference |                BootTimeRandom                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   NodePref.   |    DAGRank    |           DAGDelay            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | DIOIntDoubl.  |  DIOIntMin.   |     DAGObjectiveCodePoint     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           PathDigest                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                            DAGID                              |
       +                                                               +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   sub-option(s)...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 1: DIO Base Option

   Type: 8-bit unsigned identifying the DIO base option.  The suggested which case a local confidence value is 140 will
   assist in ordering neighbors to be confirmed by determine which ones should remain in
   the IANA.

   Length:  8-bit unsigned integer candidate neighbor set to 4 when there and which should be evicted.

   If Neighbor Unreachability Detection (NUD) determines that a
   candidate neighbor is no suboption.
         The length of longer reachable, then it shall be removed
   from the option (including candidate neighbor set.  In the type and length fields
         and case that the suboptions) candidate
   neighbor has associated states in units of 8 octets.

   Flag Field:  Three flags are currently defined:

         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 parent set when the DAG root or another node in the successor chain decides to trigger active DA
   entries, then the sending removal of destination advertisements in order to
               update routing state for the outward direction along candidate neighbor shall be
   coordinated with tearing down these states.  All provisioned routes
   associated with the
               DAG, as further detailed in Section 5.9.  Note that the
               use and semantics candidate neighbor should be removed.

5.2.2.  Directed Acyclic Graphs (DAGs) Data Structure

   At a given point of this flag are still under
               investigation.

         Destination Advertisement Supported (A) :  The Destination
               Supported (A) bit time, a DAG Iteration is set when uniquely identified by
   the tuple (DagID, InstanceID, DAGSequenceNumber) where a change in
   the sequence denotes the iteration of a given DAG root over time.  When a
   single device is capable to root multiple DAGs in support the collection of destination advertisement
               related routing state and enables the operation of the
               destination advertisement mechanism within the DAG.

         Unassigned bits of the Flag Field are considered as reserved.
         They an
   application need for multiple optimization objectives it MUST be set to zero on transmission produce
   a different and MUST be ignored on
         receipt.

   Sequence Number:  8-bit unsigned integer set by unique (DagID, InstanceID) pair for each of the
   multiple DAGs.

   For each DAG root,
         incremented according to that a policy provisioned at node is, or may become, a member of, the
   implementation MUST keep a DAG root,
         and propagated table with no change outwards along the DAG.  Each
         increment SHOULD have a value of 1 and may cause a wrap back to
         zero.

   DAGPreference:  8-bit unsigned integer following entries:

   o  InstanceID

   o  DAGID

   o  DAGSequenceNumber

   o  DAG Metric Container, including DAGObjectiveCodePoint

   o  A set by of Destination Prefixes offered inwards along the DAG root to its
         preference and unchanged at propagation.  DAGPreference ranges
         from 0x00 (least preferred) to 0xFF (most preferred).  The
         default is 0 (least preferred).  The

   o  A set of DAG preference provides an
         administrative mechanism parents and siblings

   o  A timer to engineer govern the self-organization sending of
         the LLN, DIO messages for example indicating the most preferred LBR.  If a
         node has the option to join DAG

   When a more preferred DAG while still
         meeting other optimization objectives, then is discovered for which no DAG data structure is
   instantiated, and the node will seek wants to join the more preferred DAG.

   BootTimeRandom:  A random value computed at boot time and recomputed
         in case of a duplication with another node.  The concatenation
         of the NodePreference and join, then the BootTimeRandom is a 32-bit
         extended preference that is used to resolve collisions.  It is
         set by each node at propagation time.

   NodePreference:  The administrative preference of that LLN Node.
         Default is 0. 255 DAG data structure
   is the highest possible preference.  Set by
         each LLN Node at propagation time.  Forms a collision
         tiebreaker in combination with BootTimeRandom.

   DAGRank:  8-bit unsigned integer indicating instantiated.

   When the DAG rank of the node
         sending the RA-DIO message.  The DAGRank of parent set is depleted (i.e. the last DAG root is
         typically 1.  DAGRank is further described in Section 5.3.

   DAGDelay:  16-bit unsigned integer set by removed),
   then the DAG root indicating data structure SHOULD be suppressed after the expiration
   of an implementation-specific local timer.  An implementation SHOULD
   delay before changing deallocating the DAG configuration, data structure in TBD-units.  A
         default value is TBD.  It is expected to be an order of
         magnitude smaller than the RA-interval.  It is also expected to
         be an order of magnitude longer than the typical propagation
         delay inside the LLN.

   DIOIntervalDoublings:  8-bit unsigned integer.  Configured on the DAG
         root and used to configure observe
   that the trickle timer governing when RA-
         DIO message DAGSequenceNumber has incremented should be sent within any new DAG parents
   appear for the DAG.
         DIOIntervalDoublings

5.2.2.1.  DAG Parents/Siblings Structure

   When the DAG is self-rooted, the number set of times that the
         DIOIntervalMin DAG parents/siblings is allowed to be doubled during
   empty.

   In all other cases, for each node in the trickle
         timer operation.

   DIOIntervalMin:  8-bit unsigned integer.  Configured on set, the DAG root
         and used implementation MUST
   keep a record of:

   o  a reference to configure the trickle timer governing when RA-DIO
         message should be sent within the DAG.  The minimum configured
         interval for the RA-DIO trickle timer in units of ms is
         2^DIOIntervalMin.  For example, a DIOIntervalMin value of 16ms
         is expressed as 4.

   DAGObjectiveCodePoint:  The DAG Objective Code Point neighboring device which is used to
         indicate the cost metrics, objective functions, and methods DAG parent or
      sibling

   o  a record of
         computation and comparison for DAGRank in use in most recent information taken from the DAG.  The DAG OCP is set by Information
      Object last processed from the DAG root.  (Objective Code Points are to parent

   DAG parents may be further defined in [I-D.ietf-roll-routing-metrics].

   PathDigest:  32-bit unsigned integer CRC, updated by each LLN Node.
         This is ordered, according to the result of a CRC-32c computation on a bit string
         obtained by appending OF.  When ordering DAG
   parents, in consultation with the received value and OF, the ordered set of most preferred DAG parent
   may be identified.  All current DAG parents at the LLN Node. must have a rank less
   than self.  All current DAG roots use siblings must have a 'previous value'
         of zeroes rank equal to initially set the PathDigest.  Used self.

   When nodes are added to determine
         when something in or removed from the DAG set of successor paths has changed.

   DAGID:  128-bit unsigned integer which uniquely identify a DAG.  This
         value is set by the most
   preferred DAG root. parent may have changed.  The global IPv6 address role of all the
         DAG root can be used, however. nodes in
   the DAGID MUST list should be unique per DAG
         within the scope of the LLN. reevaluated.  In the case where particular, any nodes having a DAG root is
         rooting multiple DAGs the DAGID MUST be unique for each DAG
         rooted at
   rank greater than self after such a specific DAG root.

   The following values MUST NOT change during must be evicted from the propagation
   set.

   An implementation may choose to keep these records as an extension of RA-DIO
   messages outwards along
   the DAG: Type, Length, G, DAGPreference,
   DAGDelay and DAGID.  All other fields of Default Router List (DRL).

5.3.  DAG Rank

   Based on the RA-DIO message are
   updated at each hop selection of the propagation.

5.1.1.1. DAG Information Option (DIO) Suboptions

   In addition to Parents, the minimum options presented in metrics conveyed by the base option,
   several suboptions are defined for
   most preferred DAG parent, the RA-DIO message:

5.1.1.1.1.  Format
        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  Subopt. Type | Subopt Length | Suboption Data...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 2: DIO Suboption Generic Format

   Suboption Type:  8-bit identifier of nodes own metrics and configuration,
   and a related function defined by the type of suboption.  When
         processing OF, a RA-DIO message containing node will be able to
   compute a suboption value for which its rank as a consequence of selecting a most
   preferred DAG parent.

   The rank value feeds back into the Suboption Type DAG parent selection according to
   a loop-avoidance strategy.  Once a DAG parent has been added, and a
   rank value is not recognized by for the receiver, node within the
         receiver MUST silently ignore DAG has been computed, the unrecognized option, continue nodes
   further options with regard to process DAG parent selection and movement
   within the following suboption, correctly handling any
         remaining options in the message.

   Suboption Length:  8-bit unsigned integer, representing the length DAG are restricted in
         octets favor of loop avoidance.

   It is important to note that the suboption, DAG Rank is not including the suboption Type itself a metric,
   although its value is derived from and
         Length fields.

   Suboption Data:  A variable length field that contains data specific influenced by the use of
   metrics to select DAG parents and take up a position in the option. DAG.  The following subsections specify
   only aim of the RA-DIO message suboptions which
   are currently defined for use in rank is to inform loop avoidance and detection.

   The computation of the DAG Information Option.

   Implementations Rank MUST silently ignore be done in such a way so as to
   maintain the following properties for any RA-DIO message suboptions
   options nodes M and N that they do not understand.

   RA-DIO message suboptions may have alignment requirements.  Following
   the convention in IPv6, these options are aligned
   neighbors in the LLN:

   DAGRank(M) is less than DAGRank(N):  In this case, M is probably
           located in a packet such
   that multi-octet values within more preferred position than N in the Option Data field of each option
   fall on natural boundaries (i.e., fields of width n octets are placed
   at an integer multiple 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 n octets from creating a loop.
           Further, for a node N, all parents in the start DAG parent set must
           be of rank less than self's DAGRank(N).  In other words, the header, for
   n = 1, 2, 4, or 8).

5.1.1.1.2.  Pad1

   The Pad1 suboption does not have
           rank presented by a node N MUST be greater (deeper) than that
           presented by any alignment requirements.  Its
   format is as follows:

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

                              Figure 3: Pad 1

   NOTE! the format of the Pad1 option is a special its parents.

   DAGRank(M) equals DAGRank(N):  In this case - it has
   neither Option Length nor Option Data fields.

   The Pad1 option is used to insert one octet M and N are located
           positions of padding in relatively the RA-DIO
   message to enable suboptions alignment.  If more than one octet of
   padding is required, same optimality within the PadN option, described next, should DAG.
           In some cases, Node M may be used
   rather 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 multiple Pad1 options.

5.1.1.1.3.  PadN

   The PadN option does not have any alignment requirements.  Its format DAGRank(N):  In this case, then node M is as follows:

        0                   1
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
       |   Type = 1    | Subopt Length | Subopt Data
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -

                              Figure 4: Pad
           located in a less preferred position than N

   The PadN option is used to insert two or more octets of padding in the RA-DIO message DAG with
           respect to enable suboptions alignment.  For N (N > 1)
   octets of padding, the Option Length field contains the value N-2, metrics and the Option Data consists of N-2 zero-valued octets.  PadN Option
   data MUST be ignored optimizations defined by the receiver.

5.1.1.1.4.  DAG Metric Container

   The DAG Metric Container suboption
           objective code point.  Further, Node (M) may in fact be aligned as necessary to
   support its contents.  Its format in
           Node (N)'s sub-DAG.  There is a higher risk to Node (N)
           selecting Node (M) as follows:

        0                   1
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
       |   Type = 2    | Container Len | DAG Metric Data
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -

                      Figure 5: DAG Metric Container

   The a DAG Metric Container is used to report aggregated path metrics
   along parent, as such a selection may
           create a loop.

   As an example, the DAG.  The DAG Metric Container may contain Rank could be computed in such a number of
   discrete node, link, and aggregate path metrics way so as chosen by to
   closely track ETX when the
   implementer.  The Container Length field contains objective function is to minimize ETX, or
   latency when the length objective function is to minimize latency, or in
   octets of a
   more complicated way as appropriate to the objective code point being
   used within the DAG.

5.4.  DAG Metric Data.  The order, content, Discovery and coding of the Maintenance

   DAG Metric Container data is discovery locates the nearest sink (aka root), as specified in

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

   The processing determined
   according to some metrics and propagation constraints, and forms a Directed
   Acyclic Graph towards that sink, by identifying a set of the DAG Metric Container is
   governed by implementation specific policy functions.

5.1.1.1.5.  Destination Prefix

   The Destination Prefix suboption has an alignment requirement of
   4n+1.  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     | Prefix Length |Resvd|Prf|Resvd|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Prefix Lifetime                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Destination Prefix (Variable Length)              |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 6: parents.
   During this process DAG Destination Prefix

   The Destination Prefix suboption is discovery also identifies siblings, which may
   be used when later to provide additional path diversity towards the DAG root, or
   another node located inwards along the
   root.  DAG on the path discovery enables nodes to implement different policies
   for selecting their DAG parents in the DAG
   root, needs by using implementation
   specific policy functions.  DAG discovery specifies a set of rules to indicate
   be followed by all implementations in order to ensure interoperation.
   DAG discovery also standardizes the format that it offers connectivity is used to destination
   prefixes other than advertise
   the default.  This may be useful in cases where
   more than one LBR most common information that is operating within used in order to select DAG
   parents.

   One of these information, the LLN and offering
   connectivity DAG rank, is used by DAG discovery to
   provide loop avoidance even if nodes implement different administrative domains, e.g. a home network
   and a utility network.  In such cases, upon observing policies.
   The DAG Rank is computed as specified by the Destination
   Prefixes offered OF in use by a particular the DAG, a node MAY decide to join
   multiple DAGs
   demonstrating the properties described in support of a particular application. Section 5.3.  The Length is coded as the length of the suboption rank
   should be computed in octets,
   excluding such a way so as to provide a comparable basis
   with other nodes which may not use the Type and Length fields. same metric at all.

   The Prefix Length is an 8-bit unsigned integer that indicates the DAG discovery procedures take into account a number of leading bits in the destination prefix.  Prf is the Route
   Preference as in [RFC4191].  The reserved fields MUST be set to zero factors,
   including:

   o  RPL rules for loop avoidance based on transmission DAGs and MUST be ignored on receipt. ranks

   o  The Prefix Lifetime is Objective Function

   o  The advertised metrics

   o  Local policy functions (e.g. a 32-bit unsigned integer representing the
   length bounded number of time in seconds (relative candidate
      neighbors).

5.4.1.  DAG Discovery Rules

   In order to organize and maintain loopless structure, the time DAG
   discovery implementation in the packet is sent)
   that nodes MUST obey to the Destination Prefix is valid following
   rules and definitions:

5.4.1.1.  DAGs

   1.  DAG discovery instantiates LLN topologies that are each optimized
       for route determination.  A
   value of all one bits (0xFFFFFFFF) represents infinity. specific constraints and goals.  A value topology assumes the shape
       of
   all zero bits (0x00000000) indicates a loss DAG, and a DAG Instance is uniquely identified by its
       instanceID.

   2.  For reasons of reachability.

   The Destination Prefix contains Prefix Length significant bits scalability and operations of the
   destination prefix.  The remaining bits protocol, a DAG
       Instance is partitioned into a set of the DAGs rooted at a
       destination, aka Destination Prefix, as
   required to complete the trailing octet, are set to 0.

   In Oriented DAGs.  A destination is
       uniquely identified by a DAGID so a DAG rooted at a destination
       is uniquely identified by the event that pair (InstanceID, DAGID).

   3.  A Destination Oriented DAG is periodically reconstructed from the
       root, by incrementing a RA-DIO message may need to specify connectivity 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 more than one destination, as the Destination Prefix suboption may be
   repeated.

5.2.  Conceptual Data Structures
       DAG Iteration, or in short, the DAG.

   4.  The RPL implementation MUST maintain rank is defined within the following conceptual data
   structures in support scope of a DAG discovery:

   o  A set Iteration as an
       abstract coordinate to compare the relative position of candidate neighbors

   o  For each DAG:

      *  A set nodes and
       ensure forward progress of candidate DAG parents

      * the traffic.

   5.  A set of DAG parents (which are a subset of candidate node MUST belong at most to one DAG Iteration per InstanceID
       and MUST select all its parents and may be implemented as such)

5.2.1.  Candidate Neighbors Data Structure siblings within that same DAG
       Iteration.

5.4.1.2.  DAG Sequence Number

   1.  The set of candidate neighbors DAGSequenceNumber is to be populated by neighbors who
   are discovered incremented by the neighbor discovery mechanism root and further
   qualified as statistically stable as per the mechanisms discussed in
   [I-D.ietf-roll-routing-metrics]. flooded
       through DIOs.

   2.  The candidate neighbors, and
   related metrics, should demonstrate stability/reliability beyond root floods a
   certain threshold, and it is recommended that new DAGSequenceNumber periodically, at a local confidence
   value be maintained with respect to rate
       that depends on the neighbor in order to track
   this.  Implementations MAY choose deployment.  This rate can be set to bound the maximum size of 0 if
       other methods such as loop detection are considered sufficient to
       solve the
   candidate neighbor set, routing issues in which case that deployment.

   3.  The root MAY also flood a local confidence value will
   assist in ordering neighbors new DAGSequenceNumber on-demand.  The
       details of the mechanism to determine which ones should remain in signal the candidate neighbor set and which should root to do so are to be evicted.

   If Neighbor Unreachability Detection (NUD) determines that
       specified in a
   candidate neighbor is no longer reachable, then it shall be removed
   from the candidate neighbor set.  In the case future revision of this document.

   4.  A parent that advertises the candidate
   neighbor has associated states in the DAG parent set or active DA
   entries, then new DAGSequenceNumber can not
       possibly belong to the removal sub-DAG 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

   A DAG may be uniquely identified by within the LLN by its unique
   DAGID.  When a single device is capable node that still advertises an
       older DAGSequenceNumber.  The node MAY thus attach to root multiple DAGs in
   support that parent
       regardless of an application need for multiple optimization objectives
   it the relative rank, and this situation is expected equivalent
       to produce jumping onto a different and unique DAGID for each of
   the multiple DAGs.

   For each Destination Oriented DAG.

   5.  Thus, as a new DAGSequenceNumber spreads, a new DAG Iteration
       forms that supersedes the previous one.  During a node is, or may become,
       DAGSequenceNumber transition, a member of, node MAY decide to forward
       packets via 'future parents' that belong to the
   implementation MUST keep a DAG table with the following entries:

   o  DAGID

   o  DAGObjectiveCodePoint

   o  A set of same Destination Prefixes offered inwards along the
       Oriented DAG

   o  A set of candidate (same InstanceID and DagID), but a more recent
       (incremented) DAGSequenceNumber.

5.4.1.3.  DAG parents

   o Root

   1.  A timer to govern node that does not have any DAG parent MAY become the sending root of RA-DIO messages for the DAG

   o  DAGSequenceNumber

   When a DAG
       its own floating DAG.  It's rank is discovered for which no DAG data structure ROOT_RANK.

   2.  A (non-LLN) router is
   instantiated, and the node wants considered connected to join (i.e. the neighbor a grounded
       infrastructure at rank BASE_RANK.  A LLN node that is attached to
   become a candidate DAG parent in the Held-Up state), then the DAG
   data structure
       such an infrastructure router is instantiated.

   When the candidate DAG parent set root of its own grounded
       DAG.  It's rank is depleted (i.e. the last
   candidate DAG parent has timed out 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 Held-Down state), then the
   DAG data structure SHOULD be suppressed after the expiration backbone and use
       one router as a backbone root.  The backbone root exposes a rank
       of an
   implementation-specific local timer.  An implementation SHOULD delay
   before deallocating BASE_RANK over the DAG data structure in order backbone.  All the LLN roots that are
       parented to observe that backbone root, including the DAGSequenceNumber has incremented should any new candidate DAG
   parents appear 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 DAG.

5.2.2.1.  Candidate
       backbone root.

   4.  The DAG Parents Structure

   When root exposes the DAG is self-rooted, in the set of candidate DAG parents is
   empty.

   In all other cases, for each candidate 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 in the set, selection within the
   implementation MUST keep same
       DAG Iteration.  When a record of:

   o node moves (within its DAG) in a reference fashion
       that cause its rank to decrease, the neighboring device which is the DAG parent

   o node MUST abandon all
       parents and siblings with a record of most recent information taken from 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 Information
      Object last processed from when
       the candidate move does not cause the node to increase its own DAG parent

   o  a state associated with rank, as
       per the role of rank calculation indicated by the candidate as a potential
      DAG parent {Current, Held-Up, Held-Down, Collision}, further
      described in Section 5.7

   o  A DAG Hop Timer, if instantiated

   o OF.

   3.  A Held-Down Timer, if instantiated

5.2.2.1.1. node MUST NOT move outwards along a DAG Parents

   Note that it is attached to,
       causing the subset of candidate DAG parents in the `Current' state
   comprises rank to increase.  If a node cannot stay within
       the set of DAG parents, i.e. the nodes actively acting without a rank increase, then it MUST poison its routes
       as
   parents described in Section 5.4.1.6.

   4.  When DIO messages are received from other routers located at
       lesser rank in the DAG. same DAG, those routers are eligible for
       consideration as DAG parents may be ordered, according to parents.  DIO messages received from other
       routers located at the OCP.  When ordering DAG
   parents, same rank in consultation with the OCP, the most preferred same DAG parent may be identified.  All current DAG parents must have a rank less
   than or equal to
       considered as coming from siblings.  DIO messages that of the most preferred DAG parent.

   When nodes are added to or removed
       received from other routers located at greater rank within the
       same DAG parent set the most
   preferred DAG parent may have changed might cause greedy behaviors and should be reevaluated.  Any
   nodes having a rank greater than self after loops; such a change DIO is
       ignored unless:

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

       2.  The DIO comes from a node that has better OF ratings than any
           parent known at this point; in the Held-Down state and evicted as per the procedures
   described that case, this potential
           parent MAY be remembered in Section 5.7

   An implementation may choose order to keep these records as an extension of jump at a better
           position when the Default Router List (DRL).

5.3. next sequence is flooded.

5.4.1.5.  Jumping Onto Another DAG Discovery and Maintenance

   1.  A node jumps when it performs a new parent selection whereby its
       DAG discovery locates Iteration changes within the nearest sink, as determined according to
   some metrics and constraints, and forms same DAG Instance.  When a Directed Acyclic Graph
   towards that sink, by identifying node
       jumps onto a set of DAG parents.  During this
   process new DAG discovery also identifies siblings, which may be used
   later to provide additional path diversity towards the Iteration, it MUST abandon all parents and
       siblings from its previous position.

   2.  A node MAY jump from its current DAG root. onto any other DAG
   discovery enables nodes to implement different policies that
       provides service for selecting
   their DAG parents in the DAG by using implementation specific policy
   functions.  DAG discovery specifies a set of rules to be followed same InstanceID if it is preferred by
   all implementations in order to ensure interoperation.
       the OF, for example for reasons such as connectivity, configured
       preference, free medium time, size, security, bandwidth, DAG discovery
   also standardizes
       rank, or whatever metrics the format that LLN uses.  This is used to advertise allowed
       regardless of the most
   common information rank that is used the node reaches in order the new DAG.

   3.  A node that jumps should attempt to select DAG parents.

   One transmit all the packets
       received as part of these information, the previous DAG rank, 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 used by exhausted.

   4.  Jumping back onto a previous DAG discovery is equivalent to
   provide loop avoidance even if nodes implement different policies.
   The moving inside
       that DAG Rank is computed as specified by and obeys the Objective Code Point in
   use same rules.  To satisfy this, a node
       detaching from a DAG SHOULD remember its DAG as identified by the DAG, demonstrating the properties described in
   Section 3.2.1.7.  The
       tuple (InstanceID, DagID, DAGSequenceNumber) as well as its rank should be computed in such a way so
       within that DAG for long as to
   provide that DAG exists.

5.4.1.6.  Poisoning a comparable basis with other nodes which may not use Broken Path

   1.  A node SHOULD poison its inwards routes when it looses all of its
       current feasible parents, i.e. the
   same metric at all.

   The set of DAG discovery procedures take into account parents becomes
       depleted, and it can not jump onto an alternate DAG.

   2.  In order to poison its inwards routes, a number of factors,
   including:

   o  RPL rules for loop avoidance based on rank

   o  The OCP function

   o  The advertised metrics

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

5.3.1. node MAY stay at its
       position within its DAG Discovery Rules

   In order to organize and (that is maintain loopless structure, the DAG
   discovery implementation its InstanceID, DagID,
       DAGSequenceNumber and Rank) but it SHOULD immediately advertise a
       rank of INFINITE_RANK in the nodes MUST obey a DIO so as to the following
   rules force all its children to
       remove it from their parent list and definitions:

   1.   A try an alternate path.  The
       node that does not have any DAG parents in SHOULD then wait for a new DAG is Iteration (DAGSequenceNumber
       increment) before resuming its operation in the root
        of same Destination
       Oriented DAG.

   3.  Alternatively, a node MAY detach from its own floating DAG.  It's rank is 1.  A node will end up in that situation when it looses all
       detaches becomes root of its current feasible
        parents, i.e. the set of own floating DAG parents becomes depleted.  In that
        case, the node SHOULD remember the DAGID and MUST
       immediately advertise its new situation in a DIO.

   4.  Either way, the sequence
        counter of route poisoning will recursively be flooded
       throughout the impacted sub-DAG as children lose their last RA-DIO message from
       parent in the lost parents for a
        period original DAG.

   5.  The loss of time which covers multiple RA-DIO messages.  This is
        done so that if the node does encounter another possible
        attachment point to the lost DAGID within a period of time, the
        node DIO message may observe a sequence counter change by comparing interrupt the
        observed sequence counter to flooding.  This can
       be compensated by cheer repetition through the last observed sequence counter
        and thus verify trickle algorithm.
       If that also fails, packet loops will be prevented by the new attachment point is
       detection mechanism described in Section 5.11.

5.4.1.7.  Following a Parent

   1.  If a viable and
        independent alternative to attach back to the lost DAGID.

   2.   A node that is attached to an infrastructure receives a DIO from one of its DAG parents
       indicating that does not
        support RA-DIO messages, is the DAG root of parent has left the DAG, it may either follow
       that parent or stay in its own grounded
        DAG.  It's rank is 1.  (For example current DAG through an LBR alternate DAG
       parent if that is in
        communication with a non-LLN router not running RPL).

   3.   A (non-LLN) router sending a RA messages without DIO is
        considered possible.

   2.  If a grounded infrastructure at DAG parent increases its rank 0.  (For example, a
        router that is in communication with an LLN node but not running
        RPL such as a non-LLN public Internet router in communication
        with an LBR)

   4.   The DAG root exposes the DAG in that the RA-DIO message node rank would
       have to change, and nodes
        propagate the RA-DIO message outwards along the DAG with if the RAs
        that they forward over their LLN links.

   5.   A node MAY move at any time, with no delay, within its DAG when
        the move does not cause the node wish to increase its own follow (e.g. it
       has alternate options), then the DAG rank,
        as per parent SHOULD be evicted
       from the rank calculation indicated by DAG parent set.  If the OCP.

   6.   A node MUST NOT move outwards along a DAG that it parent is attached
        to, causing the DAG rank to increase, except last in a special case
        where the
       DAG parent set, then the node MAY choose SHOULD chose to follow the last it.

5.4.1.8.  DAG parent in the
        set of DAG parents.  In the general case, if a node is required
        to move such that it cannot stay within the DAG without a rank
        increase, then it needs to first leave the DAG.  In other words Inconsistency

   1.  When a node that is already part of a DAG MAY move detects or follow causes a DAG
        parent at any time and with no delay inconsistency, as described
       in order Section 5.4.4.2, then the node SHOULD send an unsolicited DIO
       message to be closer, or
        stay as close, its one-hop neighbors.  The DIO is updated to
       propagate the new DAG root of its current DAG as it already
        is, but may not move outwards.  RAs received from other routers
        located at lesser rank in information.  Such an event MUST also cause
       the same DAG may trickle timer governing the periodic sending of DIO messages
       to be considered as
        coming from candidate parents.  RAs reset.

5.4.2.  Reception and Processing of DIO messages

   When an DIO message is received from other routers
        located at a source device named SRC, the same rank in
   receiving node must first determine whether or not the same DAG may DIO message
   should be considered as
        coming from siblings.  Nodes MUST ignore RAs that are received
        from other routers located at greater rank within accepted for further processing, and subsequently present
   the same DAG.

   7.   A node may jump from its current DAG into any different DAG DIO message for further processing if
        it eligible.

   1.  If the DIO message is preferred malformed, then the DIO message is not
       eligible for reasons further processing and is silently discarded.  A RPL
       implementation MAY log the reception of connectivity, configured
        preference, free medium time, size, security, bandwidth, DAG
        rank, or whatever metrics the LLN cares to use.  A node may jump
        at any time and to whatever rank it reaches in the new DAG, but
        it may have to wait for a DAG Hop timer to elapse in order to do
        so.  This allows malformed DIO message.

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

   3.  If the DIO message advertises a DAG to move first, thus allowing stepped DAG
        reconfigurations and limiting relative movements.  A that the node SHOULD
        NOT join is already a previous DAG (identified by its DAGID) unless
       member of, then:

       *  If the
        sequence number rank of SRC as reported in the RA-DIO DIO message has incremented since the
        node left that DAG.  A newer sequence number indicates is lesser
          than that of the
        candidate parents were not attached behind this node, as they
        kept getting subsequent RA-DIO messages with new sequence
        numbers from the same DAG.  In the event that old sequence
        numbers (two or more behind node within the present value) are encountered
        they are considered stale and DAG, then the corresponding parent SHOULD DIO message
          MUST be
        removed from the set.

   8. considered for further processing.

       *  If a node has selected a new set the rank of DAG parents but has not
        moved yet (because it SRC as reported in the DIO message is waiting for DAG Hop timer equal to elapse),
          that of the node within the DAG, then SRC is unstable MUST NOT send RA-DIOs for that DAG.

   9.   If a node receives marked as a RA-DIO from one of its DAG parents,
          sibling and if the parent contains a different DAGID, indicating DIO message is not eligible for further
          processing.

       *  If the rank of SRC as reported in the DIO message is higher
          than that of the
        parent has left node within the DAG, and if SRC is not a DAG
          parent, then the node can remain in 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 through an alternate parent for any other DAG parent, then that the node
        SHOULD remove is
       attached to, then the DIO message MUST be considered for further
       processing (the DAG parent which has joined may have jumped).

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

5.4.2.1.  Overview of DIO Message Processing

      If there the received DIO message is
        no alternate parent for the DAG, then a new/alternate DAG:

         If the node SHOULD follow
        that parent into has sent an DIO message within the new DAG.

   10.  When a node detects or causes a DAG inconsistency, risk window as
         described in Section 5.3.4.2, 5.8 then a collision has occurred; do not
         process the node SHOULD send an unsolicited RA- DIO message to its one-hop neighbors.  The RA-DIO is updated to
        propagate any further.

         If the new DAG information.  Such an event MUST SRC node is also
        cause the trickle timer governing the periodic sending of RA-DIO
        messages to be reset.

   11.  If a DAG parent increases its rank such for another DAG that the
         node rank would
        have to change, is a member of, and if the node does not wish to follow (e.g. it
        has alternate options), new/alternate DAG is the same
         InstanceID as the other DAG, then the DAG parent SHOULD be evicted is known to
         have jumped.

            Remove SRC as a DAG parent from the other DAG parent set.

            If the other DAG parent is the last in the
        DAG parent set, now empty of candidate parents, then the node SHOULD chose
            prepare to directly follow it.

5.3.2.  Reception and Processing of RA-DIO messages

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

5.3.2.1.  Determination of Eligibility new DAG by adding it
            as a DAG parent for DIO Processing

      If the RA-DIO message is malformed, then new DAG, else ignore the RA-DIO DIO message is
            (do not
      eligible for further processing and is silently discarded.  A RPL
      implementation MAY log follow the reception of parent).

         Instantiate a malformed RA-DIO
      message. data structure for the new/alternate DAG if
         necessary

         If SRC is not the new/alternate DAG offers a member of better solution to the candidate neighbor set,
         optimization objectives, then jump: copy the RA- DIO is not eligible for further processing.  (Further evaluation/
      confidence of this information
         place the neighbor is necessary)

      If into the RA-DIO message advertises a DAG that the node is already a
      member of, then: parent set.

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

         If a known/existing DAG:

         Process the rank of SRC DIO message as reported in per the RA-DIO message is equal
         to that of the node within the DAG, then SRC is marked as a
         sibling and rules in Section 5.4

   As DIO messages are received from candidate neighbors, the RA-DIO message is not eligible for further
         processing.

         If neighbors
   may be promoted to DAG parents by following the rank rules of SRC DAG
   discovery as reported described in the RA-DIO message is higher
         than that of the Section 5.4.  When a node within the DAG, and SRC is not places a DAG
         parent, then neighbor
   into the RA-DIO message MUST NOT be considered for
         further processing

      If SRC is a DAG parent for any other DAG that Parent set, the node is becomes attached
      to, then to the RA-DIO message MUST be considered for further
      processing (the DAG through
   the new parent may have jumped).

      If node.

   In the RA-DIO message advertises a DAG that offers a better (new
      or alternate) solution to an optimization objective desired by the
      node, then discovery implementation, the RA-DIO message MUST most preferred parent should
   be considered for further
      processing.

5.3.2.2.  Overview of RA-DIO Message Processing

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

         Instantiate a data structure for the new/alternate used to restrict which other nodes may become DAG if
         necessary

         Place the neighbor parents.  Some
   nodes in the candidate DAG parent set

         If may be of a rank less than or equal to
   the node has sent most preferred DAG parent.  (This case may occur, for example, if
   an RA message within the risk window energy constrained device is at a lesser rank but should be
   avoided as
         described in Section 5.7.3 then perform the collision detection
         described per an optimization objective, resulting in Section 5.7.3.  If a collision occurs, place the
         candidate DAG more
   preferred parent in the collision state and do not process
         the RA-DIO message any further as described in Section 5.7.

         If the SRC at a greater rank).

5.4.3.  DIO Transmission

   Each node is also maintains a DAG parent for another DAG timer that the
         node governs when to multicast DIO
   messages.  This timer is implemented as a member of, and if trickle timer operating
   over a variable interval.  Trickle timers are further detailed in
   Section 5.4.4.  The governing parameters for the new/alternate DAG satisfies an
         equivalent optimization objective as timer should be
   configured consistently across the other DAG, then and are provided by the DAG parent is known
   root in the DIO message.  In addition to periodic DIO messages, each
   node may respond to have jumped.

            Remove SRC as a DAG parent from the other DAG (place DIS message with a DIO message.

   o  When a node detects an inconsistency, it in
            the held-down state)

            If SHOULD reset the other DAG is now empty interval
      of candidate parents, then
            directly follow SRC into the new DAG by adding it as trickle timer to a DAG
            parent in the Current state, else ignore the RA-DIO message
            (do not follow the parent).

         If the new/alternate DAG offers minimum value, causing DIO messages to
      be emitted more frequently as part of a better solution strategy to quickly
      correct the
         optimization objectives, then prepare inconsistency.  Such inconsistencies may be, for
      example, an update to jump: copy a key parameter (e.g. sequence number) in
      the DIO
         information into the record for the candidate DAG parent, place
         the candidate DAG parent into the Held-Up state, and start the
         DAG Hop timer as per Section 5.7.1.

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

         Process loop detected when a node located inwards
      along the RA-DIO message as per the rules DAG forwards traffic outwards.  Inconsistencies are
      further detailed in Section 5.3

   As candidate 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 identified, they consistent and no
      other inconsistencies are detected, it may subsequently be
   promoted begin to DAG parents by following open up the rules
      interval of DAG discovery as
   described in Section 5.3.  When the trickle timer towards a node adds another node maximum value, causing DIO
      messages to its set
   of candidate parents, the be emitted less frequently, thus reducing network
      maintenance overhead and saving energy consumption.

   o  When a node becomes attached to the DAG through
   the parent node.

   In the DAG discovery implementation, the most preferred parent should is initialized, it MAY be used configured to restrict which other nodes may become remain silent
      and not multicast any DIO messages until it has encountered and
      joined a DAG parents.  Some
   nodes in the (perhaps initially probing for a nearby DAG parent set with an
      DIS message).  Alternately, it may be of a rank less than or equal choose to
   the most preferred root its own floating
      DAG parent.  (This and begin multicasting DIO messages using a default trickle
      configuration.  The second case may occur, for example, be advantageous if
   an energy constrained device it is at a lesser rank but should be
   avoided as per an optimization objective, resulting
      desired for independent nodes to begin aggregating into scattered
      floating DAGs in the absence of a more
   preferred parent at a greater rank).

5.3.3.  RA-DIO Transmission

   Each node maintains a timer grounded node, for example in
      support of LLN installation and commissioning.

   Note that governs when to multicast RA
   messages.  This timer is implemented as a trickle timer operating
   over a variable interval.  Trickle timers if multiple DAG roots are further detailed participating in
   Section 5.3.4.  The governing parameters for the timer should be
   configured consistently across the same DAG, and are provided by the DAG
   root in
   i.e. offering DIO messages with the RA-DIO message.  In addition to periodic RA messages, same DAGID, then they must
   coordinate with each LLN node will respond other to Router Solicitation (RS) ensure that their DIO messages
   according 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 [RFC4861].

   o  When the
   Sequence number should be issued at the same time.  The specific
   mechanism of this coordination, e.g. along a node is unstable, because any non-LLN network between
   DAG Hop timer roots, is running in
      preparation beyond the scope of this specification.

5.4.4.  Trickle Timer for DIO Transmission

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

   For each DAG that a node MUST NOT transmit
      unsolicited RA-DIOs (i.e. is part of, the node will remain silent when the
      timer expires).

   o  When must maintain a node detects an inconsistency, it SHOULD reset single
   trickle timer.  The required state contains the interval following conceptual
   items:

   I:    The current length of the trickle communication interval

   T:    A timer to with a minimum value, causing RA messages duration set to be
      emitted more frequently as part of a strategy to quickly correct random value in the inconsistency.  Such inconsistencies may be, for example, an
      update to a key parameter (e.g. sequence number) range
         [I/2, I]

   C:    Redundancy Counter

   I_min:  The smallest communication interval in milliseconds.  This
         value is learned from the RA-DIO DIO message or a loop detected when a node located inwards along the
      DAG forwards traffic outwards.  Inconsistencies are further
      detailed in Section 5.3.4.2.

   o  When a node enters a mode as (2^DIOIntervalMin)ms.
         The default value is DEFAULT_DIO_INTERVAL_MIN.

   I_doublings:  The number of consistent operation within times I_min should be doubled before
         maintaining a DAG, constant rate, i.e.  RA-DIO messages  I_max = I_min *
         2^I_doublings.  This value is learned from its DAG parents are consistent 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 no
      other inconsistencies are detected, it may begin I_doublings to open up the
      interval of values learned from the DIO
       message.

   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 towards a maximum value, causing RAs to be emitted less frequently, thus reducing network maintenance
      overhead and saving energy consumption (which is of utmost
      importance for battery-operated nodes).

   o  When expire after a duration T

   When node is initialized, it MAY be configured to remain silent learns about a DAG through a DIO message and not multicast any RA messages until makes the
   decision to join it, it has encountered initializes the state of the trickle timer by
   resetting the trickle timer and
      joined listening.  Each time it hears a DAG (perhaps initially probing
   consistent DIO message for this DAG from a nearby DAG with an
      RS message).  Alternately, it may choose to root its own floating
      DAG and begin multicasting RAs using a default trickle
      configuration.  The second case may be advantageous if parent, it is
      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 MAY
   increment C.

   When the same DAG,
   i.e. offering RA-DIO messages with timer fires at time T, the same DAGID, then they must
   coordinate with each other node compares C to ensure that their RA-DIO messages are
   consistent when they emit RA-DIO messages.  In particular the
   Sequence number must be identical from each DAG root, regardless of
   which of the multiple DAG roots issues redundancy
   constant, DEFAULT_DIO_REDUNDANCY_CONSTANT.  If C is less than that
   value, the RA-DIO message, node generates a new DIO message and
   changes to the Sequence number should be issued at multicasts it.  When
   the same time.
   The specific mechanism of this coordination, e.g. along a non-LLN
   network between DAG roots, is beyond communication interval I expires, the scope of this specification.

5.3.4.  Trickle Timer for RA Transmission

   RPL treats node doubles the construction of a DAG interval I
   so long as a consistency problem, it has previously doubled it fewer than I_doubling times,
   resets C, and
   uses chooses a new T value.

5.4.4.2.  Determination of Inconsistency

   The trickle timer [Levis08] to control the rate of control
   broadcasts.

   For each DAG that a node is part of, reset whenever an inconsistency is detected
   within the DAG, for example:

   o  The node must maintain joins a single
   trickle timer. new DAGID

   o  The required state contains the following conceptual
   items:

   I: node moves within a DAGID

   o  The current length of the communication interval

   T: node receives a modified DIO message from a DAG parent

   o  A timer with DAG parent forwards a duration set packet intended to a random value in the range
         [I/2, I]

   C:    Redundancy Counter

   I_min:  The smallest communication interval move inwards,
      indicating an inconsistency and possible loop.

   o  A metric communicated in milliseconds.  This
         value is learned from the RA-DIO DIO message as
         (2^DIOIntervalMin)ms.  The default value is
         DEFAULT_DIO_INTERVAL_MIN.

   I_doublings: determined to be
      inconsistent, as according to a implementation specific path
      metric selection engine.

   o  The number rank 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 RA-DIO message
         as DIOIntervalDoublings. DAG parent has changed.

5.5.  DAG Sequence Number Increment

   The default value is
         DEFAULT_DIO_INTERVAL_DOUBLINGS.

5.3.4.1.  Resetting DAG root makes the Trickle Timer

   The trickle timer for a DAGID is reset by:

   1.  Setting I_min and I_doublings sole determination of when to revise the values learned from
   DAGSequenceNumber by incrementing it upwards.  When the RA-
   DAGSequenceNumber is increased an inconsistency results, causing DIO message.

   2.  Setting C to zero.

   3.  Setting I
   messages to I_min.

   4.  Setting T be sent back outwards along the DAG to a random value as described above.

   5.  Restarting convey the trickle timer change.
   The degree to expire after a duration T

   When node learns about which this mechanism is relied on may be determined by
   the implementation- on one hand it may serve as a periodic heartbeat,
   refreshing the DAG through a RA-DIO message states, and makes on the
   decision to join it, other hand it initializes the state of the trickle timer by
   resetting the trickle timer and listening.  Each time it hears a
   consistent RA for this DAG from may result in 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
   constant steady-state control cost overhead which is less than that
   value, the node generates not desirable.

   Some implementations may provide an administrative interface, such as
   a new RA and broadcasts it.  When command line, at the
   communication interval I expires, DAG root whereby the node doubles DAGSequenceNumber may be
   caused to increment in response to some policy outside of the interval I so
   long as it has previously doubled it fewer than I_doubling times,
   resets C, and chooses a new T value.

5.3.4.2.  Determination scope
   of Inconsistency

   The trickle RPL.

   Other implementations may make use of a periodic timer is reset whenever an inconsistency is detected
   within to
   automatically increment the DAG, for example:

   o  The node joins a new DAGID

   o  The node moves within a DAGID

   o  The node receives a modified RA-DIO message from DAGSequenceNumber, resulting in a
   periodic DAG parent

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

   o  A metric communicated in the RA-DIO message is determined application and
   implementation.  Other automated mechanisms to be
      inconsistent, determine
   DAGSequenceNumber increments are also possible as according appropriate to a implementation specific path
      metric selection engine.

   o  The rank of a DAG parent has changed.

5.4.
   deployment.

5.6.  DAG Heartbeat Selection

   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 RA-
   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 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 Heartbeat at a rate appropriate to the application and
   implementation.

5.5.  DAG Selection

   The DAG selection 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.  Nodes
   MAY place the failed candidate parent in a Hold Down mode that
   ensures that the candidate parent will not be reused for a given
   period of time.

   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.

5.6.

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 OCP OF in order to expose an exaggerated rank.

5.7.  Candidate DAG Parent States

5.8.  Collision

   A race condition occurs if 2 nodes send DIO messages at the same time
   and Stability

   Candidate DAG parents may or may not be eligible then attempt to join each other.  This might happen, for example,
   between nodes which act as DAG
   parents depending on runtime conditions.  The following states are
   defined:

   Current     This candidate parent is in the set root of DAG parents and
               may be used for forwarding traffic inward along their own DAGs.  In order to
   detect the DAG.
               When a candidate parent is placed into situation, LLN Nodes time stamp the Current state,
               or taken out sending of the Current state, it DIO
   message.  Any DIO message received within a short link-layer-
   dependent period introduces a risk.  It is necessary up to re-
               evaluate which the implementation
   to define the duration of the remaining DAG parents risk window.

   There is the most
               preferred DAG parent and its rank.  At that time any
               remaining DAG parents risk of greater rank than this a collision when a node must
               be placed in the Held-Down state, receives and processes a DIO
   within the hold-down timer
               started, 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 order both nodes simultaneously deciding to be evicted attach to each other.
   As a remedy, in the face of a potential collision, as DAG parents.  In determined by
   receiving a DIO within the
               same fashion, siblings must also be reevaluated.

   Held-Up     This parent can risk window, the DIO message is not be used until
   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 hop timer
               elapses.

   Held-Down   This candidate parent can not be to join,
   and a number of peers in that DAG as parents.  The OF is used till hold down
               timer elapses.  At to
   compute an ordered list of parents.  The OF is also responsible to
   compute the end rank of the hold-down period, device within the
               candidate DAG.

   The Objective Function is removed from specified in the candidate DIO message within a DAG parent set,
   Metric Container using an Objective Code Point (OCP), as specified in
   [I-D.ietf-roll-routing-metrics], and may be reinserted if it appears again with a RA-DIO
               message.

   Collision   This candidate parent can not indicates the method that must
   be used till its next RA-
               DIO message.

5.7.1.  Held-Up

   This state is managed by to compute the DAG Hop timer, it serves 2 purposes:

      Delay (e.g. "minimize the reattachment of a sub-DAG that has been forced to
      detach.  This is not as safe as path cost using the use
   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 sequence, but still
      covers that when a sub-DAG has detached, case where the RA-DIO message that
      is initiated by DIO does not include an OCP
   specification in the new DAG root has a chance Metric Container, OF0 MAY be presumed.

   Most Objective Functions are expected to spread outward
      along the sub-DAG, ideally forming a frozen sub-DAG that is aware
      of follow the DAG change, such that two different DAGs have formed prior
      to an attempted reattachment.

      Limit RA-DIO message storms (control cost / churn) when two DAGs
      collide/merge. same abstract
   behavior:

   o  The idea parent selection is triggered each time an event indicates
      that between the nodes from DAG A that
      decide to move to DAG B, those that see the highest place (closer
      to the DAG root) in DAG B will move first and advertise their new
      locations before other nodes from DAG A actually move.

   A new DAG a potential next hop information is discovered updated.  This might
      happen upon receiving the reception of a RA message with DIO message, a timer elapse, or without a
   DIO.  The node joins the DAG by selecting
      trigger indicating that the source state of the RA
   message as a DAG parent (and possibly installing candidate neighbor has
      changed.

   o  An OF scans all the DAG parent as a
   default gateway).  The node is then a member of interfaces on the DAG and may begin 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 multicast RA-DIO messages containing the DIO be usable or not for the DAG.

   When RPL operation.  An interface
      can also be configured with a new DAG is discovered, the candidate parent preference or dynamically learned to
      be better than another by some heuristics that advertises
   the new DAG is placed in might be link-layer
      dependent and are out of scope.  Finally an interface might or not
      match a held up state required criterion for the duration an Objective Function, for instance
      a degree of security.  As a DAG
   Hop timer.  If result some interfaces might be
      completely excluded from the resulting new set of DAG parents is computation, while others might be
      more
   preferable than the current one, or if the node is intending to
   maintain a membership in the new DAG in addition to its current DAG, less preferred.

   o  An OF scans all the node expects to jump and becomes unstable.

   A node that is unstable may discover other candidate parents from the
   same new DAG during neighbors on the instability phase.  It needs possible interfaces
      to start check whether they can act as a new
   DAG Hop timer for all these.  The first timer that elapses router for a
   given new DAG clears them all for that DAG, allowing the node to jump
   to the highest position available in the new DAG.

   The duration of the DAG Hop timer depends on the DAG Delay  There might
      be multiple of the new
   DAG them and on the rank of a candidate parent that triggers it: (candidates
   rank + random) * candidate's DAG_delay (where 0 <= random < 1).  It
   is randomized in order neighbor might need to limit collisions and synchronizations.

5.7.2.  Held-Down

   When a neighboring node is 'removed' from the Default Router List, pass
      some validation tests before it
   is actually held down for can be used.  In particular, some
      link layers require experience on the activity with a hold down timer period, in order router to
   prevent flapping.  This happens when
      enable the router as a node disappears (upon
   expiration timer).

   When next hop.

   o  An OF computes self's rank by adding the hold down timer elapses, step of rank to that
      candidate to the node rank of that candidate.  The step of rank is removed from the
   candidate DAG parent set.

5.7.3.  Collision

   A race condition occurs if 2 nodes send RA-DIO messages at
      computed by estimating the same
   time and then attempt to join each other.  This might happen, for
   example, between nodes which act link as DAG root follows:

      *  The step of their own DAGs.  In
   order rank might vary from 1 to detect the situation, LLN Nodes time stamp the sending of
   RA-DIO message.  Any RA-DIO message received within 16.

         +  1 indicates a short link-
   layer-dependent period introduces unusually good link, for instance a risk.  To resolve the collision, link
            between powered devices in a 32bits extended preference is constructed from the RA-DIO message mostly battery operated
            environment.

         +  4 indicates a `normal'/typical link, as qualified by concatenating the NodePreference with the BootTimeRandom.

   A node that decides to add
            implementation.

         +  16 indicates a candidate link that can hardly be used to its DAG parents will do so
   between (candidate rank) and (candidate rank + 1) times the candidate
   DAG Delay.  But since forward any
            packet, for instance a node radio link with quality indicator or
            expected transmission count that is unstable as soon as it receives the
   RA-DIO message from close to the desired candidate, it will restrain from
   sending a RA-DIO message between the time it receives the RA and the
   time it actually jumps.  So acceptable
            threshold.

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

   o  Candidate neighbors that advertise an OF incompatible with the crossing set
      of RA may only happen during OF specified by the propagation time between policy functions are ignored.

   o  As it scans all the candidate and neighbors, the node, plus some
   internal queuing and processing time within each machine.  It is
   expected that one DAG delay normally covers that interval, but
   ultimately it is up to OF keeps the implementation current
      best parent and compares its capabilities with the configuration of
   the current
      candidate parent neighbor.  The OF defines a number of tests that are
      critical to define reach the duration of risk window.

   There is risk of a collision when a node receives objective.  A test between the routers
      determines an RA, order relation.

      *  If the routers are roughly equal for another
   candidate that relation then the
         next test is more preferable than attempted between the routers,

      *  Else the best of the 2 becomes the current candidate, within best parent and the risk window.  In
         scan continues with the face of next candidate neighbor

      *  Some OFs may include a potential collision, test to compare the ranks that would
         result if the node with
   lowest extended preference processes joined either router

   o  When the RA-DIO message normally,
   while scan is complete, the router with preferred parent is elected and
      self's rank is computed as the highest extended preference places preferred parent rank plus the
   other step
      in collision state, does 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 start in the same DAG hop timer, and does
   not become instable.  It is expected are ignored

      *  Candidate neighbors that next RAs between the two
   will not cross anyway.

   For example, consider are of greater rank than self are
         ignored

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

      *  Candidate neighbors of a case where two nodes lesser rank than self (non-siblings)
         are each rooting their
   own transient floating DAGs and multicast RA-DIO messages towards
   each other in preferred

5.9.2.  Objective Function 0 (OF0)

   This document specifies a close enough interval that the RA-DIO messages
   `cross'.  Then each node may receive the RA-DIO message from default objective function, called OF0,
   indicated by an OCP value of 0x0000.  OF0 is the
   other node, default objective
   function of RPL, and in some scenario decide to join each others DAG.  RPL
   avoids this deadlock scenario via can be used if allowed by the collision mechanism described
   above - after each policy of the
   processing node sends when the OF indicated in the RA-DIO DIO message they will enter is unknown
   to the
   risk window.  When node.  If not allowed, then the peer RA-DIO DIO message is received in the risk
   window, simply ignored
   and not processed by the nodes will calculate 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 extended preferences DIO message such
   as describe
   above rank and administrative preference.

   OF0 favors connectivity.  That is, the node with the lowest extended preference will proceed
   to process the RA-DIO message, while the other node will defer,
   avoiding the deadlock scenario.

5.7.4.  Instability

   A node is instable when it Objective Function is prepared to shortly replace a set of
   DAG parents in order to jump designed
   to a different DAGID.  This happens
   typically when find the node has selected a more preferred candidate
   parent in nearest sink into a different DAG 'grounded' topology, and has to wait for the DAG hop timer to
   elapse before adjusting the DAG parent set.  Instability may also
   occur when if there is
   none then join any network per order of administrative preference.
   The metric in use is the entire current DAG rank.

   OF0 selects a preferred parent set is lost and the a backup next
   best candidates are still held up.  Instability hop if one is resolved when the
   DAG
   available.  The backup next hop timer of all the candidate(s) causing instability elapse.
   Such candidates then change state to Current might be a parent or Held- Down.

   Instability a sibling.  All
   the traffic is transient (in routed via the order of DAG hop timers). preferred parent.  When a
   node is unstable, it MUST NOT send RAs with the DIO message.  This
   avoids loops when node A decides to attach to node B and node B
   decides to attach to node A. Unless RAs cross (see Collision
   section), a node receives RA-DIO messages from stable candidate
   parents, which link
   conditions do not plan to attach let a packet through to the node, so preferred parent, the node can
   safely attach
   packet is passed to them.

5.8.  Guidelines for Objective Code Points

5.8.1.  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. backup next hop.

   The OF is used to
   compute an ordered list step of parents and provides load balancing
   guidance.  The OF is also responsible to compute the rank is 4 for each hop.

5.9.2.1.  Selection of the
   device within Preferred Parent

   As it scans all the DAG.

   The Objective Function candidate neighbors, OF0 keeps the parent that is specified in
   the RA-DIO message using an
   objective code point (OCP) best for the following criteria (in order):

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

   2.   A candidate that
   has been used to compute the DAG (e.g. "minimize would cause the path cost using node to augment the ETX metric and avoid `Blue' links").  The objective code points
   are specified rank in [I-D.ietf-roll-routing-metrics].  This document
   specifies the OCP 0, in support of default operation.

   Most Objective Functions
        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 expected grounded then a DAG with a more preferred
        administrative preference (DAGPreference) is better.

   5.   A router that offers connectivity to follow 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 same abstract
   behavior:

   o backup next
        hop while assuming that this router won.

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

   9.   The preferred parent selection that was in use already is triggered each time an event indicates better.

   10.  A router that has announced a potential next_hop information DIO message more recently is updated.  This might
      happen upon the reception
        preferred.

5.9.2.2.  Selection of a RA-DIO message, a timer elapse, or
      a trigger indicating that the state of a candidate neighbor has
      changed. Backup Next Hop

   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  The interface must be usable or not for RPL operation.  An interface
      can also be configured with a and the administrative preference or dynamically learned to
      be better than another by some heuristics (if
      any) applies first.

   o  The preferred parent is ignored.

   o  Candidate neighbors 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 in the same DAG are ignored.

   o  Candidate neighbors with a degree higher rank are ignored.

   o  Candidate neighbors of security.  As a result some interfaces might be
      completely excluded from the computation, while others might be
      more or less better rank than self (non-siblings) are
      preferred.

   o  The OF scans all the candidate neighbors on the possible
      interfaces to check whether they can act as an attachment  A router
      for a DAG.  There might be multiple of them and that has been validated as usable, e.g. with a candidate
      neighbor might need to pass some validation tests before it can be
      used.  In particular, local
      confidence that has exceeded some link layers require experience on the
      activity pre-configured threshold, is
      better.

   o  The router with a better router to enable the router as a next_hop. preference wins.

   o  The OF computes self's rank by adding the step of rank to backup next hop that
      candidate to was in use already is better.

5.10.  Establishing Routing State Outward Along the rank of that candidate. DAG

   The step destination advertisement mechanism supports the dissemination of rank is
      estimated as follows:

      *  The step of rank might vary from 1
   routing state required 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 support traffic flows outward along the
            implementation.

         +  16 indicates
   DAG, from the DAG root toward nodes.

   As a link that can hardly be used to forward any
            packet, for instance result of destination advertisement operation:

   o  DAG discovery establishes a radio link with quality indicator or
            expected transmission count that is close to DAG oriented toward a DAG root along
      which inward routes toward the acceptable
            threshold.

      *  Candidate neighbors that would cause self's rank to increase DAG root are ignored set up.

   o  Candidate neighbors  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 advertise an OF incompatible do not retain state.

   Destinations disseminated with the set destination advertisement
   mechanism may be prefixes, individual hosts, or multicast listeners.
   The mechanism supports nodes of OF specified by the policy functions are ignored. varying capabilities as follows:

   o  As it scans all the candidate neighbors, the OF keeps the current
      best parent  When nodes are capable of storing routing state, they may inspect
      destination advertisements and compares its capabilities learn hop-by-hop routing state
      toward destinations by populating their routing tables with the current
      candidate neighbor.  The OF defines a number
      routes learned from nodes in their sub-DAG.  In this process they
      may also learn necessary piecewise source routes to traverse
      regions of tests the LLN that do not maintain routing state.  They may
      perform route aggregation on known destinations before emitting
      Destination Advertisements.

   o  When nodes are
      critical to reach incapable of storing routing state, they may
      forward destination advertisements, recording the Objective.  A test between reverse route as
      the routers
      determines an go in order relation.

      *  If to support the routers are roughly equal for construction of piecewise source
      routes.

   Nodes that relation then the
         next test is attempted between the routers,

      *  Else the best are capable of the 2 becomes the current best parent storing routing state, and finally the
         scan continues with the next candidate neighbor

      *  Some OFs may include a test DAG
   roots, are able to compare learn which destinations are contained in the ranks that would
         result if sub-
   DAG below the node joined either router

   o  When node, and via which next-hop neighbors.  The
   dissemination and installation of this routing state into nodes
   allows for Hop-By-Hop routing from the scan is complete, DAG root outwards along the preferred parent is elected and
      self's rank
   DAG.  The mechanism is computed as the preferred parent rank plus further enhance by supporting the step
      in rank with that parent.

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

      *  Candidate neighbors that are not source routes across stateless `gaps' in the same DAG are ignored

      *  Candidate neighbors that DAG, where nodes are
   incapable of worse rank than self are
         ignored

      *  Candidate neighbors storing additional routing state.  An adaptation of a better rank than self (non-siblings)
         are preferred

5.8.2.  Objective Code Point 0 (OCP 0)

   Here follows this
   mechanism allows for the specification for implementation of loose-source routing.

   A special case, the default Objective Function
   corresponding reception of a destination advertisement
   addressed to OCP codepoint 0.  This is a very simple reference link-local multicast address, allows for a node to
   help
   learn destinations directly available from its one-hop neighbors.

   A design more complex Objective Functions.  In particular, the
   Objective Function described here does choice behind advertising routes via destination
   advertisements is not use physical metrics as
   described in [I-D.ietf-roll-routing-metrics], but are only based on
   abstract information from to synchronize the RA-DIO message such as rank and
   administrative preference.

   This document specifies a default objective metric, called OF0, parent and
   using children
   databases along the OCP 0.  OF0 is DAG, but instead to update them regularly to
   recover from the default objective function loss of RPL, and packets.  The rationale for that choice is
   time variations in connectivity across unreliable links.  If the
   topology can be used if allowed by the policy expected to change frequently, synchronization might
   be an excessive goal in terms of the processing node when no
   objective function is included 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 RA-DIO message, 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 if all of the OF
   indicated
   feasible parents in the RA-DIO message is unknown DAG may be selected to receive prefix
   information from the node.  If not
   allowed, then destination advertisement mechanism.  This
   subset of DAG parents shall be designated the RA-DIO message is simply ignored and not processed
   by the node.

5.8.2.1.  OCP 0 Objective Function (OF0)

   OF0 favors the connectivity.  That is, the Objective Function is
   designed to find set of DA parents.

   As DAO messages for particular destinations move inwards along the nearest sink into
   DAG, a 'grounded' topology, and if
   there sequence counter is none then join any network per order of administrative
   preference. used to guarantee their freshness.  The metric in use
   sequence counter is incremented by the rank.

   OF0 selects a preferred parent and source of the DAO message (the
   node that owns the prefix, or learned the prefix via some other
   means), each time it issues a backup next_hop DAO message for its prefix.  Nodes that
   receive the DAO message and, if one is
   available.  The backup next_hop might scope allows, will be forwarding a parent or
   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 sibling.  All DAO message, and if
   the traffic DAO is routed via unchanged (the sequence counter has not changed), then the preferred parent.  When
   DAO message will be discarded without additional processing.
   Further, if the link
   conditions do not let a packet through DAO message appears to be out of synch (the sequence
   counter is 2 or more behind the preferred parent, present value) then the
   packet DAO state is passed
   considered to be stale and may be purged, and the backup next_hop.

   The step of rank DAO message is 4
   discarded.  A depth is also added for each hop.

5.8.2.2.  Selection of tracking purposes; the Preferred Parent

   As it scans all depth is
   incremented at each hop as the candidate neighbors, OF0 keeps DAO message is propagated up the parent DAG.

   Nodes that is are storing routing state may use the best depth to determine
   which possible next-hops for the following criteria (in order):

   1.   The interface must be usable and destination are more optimal.

   If destination advertisements are activated in the administrative preference
        (if any) applies first.

   2.   A candidate that would cause DIO message as
   indicated by the `D' bit, the node sends unicast destination
   advertisements to augment the rank in the
        current DAG is not considered.

   3.   A router one of its DA parents, that has been validated is selected as usable, e.g. 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 local
        confidence that has exceeded some pre-configured threshold, DAG parent stimulates the sending of a delayed destination
   advertisement back, with the collection of all known prefixes (that
   is
        better.

   4. the prefixes learned via destination advertisements for nodes
   lower in the DAG, and any connected prefixes).  If none are grounded the Destination
   Advertisement Supported (A) bit is set in the DIO message for the
   DAG, then a DAG with a more preferred
        administrative preference destination advertisement is better.

   5.   A router that offers connectivity also sent 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
   once it has been added to the backup
        next_hop while assuming that this router won.

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

   9.   The router with a better router preference wins.

   10.  The preferred DA parent that was in use already is better.

   11.  A router that has announced set after a RA-DIO message more recently is
        preferred.

5.8.2.3.  Selection movement, or when
   the list of advertised prefixes has changed.

   A node that modifies its DAG Parent set may set the Backup next_hop

   o  The interface must `D' bit in
   subsequent DIO propagation in order to trigger destination
   advertisements to be usable updated to its DAG Parents and other inward
   nodes on the administrative preference (if
      any) applies first.

   o  The preferred parent is ignored.

   o  Candidate neighbors that are not in DAG.  Additional recommendations and guidelines
   regarding the same DAG use of this mechanism are ignored.

   o  Candidate neighbors with still under consideration and
   will be elaborated in a higher rank are ignored.

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

   o prefix below.  This is
   discovered by timing out after a request (a DIO message) or by
   receiving a no-DAO.  A router that has been validated no-DAO is a conveyed as usable, e.g. with a local
      confidence that has exceeded some pre-configured threshold, is
      better.

   o  The router DAO message with a better router preference wins.

   o  The backup next_hop
   DAO Lifetime of ZERO_LIFETIME.

   A node that was in use already is better.

5.9.  Establishing Routing State Outward Along the DAG

   The destination advertisement mechanism supports the dissemination capable of
   routing recording the state required to support traffic flows outward along information conveyed in
   a unicast DAO message will do so upon receiving and processing the
   DAG, from
   DAO message, thus building up routing state concerning destinations
   below it in the DAG root toward nodes.

   As DAG.  If a result node capable of destination advertisement operation:

   o  DAG discovery establishes recording state
   information receives a DAG oriented toward DAO message containing a DAG root using
      extended Neighbor Discovery RS/RA flows, along which inward routes
      toward Reverse Route Stack,
   then the DAG root are set up.

   o  Destination advertisement extends Neighbor Discovery in order to
      establish outward routes along node knows that the DAG.  Such paths consist of:
      *  Hop-By-Hop routing state within islands of `stateful' nodes.
      *  Source Routing `bridges' across DAO message has traversed one or more
   nodes who do that did 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 any routing state
      toward destinations by populating their routing tables with as it traversed the
      routes learned path
   from nodes in their sub-DAG.  In this process they
      may also learn necessary piecewise the DAO 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 node.  The node may
      forward destination advertisements, recording then extract the reverse route as
   Reverse Route Stack and retain the go included state in order to support specify
   Source Routing instructions along the construction of piecewise source
      routes.

   Nodes that are capable of storing routing state, return path towards the
   destination.  The node MUST set the RRCount back to zero and finally clear
   the DAG
   roots, are able Reverse Route Stack prior to learn which destinations are contained in passing the sub-
   DAG below DAO message information
   on.

   A node that is unable to record the node, and via which next-hop neighbors.  The
   dissemination and installation of this routing state into nodes
   allows for Hop-By-Hop routing from information conveyed in the DAG root outwards along
   DAO message will append the
   DAG.  The mechanism is further enhance by supporting next-hop address to the construction
   of source routes across stateless `gaps' in Reverse Route
   Stack, increment the DAG, where nodes are
   incapable of storing RRCount, and then pass the destination
   advertisement on without recording any additional routing state.  An adaptation of  In this
   mechanism allows for way
   the implementation Reverse Route Stack will contain a vector of loose-source routing.

   A special case, next hops that must
   be traversed along the reverse path that the DAO message has
   traveled.  The vector will be ordered such that the reception of a destination advertisement
   addressed to a link-local multicast address, allows for a node closest to
   learn destinations directly available from its one-hop neighbors.

   A design choice behind advertising routes via
   the destination
   advertisements will appear first in the list.  In such cases, if it
   is not useful to synchronize the parent implementation to try and children
   databases along build up redundant paths,
   the DAG, but instead to update them regularly node may choose to
   recover from convey the loss destination advertisement to one or
   more DAG parents in order 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 preference as guided by an excessive goal in terms of exchanges and protocol complexity.
   The approach used here results in
   implementation specific policy.

   In some cases (called hybrid cases), some nodes along the path a simple protocol with no real
   peering.
   destination advertisement follows inward along the DAG may store
   state and some may not.  The destination advertisement mechanism hence provides
   allows for
   periodic updates of the provisioning of routing state, as cued by occasional RAs and
   other mechanisms, similarly to other protocols state such as RIP [RFC2453].

5.9.1.  Destination Advertisement Message Formats

5.9.1.1.  DAO Option

   RPL extends Neighbor Discovery [RFC4861] and RFC4191 [RFC4191] to
   allow a node to include that when a destination advertisement option, which
   includes prefix information, in the Neighbor Advertisement (NA)
   messages.  A prefix option is normally present in RA messages only,
   but the NA packet
   is augmented with this option in order to propagate
   destination information inwards traversing outwards along the DAG.  The option is named DAG, some nodes may be able to
   directly forward to the Destination Advertisement Option (DAO), next hop, and an NA message
   containing this option other nodes may be referred able to as
   specify a destination
   advertisement, or NA-DAO.  The RPL use piecewise source route in order to bridge spans of destination advertisements
   allows the
   stateless nodes in within the DAG path on the way to build up the desired
   destination.

   In the case where no node is able to store any routing state for nodes
   contained in the sub-DAG in support of traffic flowing outward along as
   destination advertisements pass by, and 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |    Length     | Prefix Length |    RRCount    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          DAO Lifetime                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Route Tag                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   DAO Depth   |   Reserved    | DAG root ends up with DAO Sequence          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Prefix (Variable Length)                    |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Reverse Route Stack (Variable Length)             |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 7: The Destination Advertisement Option (DAO)

   Type: 8-bit unsigned identifying the Destination Advertisement
         option.  IANA had defined the IPv6 Neighbor Discovery Option
         Formats registry.  The suggested type value for the Destination
         Advertisement Option carried within
   messages that contain a NA message is 141, completely specified route back to be
         confirmed by IANA.

   Length:  8-bit unsigned integer.  The length of the option (including
         the Type and Length fields) in units of 8 octets.

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

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

   DAO Lifetime:  32-bit unsigned integer.  The length of time information in
         seconds (relative this case.

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

   o  A reference to the packet is sent) ND entry that was created for the
         prefix is valid advertising
      neighbor.

   o  The IPv6 address and interface for route determination.  A value of all one
         bits (0xFFFFFFFF) represents infinity.  A value of all zero
         bits (0x00000000) indicates a loss of reachability.

   Route Tag:  32-bit unsigned integer. the advertising neighbor.

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

   o  A 'reported' Boolean to
         give a priority keep track whether this prefix was
      reported already, and to prefixes that should be stored.  This may be
         useful in cases where intermediate nodes are capable of storing
         a limited amount which of routing state.  The further specification the DA parents.

   o  A counter of this field and its use is under investigation.

   DAO Depth:  Set retries to 0 by count how many DIO messages were sent on
      the node that owns interface to the prefix and first
         issues advertising neighbor without reachability
      confirmation for the NA-DAO message.  Incremented by all LLN nodes prefix.

   Note that
         propagate nodes may receive multiple information from different
   neighbors for a specific destination, as different paths through the NA-DAO message.

   Reserved:  8-bit unused field.  The reserved field MUST be set to
         zero on transmission and MUST
   DAG may be ignored on receipt.

   DAO Sequence:  Incremented by propagating information inwards along the DAG for the same
   destination.  A node that owns is recording routing state will keep track
   of the prefix for information from each
         new NA-DAO neighbor independently, and when it
   comes time to propagate the DAO message for that prefix.

   Prefix:  Variable-length field containing an IPv6 address or a particular prefix
         of an IPv6 address.  The Prefix Length field contains to
   the
         number of valid leading bits in DA parents, then the prefix. DAO information will be selected from among
   the advertising neighbors who offer the least depth to the
   destination.

   The bits in destination advertisement mechanism stores the prefix after entries in
   one of 3 abstract lists; the prefix length (if any) are reserved Connected, the Reachable and MUST
         be set the
   Unreachable lists.

   The Connected list corresponds to zero on transmission the prefixes owned and MUST be ignored on receipt.

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

5.9.2.  Destination Advertisement Operation

5.9.2.1.  Overview

   According to implementation specific policy, a subset or all process of the
   feasible parents being deleted, in the DAG may be selected order to send DAO
   messages with zero lifetime (also called no-DAO) to receive prefix
   information from the DA parents.

5.10.1.1.1.  Destination Advertisement Timers

   The destination advertisement mechanism.  This
   subset of DAG parents shall be designated mechanism requires 2 timers; the set of DA parents.

   As NA-DAO messages for particular destinations move inwards along
   DelayDAO timer and the
   DAG, a sequence counter RemoveTimer.

   o  The DelayDAO timer is used armed upon a stimulation to guarantee their freshness.  The
   sequence counter send a
      destination advertisement (such as a DIO message from a DA
      parent).  When the timer is incremented by armed, all entries in the source 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 NA-DAO message
   (the node that owns DAG rank of the prefix, or learned node.  The intention is that
      nodes located deeper in the prefix via some other
   means), each time it issues DAG should have a NA-DAO message 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.

   o  The RemoveTimer is used to clean up entries for its prefix.  Nodes
   who receive which DAO messages
      are no longer being received from the NA-DAO message and, if scope allows, will be
   forwarding sub-DAG.

      *  When a NA-DAO DIO message for the unmodified is sent that is requesting destination inwards
   along the DAG, will leave
         advertisements, a flag is set for all DAO entries in the sequence number unchanged.
   Intermediate nodes will check
         routing table.

      *  If the sequence counter before processing flag has already been set for a NA-DAO message, and if the DAO is unchanged (the sequence counter
   has not changed), then the NA-DAO message will be discarded without
   additional processing.  Further, if entry, the NA-DAO retry
         count is incremented.

      *  If a DAO message appears to be
   out of synch (the sequence counter is 2 or more behind received to confirm the present
   value) then entry, the DAO state entry is considered to be stale
         refreshed and the flag and count may be
   purged, cleared.

      *  If at least one entry has reached a threshold value and the NA-DAO message is discarded.  A depth
         RemoveTimer is also added
   for tracking purposes; not running, the depth entry is incremented at each hop as considered to be
         probably gone and the
   NA-DAO message RemoveTimer is propagated up started.

      *  When the DAG.  Nodes who RemoveTimer elapse, DAO messages with lifetime 0, i.e.
         no-DAOs, are storing
   routing state may use the depth sent to determine explicitly inform DA parents that the
         entries which possible next-hops
   for have reached the destination are more optimal.

   If destination advertisements threshold are activated in the RA-DIO message as
   indicated by the `D' bit, the node sends unicast destination
   advertisements to its DA parents, no longer
         available, and only accepts unicast
   destination advertisements from any nodes but those contained in the
   DA parent subset.

   Every NA to a DA parent MAY contain one or more DAOs.  Receiving a
   RA-DIO message with the `D' destination advertisement bit set from a
   DAG parent stimulates the sending of related routing states may be propagated and
         cleaned up.

   o  The RemoveTimer has a delayed destination
   advertisement back, with the collection duration of all known prefixes (that
   is the prefixes learned via destination advertisements for nodes
   lower in the DAG, and any connected prefixes).  If the min (MAX_DESTROY_INTERVAL,
      TBD(DIO Trickle Timer Interval)).

5.10.1.2.  Multicast Destination Advertisement Supported (A) bit Messages

   It is set in the RA-DIO message also possible for the
   DAG, then a destination advertisement is also sent node to multicast a DAG parent
   once it has been added DAO message to the DA parent set after a movement, or when
   the list of advertised prefixes has changed.  Destination
   advertisements may also
   link-local scope all-nodes multicast address FF02::1.  This message
   will be scheduled for sending when the PathDigest received by all node listening in range of the RA-DIO message has changed, indicating that some aspect of emitting node.
   The objective is to enable direct P2P communication, between
   destinations directly supported by neighboring nodes, without needing
   the
   inwards paths along RPL routing structure to relay the DAG has been modified.

   Destination advertisements may advertise positive (prefix is present)
   or negative (removed) NA-DAO messages, termed as no-DAOs. packets.

   A no-DAO
   is stimulated by multicast DAO message MUST be used only to advertise information
   about self, i.e. prefixes in the disappearance of a prefix below.  This is
   discovered by timing out after a request (a RA-DIO message) Connected list or addresses owned by
   receiving
   this node.  This would typically be a no-DAO.  A no-DAO multicast group that this node
   is listening to or a conveyed global address owned by this node, though it can
   be used to advertise any prefix owned by this node as a NA-DAO message with
   a DAO Lifetime of 0. well.  A node who
   multicast DAO message is capable of recording not used for routing and does not presume
   any DAG relationship between the state emitter and the receiver; it MUST
   NOT be used to relay information conveyed 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 NA-DAO message will do so upon DAOs.

   A node receiving and processing a multicast DAO message addressed to FF02::1 MAY
   install prefixes contained in the
   NA-DAO message, thus building up routing state concerning
   destinations below it DAO message in the DAG.  If routing table
   for local use.  Such a node capable of recording
   state information receives a NA-DAO MUST NOT perform any other processing on
   the DAO message containing (i.e. such a Reverse
   Route Stack, then the node knows that the NA-DAO message has
   traversed one or more nodes that did does not retain any routing state as presume it traversed the path is a DA
   parent).

5.10.1.3.  Unicast Destination Advertisement Messages from the DAO source Child to the node.  The
           Parent

   When sending a destination advertisement to a DA parent, a node may
   then extract
   includes the Reverse Route Stack and retain DAOs for prefix entries not already reported (since the included state
   last DA Trigger from an DIO message) in
   order to specify Source Routing instructions along the return path
   towards Reachable and Connected
   lists, as well as no-DAOs for all the destination.  The node MUST set entries in the RRCount back to zero Unreachable
   list.  Depending on its policy and clear the Reverse Route Stack prior ability to passing retain routing state,
   the NA-DAO message
   information on.

   A receiving node who is unable to SHOULD keep a record of the state information conveyed in reported DAO message.
   If the
   NA-DAO DAO message will append offers the next-hop address best route to the Reverse Route
   Stack, increment the RRCount, prefix as determined
   by policy and then pass the destination
   advertisement on without recording any additional state.  In this way other prefix records, the Reverse Route Stack will contain node SHOULD install a vector of next hops that must
   be traversed along route
   to the reverse path that prefix reported in the NA-DAO DAO message has
   traveled.  The vector will be ordered such that via the node closest to link local address
   of the destination will appear first reporting neighbor and it SHOULD further propagate the
   information in a DAO message.

   The DIO message from the list.  In such cases, if it DAG root is useful used to synchronize the implementation to try and build whole
   DAG, including the periodic reporting of destination advertisements
   back up redundant paths, the node may choose DAG.  Its period is expected to convey vary, depending on the destination advertisement to one or
   more DAG parents in order
   configuration of preference as guided by the trickle timer that governs the RAs.

   When a node receives a DIO message over an
   implementation specific policy.

   In some cases (called hybrid cases), some nodes along LLN interface from a DA
   parent, the path DelayDAO is armed to force a
   destination advertisement follows inward along full update.

   When the DAG may store
   state and some may not.  The destination advertisement mechanism
   allows node broadcasts a DIO message on an LLN interface, for the provisioning of routing state such all
   entries on that when a packet interface:

   o  If the entry is traversing outwards along CONFIRMED, it goes PENDING with the DAG, some nodes may be able to
   directly forward retry count
      set to 0.

   o  If the next hop, and other nodes may be able to
   specify entry is PENDING, the retry count is incremented.  If it
      reaches a piecewise source route in order to bridge spans of
   stateless nodes within maximum threshold, the path on entry goes ELAPSED If at least
      one entry is ELAPSED at the way to end of the desired
   destination.

   In process: if the case where no node RemoveTimer
      is able to store any routing state as
   destination advertisements pass by, and not running then it is armed with a jitter.

   Since the DAG root ends up DelayDAO timer has a duration that decreases with NA- the
   depth, it is expected to receive all DAO messages that contain a completely specified route back to the
   originating node in from all children
   before the form of timer elapses and 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 full update is not able sent to store the Reverse
   Route Stack information in this case.

   The destination advertisement mechanism requires stateful nodes to
   maintain lists of known prefixes.  A DA
   parents.

   Once the RemoveTimer is elapsed, the prefix entry contains the
   following abstract information:

   o  A reference is scheduled to be
   removed and moved to the ND entry Unreachable list if there are any DA parents
   that was created for need to be informed of the advertising
      neighbor.

   o  The IPv6 address and interface change in status for the advertising neighbor.

   o  The logical equivalent of the full destination advertisement
      information (including prefix,
   otherwise 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 entry is cleaned up right away.  The prefix
   entry is removed from the Unreachable list when no more DA parents.

   o  A counter of retries parents
   need to count how many RA-DIO messages were be informed.  This condition may be satisfied when a no-DAO
   is sent
      on the interface to all current DA parents indicating the advertising neighbor without reachability
      confirmation for loss of the prefix.

   Note prefix,
   and noting that nodes in some cases parents may receive multiple information have been removed from different
   neighbors for a specific destination, as different paths through the
   DAG may be propagating information inwards along
   set of DA parents.

5.10.1.4.  Other Events

   Finally, the DAG for the same
   destination.  A node who is recording routing state will keep track
   of the information from each neighbor independently, and when it
   comes time to propagate the NA-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 responds to a series
   of events, such as:

   o  Destination advertisement operation stopped: All entries in
   one of 3 the
      abstract lists; lists are freed.  All the Connected, routes learned from DAO
      messages are removed.

   o  Interface going down: for all entries in the Reachable and the
   Unreachable lists.

   The Connected list corresponds to on
      that interface, the prefixes owned associated route is removed, and managed by the local node.

   The Reachable list contains prefixes for which entry is
      scheduled to be removed.

   o  Loss of routing adjacency: When the node keeps
   receiving NA-DAO messages, and routing adjacency for those prefixes which have not yet
   timed out.

   The Unreachable list keeps track of prefixes which are no longer
   valid and a
      neighbor is lost, as per the procedures described in Section 5.13,
      and if the process of being deleted, associated entries are in order to send NA-DAO
   messages with zero lifetime (also called no-DAO) to the DA parents.

5.9.2.1.1.  Destination Advertisement Timers

   The destination advertisement mechanism requires 2 timers; Reachable list, the
   DelayNA timer
      associated routes are removed, and the RemoveTimer. entries are scheduled to be
      destroyed.

   o  The DelayNA timer is armed upon a stimulation  Changes to send a
      destination advertisement (such as a RA-DIO message from a DA
      parent).  When the timer is armed, parent set: 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 DelayNA timer has a duration that 'reported' and DelayDAO is DEF_NA_LATENCY divided armed.

5.10.1.5.  Aggregation of Prefixes by a multiple 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 DAG rank sub-DAG of the node.  The intention nodes
       it is aggregating for.

   2.  A node that
      nodes located deeper in the DAG should have a shorter DelayNA
      timer, allowing NA-DAO messages a chance is to be reported from
      deeper aggregated for is located somewhere else
       within the DAG, not in the DAG and potentially aggregated along sub-DAGs before
      propagating further inwards.

   o  The RemoveTimer sub-DAG of the aggregating node.

   3.  A node that is used to clean up entries be aggregated for which NA-DAO
      messages are no longer being received from is located somewhere else in
       the sub-DAG.

      *  When LLN.

   Consider a RA-DIO message is sent node M that is requesting destination
         advertisements, performing an aggregation, and a flag node N
   that is set for all DAO entries to be a member of the aggregation group.  A node Z situated
   above the node M in the
         routing table.

      *  If DAG, but not above node N, will see the flag has already been set
   advertisements for a DAO entry, the retry
         count is incremented.

      *  If a NA-DAO message is received to confirm aggregation owned by M but not that of the entry,
   individual prefix for N. Such a node Z will route all the entry
         is refreshed and packets for
   node N towards node M, but node M will have no route to the flag node N
   and count will fail to forward.

   Additional protocols may be cleared.

      *  If at least one entry has reached a threshold value and the
         RemoveTimer is not running, applied beyond the entry is considered scope of this
   specification to be
         probably gone dynamically elect/provision an aggregating node and the RemoveTimer is started.

      *  When the RemoveTimer elapse, NA-DAO messages with lifetime 0,
         i.e. no-DAOs, are sent
   groups of nodes eligible to explicitly inform DA parents that the
         entries who have reached the threshold be aggregated in order to provide route
   summarization for a sub-DAG.

5.11.  Loop Detection

   RPL loop avoidance mechanisms are no longer available, kept simple and the related routing states may be propagated designed to
   minimize churn and cleaned
         up.

   o  The RemoveTimer has a duration of min (MAX_DESTROY_INTERVAL,
      RA_INTERVAL).

5.9.2.2.  Multicast Destination Advertisement messages

   It is also possible states.  Loops may form for a node number of reasons,
   from control packet loss to multicast sibling forwarding.  RPL includes a NA-DAO message to the
   link-local scope all-nodes multicast address FF02::1.  This message
   will be received by all node listening in range
   reactive loop detection technique that protects from meltdown and
   triggers repair of the emitting node.
   The objective is to enable direct P2P communication, between
   destinations directly supported by neighboring nodes, without needing
   the broken paths.

   RPL routing structure to relay the packets.

   A multicast NA-DAO message MUST be used only to advertise loop detection uses information
   about self, i.e. prefixes that is placed into the packet in
   the Connected list or addresses owned by
   this node.  This would typically be a multicast group flow label.  It assumes that the flow label may be overloaded for
   this node purpose.  The flow label is listening 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 a global address owned by this node, though it can
   be used to advertise any prefix owned by this node as well. outwards.  A
   multicast NA-DAO message router sets the
         'O' bit when the packet is not used for routing expect to progress outwards (using
         DAO routes), and does not presume
   any DAG relationship between resets it when forwarding towards the emitter and root of
         the receiver; it DAG.  A host MUST
   NOT be used set the bit to relay information learned (e.g. information in 0.

   Sibling 'S' bit:  1-bit flag indicating whether the
   Reachable list) from another node; information obtained from packet has been
         forwarded via a
   multicast NA-DAO MAY be installed in sibling at the routing table present rank, and MAY be
   propagated by denotes a router in unicast NA-DAOs.

   A node receiving risk
         of a multicast NA-DAO message addressed sibling loop.  A host sets the bit to FF02::1 MAY
   install prefixes contained 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 NA-DAO message relative ranks and the direction as indicated in the routing table
   for local use.  Such a node 'O'
         bit.  A host MUST NOT perform any other processing on set the NA-DAO message (i.e. such a node does not presume it is bit to 0.

   DAO-Error 'D' bit:  1-bit flag indicating whether a DA
   parent).

5.9.2.3.  Unicast Destination Advertisement messages from child to
          parent

   When sending a destination advertisement DAO error was
         detected.  An undetected DAO error would have resulted in an
         inward to a DA parent, a node
   includes the DAOs for prefix entries outward transition that is not already reported (since the
   last DA Trigger from an RA-DIO message) in the Reachable and
   Connected lists, as well as no-DAOs for all the entries in expected with this
         spec.  A host MUST set the
   Unreachable list.  Depending on its policy and ability bit to retain
   routing state, 0.

   SenderRank:  8-bit field indicating the receiving node SHOULD keep a record rank of the
   reported NA-DAO message.  If the NA-DAO message offers sender.  A host
         MUST set the best route rank to INFINITE_RANK.  A router MUST place its
         own rank in the prefix as determined by policy and other prefix records, flow label when forwarding.

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

5.11.1.  Host Basic Operation

   It is expected that a route host that does not participate to the prefix reported RPL in any
   fashion is configured to set the NA-DAO
   message via the link local address flow label to all zeroes in its
   outgoing packets.  The host MAY send a packet to any router
   regardless of the reporting neighbor DAG and it RPL operations at large.

   A host that participates to RPL SHOULD further propagate zero out all the information in a NA-DAO message.

   The RA-DIO message from flags, and it
   MUST set the DAG root is used sender rank to synchronize INFINITE_RANK.  If the whole
   DAG, including host can map a
   flow to a given InstanceID then it MUST set the periodic reporting of destination advertisements
   back up flow label
   accordingly.  Forwarding rules are the DAG.  Its period same for this host and a
   router, and are described in the next section.

5.11.2.  Instance Forwarding

   Instance IDs is expected used to vary, depending on the
   configuration of the trickle timer that governs the RAs.

   When a node receives a RA-DIO message over an LLN interface 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 DA
   parent, packet along the DelayNA DAG that is armed
   associated to force a full update.

   When given instance.

   The InstanceID is placed by the node broadcasts a RA-DIO message on an LLN interface, for
   all entries on that interface:

   o  If source in the entry flow label.  It is CONFIRMED, it goes PENDING with not
   meaningful if the retry count packet has the flow label set to 0.

   o  If the entry is PENDING, the retry count is incremented.  If all zeroes.
   Otherwise it
      reaches a maximum threshold, the entry goes ELAPSED If at least
      one entry is ELAPSED at the end of MUST match the process: if DAG instance onto which the Destroy
      timer packet is not running then
   placed by any node, be it is armed with a jitter.

   Since the DelayNA timer has host or router.

   When a duration router receives a packet that decreases with the depth,
   it is expected to receive all NA-DAO messages from all children
   before the timer elapses flagged with a given instance
   ID and the full update is sent to the DA
   parents.

   Once node can forward the RemoveTimer is elapsed, packet along the prefix entry is scheduled DAG associated to be
   removed
   that instance, then the router MUST do so and moved to leave the Unreachable list if there are instance ID
   flag unchanged.

   If any DA parents
   that need node can not forward a packet along the DAG associated to be informed of the change
   instance ID in status for the prefix,
   otherwise flow label, then the prefix entry is cleaned up right away.  The prefix
   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.

5.9.2.4.  Other events

   Finally, node MAY either change the destination advertisement mechanism responds
   InstanceID to match a series
   of events, such as:

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

   o  Interface going down: for all entries in the Reachable list on DAG that interface, the associated route it is removed, and using for this packet or discard
   the entry packet.  That decision is
      scheduled to be removed.

   o  Loss of routing adjacency: When the routing adjacency for based on a
      neighbor policy.

   The default policy is lost, as per the procedures described in Section 5.11,
      and follows: if the associated entries are in the Reachable list, node can forward along the
   DAG 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 DelayNA 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 armed.

5.9.2.5.  Aggregation inconsistent is the direction of prefixes by a node

   There may be number of cases where packet does not match
   the rank relationship.  A receiver detects an inconsistency if it
   receives a aggregation may be shared within packet with either:

      the 'O' bit set (to outwards) from a group node of nodes.  In such a case, 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 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 a router to forward a packet to a
   future parent (same instance, same DAGID, higher sequence) without a
   loop, regardless of the sub-DAG rank of that parent.  In that case, the nodes
   sending router MUST present itself as a host on the future DAG and
   use a rank of INFINITE_RANK as it is aggregating for.

   2.  A node that is forwards the packets via a future
   parent to be aggregated for is located somewhere else
       within avoid a false positive.

   One inconsistency along the DAG, path is not in considered as a critical
   error and the sub-DAG packet may continue.  But a second detection along the
   path of a same packet should not occur and the aggregating node.

   3.  A node that packet is to be aggregated for dropped.

   This process is located somewhere else controlled by the Rank-Error bit in the LLN.

   Consider a node M who is performing Flow Label.
   When an aggregation, and a node N who inconsistency, is to be detected on a member of the aggregation group.  A node Z situated above
   the node M in packet, if the DAG, but Rank-Error bit
   was not above node N, will see set then the
   advertisements for Rank-Error bit is set.  If it was set the aggregation owned by M but not that of packet
   is discarded and the
   individual prefix for N. Such a node Z will route all the packets trickle timer is reset.

5.11.4.  Sibling Loop Avoidance

   When a packet is forwarded along siblings, it cannot be checked for
   node N towards node M, but node M will have no route to the node N
   forward progress and will fail to forward.

   Additional protocols may loop between siblings.  Experimental
   evidence has shown that one sibling hop can be applied beyond the scope of very useful but is
   generally sufficient to avoid loops.  Based on that evidence, this
   specification to dynamically elect/provision an aggregating node and
   groups of nodes eligible to be aggregated enforces the simple rule that a packet may not make 2
   sibling hops in order to provide route
   summarization for a sub-DAG.

5.9.2.6.  Default Values

   DEF_NA_LATENCY = To Be Determined

   MAX_DESTROY_INTERVAL = To Be Determined

5.10.  Multicast Operation

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

   As is traditional, row.

   When a listener uses host issues a protocol such as MLD with packet or when a router to register forwards a packet to a multicast group.

   Along
   non sibling, the path between Sibling bit in the packet must be reset.  When a
   router and the root of forwards to a sibling: if the DAG, MLD
   requests are mapped and transported as NA-DAO messages within Sibling bit was not set then the RPL
   protocol; each hop coalesces
   Sibling bit is set.  If the multiple requests for a same group
   as a single NA-DAO message to Sibling bit was set then the parent(s), in packet is
   discarded.  This does not denote a fashion similar to
   proxy IGMP, graph inconsistency but recursively between child router indicates
   that a new graph should probably be formed with a new sequence.

5.11.5.  DAO Inconsistency Loop Detection and parent up to the
   root. Recovery

   A DAO inconsistency happens when router might select to pass that has an outwards DAO
   route via a listener registration NA-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 child that link.  Alternatively the router might select to copy
   additional parents as it would do for NA-DAO messages advertising
   unicast destinations, in which case there might be duplicates is a remnant from an obsolete state that is
   not matched in the router will need child.  With DAO inconsistency loop recovery, a
   packet can be used to prune.

   As recursively explore and cleanup the obsolete
   DAO states along a result, multicast sub-DAG.

   In a general manner, a packet that goes outwards should never go
   inwards again.  So rather than routing states are installed in each router on inwards a packet with the way from
   Outwards bit set, the listeners to router MUST discard the root, enabling packet.  If DAO
   inconsistency loop recovery is applied, then the router SHOULD send
   the root to copy a
   multicast packet to all its children routers the parent that had issued passed it with the DAO-Error bit set.

   Upon a NA-DAO
   message including packet with a DAO for that multicast group, as well as all bit set, the
   attached nodes parent MUST remove the routing
   states that registered over MLD.

   For unicast traffic, it is expected caused forwarding to that the grounded root of an RPL
   DAG terminates RPL child, clear DAO-Error bit and MAY redistribute the RPL routes over
   send the
   external infrastructure using whatever routing protocol is used
   there.  For multicast traffic, the root MAY proxy MLD for all the
   nodes attached packet again.  The packet will make its way either to an
   alternate child or inwards to the RPL routers (this would be needed if the
   multicast source is located in the external infrastructure).  For
   such a source, parent.  If that parent still has an
   inconsistent DAO state via self, the packet process will recurse and that
   state will be replicated cleaned up as it flows outwards
   along the DAG based on well.

5.12.  Multicast Operation

   This section describes further the multicast routing table entries installed
   from the NA-DAO message.

   For a source inside the DAG, the packet is passed to the preferred
   parents, operations over
   an IPv6 RPL network, and if that fails then to the alternates in the DAG.  The
   packet is also copied specifically how unicast DAOs can be used to all the registered children, except for
   relay group registrations inwards.  Wherever the following text
   mentions MLD, one that passed the packet.  Finally, if there can read MLDv2 or v3.

   As is traditional, a listener in the
   external infrastructure then the DAG root has uses a protocol such as MLD with a
   router to further propagate register to a multicast group.

   Along the packet into path between the external infrastructure.

   As a result, router and the DAG Root acts as an automatic proxy Rendez-vous
   Point for root of the RPL network, DAG, MLD
   requests are mapped and transported as source towards the Internet for all
   multicast flows started in DAO messages within the RPL LLN.  So regardless of whether
   protocol; each hop coalesces the
   root is actually attached multiple requests for a same group
   as a single DAO message to the Internet, and regardless of whether
   the DAG is grounded or floating, parent(s), in a fashion similar to
   proxy IGMP, but recursively between child router and parent up to the root can serve inner
   root.

   A router might select to pass a listener registration DAO message to
   its preferred parent only, in which case multicast
   streams at packets coming
   back might be lost for all times.

5.11.  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 its sub-DAG if the formation of routing adjacencies transmission fails
   over that require maintenance.

   In IGPs such as OSPF [RFC4915] or IS-IS [RFC5120], the maintenance of
   a routing adjacency involves link.  Alternatively 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 router might select to copy
   additional parents as LLN and it would lead to excessive control traffic do for DAO messages advertising
   unicast destinations, in light of which case there might be duplicates that
   the data traffic with a negative impact on both link
   loads and nodes resources.  Overhead router will need to maintain the prune.

   As a result, multicast routing
   adjacency should be minimized.  Furthermore, it is not always
   possible to rely states are installed in each router on
   the link or transport layer way from the listeners to provide
   information of the associated link state.  The network layer needs root, enabling the root to
   fall back on its own mechanism.

   Thus RPL makes use of copy a different approach consisting of probing the
   neighbor using
   multicast packet to all its children routers that had issued a Neighbor Solicitation DAO
   message (see [RFC4861]).  The
   reception of including a Neighbor Advertisement (NA) message with DAO for that multicast group, as well as all the
   "Solicited Flag" set
   attached nodes that registered over MLD.

   For unicast traffic, it is used to verify expected that the validity grounded root of an RPL
   DAG terminates RPL and MAY redistribute the RPL routes over the
   external infrastructure using whatever routing
   adjacency.  Such mechanism MAY be protocol is used prior to sending a data
   packet.  This allows
   there.  For multicast traffic, the root MAY proxy MLD for detecting whether or not all the routing
   adjacency is still valid, and should it not
   nodes attached to the RPL routers (this would be needed if the case, select
   another feasible successor to forward
   multicast source is located in the packet.

5.12.  Packet Forwarding

   When forwarding external infrastructure).  For
   such a source, the packet to a destination, precedence is given to
   selection of a next-hop successor will be replicated as follows:

   1.  It is preferred to select a successor it flows outwards
   along the DAG based on the multicast routing table entries installed
   from the DAO message.

   For a DAG who source inside the DAG, the packet is
       supporting an OCP passed to the preferred
   parents, and related optimization if that maps fails then to an
       objective marked in the IPv6 header of alternates in the DAG.  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 also copied to all the
       destination that has been learned from a multicast destination
       advertisement (e.g. registered children, except for the destination is a one-hop neighbor), then
       use
   one that successor.

   4.  If passed the packet.  Finally, if there is an entry a listener in the routing table matching
   external infrastructure then the
       destination that DAG root has been learned from a unicast destination
       advertisement (e.g. to further propagate
   the destination is located outwards along packet into the
       sub-DAG), then use that successor.

   5.  If there is a DAG offering a route to external infrastructure.

   As a prefix matching result, the
       destination, then select one of those DAG parents as a successor.

   6.  If there is a DAG offering a default route with a compatible OCP,
       then select one of those DAG parents Root acts as a successor.

   7.  If there is a DAG offering a route to a prefix matching an automatic proxy Rendezvous Point
   for the
       destination, but all DAG parents have been tried RPL network, 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, source towards the packet is dropped.
       ICMP Destination Unreachable may be invoked.  An inconsistency is
       detected.

   TTL MUST be decremented when forwarding.  If Internet for all
   multicast flows started in the packet is being
   forwarded via a sibling, then RPL LLN.  So regardless of whether the TTL MAY be decremented more
   aggressively (by more than one)
   root is actually attached to limit the impact Internet, and regardless of possible
   loops.

   Note that whether
   the chosen successor MUST NOT be DAG is grounded or floating, the neighbor who was the
   predecessor root can serve inner multicast
   streams at all times.

5.13.  Maintenance of Routing Adjacency

   The selection of successors, along the packet (split horizon), except in default paths inward along the case where
   it is intended for
   DAG, or along the packet to change paths learned from an inward to an destination advertisements
   outward
   flow, such as switching from DIO routes along the DAG, leads to DAO routes as the
   destination is neared.

6.  RPL Variables

   DIO Timer  One instance per DAG formation of routing adjacencies
   that require maintenance.

   In IGPs such as OSPF [RFC4915] or IS-IS [RFC5120], the maintenance of
   a node 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 a member of.  Expiry
         triggers RA-DIO message transmission.  Trickle timer with
         variable interval not desirable in [0,
         DIOIntervalMin..2^DIOIntervalDoublings].  See Section 5.3.4

   DAG Hop Timer  Up constrained
   environments such as LLN and would lead to one instance per candidate DAG parent excessive control traffic
   in light of the
         `Held-Up' state per DAG that data traffic with a node is going to jump to.
         Expiry triggers candidate DAG parent negative impact on both link
   loads and nodes resources.  Overhead to become a DAG parent in maintain the `Current' state, as well as cancellation of any other DAG
         Hop timers associated with other DAG parents for that DAG.
         Duration routing
   adjacency should be minimized.  Furthermore, it is computed based not always
   possible to rely on the rank link or transport layer to provide
   information of the candidate DAG
         parent and DAG delay, as (candidates rank + random) *
         candidate's DAG_delay (where 0 <= random < 1).  See
         Section 5.7.1.

   Hold-Down Timer  Up to one instance per candidate DAG parent in the
         `Held-Down' state per DAG.  Expiry triggers associated link state.  The network layer needs to
   fall back on its own mechanism.

   Thus RPL makes use of a different approach consisting of probing the eviction
   neighbor using a Neighbor Solicitation message (see [RFC4861]).  The
   reception of a Neighbor Advertisement (NA) message with the
         candidate DAG parent from
   "Solicited Flag" set is used to verify the candidate DAG parent set.  The
         interval should validity of the routing
   adjacency.  Such mechanism MAY be chosen as appropriate to prevent flapping.
         See Section 5.7.

   DAG Heartbeat Timer  Up used prior to one instance per DAG that sending a data
   packet.  This allows for detecting whether or not the node routing
   adjacency is
         acting as DAG root of.  May still valid, and should it not be supported in all
         implementations.  Expiry triggers revision of
         DAGSequenceNumber, causing the case, select
   another feasible successor to forward the packet.

5.14.  Packet Forwarding

   When forwarding a new series of updated RA-DIO
         message packet to be sent.  Interval should be chosen appropriate a destination, precedence is given to
         propagation time
   selection of DAG and a next-hop successor as appropriate to application
         requirements (e.g. response time vs. overhead).  See
         Section 5.4

   DelayNA Timer  Up to one instance per DA parent (the subset of DAG
         parents chosen to receive destination advertisements) per DAG.
         Expiry triggers sending of NA-DAO message to follows:

   1.  In the DA parent.
         The interval scope of this specification, it is preferred to be proportional to DEF_NA_LATENCY/(node
         rank), such select a
       successor from a DAG that nodes of greater rank (further outward along matches the DAG) expire first, coordinating InstanceID marked in the sending of NA-DAO
         messages to allow for a chance
       IPv6 header of aggregation.  See
         Section 5.9.2.1.1

   DestroyTimer  Up to one instance per DA entry per neighbor (i.e.
         those neighbors who have given NA-DAO messages to this node as the packet being forwarded.

   2.  If a DAG parent) Expiry triggers local administrative preference favors a change route that has been
       learned from a different routing protocol than RPL, then use that
       successor.

   3.  If there is an entry in state for the DA
         entry, setting up to do unreachable (No-DAO) advertisements or
         immediately deallocating routing table matching the DA entry if there are no DA
         parents.  The interval
       destination that has been learned from a multicast destination
       advertisement (e.g. the destination is min(MAX_DESTROY_INTERVAL,
         RA_INTERVAL).  See Section 5.9.2.1.1

7.  Manageability Considerations

   The aim of this section a one-hop neighbor), then
       use that successor.

   4.  If there is to give consideration to the manageability
   of RPL, and how RPL will be operated an entry in LLN beyond the use of routing table matching the
       destination that has been learned from a MIB
   module.  The scope of this section 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 consider a prefix matching the following
   aspects of manageability: fault management, configuration, accounting
   and performance.

7.1.  Control
       destination, then select one of Function and Policy

7.1.1.  Initialization Mode

   When those DAG parents as a node successor.

   6.  If there is first powered up, it may either choose to stay silent
   and not send any multicast RA-DIO message until it has joined a DAG,
   or to immediately root DAG parent offering a transient default route then select
       that DAG and start sending multicast
   RA-DIO messages.  A RPL implementation SHOULD allow configuring
   whether the node should stay silent or should start advertising RA-
   DIO messages.

   Furthermore, the implementation SHOULD to allow configuring whether
   or not the node should start sending an RS message parent 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
   protocol parameters, which are further described in Section 5.1.1:

   DAGPreference

   NodePreference

   DAGDelay

   DIOIntervalDoublings

   DIOIntervalMin:

   DAGObjectiveCodePoint

   PathDigest

   DAGID

   Destination Prefixes

   DAG Root behavior:  In some cases, a node may not want to permanently
         act as successor.

   7.  If there is a DAG root if it cannot join a grounded DAG.  For
         example offering a battery-operated node may not want route to act as a prefix matching the
       destination, but all DAG
         root for a long period of time.  Thus a RPL implementation MAY
         support parents have been tried and are
       temporarily unavailable (as determined by the ability to configure whether or not a node could
         act as forwarding
       procedure), then select a DAG root for sibling as a configured period of time. successor.

   8.  Finally, if no DAG Hop Timer:  A RPL implementation siblings are available, the packet is dropped.
       ICMP Destination Unreachable may be invoked.  An inconsistency is
       detected.

   TTL MUST provide be decremented when forwarding.  If the ability packet is being
   forwarded via a sibling, then the TTL MAY be decremented more
   aggressively (by more than one) to
         configure limit the value impact of possible
   loops.

   Note that the DAG Hop Timer, expressed in ms.

   DAG Table Entry Suppression  A RPL implementation SHOULD provide chosen successor MUST NOT be the
         ability to configure a timer after neighbor that was the expiration
   predecessor of which the
         DAG table that contains all packet (split horizon), except in the records about a DAG case where
   it is
         suppressed, intended for the packet to be invoked if change from an inward to an outward
   flow, such as switching from DIO routes to DAO routes as the DAG parent set becomes empty.

7.1.3.  Trickle Timers

   A
   destination is neared.

6.  RPL implementation makes use of trickle timer to govern Constants and Variables

   ZERO_LIFETIME  This is the sending special value of RA-DIO message.  Such an algorithm is determined a by lifetime that indicates
         immediate death and removal.  ZERO_LIFETIME has a set value of
   configurable parameters that are then advertised by 0.

   BASE_RANK  This is the DAG rank for a virtual root
   along the DAG in RA-DIO messages.

   For each DAG, that might be used to
         coordinate multiple roots.  BASE_RANK has a RPL implementation MUST allow for the monitoring value of 0.

   ROOT_RANK  This is the following parameters, further described in Section 5.3.4:

   I

   T

   C

   I_min

   I_doublings:

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

7.1.4. DAG Heartbeat

   A RPL implementation may allow by configuration at root.  ROOT_RANK has a value of
         1.

   INFINITE_RANK  This is the DAG root to
   refresh constant maximum for the DAG states by updating rank.
         INFINITE_RANK has a value of 0xFF.

   RPL_DEFAULT_INSTANCE  This is the DAGSequenceNumber.  A RPL
   implementation SHOULD allow configuring whether or not periodic or
   event triggered mechanism are instance ID that is used by the DAG root this
         protocol by a node without a policy to control
   DAGSequenceNumber change.

7.1.5.  The Destination Advertisement Option

   The following set of parameters know any better.
         RPL_DEFAULT_INSTANCE has a value of the NA-DAO messages SHOULD 0.

   DEFAULT_DIO_INTERVAL_MIN  To be
   configurable:

   o  The DelayNA timer

   o  The Remove 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

7.1.6.  Policy Control with variable
         interval in [0, DIOIntervalMin..2^DIOIntervalDoublings].  See
         Section 5.4.4

   DAG discovery enables nodes Sequence Number Increment Timer  Up to implement different policies for
   selecting their one instance per DAG parents.

   A RPL implementation SHOULD allow configuring that
         the set node is acting as DAG root of.  May not be supported in all
         implementations.  Expiry triggers revision of acceptable
   or preferred Objective Functions (OF) referenced by their Objective
   Codepoints (OCPs) for
         DAGSequenceNumber, causing a node new series of updated DIO message
         to join a DAG, and what action be sent.  Interval should be
   taken if none chosen appropriate to
         propagation time of a node's candidate neighbors advertise DAG and as appropriate to application
         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
   configured allowable Objective Functions.

   A node in an LLN may learn routing information from different routing
   protocols including RPL.  It DA parent.  The
         interval is in this case desirable to control via
   administrative preference which route should be favored.  An
   implementation SHOULD 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 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 NA-DAO messages according to a set of rules defined
   by policy.

7.1.7.  Data Structures

   Some RPL implementation may limit the size chance of the candidate aggregation.  See Section 5.10.1.1.1

   RemoveTimer  Up to one instance per DA entry per neighbor
   list in order (i.e. those
         neighbors that have given DAO messages to bound the memory usage, this node as a DAG
         parent) Expiry triggers a change 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 state for the size of DA entry,
         setting up to do unreachable (No-DAO) advertisements or
         immediately deallocating the
   candidate neighbor list.

7.2.  Information and Data Models DA entry if there are no DA
         parents.  The information and data models necessary for 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 operation manageability
   of RPL, and how RPL will be defined operated in a separate document specifying LLN beyond the RPL SNMP MIB.

7.3.  Liveness Detection and Monitoring use of a MIB
   module.  The aim scope of this section is to describe the various RPL mechanisms
   specified to monitor consider the protocol.

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

   o  The candidate neighbors data structure

   o  For each DAG:

      *  A set manageability: fault management, configuration, accounting
   and performance.

7.1.  Control of candidate DAG parents

      *  A set of DAG parents (which are a subset of candidate DAG
         parents Function and may be implemented as such)

7.3.1.  Candidate Neighbor Data Structure

   A node in the candidate neighbor list is Policy

7.1.1.  Initialization Mode

   When a node discovered by the
   some means and qualified is first powered up, it may either choose to potentially become of neighbor stay silent
   and not send any multicast DIO message until it has joined a DAG, or
   to immediately root a
   sibling (with high enough local confidence). transient DAG and start sending multicast DIO
   messages.  A RPL implementation SHOULD provide a way monitor allow configuring whether the candidate neighbors list with some
   metric reflecting local confidence (the degree of stability of
   node should stay silent or should start advertising DIO messages.

   Furthermore, the
   neighbors) measured by some metrics.

   A RPL implementation MAY provide a counter reporting SHOULD to allow configuring whether
   or not the number 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
   times existing DAGs.

7.1.2.  DIO Base option

   RPL specifies 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 protocol parameters.

   A RPL implementation MUST keep track of SHOULD allow configuring the following
   DAG table values:

   o  DAGID

   o routing
   protocol parameters, which are further described in Section 5.1.3.1:

   DAGPreference
   InstanceID
   DAGObjectiveCodePoint

   o  A set of
   DAGID
   Destination Prefixes offered inwards along the DAG

   o  A set of candidate DAG Parents
   o  timer to govern the sending of RA-DIO messages for the
   DIOIntervalDoublings
   DIOIntervalMin

   DAG

   o  DAGSequenceNumber

   The set of candidate DAG parents structure is itself Root behavior:  In some cases, a table with the
   following entries:

   o  A reference node may not want to the neighboring device which is the DAG parent

   o  A record of most recent information taken from the permanently
         act as a DAG Information
      Object last processed from the candidate root if it cannot join a grounded DAG.  For
         example a battery-operated node may not want to act as a DAG Parent

   o
         root for a state associated with the role long period of time.  Thus a RPL implementation MAY
         support the candidate ability to configure whether or not a node could
         act as a potential
      DAG Parent {Current, Held-Up, Held-Down, Collision}, further
      described in Section 5.7

   o  A DAG Hop Timer, if instantiated

   o  A Held-Down Timer, if instantiated

   o  A flag reporting if the Parent is root for a DA Parent as described in
      Section 5.9

7.3.3.  Routing configured period of time.

   DAG Table

   To be completed.

7.3.4.  Other RPL Monitoring Parameters Entry Suppression  A RPL implementation SHOULD provide the
         ability to configure a counter reporting timer after the number expiration of
   a times which the node has detected an inconsistency with respect to
         DAG table that contains all the records about a DAG
   parent, e.g. is
         suppressed, to be invoked if the DAGID has changed. DAG parent set becomes empty.

7.1.3.  Trickle Timers

   A RPL implementation MAY log makes use of trickle timer to govern the reception sending
   of DIO message.  Such an algorithm is determined a malformed RA-DIO
   message along with the neighbor identification if avialable.

7.3.5.  RPL Trickle Timers

   A RPL implementation operating on by a set of
   configurable parameters that are then advertised by the DAG root MUST allow for the
   configuration of
   along the following trickle parameters:

   o  The DIOIntervalMin expressed DAG in ms

   o  The DIOIntervalDoublings 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 MAY SHOULD provide a counter reporting the number of
   times command (for example via API,
   CLI, or SNMP MIB) whereby any procedure that detects an inconsistency (and thus
   may cause the trickle timer has been reset).

7.4.  Verifying Correct Operation

   This section has to be completed in further revision of this document reset.

7.1.4.  DAG Sequence Number Increment

   A RPL implementation may allow by configuration at the DAG root to list potential Operations and Management (OAM) tools that could be
   used for verifying
   refresh the correct operation of RPL.

7.5.  Requirements on Other Protocols and Functional Components DAG states by updating the DAGSequenceNumber.  A RPL does
   implementation SHOULD allow configuring whether or not have any impact on periodic or
   event triggered mechanism are used by the operation DAG root to control
   DAGSequenceNumber change.

7.1.5.  Destination Advertisement Timers

   The following set of existing protocols.

7.6.  Impact on Network Operation

   To parameters of the DAO messages SHOULD be completed.

8.  Security Considerations

   Security Considerations
   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 are 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 developed
   taken if none of a node's candidate neighbors advertise one of the
   configured allowable Objective Functions.

   A node in accordance
   with recommendations laid out in, for example,
   [I-D.tsao-roll-security-framework].

9.  IANA Considerations

9.1.  DAG Information Option (DIO) Base Option

   The DAG Information Option is a container option carried within an
   IPv6 Router Advertisement message as defined in [RFC4861], which
   might contain a number of suboptions.  The base option regroups the
   minimum LLN may learn routing information set that from different routing
   protocols including RPL.  It is mandatory in all cases.

   IANA had defined the IPv6 Neighbor Discovery Option Formats registry.
   The suggested type value for the DAG Information Option (DIO) Base
   Option is 140, this case desirable to control via
   administrative preference which route should be confirmed by IANA.

9.2.  New Registry favored.  An
   implementation SHOULD allow for specifying an administrative
   preference for the Flag Field routing protocol from which the route was learned.

   A RPL implementation SHOULD allow for the configuration of the DIO Base Option

   IANA is requested "Route
   Tag" field of the DAO messages according to create a registry for set of rules defined by
   policy.

7.1.7.  Data Structures

   Some RPL implementation may limit the Flag field size of the DIO
   Base Option.

   New bit numbers candidate neighbor
   list in order to bound the memory usage, in which case some otherwise
   viable candidate neighbors may not be allocated only by considered and simply dropped
   from the candidate neighbor list.

   A RPL implementation MAY provide an IETF Consensus action.
   Each bit should be tracked with indicator on the following qualities:

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

   o  Capability description
   o  Defining RFC

   Three flags are currently defined:

       +-----+-------------------------------------+---------------+
       | Bit | Description                         | Reference     |
       +-----+-------------------------------------+---------------+
       |  0  | Grounded DAG                        | This document |
       |  1  | Destination Advertisement Trigger   | This document |
       |  2  | Destination Advertisement Supported | This document |
       +-----+-------------------------------------+---------------+

                           DIO Base Option Flags

9.3.  DAG
   candidate neighbor list.

7.2.  Information Option (DIO) Suboption

   IANA is requested to create a registry and Data Models

   The information and data models necessary for the DIO Base Option
   Suboptions

         +-------+------------------------------+---------------+
         | Value | Meaning                      | Reference     |
         +-------+------------------------------+---------------+
         |   0   | Pad1 - DIO Padding           | This document |
         |   1   | PadN - DIO suboption padding | This operation of RPL
   will be defined in a separate document |
         |   2   | DAG Metric Container         | This Document |
         |   3   | Destination Prefix           | This Document |
         +-------+------------------------------+---------------+

            DAG Information Option (DIO) Base Option Suboptions

9.4.  Destination Advertisement Option (DAO) Option

   The specifying the RPL protocol extends Neighbor Discovery [RFC4861] SNMP MIB.

7.3.  Liveness Detection and [RFC4191] Monitoring

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

   As specified in Section 5.2, an implementation must maintain a Destination Advertisement Option, which
   includes prefix information set of
   data structures in the Neighbor Advertisements messages. support of DAG discovery:

   o  The candidate neighbors data structure

   o  For each DAG:

      *  A set of DAG parents

7.3.1.  Candidate Neighbor Advertisement messages are augmented with the
   Destination Advertisement Option (DAO).

   IANA had defined the IPv6 Neighbor Discovery Option Formats registry.
   The suggested type value for Data Structure

   A node in the Destination Advertisement Option
   carried within a Neighbor Advertisement message candidate neighbor list is 141, to be
   confirmed a node discovered by IANA.

9.5.  Objective Code Point

   This specification requests that an Objective Code Point registry, as the
   some means and qualified to be specified in [I-D.ietf-roll-routing-metrics], reserve potentially become of neighbor or a
   sibling (with high enough local confidence).  A RPL implementation
   SHOULD provide a way monitor the
   Objective Code Point value 0x0000, for candidate neighbors list with some
   metric reflecting local confidence (the degree of stability of the purposes designated as OCP
   0 in this document.

10.  Acknowledgements

   The ROLL Design Team would like to acknowledge
   neighbors) measured by some metrics.

   A RPL implementation MAY provide a counter reporting the review, feedback,
   and comments from Dominique Barthel, Yusuf Bashir, Mathilde Durvy,
   Manhar Goindi, Mukul Goyal, Quentin Lampin, Philip Levis, Jerry
   Martocci, Alexandru Petrescu, and Don Sturek.

   The ROLL Design Team would like 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 acknowledge govern the guidance and input
   provided by sending of DIO messages for the ROLL Chairs, David Culler and JP Vasseur. DAG
   o  DAGSequenceNumber

   The ROLL Design Team would like set of DAG parents structure is itself a table with the following
   entries:

   o  A reference to acknowledge prior contributions the neighboring device which is the DAG parent

   o  A record 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

   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
   USA

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

   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.

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-indus-routing-reqs]
              Networks, D., Thubert, P., Dwars, S., and T. Phinney,
              "Industrial Routing Requirements in Low Power and Lossy
              Networks", draft-ietf-roll-indus-routing-reqs-06 (work in
              progress), June 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-00 (work in progress),
              April 2009.

   [I-D.ietf-roll-terminology]
              Vasseur, J., "Terminology in Low power And Lossy
              Networks", draft-ietf-roll-terminology-01 (work in
              progress), May 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>.

   [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.

   [RFC4461]  Yasukawa, S., "Signaling Requirements for Point-to-
              Multipoint Traffic-Engineered MPLS Label Switched Paths
              (LSPs)", RFC 4461, April 2006.

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

   [RFC4875]  Aggarwal, R., Papadimitriou, D., and S. Yasukawa,
              "Extensions to Resource Reservation Protocol - Traffic
              Engineering (RSVP-TE) for Point-to-Multipoint TE Label
              Switched Paths (LSPs)", RFC 4875, May 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.

Appendix A.  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 8.  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 9, 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 RAs containing
   DIO, 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 advertise RA-DIO multicasts.  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
   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 8: 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 8.  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 9: Example DAG

B.1.  Moving Down a DAG

   Consider node (56) in the example of Figure 8.  In the unmodified
   example, node (56) is at rank 6 with one DAG parent, {(43)}, and one
   sibling (55).  Suppose, for example, that node (56) wished to expand
   its DAG parent set to contain node (55), as {(43), (55)}.  Such a
   change would require node (56) to detach from the DAG, to defer
   reattachment until a loop avoidance algorithm has completed, and to
   then reattach to the DAG with {(43), (55)} as it's DAG parents.  When
   node (56) detaches from the DAG, it is able to act as the root of its
   own floating DAG and establish its frozen sub-DAG (which is empty).
   Node (56) can then observe that Node (55) is still attached to the
   original DAG, that its sequence number is able to increment, and
   deduce that Node (55) is safely not behind Node (56).  There is then
   little change for a loop, and Node (56) may safely reattach to the
   DAG, with parents {(43), (55)}.  At reattachment time, node (56)
   would present itself with a rank deeper than that of its deepest DAG
   parent (node (55) at rank 6), rank 7.

B.2.  Link Removed

   Consider the example of Figure 8 when link (13)-(24) goes down.

   o  Node (24) will detach and become the root of its own floating DAG

   o  Node (34) will learn that its DAG parent is now part of its own
      floating DAG, will consider that it can remain a part of the DAG
      rooted at node (LBR) via node (33), and will initiate procedures
      to detach from DAG (LBR) in order to re-attach at a lower rank.

   o  Node (45) will similarly make preparations to remain attached to
      the DAG rooted at (LBR) by detaching from Node (34) and re-
      attaching at a lower rank to node (44).

   o  Node (34) will complete re-attachment to Node (33) first, since it
      is able to attach closer to the root of the DAG.

   o  Node (45) will cancel plans to detach/reattach, keep node (34) as
      a DAG parent, and update its dependent rank accordingly.

   o  Node (45) may now anyway add node (44) to its set of DAG parents,
      as such an addition does not require any modification to its own
      rank.

   o  Node (24) will observe that it may reattach to the DAG rooted at
      node (LBR) by selecting node (34) as its DAG parent, thus
      reversing the relationship that existed in the initial state.

B.3.  Link Added

   Consider most recent information taken from the example of Figure 8 when link (12)-(42) appears.

   o  Node (42) will see a chance to get closer to DAG Information
      Object last processed from the LBR by adding
      (12) to its set of DAG parents, {(32), (12)} Parent

   o  Node (42) may be content to leave its advertised rank at 5,
      reflecting  A flag reporting if the Parent is a rank deeper than its deepest parent (32).

   o  Node (42) may now choose to remain where it is, with two parents
      {(12), (32)}.  Should there be DA Parent as described in
      Section 5.10

7.3.3.  Routing Table

   For each route provisioned by RPL operation, a reason for Node (42) to evict
      Node (32) from its set RPL implementation
   MUST keep track of DAG parents, Node (42) would then
      advertise itself at rank 2, thus moving up the DAG.  In this case,
      Node (53), (54), and (55) may similarly follow and advertise
      themselves at rank 3.

B.4.  Node Removed

   Consider the example of Figure 8 when node (41) disappears. following:

   o  Node (51) and (52) will now have empty DAG parent sets and be
      detached from the DAG rooted by (LBR), advertising themselves as
      the root of their own floating DAGs.  Destination Prefix

   o  Node (52) would observe a chance to reattach to  Destination Prefix Length

   o  Lifetime Timer

   o  Next Hop

   o  Next Hop Interface

   o  Flag indicating that the DAG rooted at
      (LBR) by adding Node (53) to its set 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 DAG parents, after
   a times the node has detected an
      appropriate delay inconsistency with respect to avoid creating loops.  Node (52) will then
      advertise itself in the a DAG rooted at (LBR) at rank 7.

   o  Node (51) will then be able to reattach to
   parent, e.g. if the DAG rooted at (LBR)
      by adding Node (52) to its set of DAG parents and advertising
      itself at rank 8.

B.5.  New LBR Added

   Consider DAGID has changed.

   A RPL implementation MAY log the example reception of Figure 8 when a new LBR, (LBR2) appears, malformed DIO message
   along with
   connectivity (LBR2)-(52), (LBR2)-(53).

   o  Nodes (52) and Node (53) will see a chance to join the neighbor identification if avialable.

7.3.5.  RPL Trickle Timers

   A RPL implementation operating on a new DAG
      rooted at (LBR2) with a rank of 2.  Node (52) and (53) may take
      this chance immediately, as there is no risk root MUST allow for the
   configuration of forming loops when
      joining the following trickle parameters:

   o  The DIOIntervalMin expressed in ms

   o  The DIOIntervalDoublings

   A RPL implementation MAY provide a DAG that counter reporting the number of
   times an inconsistency (and thus the trickle timer has never before been encountered.  Note 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 nodes may choose 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 join the new DAG rooted at (LBR2) if and
      only if (LBR2) offers more optimum properties be developed in line accordance
   with the
      implementation specific local policy.

   o  Nodes (52) and (53) begin to send RA-DIO messages advertising
      themselves at rank 2 in the DAGID (LBR2).

   o  Nodes (51), (41), (42), and (54) may then choose 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 join the new be used carry DAG at rank 3, possibly to get closer to the Information Objects, DAG root.  Note that Information
   Solicitations, and Destination Advertisement Objects in a more advanced case, these nodes also remain members support of
   RPL operation.

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

9.2.  New Registry for example in support of different
      constraints RPL Control Codes

   IANA is requested to create a registry, RPL Control Codes, for different types the
   Code field of traffic.

   o  Node (55) the ICMPv6 RPL Control Message.

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

   o  Code

   o  Description

   o  Defining RFC

   Three codes are currently defined:

        +------+----------------------------------+---------------+
        | Code | Description                      | Reference     |
        +------+----------------------------------+---------------+
        | 0x01 | DAG at rank 4, possibly to get
      closer to the Information Solicitation     | This document |
        | 0x02 | DAG root.

   o  The remaining nodes may choose to remain in their current
      positions within Information Object           | This document |
        | 0x04 | Destination Advertisement Object | This document |
        +------+----------------------------------+---------------+

                             RPL Control Codes

9.3.  New Registry for the Control Field of the DAG rooted at node (LBR), since there DIO Base Option

   IANA is no
      clear advantage requested to create a registry for the Control field of the
   DIO Base Option.

   New bit numbers may be gained allocated only by moving to 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:

      +-------+-------------------------------------+---------------+
      |  Bit  | Description                         | Reference     |
      +-------+-------------------------------------+---------------+
      |   0   | Grounded DAG (LBR2).

B.6.                        | This document |
      |   1   | Destination Advertisement

   Consider the example Trigger   | This document |
      |   2   | Destination Advertisement Supported | This document |
      | 5,6,7 | DAG depicted in Figure 9.  Suppose that Nodes
   (22) and (32) are unable to record routing state.  Suppose that Node
   (42) Preference                      | This document |
      +-------+-------------------------------------+---------------+

                           DIO Base Option Flags

9.4.  DAG Information Object (DIO) Suboption

   IANA is able requested to perform prefix aggregation on behalf of Nodes (53),
   (54), and (55).

   o  Node (53) 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 send a NA-DAO message like to Node (42), indicating acknowledge the
      availability of destination (53).

   o  Node (54) review, feedback, and Node (55)
   comments from Emmanuel Baccelli, Dominique Barthel, Yusuf Bashir,
   Mathilde Durvy, Manhar Goindi, Mukul Goyal, Anders Jagd, Quentin
   Lampin, Jerry Martocci, Alexandru Petrescu, and Don Sturek.

   The authors would similarly send NA-DAO messages like to
      Node (42) indicating their own destinations.

   o  Node (42) would collect acknowledge the guidance and store input provided
   by the routing state for
      destinations (53), (54), ROLL Chairs, David Culler and (55).

   o  In this example, Node (42) may then be capable JP Vasseur.

   The authors would like to acknowledge prior contributions of representing
      destinations (42), (53), (54), 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 (55) in the aggregation (42').

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

   o  Node (32) does not want to maintain any routing state, so it adds
      onto Arsalan Tavakoli, which have provided useful design
   considerations to RPL.

11.  Contributors

   RPL is the Reverse Route Stack in result of the NA-DAO message and passes
      it on to Node (22) as (42'):[(42)].  It may send a separate NA-DAO
      message to indicate destination (32).

   o  Node (22) does not want to maintain any routing state, so it adds
      on to contribution of the Reverse Route Stack in following members of the
   ROLL Design Team, including the NA-DAO message editors, and passes it
      on to Node (12) as (42'):[(42), (32)].  It also relays the NA-DAO
      message containing destination (32) to Node 12 additional contributors
   as (32):[(32)], and
      finally may send a NA-DAO message 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
   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 itself indicating
      destination (22).

   o  Node (12) is capable use in RFCs to maintain routing state again, Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

12.2.  Informative References

   [I-D.ietf-bfd-base]
              Katz, D. and receives
      the NA-DAO messages from Node (22).  Node (12) then learns:
      *  Destination (22) is available via Node (22)
      *  Destination (32) is available via Node (22) 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 the piecewise
         source route to (32)
      *  Destination (42') is available via Node (22) 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 the piecewise
         source route to (32), (42').

   o  Node (12) sends NA-DAO messages to (LBR), allowing (LBR) to learn
      routes to the destinations (12), (22), (32),
              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 (42'). (42),
      (53), (54), G. Porcu, "Home Automation
              Routing Requirements in Low Power and (55) are available via the aggregation (42').  It
      is not necessary 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 Node (12) to propagate the piecewise source
      routes to (LBR).

B.7.  Example: DAG Parent Selection

   For example, suppose that a node (N) is not attached to any DAG,
              Path Calculation in Low Power and
   that it is Lossy Networks",
              draft-ietf-roll-routing-metrics-01 (work in range of nodes (A), (B), (C), (D), 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 (E).  Let all
   nodes be configured to use an OCP which defines a policy such that
   ETX is to be minimized A.
              Lozano, "A Security Framework for Routing over Low Power
              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) 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 its neighbors (A-E) all have an ETX A.
              Woo, "The Emergence of 1 (which is
   learned by node (N) through some implementation specific method).
   Let node (N) be configured to send IPv6 Router Solicitation (RS)
   messages to probe for nearby DAGs.

   o  Node (N) transmits a Router Solicitation.

   o  Node (B) responds.  Node (N) investigates Networking Primitive in Wireless
              Sensor Networks", Communications of the RA-DIO message, ACM, v.51 n.7,
              July 2008,
              <http://portal.acm.org/citation.cfm?id=1364804>.

   [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
      learns that Node (B) is a member of DAGID 1 at rank 4, L.
              Wood, "Advice for Internet Subnetwork Designers", BCP 89,
              RFC 3819, July 2004.

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

   [RFC4191]  Draves, R. and Node (E) at rank 4.

   o  Node (D) responds.  Node (D) has a RA-DIO message that indicates
      that it is a member of DAGID 1 at rank 2, but it carries 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
      attribute `Blue'.  Node (N)'s policy function rejects Node (D), 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 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) N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to its set of DAG parents
              Intermediate Systems (IS-ISs)", RFC 5120, February 2008.

   [RFC5548]  Dohler, M., Watteyne, T., Winter, T., and D. Barthel,
              "Routing Requirements for
      DAGID 1.  Following 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 mechanisms following properties, consistent with the
   requirements specified by the OCP, and given
      that the ETX application-specific requirements
   documents.

A.1.1.  IPv6 Architecture

   RPL is 1 for the link between (N) and (E), Node (N) strictly compliant with layered IPv6 architecture.

   Further, RPL is
      now at rank 5 in DAGID 1.

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

   o  Node (N) is a sibling the practical support
   and implementation of Node (C), both are at rank 5.

   o  Node (N) IPv6 architecture on devices which may now forward traffic intended for the default
      destination inward along DAGID 1 via nodes (B) 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 (E).  In some
      cases, e.g. if 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 (B) within the LLN from and (E) to egress points are tried very
   common in LLNs.  Low power and fail, node (N) lossy network Border Router (LBR)
   nodes may
      also choose to forward traffic to its sibling node (C), without
      making inward progress but with typically be at the intention that node (C) or a
      following successor can make inward progress.  Should Node (C) root of such flows, although such flows
   are not
      have a viable parent, it should never send 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 packet back to Node
      (N) (to avoid a 2-node loop).

B.8.  Example: DAG Maintenance

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

              -1-                    -2-                    -3-

                        Figure 10: DAG Maintenance

   Consider aforementioned routing requirements documents, RPL
   supports the example depicted in Figure 10-1.  In this example, Node
   (A) is attached installation of multiple paths.  The use of multiple
   paths include sending duplicated traffic along diverse paths, as well
   as to a DAG at some rank d.  Node (A) is a DAG parent support advanced features such as Class of
   Nodes (B) and (C).  Node (C) is Service (CoS) based
   routing, or simple load balancing among a DAG parent set 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 paths (which could
   be useful for the LLN to Node (B) in some cases, 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 link (C)->(A) construction of which is
   temporarily unavailable, but with some chance governed by
   different Objective Functions (OF).  Details of creating a loop
   (e.g. if multiple nodes RPL operation in a set
   support of siblings start forwarding
   `sideways' in a cycle) and requiring multiple instances are beyond the intervention scope of additional
   mechanisms to detect and break the loop.

   Consider the case where Node (C) hears a RA-DIO message from a Node
   (Z) at present
   specification.

A.1.3.  Constraint Based Routing

   The RPL design supports constraint based routing, based on 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 of
   routing metrics and attach directly to Node (Z) without creating constraints.  The routing metrics and constraints
   for links and nodes with capabilities supported by RPL are specified
   in a
   loop, because its rank will decrease.

   Now consider the case where companion document to this specification,
   [I-D.ietf-roll-routing-metrics].  RPL signals the link (C)->(A) becomes nonviable, metrics,
   constraints, and
   node (C) must move to related Objective Functions (OFs) in use in a deeper rank within the DAG:

   o  Node (C) must first detach from the DAG
   particular implementation by removing Node (A) from
      its DAG parent set, leaving means of an empty DAG parent set.  Node (C)
      becomes Objective Code Point (OCP).
   Both the root routing metrics, constraints, and the OF help determine the
   construction of its own floating, less preferred, DAG.

   o  Node (D), hearing a modified RA-DIO message from Node (C), follows
      Node (C) into the floating DAG.  This Directed Acyclic Graphs (DAG) using a distributed
   path computation algorithm.

A.2.  Deferred Requirements

   NOTE: RPL is depicted 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 10-2. 11.  In general, any node with no other options in this
   example the sub-DAG of Node
      (C) will follow Node (C) into links depicted are all usable L2 links.  Suppose that all
   links are equally usable, and that the floating DAG, maintaining implementation specific policy
   function is simply to minimize hops.  This LLN physical topology then
   yields the
      structure of DAG depicted in Figure 12, where the sub-DAG.

   o  Node (C) hears 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 RA-DIO message from Node (B) and determines it is
      able trickle timer.  Nodes (11), (12), (13),
   have selected (LBR) as their only parent, attached to rejoin the grounded DAG by reattaching at a deeper rank to
      Node (B).  Node (C) starts a DAG Hop timer to coordinate this
      move.

   o  The timer expires
   2, and periodically multicast DIOs.  Node (C) adds Node (B) to (22) has selected (11) and
   (12) in its DAG parent
      set. set, and advertises itself at rank 3.  Node (C)
   (22) thus has now safely moved deeper within the grounded a set of DAG
      without creating any loops.  Node (D), parents {(11), (12)} and any other sub-DAG of
      Node (C), will hear 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 modified RA-DIO message sourced from links depicted represent the usable L2 connectivity
   available in the LLN.  For example, Node
      (C) (31) can communicate
   directly with its neighbors, Nodes (21), (22), (32), and follow (41).  Node (C) in a coordinated manner to reattach to the
      grounded DAG.  The final DAG is depicted in Figure 10-3

B.9.  Example: Greedy Parent Selection
   (31) cannot communicate directly with any other nodes, e.g. (33),
   (23), (42).  In this example these links offer bidirectional
   communication, and Instability

         (A)                    (A)                    (A)
          |\                     |\                     |\ `bad' links are not depicted.

                      Figure 11: Example LLN Topology

                                     (LBR)
                                     / | `-----. \
                                .---`  | `-----.  `----.
                               /       | `-----.        \
                            (11)      (12)      (13)
                             | \       | \       | \
         (B)       (C)          (B)
                             |  `----. |  `----. |  `----.
                             |        \|        \|        \
                            (21)      (22)      (23)      (24)
                             |        /|        /|         |
                             |  .----` |  .----` |         |
                             | /       | /       |         |
                            (31)      (32)      (33)      (34)
                             |        /| \       | \       |        (C) \
                             |  .----` |  `----. |  `----. |  `----.
                             | /
                                   `-----.       |        \|        \|        \
                   .--------(41)      (42)      (43)      (44)      (45)
                  /         /         /| \       | \
            .----`    .----`    .----` |  `----. |  `----.
           /         /         /       | .-----`        \|            |/
                                          (C)          (B)

              -1-                    -2-                    -3-

                  Figure 11: Greedy        \
        (51)      (52)      (53)      (54)      (55)      (56)

   Note that the links depicted represent directed links in the DAG Parent Selection

   Consider
   overlaid on top of the example physical topology depicted in Figure 11.  A DAG is  As
   such, the depicted in 3
   different configurations.  A usable link edges represent the relationship between (B) and (C) exists
   in all 3 configurations.  In Figure 11-1, Node (A) is a DAG parent
   for Nodes (B) and (C), nodes and (B)--(C) is a sibling link.  In
   Figure 11-2, Node (A) is a
   their DAG parent for Nodes (B) and (C), parents, wherein all depicted edges are directed and Node
   (B) is also a
   oriented `up' on the page toward the DAG parent for Node (C).  In Figure 11-3, Node (A) is a root (LBR).  The DAG parent for Nodes (B) and (C), may
   provide default routes within the LLN, and Node (C) is also a serves as the foundation
   on which RPL builds further routing structure, e.g. through the
   destination advertisement mechanism.

                          Figure 12: Example DAG parent
   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.

B.1.  Destination Advertisement

   Consider the example DAG illustrated depicted in Figure 11-1.
   In this example, 12.  Suppose that Nodes (B)
   (22) and (C) may most prefer Node (A) as a DAG
   parent, but (32) 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 unable 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 11-1 be the initial condition.

   o record routing state.  Suppose that Node (C) first
   (42) is able to leave the DAG perform prefix aggregation on behalf of Nodes (53),
   (54), and rejoin at (55).

   o  Node (53) would send a
      lower rank, taking both Nodes (A) DAO message to Node (42), indicating the
      availability of destination (53).

   o  Node (54) and (B) as DAG parents as
      depicted in Figure 11-2.  Now Node (C) is deeper than both Nodes
      (A) (55) would similarly send DAO messages to Node
      (42) indicating their own destinations.

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

   o  In this example, Node (C) is satisfied (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 have 2 DAG parents.
      Node 32.

   o  Suppose  Node (B), in its greediness, is willing (32) does not want to receive maintain any routing state, so it adds
      onto to the Reverse Route Stack in the DAO message and
      process passes it
      on to Node (22) as (42'):[(42)].  It may send a RA-DIO separate DAO
      message from to indicate destination (32).

   o  Node (C) (against (22) does not want to maintain any routing state, so it adds
      on to the rules of RPL),
      and then Node (B) leaves Reverse Route Stack in the DAG and rejoins at a lower rank,
      taking both Nodes (A) DAO message and (C) passes it on
      to Node (12) as DAG parents.  Now (42'):[(42), (32)].  It also relays the DAO
      message containing destination (32) to Node (B) is
      deeper than both Nodes (A) and (C) 12 as (32):[(32)], and is satisfied with 2 DAG
      parents.
      finally may send a DAO message for itself indicating destination
      (22).

   o  Then  Node (C), because it (12) is also greedy, will leave and rejoin
      deeper, capable to again get 2 parents maintain routing state again, and have a lower rank receives
      the DAO messages from Node (22).  Node (12) then both of
      them.

   o  Next learns:
      *  Destination (22) is available via Node (B) will again leave and rejoin deeper, to again get 2
      parents

   o  And again (22)
      *  Destination (32) is available via Node (C) leaves and rejoins deeper...

   o  The process will repeat, and the DAG will oscillate between
      Figure 11-2 (22) and Figure 11-3 until the nodes count piecewise
         source route to infinity (32)
      *  Destination (42') is available via Node (22) and
      restart the cycle again. piecewise
         source route to (32), (42').

   o  This cycle can be averted through mechanisms in RPL:

      *  Nodes (B) and (C) stay at a rank sufficient  Node (12) sends DAO messages to attach (LBR), allowing (LBR) to their
         most preferred parent (A) learn
      routes to the destinations (12), (22), (32), and don't go for any deeper (worse)
         alternate parents (Nodes are not greedy)

      *  Nodes (B) (42'). (42),
      (53), (54), and (C) do not process RA-DIO messages from nodes
         deeper than themselves (because such nodes (55) are possibly in
         their own sub-DAGs)

B.10. available via the aggregation (42').  It
      is not necessary for Node (12) to propagate the piecewise source
      routes to (LBR).

B.2.  Example: DAG Merge

                                :
                                :
                               (A)       (D)
                                |         |
                                |         |
                                |         |
                               (B)       (E)
                                |         |
                                |         |
                                |         |
                               (C)       (F)

                          Figure 12: Merging DAGs

   Consider the example depicted Parent Selection

   For example, suppose that a node (N) is not attached to any DAG, and
   that it is in Figure 12.  Nodes range of nodes (A), (B), (C), (D), and (C)
   are part (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 larger grounded DAG, where implementation specific method).
   Let node (N) be configured to send RPL DIS messages to probe for
   nearby DAGs.

   o  Node (A) is at (N) transmits a rank of
   d, RPL DIS message.

   o  Node (B) at d+1, and responds.  Node (C) at d+2.  The DAG comprised of Nodes
   (D), (E), (N) investigates the DIO message, and (F)
      learns that Node (B) is a floating, less preferred, DAG, with member of DAGID 1 at rank 4, and not
      `Blue'.  Node (D)
   as the DAG root.  This floating DAG may have been formed, for
   example, in the absence (N) takes note of a grounded DAG or when this, but is not yet confident.

   o  Similarly, Node (D) had to
   detach (N) hears from a grounded DAG Node (A) at rank 9, Node (C) at
      rank 5, and Node (E) and (F) followed.  All nodes are
   using compatible objective code points.

   Nodes (D), (E), and (F) would rather join the more preferred grounded
   DAG if they are able than to remain in the less preferred floating
   DAG.

   Next, let links (C)--(D) and (A)--(E) become viable.  The following
   sequence of events may then occur in a typical case: at rank 4.

   o  Node (D) will receive and process responds.  Node (D) has a RA-DIO DIO message from Node (C)
      on link (C)--(D). that indicates that
      it is a member of DAGID 1 at rank 2, but it carries the attribute
      `Blue'.  Node (D) will consider (N)'s policy function rejects Node (C) a candidate
      neighbor (D), and no
      further consideration is given.

   o  This process the RA-DIO message since continues until Node (C) belongs (N), based on implementation
      specific policy, builds up enough confidence to trigger a different DAG (different DAGID) than decision
      to join DAGID 1.  Let Node (D). (N) determine its most preferred parent
      to be Node (D) will
      note that (E).

   o  Node (C) is in a grounded (N) adds Node (E) (rank 4) to its set of DAG at rank d+2, parents for
      DAGID 1.  Following the mechanisms specified by the OCP, and will
      begin given
      that the process to join ETX is 1 for the grounded DAG link between (N) and (E), Node (N) is
      now at rank d+3. 5 in DAGID 1.

   o  Node (D)
      will start a DAG Hop timer, logically associated with the grounded
      DAG at (N) adds Node (C), (B) (rank 4) to coordinate the jump.  The its set of DAG Hop timer will
      have a duration proportional to d+2. parents for
      DAGID 1.

   o  Similarly,  Node (E) will receive and process (N) is a RA-DIO message from sibling of Node (A) on link (A)--(E). (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) will consider Node (A) a
      candidate neighbor, will note 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 (A) is in (C) not
      have a grounded DAG
      at rank d, and will begin viable parent, it should never send the process packet back to join the grounded DAG at
      rank d+1. Node (E) will start
      (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 Hop timer, logically
      associated with Maintenance

   Consider the grounded DAG at example depicted in Figure 13-1.  In this example, Node (A),
   (A) is attached to coordinate the
      jump.  The DAG Hop timer will have a duration proportional to d.

   o  Node (F) takes no action, for Node (F) has observed nothing new to
      act on.

   o  Node (E)'s DAG Hop timer for the grounded DAG at some rank d.  Node (A) expires
      first.  Node (E), upon the DAG Hop timer expiry, removes Node (D)
      as its parent, thus emptying the is a DAG parent set for the floating
      DAG, of
   Nodes (B) and leaving the floating DAG. (C).  Node (E) then jumps to the
      grounded (C) is a DAG by entering Node (A) into the set parent of DAG parents for
      the grounded DAG. Node (E) (D).  There is now in the grounded DAG at rank
      d+1.  Node (E), by jumping into the grounded DAG, has created
   also an
      inconsistency by changing its DAGID, undirected sibling link between Nodes (B) and will begin (C).

   In this example, Node (C) may safely forward to emit RA-DIO
      messages more frequently.

   o Node (F) will receive and process (A) without
   creating a RA-DIO message from loop.  Node (E). (C) may not safely forward to Node (F) will observe that (D),
   contained within it's own sub-DAG, without creating a loop.  Node (E) has changed its DAGID and will
      directly follow (C)
   may forward to Node (E) into (B) in some cases, e.g. the grounded DAG.  Node (F) link (C)->(A) is now a
      member of the grounded DAG at rank d+2.  Note that any additional
      sub-DAG
   temporarily unavailable, but with some chance of Node (E) would continue to join into the grounded DAG creating a loop
   (e.g. if multiple nodes in this coordinated manner.

   o  Node (D) will receive a RA-DIO message from Node (E).  Since Node
      (E) is now set of siblings start forwarding
   `sideways' in a different DAG, Node (D) may process cycle) and requiring the RA-DIO
      message from Node (E).  Node (D) will observe that, via node (E),
      it could attach intervention of additional
   mechanisms to detect and break the grounded DAG at rank d+2.  Node (D) will
      start another DAG Hop timer, logically associated with loop.

   Consider the
      grounded DAG at case where Node (E), with (C) hears a DIO message from a duration proportional to d+1. Node (D) now is running two DAG hop timers, one which was started
      with duration proportional to d+1 (Z)
   at a lesser rank and one proportional to d+2.

   o  Generally, Node (D) will expire the timer associated with the jump
      to superior position in the grounded DAG at than node (E) first. (A).
   Node (D) (C) may then jump to safely undergo the grounded DAG by entering Node (E) into process to evict node (A) from its
   DAG parent set for
      the grounded DAG.  Node (D) is now in the grounded DAG at rank
      d+2.

   o  In this way RPL has coordinated a merge between the more preferred
      grounded DAG and the less preferred floating DAG, such that the
      nodes within the two DAGs come together in attach directly to Node (Z) without creating a generally ordered
      manner, avoiding the formation of loops in the process.

Appendix C.  Additional Examples

   Consider
   loop, because its rank will decrease.

   Now consider the expanded example LLN physical topology in Figure 13.  In
   this example an additional LBR is added.  Suppose that all nodes are
   configured with an implementation specific policy function that aims
   to minimize case where the number of hops, link (C)->(A) becomes nonviable, and that both LBRs are configured
   node (C) must move to
   root different DAGIDs.  We may now walk through a deeper rank within the formation of DAG:

   o  Node (C) must first detach from the
   two DAGs.

                                     (LBR)                    (LBR2)
                                     / | \                    /    \
                                .---`  |  `----.             /      \
                               /       |        \            |      |
                            (11)------(12)------(13)      (14)      (15)
                             | \       | \       | \       |        /|
                             |  `----. |  `----. |  `----. |  .----` |
                             |        \|        \|        \| /       |
                            (21)------(22)------(23)      (24)      (25)
                             |        /|        /|         |        / /
                             |  .----` |  .----` |  .-----]|[------` /
                             | /       | /       | /       |        /
                            (31)------(32)------(33)------(34)-----`
                             |        /| \       | \       | \
                             |  .----` |  `----. |  `----. |  `----.
                             | /       |        \|        \|        \
                   .--------(41)      (42)      (43)------(44)------(45)
                  /         /         /| \       | \
            .----`    .----`    .----` |  `----. |  `----.
           /         /         /       |        \|        \
        (51)------(52)------(53)------(54)------(55)------(56)

                     Figure 13: Expanded LLN Topology
                                     (LBR)                    (LBR2)
                                     / | \                    /    \
                                .---`  |  `----.             /      \
                               /       |        \            |      |
                            (11)      (12)      (13)      (14)      (15)

                            (21)      (22)      (23)      (24)      (25)

                            (31)      (32)      (33)      (34)

                            (41)      (42)      (43)      (44)      (45)

        (51)      (52)      (53)      (54)      (55)      (56)

                    Figure 14: DAG Construction Step 1

                                     (LBR)                    (LBR2)
                                     / | \                    /    \
                                .---`  |  `----.             /      \
                               /       |        \            |      |
                            (11)      (12)      (13)      (14)      (15)
                             | \       | \       |         |        /|
                             |  `----. |  `----. |         |  .----` |
                             |        \|        \|         | /       |
                            (21)      (22)      (23)      (24)      (25)

                            (31)      (32)      (33)      (34)

                            (41)      (42)      (43)      (44)      (45)

        (51)      (52)      (53)      (54)      (55)      (56) 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 15: 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 Construction Step 2

                                     (LBR)                    (LBR2)
                                     / | \                    /    \
                                .---`  |  `----.             /      \
                               /       |        \            |      |
                            (11)      (12)      (13)      (14)      (15)
                             | \       | \       |         |        /|
                             |  `----. |  `----. |         |  .----` |
                             |        \|        \|         | /       |
                            (21)      (22)      (23)      (24)      (25)
                             |        /|        /          |        / /
                             |  .----` |  .----`    .-----]|[------` /
                             | /       | /         /       |        /
                            (31)      (32)      (33)      (34)-----`

                            (41)      (42)      (43)      (44)      (45)

        (51)      (52)      (53)      (54)      (55)      (56)

                    Figure 16:
      by reattaching at a deeper rank to Node (B).  Node (C) adds Node
      (B) to its DAG Construction Step 3
                                     (LBR)                    (LBR2)
                                     / | \                    /    \
                                .---`  |  `----.             /      \
                               / 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)
          |\                     |\                     |\
          |        \ `-----.              | `-----.              |
                            (11)      (12)      (13)      (14)      (15) `-----.
          |        \             |        \             |         |        /|
                             |  `----. |  `----. |         |  .----` |
                             |        \|        \|         | /       |
                            (21)      (22)      (23)      (24)      (25)
                             |        /|        /          |        / /
                             |  .----` |  .----`    .-----]|[------` /
                             | /       | /         /       |        /
                            (31)      (32)      (33)      (34)-----`
                             |        /|         |        \       |
         (B)       (C)          (B)        \            |  .----` |         |  `----.        (C)
                                  \        |  `----.            |        /
                                   `-----. |            | .-----`
                                          \|        \
                            (41)      (42)      (43)      (44)      (45)

        (51)      (52)      (53)      (54)      (55)      (56)            |/
                                          (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
   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 17: 14-1 be the initial condition.

   o  Suppose Node (C) first is able to leave the DAG Construction Step 4

                                     (LBR)                    (LBR2)
                                     / | \                    /    \
                                .---`  |  `----.             /      \
                               /       |        \            |      |
                            (11)      (12)      (13)      (14)      (15)
                             | \       | \       |         |        /|
                             |  `----. |  `----. |         |  .----` |
                             |        \|        \|         | /       |
                            (21)      (22)      (23)      (24)      (25)
                             |        /|        /          |        / /
                             |  .----` |  .----`    .-----]|[------` /
                             | /       | /         /       |        /
                            (31)      (32)      (33)      (34)-----`
                             |        /|         | \       | \
                             |  .----` |         |  `----. |  `----.
                             | /       |         |        \|        \
                   .--------(41)      (42)      (43)      (44)      (45)
                  /         /         /|         | \
            .----`    .----`    .----` |         |  `----.
           /         /         /       |         |        \
        (51)      (52)      (53)      (54)      (55)      (56) and rejoin at a
      lower rank, taking both Nodes (A) and (B) as DAG parents as
      depicted in Figure 18: 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 Construction Step 5 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)

      *  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 D. C.  Outstanding Issues

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

D.1.

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.

D.2.

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.

D.3.

C.3.  Destination Advertisement / DAO Fan-out

   When NA-DAO DAO messages are relayed to more than one DAG parent, in some
   cases a situation may be created where a large number of NA-DAO DAO messages
   conveying information about the same destination flow inward along
   the DAG.  It is desirable to bound/limit the multiplication/
   fan-out multiplication/fan-out
   of NA-DAO 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.

D.4.

   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
   triggered `D' bit further).

C.4.  Source Routing

   In support of nodes who 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.

D.5.

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: dtroll@external.cisco.com rpl-authors@external.cisco.com