Networking Working Group                                  T. Winter, Ed.
Internet-Draft
Intended status: Standards Track                         P. Thubert, Ed.
Expires: March 27, April 7, 2010                                     Cisco Systems
                                                        ROLL Design Team
                                                            IETF ROLL WG
                                                      September 23,
                                                         October 4, 2009

         RPL: Routing Protocol for Low Power and Lossy Networks
                         draft-ietf-roll-rpl-02
                         draft-ietf-roll-rpl-03

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Abstract

   Low Power and Lossy Networks (LLNs) are made largely of constrained
   nodes (with limited processing power, memory, and sometimes energy
   when they are battery operated).  These routers are interconnected by
   lossy links, most of the 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 or 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.  These characteristics offer
   unique challenges to a routing solution: the IETF ROLL Working Group
   has defined application-specific routing requirements for a Low Power
   and Lossy Network (LLN) routing protocol.  This document specifies
   the 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 as described in RFC 2119 [RFC2119].

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.1.  Design Principles  . . . . . . . . . . . . . . . . . . . .  6
     1.2.  Expectations of Link Layer Behavior  . . . . . . . . . . .  7
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Protocol Model . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Protocol Properties Overview . . . . . . . . . . . . . . .  9
       3.1.1.  IPv6 Architecture  . . . . . . . . . . . . . . . . . .  9
       3.1.2.  Typical LLN Traffic Patterns . . . . . . . . . . . . . 10
       3.1.3.  Constraint Based Routing . . . . . . . . . . . . . . . 10
     3.2.  Protocol Operation . . . . . . . . . . . . . . . . . . . . 10
       3.2.1.  DAG Construction . . . . . . . . . . . . . . . . . . . 11 12
       3.2.2.  Destination Advertisement  . . . . . . . . . . . . . . 21 19
     3.3.  Other Considerations  Loop Avoidance and Stability . . . . . . . . . . . . . . . 21
       3.3.1.  Greediness and Rank-based Instabilities  . . . . . . 23
       3.3.1. . 22
       3.3.2.  Merging DAGs . . . . . . . . . . . . . . . . . . . . . 22
       3.3.3.  DAG Rank and Loop Avoidance Loops  . . . . . . . . . . . . . . . . . . . . . . 23
       3.3.2.  DAG Parent Selection, Stability, and Greediness
       3.3.4.  DAO Loops  . . . . . . . . . . 27
       3.3.3.  Merging DAGs . . . . . . . . . . . . 23
       3.3.5.  Sibling Loops  . . . . . . . . . . . . . . 29 . . . . . . 23
     3.4.  Local and Temporary Routing Decision . . . . . . . . . . . 32 24
     3.5.  Maintenance of Routing Adjacency . . . . . . . . . . . . . 32 25
   4.  Constraint Based Routing in LLNs . . . . . . . . . . . . . . . 33 25
     4.1.  Routing Metrics  . . . . . . . . . . . . . . . . . . . . . 33 25
     4.2.  Routing Constraints  . . . . . . . . . . . . . . . . . . . 34 26
     4.3.  Constraint Based Routing . . . . . . . . . . . . . . . . . 34 26
   5.  RPL Protocol Specification . . . . . . . . . . . . . . . . . . 35 27
     5.1.  DAG Information Option . . . . . . . . . . . . . . . . . . 35 27
       5.1.1.  DAG Information Option (DIO) base option . . . . . . . 35 27
     5.2.  Conceptual Data Structures . . . . . . . . . . . . . . . . 42 34
       5.2.1.  Candidate Neighbors Data Structure . . . . . . . . . . 42 34
       5.2.2.  Directed Acyclic Graphs (DAGs) Data Structure  . . . . 43 35
     5.3.  DAG Discovery and Maintenance  . . . . . . . . . . . . . . 44 36
       5.3.1.  DAG Discovery Rules  . . . . . . . . . . . . . . . . . 45 37
       5.3.2.  Reception and Processing of RA-DIO messages  . . . . . 47 39
       5.3.3.  RA-DIO Transmission  . . . . . . . . . . . . . . . . . 49 41
       5.3.4.  Trickle Timer for RA Transmission  . . . . . . . . . . 50 42
     5.4.  DAG Heartbeat  . . . . . . . . . . . . . . . . . . . . . . 52 44
     5.5.  DAG Selection  . . . . . . . . . . . . . . . . . . . . . . 52 44
     5.6.  Administrative rank  . . . . . . . . . . . . . . . . . . . 53 45
     5.7.  Candidate DAG Parent States and Stability  . . . . . . . . 53 45
       5.7.1.  Held-Up  . . . . . . . . . . . . . . . . . . . . . . . 53 45
       5.7.2.  Held-Down  . . . . . . . . . . . . . . . . . . . . . . 54 46
       5.7.3.  Collision  . . . . . . . . . . . . . . . . . . . . . . 54 46
       5.7.4.  Instability  . . . . . . . . . . . . . . . . . . . . . 55 47
     5.8.  Guidelines for Objective Code Points . . . . . . . . . . . 56 48
       5.8.1.  Objective Function . . . . . . . . . . . . . . . . . . 56 48
       5.8.2.  Objective Code Point 0 (OCP 0) . . . . . . . . . . . . 58 50

     5.9.  Establishing Routing State Outward Along the DAG . . . . . 60 52
       5.9.1.  Destination Advertisement Message Formats  . . . . . . 61 53
       5.9.2.  Destination Advertisement Operation  . . . . . . . . . 63 55
     5.10. Multicast Operation  . . . . . . . . . . . . . . . . . . . 70 62
     5.11. Maintenance of Routing Adjacency . . . . . . . . . . . . . 71 63
     5.12. Packet Forwarding  . . . . . . . . . . . . . . . . . . . . 72
       5.12.1. Loop Taxonomy  . . . . . . . . . . . . . . . . . . . . 73 64
   6.  RPL Variables  . . . . . . . . . . . . . . . . . . . . . . . . 74 65
   7.  Manageability Considerations . . . . . . . . . . . . . . . . . 75 66
     7.1.  Control of Function and Policy . . . . . . . . . . . . . . 75 66
       7.1.1.  Initialization Mode  . . . . . . . . . . . . . . . . . 75 66
       7.1.2.  DIO Base option  . . . . . . . . . . . . . . . . . . . 76 66
       7.1.3.  Trickle Timers . . . . . . . . . . . . . . . . . . . . 77 67
       7.1.4.  DAG Heartbeat  . . . . . . . . . . . . . . . . . . . . 77 68
       7.1.5.  The Destination Advertisement Option . . . . . . . . . 78 68
       7.1.6.  Policy Control . . . . . . . . . . . . . . . . . . . . 78 68
       7.1.7.  Data Structures  . . . . . . . . . . . . . . . . . . . 78 68
     7.2.  Information and Data Models  . . . . . . . . . . . . . . . 78 69
     7.3.  Liveness Detection and Monitoring  . . . . . . . . . . . . 79 69
       7.3.1.  Candidate Neighbor Data Structure  . . . . . . . . . . 79 69
       7.3.2.  Directed Acyclic Graph (DAG) Table . . . . . . . . . . 79 69
       7.3.3.  Routing Table  . . . . . . . . . . . . . . . . . . . . 80 70
       7.3.4.  Other RPL Monitoring Parameters  . . . . . . . . . . . 80 70
       7.3.5.  RPL Trickle Timers . . . . . . . . . . . . . . . . . . 80 70
     7.4.  Verifying Correct Operation  . . . . . . . . . . . . . . . 80 71
     7.5.  Requirements on Other Protocols and Functional
           Components . . . . . . . . . . . . . . . . . . . . . . . . 81 71
     7.6.  Impact on Network Operation  . . . . . . . . . . . . . . . 81 71
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 81 71
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 81 71
     9.1.  DAG Information Option (DIO) Base Option . . . . . . . . . 81 71
     9.2.  New Registry for the Flag Field of the DIO Base Option . . 81 71
     9.3.  DAG Information Option (DIO) Suboption . . . . . . . . . . 82 72
     9.4.  Destination Advertisement Option (DAO) Option  . . . . . . 82 72
     9.5.  Objective Code Point . . . . . . . . . . . . . . . . . . . 82 72
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 83 73
   11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 83 73
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 84 74
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 84 74
     12.2. Informative References . . . . . . . . . . . . . . . . . . 84 74
   Appendix A.  Deferred Requirements . . . . . . . . . . . . . . . . 86 76
   Appendix B.  Examples  . . . . . . . . . . . . . . . . . . . . . . 87 77
     B.1.  Moving Down a DAG  . . . . . . . . . . . . . . . . . . . . 88 78
     B.2.  Link Removed . . . . . . . . . . . . . . . . . . . . . . . 89 79
     B.3.  Link Added . . . . . . . . . . . . . . . . . . . . . . . . 89 79
     B.4.  Node Removed . . . . . . . . . . . . . . . . . . . . . . . 90 80
     B.5.  New LBR Added  . . . . . . . . . . . . . . . . . . . . . . 90 80
     B.6.  Destination Advertisement  . . . . . . . . . . . . . . . . 91
   Appendix C.  Additional Examples 81
     B.7.  Example: DAG Parent Selection  . . . . . . . . . . . . . . 82
     B.8.  Example: DAG Maintenance . . . 92
   Appendix D.  Outstanding Issues . . . . . . . . . . . . . . 83
     B.9.  Example: Greedy Parent Selection and Instability . . . 96
     D.1.  Additional Support for P2P Routing . . 84
     B.10. Example: DAG Merge . . . . . . . . . . 96
     D.2.  Loop Detection . . . . . . . . . . 86
   Appendix C.  Additional Examples . . . . . . . . . . . . 96
     D.3.  Destination Advertisement / . . . . . 88
   Appendix D.  Outstanding Issues  . . . . . . . . . . . . . . . . . 92
     D.1.  Additional Support for P2P Routing . . . . . . . . . . . . 92
     D.2.  Loop Detection . . . . . . . . . . . . . . . . . . . . . . 92
     D.3.  Destination Advertisement / DAO Fan-out  . . . . . . . . . 96 92
     D.4.  Source Routing . . . . . . . . . . . . . . . . . . . . . . 96 92
     D.5.  Address / Header Compression . . . . . . . . . . . . . . . 97 93
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 97 93

1.  Introduction

   Low Power and Lossy Networks (LLNs) are made largely of constrained
   nodes (with limited processing power, memory, and sometimes energy
   when they are battery operated).  These routers are interconnected by
   lossy links, most of the 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 or 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.  These characteristics offer
   unique challenges to a routing solution: the IETF ROLL Working Group
   has defined application-specific routing requirements for a Low Power
   and Lossy Network (LLN) routing protocol, specified in
   [I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs],
   [I-D.ietf-roll-indus-routing-reqs], and [RFC5548].  This document
   specifies the Routing Protocol for Low Power and Lossy Networks
   (RPL).

1.1.  Design Principles

   RPL was designed with the objective to meet the requirements spelled
   out in [I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs],
   [I-D.ietf-roll-indus-routing-reqs], 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 of the protocol to be optimized in terms of
   required code space.  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

   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
   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 in this document is consistent with and
   incorporates that 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 root node (a DAG root, or sink-
         typically a Low Power and Lossy Network Border Router (LBR)).

   DAGID:  DAG Identifier.  A globally unique identifier for a DAG.  All
         nodes who are part of a given DAG have knowledge of the DAGID.
         This knowledge is used to identify peer nodes within the DAG in
         order to coordinate DAG maintenance while avoiding loops.

   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 within the DAG.  All paths in the DAG
         terminate at a DAG root, and all DAG edges contained in the
         paths terminating at a DAG root are oriented toward the DAG
         root.  There must be at least one DAG root per DAG, and in some
         cases there may be more than one.  In many use cases, source-
         sink represents a dominant traffic flow, where the sink is a
         DAG root or is located behind the DAG root.  Maintaining routes
         towards DAG roots is therefore a prominent functionality for
         RPL.

   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 to traffic between one node
         and a set of nodes.  This is similar to the P2MP concept in
         Multicast or MPLS Traffic Engineering ([RFC4461] 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 use in a DAG.  Instances of the
         Objective Code Point are 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
   and implementation of IPv6 architecture on devices which may operate
   under severe resource constraints, including but not limited to
   memory, processing power, energy, and communication.  The RPL design
   does not presume high quality reliable links, and operates over lossy
   links (usually low bandwidth with low packet delivery success rate).

3.1.2.  Typical LLN Traffic Patterns

   Multipoint-to-point

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

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

3.1.3.  Constraint Based Routing

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

   RPL supports the computation and installation of different paths in
   support of and optimized for a set of application and implementation
   specific constraints, as guided by an OCP.  Traffic may subsequently
   be directed along the appropriate constrained path based on traffic
   marking within the IPv6 header.  For more details on the approach
   towards constraint-based routing, see Section 4.

3.2.  Protocol Operation

   A LLN nodes running RPL deployment will construct Directed Acyclic Graphs (DAGs)
   rooted at designated consist of a number of nodes that generally have some application
   significance, such as providing connectivity to an external routed
   infrastructure.  The term "external" is used top refer to the public
   Internet or and a core private (non-LLN) IP network.  The DAG is
   sufficient number of
   edges (links) between them, whose characteristics will depend on
   implementation and link layer (L2) specifics.  Due to support inward MP2P traffic flows, flowing inward along the LLN towards a sink (DAG root), which is one nature of
   the dominant
   traffic flows described in LLN environment the requirements documents
   ([I-D.ietf-roll-building-routing-reqs],

   [I-D.ietf-roll-home-routing-reqs],
   [I-D.ietf-roll-indus-routing-reqs], L2 links are expected to demonstrate a large
   degree of variance as to their availability, quality, and [RFC5548]).

   By utilizing other
   related parameters.  Certain links, demonstrating a DAG viability above a
   confidence threshold for dominant MP2P flows, RPL allows each particular node to
   select and maintain potentially multiple successors capable of
   forwarding traffic inwards towards the root.  The DAG does not
   present as many single points of failure link metrics, as a tree, based
   on guidelines from [I-D.ietf-roll-routing-metrics], will be extracted
   from the L2 graph, and in addition
   can offer a node a set of pre-computed successors in support of, e.g.
   local route repair in case of a temporary failure, load balancing, or
   short term fluctuations in link characteristics.

   A DAG also serves to restrict the routing problem on the nodes when
   it is used as a reference topology.  This allows nodes to determine
   their positions in a DAG relative to each other and provides a means
   to coordinate route repair in a way that endeavors to avoid loops.
   These mechanisms resulting graph will be described in more detail later in this
   specification.

   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 used as a reference,
   distributes routing information across intermediate nodes (between
   the DAG leaves and the root), guided along basis
   on which to operate the DAG, such routing protocol.  Note that as the
   routes toward destination prefixes in the outward direction may be
   set up.  As
   characteristics of the DAG undergoes modification during DAG maintenance, L2 topology vary over time the destination advertisement mechanism can be triggered set of viable
   links is to update be updated and the outward routing state.

   Arbitrary P2P traffic may flow inward along protocol thus continues to
   evaluate the DAG until LLN.  In RPL this process happens in a common
   parent is reached who has stored an entry for distributed
   manner, and from the destination perspective of a single node running RPL this
   process results in its
   routing table a set of candidate neighbors, with associated node
   and is capable link metrics as well as confidence values.

   Many of directing the dominant traffic outward along
   the correct outward path.  In flows in support of the present specification 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.

3.2.1.  DAG Construction

3.2.1.1.  Overview of a Typical Case

   RPL constructs one or more DAGs, over gradients defined by optimizing
   cost metrics along paths 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], and [RFC5548]).  These flows are
   rooted at designated nodes.

   DAGs may be grounded, in which case the DAG root (e.g. an LBR) is
   offering nodes that have some application significance,
   such as providing connectivity to an external routed infrastructure such as infrastructure.
   The term "external" is used top refer to the public Internet or a private
   core private (non-LLN) IP network.  A
   typical goal for a node participating in DAG construction may be to
   find and join a grounded DAG.  Any DAG which is not grounded is
   floating,  In support of this dominant flow
   RPL constructs Directed Acyclic Graphs (DAGs) on top of the viable
   LLN topology, selecting and routes may still be provisioned orienting links among candidate neighbors
   toward the DAG roots which root
   although with no expectations of reaching an external infrastructure.

   In the context of a particular MP2P flows.

   LLN application one or more nodes running RPL will
   be capable of, e.g. serving construct Directed Acyclic Graphs (DAGs)
   rooted at designated nodes that generally have some application
   significance, such as providing connectivity to an LBR external routed
   infrastructure.  The term "external" is used top refer to the public
   Internet or acting as a data collection
   point, and thus be provisioned to act as core private (non-LLN) IP network.  This structure
   provides the most preferred DAG
   roots.  These nodes will initiate and continue routing solution for the process of
   constructing a dominant MP2P traffic flows.
   The DAG by occasionally emitting IPv6 Router Advertisement
   (RA) messages containing the necessary information structure further provides each node potentially multiple
   successors for neighboring
   nodes MP2P flows, which may be used for, e.g., local route
   repair or load balancing.

   Nodes running RPL are able to evaluate further restrict the scope of the
   routing problem by using the DAG root as a potential DAG parent.  This
   information will include at least reference topology.  By
   referencing a DAGID, an administrative
   preference, and an Objective Code Point (OCP).  The DAGID rank property that is an
   identifier unique to the DAG.  The administrative preference offers a
   way related to engineer the formation of the DAG positions in support of the
   application, by providing
   DAG, nodes are able to determine their positions in a mechanism by which the DAG may look more
   or less attractive for other nodes relative to join.  The OCP provides
   each other.  This information as is used by RPL in part to construct
   rules for movement relative to which metrics and optimization goals are being
   employed across the DAG.

   Nodes who hear RA messages, advertising a specific DAGID, will take
   into consideration several criteria when processing the extracted DAG
   information.  A node may seek a DAG advertising a specific OCP,
   reflecting that endeavor to avoid loops.
   It is important to note that the implementation specific routing constraints understood
   by rank property is derived from
   metrics, and not directly from the node.  In particular, a node position in the DAG, as will be seeking to find
   discussed further.

   As DAGs are organized, RPL will use a least
   cost path satisfying some objective function as indicated by the OCP
   according destination advertisement
   mechanism to some build up routing metrics defined tables in
   [I-D.ietf-roll-routing-metrics].  For example, the least cost path
   may be determined in part by minimizing energy along a path, or
   latency, or avoiding the use of battery powered nodes.  A node may be
   seeking to explicitly join a grounded DAG.  Further, a node may seek
   the most desirable administrative preference when selecting a DAG,
   all else being equal.  Based on the evaluation support of such criteria, a
   node may determine if the node who emitted outward P2MP
   traffic flows.  This mechanism, using the RA message should be
   considered DAG as a potential DAG parent.  If so, then the node may add reference,
   distributes routing information across intermediate nodes (between
   the advertising node to its set of candidate DAG parents for the
   advertised DAGID, leaves and after waiting for a designated delay, the node
   may follow root), guided along the procedures to activate DAG, such that the advertising node as a DAG
   parent and
   routes toward destination prefixes in the outward direction may then be considered to have joined
   set up.  As the DAG designated
   by DAGID.

   When a node adds the first undergoes modification during DAG parent maintenance,
   the destination advertisement mechanism can be triggered to update
   the set of DAG parents outward routing state.

   A baseline support for a
   particular DAGID, the node P2P traffic in RPL is said to have joined, or attached to,
   the DAG designated provided by DAGID.  Adding additional DAG parents beyond the first simply increases path diversity inwards toward DAG, as
   P2P traffic may flow inward along the DAG
   root.  When until a node removes the last DAG common parent from the set of DAG
   parents is
   reached who has stored an entry for a particular DAGID, the node destination in its routing
   table and is said to have left, or
   detached from, capable of directing the DAG designated by DAGID. traffic outward along the
   correct outward path.  RPL will coordinate also provides support for the trivial case
   where a P2P destination may be a `one-hop' neighbor.  In the
   joining, leaving, present
   specification RPL does not specify nor preclude any additional
   mechanisms that may be capable to compute and movement of install more optimal
   routes into LLN nodes within a DAGID in such a way
   so as support of arbitrary P2P traffic according
   to avoid the formation some routing metric.

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 loops, as described further below.

   As DAG construction rules and objective functions when
   considering its role with respect to neighboring nodes join such that the
   DAG they are able advertise the fact by
   multicasting the structure emerges.

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

   The IPv6 Router Advertisement (RA) mechanism (as specified in
   [RFC4861]) is used by RPL in order to build and maintain a DAG.

   The IPv6 RA messages (to neighbors message is augmented with a
   link-local scope).  In this way, nodes are able DAG Information Option (DIO),
   forming an RA-DIO message, to join convey information about the DAG at
   ever-increasing rank outward
   including:

   o  A DAGID used to identify the DAG as sourced from the DAG root.  As nodes continue to
   receive DAG multicasts they may continue
      The DAGID must be unique to expand their set of a single DAG
   parents, while employing loop avoidance strategies as described
   below, in order to build path diversity inwards toward the DAG root.

   Using scope of the LLN.

   o  Objective Code Point (OCP) as described below.

   o  Rank information conveyed used by nodes to determine their positions in the metrics of its most preferred
      DAG parent, relative to each other.  This is not a metric, although its own metrics,
      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 conventions DAG root, used to aid in
      identification of dependent sub-DAGs and functions
   indicated by coordinate topology
      changes in a manner so as to avoid loops.

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

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

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

   The RA messages are issued whenever a change is detected to the DAG
   such that a node is able to compute determine that a rank value within region of the DAG has
   become inconsistent.  As the DAG stabilizes the period at which it will use RA
   messages occur is configured to coordinate its taper off, reducing the steady-state
   overhead of DAG maintenance.

   Once a preferred parent is selected,  The periodic issue of RA messages,
   along with the node can compute its own
   rank triggered RA 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 and determine alternate parents.  Any node inwards
   from this node, that is with identified by a lower rank than this node, can be used particular identifier (DAGID) as an alternate parent for forwarding.

   In addition to identifying well as
   its supported optimization objectives and available destination
   prefixes.  The optimization objectives are conveyed as an Objective
   Code Point (OCP) as described further below.  Available destination
   prefixes, which may include destinations available beyond the DAG parents, a
   root, multicast destinations, or IPv6 node also may hear addresses, are advertised
   outwards along the RA
   messages of other neighboring DAG and recipient nodes may then provision routing
   tables with entries inwards towards the destinations.  The RPL
   implementation at each node will be provisioned by the same rank within application
   with sufficient information to determine which objectives and
   destinations are required, and thus the DAG.
   In this way a node can discover RPL implementation may
   determine which DAG siblings.  As it selects its
   initial position within a DAG, to join.

   The decision for a node MAY increment its rank it order
   to have at least one sibling but it SHOULD NOT increase it as to
   obtain more parents.

   A node may order its set of join a DAG parents may be optimized according to some
   implementation specific preference, and it SHOULD install a DAG
   parent policy functions on the node as indicated by
   one or more specific OCP values.  For example, a default gateway.  To this list the node may also append a
   similarly ordered set of DAG siblings.  By forwarding traffic
   intended be
   configured for the default destination towards the DAG parents, the
   node is able one goal to support the main Multipoint-to-point (MP2P) traffic
   flows required by optimize a typical LLN application.  By using the ordered
   set of DAG parents bandwidth metric (OCP-1), and DAG siblings the node is able to take
   advantage of path diversity.  For example, preferring
   with a parallel goal to forward
   traffic towards parents guarantees to get optimize for a reliability metric (OCP-2).
   Thus two DAGs, with two unique DAGIDs, may be constructed and
   maintained in the traffic inwards, closer LLN: DAG-1 would be optimized according to the DAG root, by definition, regardless of which parent is
   selected.  In this example, if forwarding towards parents is not
   possible, perhaps due OCP-1,
   whereas DAG-2 would be optimized according to a transient phenomena, then a OCP-2.  A node may then
   choose to forward traffic towards siblings, moving across the
   maintain independent sets of DAG at
   the same level (neither inwards or outwards).  When receiving traffic
   forwarded from a sibling, the traffic should not be forwarded back to
   the same sibling parents and related data structures
   for each DAG.  Note that in order to avoid a 2-node loop.

   By employing this procedure, the LLN is able to set up such a path-
   constrained DAG, rooted at designated nodes, with other nodes
   organized case traffic may directed along paths leading inward toward
   the appropriate constrained DAG root.  MP2P based on traffic intended for marking within the destinations available to or through
   IPv6 header.  This specification will focus on the DAG
   root, e.g. case where the default destination or other advertised prefixes,
   flows inward along
   node only joins one DAG; further elaboration on the DAG towards proper operation
   of RPL in the root, and nodes forwarding presence of multiple DAGs, including traffic marking
   and related rules, are able to leverage the path diversity be specified further in future revisions of the DAG as
   necessary.

   Further mechanisms described below will populate the routing tables
   along the DAG
   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 P2MP and P2P traffic.

3.2.1.2.  Further Operation

   The sub-DAG of a node is the set of other an application scenario.  These nodes of greater rank in
   are designated `grounded', and may serve as the DAG roots of a
   grounded DAG.  DAGs that might use do not have a path towards the grounded DAG root that contains this
   node.  Rank in are floating
   DAGs.  In either case routes may be provisioned toward the DAG is defined such that nodes contained root,
   although in the
   sub-DAG of a specific node should have a greater rank than the node.
   This floating case there is no expectation to reach an important property
   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 is leveraged for loop avoidance-
   if a node has lesser rank then it is not some DAGs in the sub-DAG.  (An
   arbitrary node with greater rank may or LLN may not be contained in the
   sub-DAG).  Paths through siblings are not contained in this set.

   As more
   preferred over other DAGs.  For example, a further illustration, consider the DAG examples in Appendix B.
   Consider Node (24) 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 Example depicted in Figure 12.  In this
   example, the sub-DAG of Node (24)
   that is comprised only existing in support of Nodes (34), (44),
   and (45).

   A DAG repair until a permanent DAG
   is found, may also be floating.  Floating DAGs may configured to be encountered, for
   example, during coordinated reconfigurations less preferred.  The administrative
   preference offers a way to engineer the formation of the network topology
   wherein a node and its sub-DAG breaks off the DAG, temporarily
   becomes a floating DAG, and reattaches to a grounded DAG at a
   different (more optimal) location.  (Such coordination endeavors in
   support of the application.

3.2.1.5.  Objective Code Point (OCP)

   The OCP serves to
   avoid convey and control the construction of transient loops optimization objectives in
   use within the LLN).  A DAG, or a
   sub-DAG, may also become floating because DAG.  The OCP is further specified in
   [I-D.ietf-roll-routing-metrics].  Each instance of an allocated OCP
   indicates:

   o  The set of metrics used within the DAG

   o  The objective functions used for least cost path determination.

   o  The function used to compute DAG Rank

   o  The functions used to accumulate metrics for propagation within a network element
   failure.  Note
      RA-DIO message

   For example, an objective code point might indicate that in the case where a floating DAG exists is
   using the Expected Number of Transmissions (ETX) as a
   consequence of DAG repair, metric, that
   the floating optimization goal is to minimize ETX, that DAG Rank is also intended equivalent
   to be
   transient ETX, and carries a marking to make it less attractive.  Some
   specific application scenarios may employ permanent floating DAGs,
   e.g.  DAGs without connectivity that RA-DIO propagation entails adding the advertised ETX
   of the most preferred parent to an external routed infrastructure,
   as a matter the ETX of normal operation.  In such cases the floating DAG is
   likely link to have been provisioned 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
   message, RPL nodes may work to build optimized LLN using a variety of
   application with an
   administrative preference which will make it more attractive. and implementation specific metrics and goals.

   A node will generally join at least one DAG, typically (but not
   necessarily) to or through default OCP, OCP 0, is specified with a grounded DAG rooted at an LBR.  In some
   cases, as suitable well-defined default
   behavior.  OCP 0 is used to define RPL behaviors in the application scenario, case where a DAG may still
   provision the DAG parents as default gateways and
   node encounters a RA-DIO message containing a code point that it does
   not support.

3.2.1.6.  Distributed Algorithm Operation

   o  Some nodes may be connected initially provisioned to
   a non-LLN infrastructure such act as the public Internet DAG roots,
      either permanent or transient, with associated preferences.

   o  Nodes will maintain a private IP
   network. data structure containing their candidate
      (viable) neighbors, as based on guidelines in
      [I-D.ietf-roll-routing-metrics] This specification does not preclude a node data structure will also
      track DAG information as learned from joining multiple
   DAGs.  In one such case, and associated with each
      neighbor.

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

   o  Nodes who receive RA-DIO messages will take into consideration
      several criteria when processing the extracted DAG information.
      The node may discount the RA-DIO according to maintain membership loop avoidance rules
      based on rank as described further below.  Nodes will consider the
      information in multiple DAGs the RA-DIO in order to satisfy
   competing constraints, for example to support different types of
   traffic, similar determine whether or not
      that candidate neighbor offers a better attachment point to a DAG
      (which the concept of MTR (Multi-topology routing) as
   supported by other routing protocols such as IS-IS [RFC5120] node may or OSPF
   [RFC4915], although the RPL mechanisms will significantly differ from
   the ones specified for these protocols.  (Note that not all
   constrained traffic cases may require multiple DAGs).  In support of
   such cases not be a member of) according to the RPL
      implementation must independently maintain
   requisite information specific optimization goals, OCP, and state for each DAG in parallel.  In cases
   where current
      metrics.

   o  Nodes may join a competing constraints must be satisfied toward the same new DAG
   root, or move within the OCP should differ by definition and may serve current DAG, in
      response to coordinate the maintenance of the multiple DAGs.  Further, additional
   recommendations for information contained in the operation of RA-DIO message, and
      in accordance with loop avoidance/loop detection
   mechanisms avoidance rules described further in this
      specification.  For the presence of multiple DAGs are under investigation.

   An administrative preference, successors within the DAG preference, shall be associated
   with each DAG.  In cases where DAG, a RPL node has manages
      a choice set of joining
   more than one DAG to satisfy a particular constraint, Parents.  Joining, moving within, and all else
   being equal, the node will seek to join leaving the most preferred DAG as
   indicated by the administrative preference.  In practice
      is accomplished through managing this
   mechanism may be assist in engineering the construction of a DAG as
   appropriate set according to an application.  For example, the rules
      specified by RPL.

   o  As nodes that join, move within, and leave DAGs they emit updated RA-
      DIOs which are to become
   DAG roots in support of a particular application role, e.g. received and acted on by neighboring nodes.  When
      inconsistencies (such as a data
   sink caused by movement or a controller, may be provisioned such that they have are more
   preferred.  Nodes who link loss) are serving as
      detected within the DAG root of a transient DAG,
   e.g. for structure, RA-DIO messages are emitted
      more frequently.  When the DAG repair, may take on a structure becomes consistent, RA-
      DIO messages taper off.

   o  As less desirable preference value.
   Nodes will then be able to yield their transient preferred DAGs to join the encounter more preferred DAGs that are more preferred.

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

   The IPv6 Router Advertisement (RA) mechanism (as specified in
   [RFC4861]) is used by RPL offer
      equivalent or better optimization objectives, the nodes in order the
      less preferred DAGs may leave to build and maintain a DAG.

   The IPv6 RA message join the more preferred DAGs,
      finally leaving only the more preferred DAGs.  This is augmented with a DAG Information Option (DIO),
   forming an RA-DIO message, in order to facilitate the formation and
   maintenance
      illustration of DAGs.  The information conveyed in the RA-DIO message
   includes the following: mechanism by which an application may engineer
      DAG construction.

   o  A DAGID used to identify  As the DAG as sourced from construction operation proceeds, nodes accumulate onto
      the DAG root.
      Typically the (potentially compressed) IPv6 address of in progressively outward tiers, centered around the DAG
      root.

   o  The DAGID must be unique to a single DAG in the scope of nodes provision routing table entries for the LLN.  If destinations
      specified by the RA-DIO towards their DAG root is rooting multiple DAGs, each Parents.  Nodes may
      provision a DAG must
      be provisioned with their own IPv6 address and thus derive unique
      DAGIDs.

   o  Objective Code Point (OCP) Parent as described below.

   o a default gateway.

3.2.1.7.  DAG Rank information used by

   When nodes select DAG parents, they will select the most preferred
   parent according to determine their relationships implementation specific objectives, using
   the cost metrics conveyed in the RA-DIO messages along the DAG relative to each other, i.e. parents, siblings, or
      children.  This is not a metric, although its derivation is
      typically closely in
   conjunction with the related to one or more metrics objective functions 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

   Based on this selection, the metrics conveyed by the most preferred
   DAG root, used to aid in
      identification of dependent sub-DAGs parent, the nodes own metrics and configuration, and coordinate topology
      changes in a manner so as to avoid loops.

   o  Indications for related
   function defined by the DAG, e.g. grounded or floating.

   o  DAG configuration parameters.

   o  A vector of path metrics.  As discussed in
      [I-D.ietf-roll-routing-metrics] such metrics may OCP, a node will be cumulative,
      may report able to compute a maximum, minimum, or average scalar value, or value
   for its rank as a link
      property.

   o  List consequence of additional destination prefixes reachable via the selecting a most preferred DAG
      root.
   parent.

   The RA messages are issued whenever a change is detected to rank value feeds back into the DAG
   such that a node is able parent selection according to determine that
   a region of the loop-avoidance strategy.  Once a DAG parent has
   become inconsistent.  As been added, and a
   rank value for the node within the DAG stabilizes has been computed, the period at which RA
   messages occur is configured nodes
   further options with regard to taper off, reducing the steady-state
   overhead of DAG maintenance.  The periodic issue of RA messages,
   along with parent selection and movement
   within the triggered RA messages DAG are restricted in response to inconsistency, favor of loop avoidance.

   It is
   one feature that enables RPL important to operate in note that the presence of unreliable
   links.

   The RA-DIO and related mechanisms are described in more detail in
   Section 5.

3.2.1.4.  Objective Code Point (OCP)

   The OCP DAG Rank is not itself a metric,
   although its value is seeded derived from and influenced by the DAG root and serves use of
   metrics to convey select DAG parents and control
   the optimization functions used within take up a position in the DAG.  The OCP is further
   specified in [I-D.ietf-roll-routing-metrics].  Each instance of an
   allocated OCP indicates:

   o  The set of  In
   other words, routing metrics used within the DAG
   o  The objective functions and OCP (not rank directly) are used to
   determine the least cost
      constrained paths in order to optimize the DAG

   o  The function used to compute DAG Rank

   o  The functions used to construct derived metrics for propagation
      within a RA-DIO message

   For example, an objective code point might indicate that the DAG is
   using structure and consequently the Expected Number path cost.  The only
   aim of Transmissions (ETX) as a metric, that the optimization goal rank is to minimize ETX, that inform loop avoidance as explained hereafter.
   The computation of the DAG Rank is equivalent MUST be done in such a way so as to ETX, and that RA-DIO propagation entails adding
   maintain the advertised ETX
   of following properties for any nodes M and N who are
   neighbors in the LLN:

      For a node N, and its most preferred parent to M, DAGRank(N) >
      DAGRank(M) must hold.  Further, all parents in the ETX DAG parent set
      must be of a rank less than self's DAGRank(N).  In other words,
      the link to the most
   preferred parent.

   By using defined OCPs that are understood rank presented by all nodes in a
   particular implementation, and by conveying them node N MUST be greater (deeper) than that
      presented by any of its parents.

      If DAGRank(M) < DAGRank(N), then M is probably located in a more
      preferred position than N in the RA-DIO
   message, RPL nodes may work DAG with respect to build optimized LLN using a variety of
   application and implementation specific the metrics
      and goals.

   A default OCP, OCP 0, is specified with a well-defined default
   behavior.  OCP 0 is used to define RPL behaviors in optimizations defined by the case where a
   node encounters a RA-DIO message containing a objective code point that it does
   not support.

3.2.1.5.  Selection of Feasible point.  In any
      fashion, Node M may safely be a DAG Parents

   The decision parent for Node N without risk
      of creating a node to join a DAG may be optimized according to
   implementation specific policy functions on the node as indicated by
   one or more specific OCP values. loop.

         For example, a node may be
   configured for one goal to optimize Node M of rank 3 is likely located in a bandwidth metric (OCP-1), and
   with more
         optimum position than a parallel goal Node N of rank 5.  A packet directed
         inwards and forwarded from Node N to optimize for a reliability metric (OCP-2).
   Thus two DAGs, Node M will always make
         forward progress with two unique DAGIDs, may be constructed and
   maintained in respect to the LLN: DAG-1 DAG organization on that
         link; there is no risk of Node M at rank 3 forwarding the
         packet back into Node N's sub-DAG at rank of 5 or greater
         (which would be optimized according a sufficient condition for a loop to OCP-1,
   whereas DAG-2 would be optimized according to OCP-2.  A node may occur).

      If DAGRank(M) == DAGRank(N), then
   maintain two parallel sets of DAG parents M and related data
   structures.  Note that in such a case traffic may directed along N are located positions of
      relatively the
   appropriate constrained DAG based on traffic marking same optimality within the IPv6
   header.

   As DAG.  In some cases,
      Node M may be used as a successor by Node N, but with related
      chance of creating a loop that must be detected and broken by some
      other means.

         If Node M is at rank 3 and node hears RA messages from its neighbors it N is at rank 3, then they are
         siblings; by definition Node M and N cannot be in each others
         sub-DAG.  They may process their
   attached DIO messages.  At this time the node then forward to each other failing
         serviceable parents, making `sideways' progress (but not
         reverse progress).  If another sibling or more gets involved
         there may then be able to take
   into consideration, some chance for example, the following:

   o  Is 3 or more way loops, which is
         the neighboring risk of sibling forwarding.

      If DAGRank(M) > DAGRank(N), then node heard reliably enough, and are the metrics
      stable enough, that M is located in a local degree of confidence may be
      established less
      preferred position than N in the DAG with respect to the neighboring node?  Should metrics
      and optimizations defined by the
      neighboring node objective code point.  Further,
      Node (M) may in fact be considered in the set of candidate neighbors?

   o  In consultation with implementation specific policy (OCP goal), Node (N)'s sub-DAG.  There is
      the neighboring node a feasible parent from a constrained-path
      perspective?

   o  According no
      advantage to the implementation specific policy (OCP), does the
      neighboring node offer a better optimized position into the DAG?

   o  Does the neighboring node offer Node (N) selecting Node (M) as a DAG with parent, and such
      a more desirable
      administrative preference for an otherwise currently satisfied
      optimization objective, all else being equal?

   o  Is the neighboring node selection may create a peer (sibling) within the DAG?

   Based on such considerations, the node loop.

         For example, if Node M is of rank 3 and Node N is of rank 5,
         then by definition Node N is in a less optimum position than
         Node N. Further, Node N at rank 5 may incorporate the
   neighboring node into in fact be in Node M's
         own sub-DAG, and forwarding a packet directed inwards towards
         the set of DAG parents.  When the node inserts root from M to N will result in backwards progress and
         possibly a loop.

   As an example, the first DAG parent into Rank could be computed in such a way so as to
   closely track ETX when the empty DAG parent set, it objective function is able to
   join the DAG.  As minimize ETX, or
   latency when the DAG parent set objective function is updated, the node will
   consult to minimize latency, or in a rank computation function indicated by
   more complicated way as appropriate to the OCP for objective code point being
   used within the DAG.

   The DAG
   in order to determine its own rank value, which it will subsequently
   advertise when it emits its own RA-DIO messages.

   Following is an overview of the rules subsequently used to select restrict the options a parent (the
   detailed mode of operation node has
   for movement within the selection of the candidate DAG
   parent(s) is described in Section 5.3.  First, it is important and to
   note that coordinate movements in order to
   avoid the rank creation of the node is not directly used as a selection
   criteria. loops.

3.2.1.8.  Sub-DAG

   The metric sub-DAG of choice as indicated by the OCP advertised by
   the candidate parents a node is used to select the parent, although set of other nodes of greater rank in
   the DAG, and thus might use
   of a cumulative metric to reflect path towards the rank DAG root that contains
   this node.  This is not precluded.

   Consider an example where important property that is leveraged for loop
   avoidance- if a node N receives two RAs from node A and B
   with (rank, metric) equal to (2,4) and (5,3) respectively.  Node N
   may chose B as its parent (higher has lesser rank but smaller metric).  Once the
   parent, B, then it is selected, not in the sub-DAG.
   (An arbitrary node computes its own with greater rank according to
   the OCP.

   If may or may not be contained in
   the node receives other RA messages it cannot attach to other
   parents if choosing that parent would cause sub-DAG).  Paths through siblings are not contained in this set.

   As a further illustration, consider the nodes own rank to
   increase.  Back to DAG examples in Appendix B.
   Consider Node (24) in the previous DAG Example depicted in Figure 9.  In this
   example, suppose that a node C
   appears with a (rank, metric) equal to (5,1).  By selecting C as the
   new parent, N would have sub-DAG of Node (24) is comprised of Nodes (34), (44),
   and (45).

   A frozen sub-DAG is a rank subset of 6 (making the assumption that nodes in the
   rank is increased by sub-DAG of a value node who
   have been informed of 1 according a change to the OCP).  Although node, and choose to follow the path metric would be lower, this may lead to
   node in a DAG Loop should C
   belong to manner consistent with the sub-DAG of N as further discussed change, for example in Section 3.3.1.

   All reliable neighboring nodes of
   preparation for a lesser rank than the node may be
   considered as potential DAG parents (Note that, as coordinated move.  Nodes in the above
   example, as a consequence sub-DAG who hear of satisfying
   a particular OCP goal, change and have other options than to follow the
   most preferred parent may node do not necessarily be the potential parent have
   to become part of
   least rank, the frozen sub-DAG, for example such a potential parent of lesser rank node may also be
   an energy constrained device that is
   able to remain attached to generally be avoided and thus
   not the most preferred).  No nodes of greater rank than self original DAG through a different DAG
   parent.  A further example may be found in Appendix B.8.

3.2.1.9.  Moving up in the DAG parent set; to allow such nodes will introduce a
   possibility to create loops (by potentially allowing a packet to make
   backwards progress as it is forwarded DAG

   A node may safely move `up' in the DAG).  All neighboring
   nodes of equal DAG, causing its DAG rank may be considered as siblings within to
   decrease and moving closer to the DAG
   (even though they may not have parents in common, they may still
   provide path diversity towards root without risking the DAG root).

   The computation
   formation of rank, and related properties, are further
   described a loop.

3.2.1.10.  Moving down in Section 3.3.1.

3.2.1.5.1.  Example

   For example, suppose that a DAG

   A node (N) is may not attached consider to any move `down' the DAG, causing its DAG rank
   to increase and
   that it is in range of nodes (A), (B), (C), (D), and (E).  Let all
   nodes be configured to use an OCP which defines moving further from the DAG root.  In the case where
   a policy such that
   ETX is node looses connectivity to be minimized and paths with the attribute `Blue' should be
   avoided.  Let DAG, it must first leave the rank computation indicated by DAG
   before it may then rejoin at a deeper point.  This allows for the OCP simply
   reflect
   node to coordinate moving down, freezing its own sub-DAG and
   poisoning stale routes to the ETX aggregated along DAG, and minimizing the path.  Let chances of re-
   attaching to its own sub-DAG thinking that it has found the links between original
   DAG again.  If a node (N) and where allowed to re-attach into its neighbors (A-E) all have an ETX of 1 (which is
   learned by node (N) through some implementation specific method).
   Let node (N) own sub-DAG
   a loop would most certainly occur, and may not be configured broken until a
   count-to-infinity process elapses.  The procedure of detaching before
   moving down eliminates the need to send IPv6 Router Solicitation (RS)
   messages count-to-infinity.

3.2.1.11.  DAG Jumps

   A jump from one DAG to another DAG is attaching to probe for nearby DAGs.

   o  Node (N) transmits a Router Solicitation.

   o  Node (B) responds.  Node (N) investigates the RA-DIO message, and
      learns new DAGID, in
   such a way that Node (B) an old DAGID is a member of replaced by the new DAGID.  In
   particular, when an old DAGID 1 at rank 4, and not
      `Blue'.  Node (N) takes note of this, but is not yet confident.

   o  Similarly, Node (N) hears from Node (A) at rank 9, Node (C) at
      rank 5, left, all associated parents are no
   longer feasible, and Node (E) at rank 4.

   o  Node (D) responds.  Node (D) has a RA-DIO message that indicates
      that it new DAGID is joined.

   When a member of DAGID 1 at rank 2, but node in a DAG follows a DAG parent, it carries means that the
      attribute `Blue'.  Node (N)'s policy function rejects Node (D),
      and no further consideration is given.

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

   o  Node (N) adds Node (E) (rank 4) to has changed its set of DAG parents for DAGID 1.  Following the mechanisms specified (e.g. by the OCP, joining a new DAG) and given that the ETX is 1 for the link between (N) and (E), Node (N) is
      now at rank 5 in
   node updates its own DAGID 1.

   o  Node (N) adds Node (B) (rank 4) in order to its set of keep the DAG parents parent.

3.2.1.12.  Floating DAGs for
      DAGID 1.

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

   o  Node (N) DAG Repair

   A DAG may now forward traffic intended also be floating.  Floating DAGs may be encountered, for
   example, during coordinated reconfigurations of the default
      destination inward along DAGID 1 via nodes (B) and (E).  In some
      cases, e.g. if nodes (B) and (E) are tried and fail, network topology
   wherein a node (N) may
      also choose to forward traffic to and its sibling node (C), without
      making inward progress but with sub-DAG breaks off the intention that node (C) or DAG, temporarily
   becomes a
      following successor can make inward progress.  Should Node (C) not
      have floating DAG, and reattaches to a viable parent, it should never send the packet back grounded DAG.  (Such
   coordination endeavors to Node
      (N) (to avoid a 2-node loop).

3.2.1.6.  DAG Maintenance

   When a node moves within a DAG, the move is defined as updating the
   set construction of DAG parents for a particular DAGID, i.e. adding or deleting
   DAG parents.  Not all moves entail changes in rank.

   A jump from one DAG to another DAG is attaching to a new DAGID, transient loops
   in
   such a way that an old DAGID is replaced by the new DAGID.  In
   particular, when an old DAGID is left, all associated parents are no
   longer feasible, and a new DAGID is joined.

   When LLN).

   A DAG, or a node in sub-DAG temporarily promoted to a DAG follows DAG, may also become
   floating because of a DAG parent, it means that network element failure.  If the DAG parent has changed its DAGID (e.g. by joining a new DAG) and that set
   of the node updates becomes completely depleted, the node will have detached
   from the DAG, and may, if so configured, become the root of its own DAGID in order to keep the
   transient floating DAG parent.

   A frozen sub-DAG is with a subset of nodes in less desirable administrative
   preference (thus beginning the sub-DAG of a node who
   have been informed process of a change to establishing the node, frozen
   sub-DAG), and choose then may reattach to follow the
   node in original DAG at a manner consistent with the change, for example in
   preparation for a coordinated move.  Nodes in the sub-DAG who hear of
   a change lower point
   if 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 have other options than to follow the node do not have are thus able to become part of send traffic inward along the frozen sub-DAG, for example such a node may be
   able
   DAG by forwarding to remain attached their selected parents.  However, this mechanism
   alone is not sufficient to support P2MP traffic flowing outward along
   the original DAG through a different from the DAG
   parent. root toward nodes.  A further example destination advertisement
   mechanism is employed by RPL to build up routing state in support of
   these outward flows.  The destination advertisement mechanism may not
   be found supported in Section 3.3.1.1.

   When all implementations, as appropriate to the node encounters new candidate neighbors
   application requirements.  A DAG root that offer higher
   positions supports using the
   destination advertisement mechanism to build up routing state will
   indicate such in the DAG, it may incorporate them directly into its set
   of RA-DIO message.  A DAG parents.  In this case root that supports using
   the node may update its choice destination advertisement mechanism must be capable of most
   preferred parent, possibly causing its own advertised rank allocating
   enough state to
   decrease, and discarding any former parents now of a deeper rank.
   This case is `moving inwards along the DAG' and does not require any
   additional coordination for loop avoidance.

   If store the DAG parent set of routing state received from the node becomes completely depleted, 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 used to convey the
   node will have detached from destination information inward
   along the DAG, and may, if so configured,
   become DAG toward the root of its own transient floating DAG with a less
   desirable administrative preference (thus beginning root.

   The information conveyed in the process of
   establishing NA-DAO message includes the frozen sub-DAG),
   following:

   o  A lifetime and then may reattach sequence counter to determine the
   original DAG at a lower point if it is able (after hearing RA-DIO
   messages from alternate attachment points).

   When freshness of the node encounters candidate parents that are in a different
   DAG, and decides
      destination advertisement.

   o  Depth information used by nodes to leave determine how far away the current DAG in order to join
      destination (the source of the
   different DAG (thus doing a jump), it may do so safely without regard
   to loop avoidance.  However, it may not return immediately destination advertisement) is

   o  Prefix information to identify the
   current DAG as such movement destination, which may result in the creation of be a DAG
   Loop, in particular if it reattaches back into its own former sub-DAG
   in
      prefix, an uncoordinated manner.

   When a node, and perhaps a related frozen sub-DAG, jumps individual host, or multicast listeners

   o  Reverse Route information to a
   different DAG, the move is coordinated by a DAG Hop timer.  The DAG
   Hop timer allows record the nodes who will attach closer to visited (along the sink of
      outward path) when the
   new DAG to `jump' first, and then drag dependent intermediate nodes behind them,
   thus endeavoring to efficiently coordinate the attachment of the
   frozen sub-DAG into along the new DAG.  A further illustration of this
   mechanism may be found in Section 3.3.3.

   Appendix B provides additional examples of DAG discovery and
   maintenance.

3.2.2. path cannot
      support storing state for Hop-By-Hop routing.

3.2.2.2.  Destination Advertisement Operation

   As RPL constructs DAGs, nodes may provision routes toward
   destinations advertised through RA-DIO messages through their
   selected parents, the DAG is constructed and maintained, nodes are thus able capable to send traffic inward along the
   DAG by forwarding emit
   NA-DAO messages to a subset, or all, of their selected DAG parents.  However,  The
   selection of this mechanism
   alone subset is not sufficient according to support P2MP traffic flowing outward along
   the DAG from the DAG root toward nodes.  A an implementation specific
   policy.

   As a special case, a node may periodically emit a link-local
   multicast IPv6 NA-DAO message advertising its locally available
   destination advertisement prefixes.  This mechanism is employed by RPL allows for the one-hop
   neighbors of a node to build up routing state learn explicitly of the prefixes on the node,
   and in some application specific scenarios this is desirable in
   support of
   these outward flows.  The provisioning a trivial `one-hop' route.  In this case,
   nodes who receive the multicast destination advertisement mechanism may use it
   to provision the one-hop route only, and not
   be supported engage in all implementations, any additional
   processing (so as appropriate not to engage the
   application requirements.  A mechanisms used by a DAG root that supports using the
   destination advertisement mechanism to build up parent).

   When a (unicast) NA-DAO message reaches a node capable of storing
   routing state will
   indicate such in state, the RA-DIO message.  A DAG root that supports using
   the destination advertisement mechanism must be capable of allocating
   enough state to store the routing state received node extracts information from the LLN.

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

   An IPv6 Neighbor Advertisement Message NA-DAO message
   and updates its local database with Destination Advertisement
   Options (NA-DAO) is used to convey the destination information inward
   along the DAG toward the DAG root.

   The information conveyed in a record of the NA-DAO message includes the
   following:

   o  A lifetime
   and sequence counter to determine who it was received from.  When the freshness of node later propagates NA-DAO
   messages, it selects the
      destination advertisement.

   o  Depth best (least depth) information used by nodes to determine how far away the
      destination (the source of the for each
   destination advertisement) is

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

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

3.2.2.2.  Destination Advertisement Operation

   As again in the DAG is constructed and maintained, nodes are capable to emit form of NA-DAO
   messages to a subset, or all, subset of their its own DAG parents.  The
   selection of  At this subset is according to an implementation specific
   policy.

   As a special case, a time the node
   may periodically emit perform route aggregation if it is able, thus reducing the
   overall number of NA-DAO messages.

   When a link-local
   multicast IPv6 (unicast) NA-DAO message advertising its locally available
   destination prefixes.  This mechanism allows for the one-hop
   neighbors of reaches a node to learn explicitly incapable 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 DAG parent).

   When a (unicast) NA-DAO message reaches a node capable of storing
   routing state, the node extracts information from the NA-DAO message
   and updates its local database with 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 a subset of its own DAG parents.  At this time the node
   may perform route aggregation if it is able, thus reducing the
   overall number of NA-DAO messages.

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

   When a node that is capable of storing routing state encounters a
   (unicast) 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.  The node is
   able to detach and store the Reverse Route State and associate it
   with the destination described by the NA-DAO message.  Subsequently
   the node may use this information to construct a source route in
   order to bridge the region of nodes that are unable to support Hop-
   By-Hop routing to reach the destination.

   In this way the destination advertisement mechanism is able to
   provision routing state in support of P2MP traffic flows outward
   along the DAG, and as according to the available resources in the
   network.

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

   A further example of the operation of the destination advertisement
   mechanism is available in Appendix B.6

3.3.  Other Considerations

3.3.1.  DAG Rank and  Loop Avoidance

   When nodes select DAG parents, they should select and Stability

   The goal of a guaranteed consistent and loop free global routing
   solution for an LLN may not be practically achieved given the most preferred
   parent according real
   behavior and volatility of the underlying metrics.  The trade offs to
   achieve a stable approximation of global convergence may be too
   restrictive with respect to their implementation specific objectives, using the cost metrics conveyed need of the LLN to react quickly in
   response to the RA-DIO messages along lossy environment.  Globally the DAG in
   conjunction with the related objective functions as specified by the
   OCP.

   Based on this selection, the metrics conveyed by the most preferred
   DAG parent, the nodes own metrics and configuration, and a related
   function defined by the objective code point, a node will LLN may be able to
   compute
   achieve a value for its rank weak convergence, in particular as a consequence of selecting a most
   preferred DAG parent.

   It is important to note that the DAG Rank is not itself a metric,
   although its value is derived from and influenced by the use of
   metrics link changes are able to select DAG parents
   be handled locally and take up a position result in the DAG.  In
   other words, routing metrics and OCP (not rank directly) are used minimal changes to
   determine the DAG structure and consequently the path cost.  The only global topology.

   RPL does not aim of the rank is to inform guarantee loop avoidance as explained hereafter.
   The computation of the DAG Rank MUST be done free path selection, or strong
   global convergence.  In order to reduce control overhead, in
   particular the expense of mechanisms such a way so as count-to-infinity, RPL
   does try to
   maintain avoid the following properties for any nodes M and N who are
   neighbors in creation of loops when undergoing topology
   changes.  Further mechanisms to mitigate the LLN:

      For impact of loops, such as
   loop detection when forwarding, are under investigation.

3.3.1.  Greediness and Rank-based Instabilities

   If a node N, is greedy and attempts to move deeper in the DAG, beyond
   its most preferred parent M, DAGRank(N) >
      DAGRank(M) must hold.  Further, all parents parent, in order to increase the size of the DAG
   parent set
      must be of set, then an instability can result.  This is illustrated in
   Figure 11.

   Suppose a rank less than self's DAGRank(N).  In other words,
      the rank presented by node is willing to receive and process a RA-DIO messages
   from a node N MUST be greater (deeper) than that
      presented by any of in its parents.  (This mechanism serves to avoid
      loops own sub-DAG, and in the case where an alternate parent is used- if no
      alternate parent is general a node deeper than the preferred parent then loops
      are avoided.  The risk of loops occurs if there is it.
   In such cases a chance for an
      alternate parent exists to forward traffic create a feedback loop, wherein two
   or more nodes continue to try and move in the DAG in order to
   optimize against each other.  In some cases this will result in an
   instability.  It is for this reason that RPL mandates that a node
   never receive and process RA-DIO messages from deeper successor, which nodes.  This
   rule creates an `event horizon', whereby a node cannot be influenced
   into an instability by the action of nodes that may be in its own
   sub-DAG.

   A further example of the sub-DAG, and traffic then makes backwards progress consequences of greedy operation, and comes back
   instability related to the node again).

      If DAGRank(M) < DAGRank(N), then M processing RA-DIO messages from nodes of
   greater rank, may be found in Appendix B.9

3.3.2.  Merging DAGs

   The merging of DAGs is probably located coordinated in a more
      optimum position than N in the DAG with respect way such as to the metrics try and
      optimizations defined by merge
   two DAGs cleanly, preserving as much DAG structure as possible, and
   in the objective code point.  In any
      fashion, Node M may safely be process effecting a DAG parent for Node N without risk clean merge with minimal likelihood of creating
   forming transient DAG loops.  The coordinated merge is also intended
   to minimize the related control cost.

   When a loop.  For example, node, and perhaps a Node M of rank 3 is located in related frozen sub-DAG, jumps to a more optimum position than
   different DAG, the move is coordinated by a Node N set of rank 5.  A packet
      directed inwards and forwarded from Node N to Node M timers (DAG Hop
   timers).  The DAG Hop timers allow the nodes who will always
      make forward progress with respect attach closer
   to the DAG organization on that
      link; there is no risk sink of Node M at rank 3 forwarding the packet
      back into Node N's sub-DAG at rank of 5 or greater (which would be
      a sufficient condition for a loop new DAG to occur).

      If DAGRank(M) == DAGRank(N), then M `jump' first, and N are located positions then drag dependent
   nodes behind them, thus endeavoring to efficiently coordinate the
   attachment of
      relatively the same optimality within frozen sub-DAG into the new DAG.  In some cases,
      Node M

   A further example of a DAG Merge operation may be used as found in
   Appendix B.10

3.3.3.  DAG Loops

   A DAG loop may occur when a successor by Node N, but with related
      chance of creating node detaches from the DAG and reattaches
   to a loop device in its prior sub-DAG that must be detected and broken by some
      other means.  If Node M is at rank 3 has missed the whole detachment
   sequence and node N is at rank 3, then
      they are siblings; by definition Node M and N cannot be in each
      others sub-DAG.  They may then forward to each other failing
      serviceable parents, making `sideways' progress (but not reverse
      progress).  If another sibling or more gets involved there kept advertising the original DAG.  This may
      then be some chance for 3 or more way loops, which is happen in
   particular when RA-DIO messages are missed.  Use of the risk DAG sequence
   number can eliminate this type of
      sibling forwarding. loop.  If DAGRank(M) > DAGRank(N), then node M the DAG sequence number
   is located in a less
      optimum position than N not in use, the protection is limited (it depends on propagation
   of RA-DIO messages during DAG with respect hop timer), and temporary loops might
   occur.  RPL will move to eliminate such a loop as soon as a RA-DIO
   message is received from a parent that appears to be going down, as
   the metrics child has to detach from it immediately.  (The alternate choice
   of staying attached and
      optimizations defined by following the objective code point.  Further, Node
      (M) may in fact be parent in Node (N)'s sub-DAG.  There is no advantage its fall would have
   counted to Node (N) selecting Node (M) infinity and led to detach as a well).

   Consider node (24) in the DAG parent, Example depicted in Figure 9, and such a
      selection may create a loop.  For example, if Node M is of rank 3 its
   sub-DAG nodes (34), (44), and Node N is (45).  An example of rank 5, then by definition Node N is in a less
      optimum position than Node N. Further, Node N at rank 5 may in
      fact DAG loop would
   be in Node M's own sub-DAG, and forwarding a packet directed
      inwards towards if node (24) were to detach from the DAG root from M rooted at (LBR), and
   nodes (34) and (45) were to N will result in backwards
      progress miss the detachment sequence.
   Subsequently, if the link (24)--(45) were to become viable and possibly a loop.

   For example, node
   (24) heard node (45) advertising the DAG Rank could be computed in such rooted at (LBR), a way so as to
   closely track ETX when the objective function is DAG loop
   (45->34->24->45) may form if node (24) attaches to minimize ETX, or
   latency node (45).

3.3.4.  DAO Loops

   A DAO loop may occur when the objective function is to minimize latency, or in parent has a
   more complicated way as appropriate to route installed upon
   receiving and processing a NA-DAO message from a child, but the objective code point being
   used within child
   has subsequently cleaned up the DAG. state.  This loop happens when a no-
   DAO was missed till a heartbeat cleans up all states.  The DAG rank DAO loop
   is subsequently used to restrict not explicitly handled by the options current specification.  Split
   horizon, not forwarding a node has
   for movement within the DAG and to coordinate movements in order packet back to
   avoid the creation of loops.

   A node it came from, may safely move `up' in
   mitigate the DAG, causing its DAG rank to
   decrease and moving closer to the DAG root without risking the
   formation of a loop.

   A node may not consider to move `down' the DAG, causing its DAG rank
   to increase and moving further from the DAG root.  Such a move will
   entail moving to a less optimum position in DAO loop in some cases, but does not eliminate it.

   Consider node (24) in the DAG Example depicted in all cases, as
   defined by the objective code point.  In the case where Figure 9.  Suppose
   node (24) has received a DA from node looses
   connectivity to the DAG, it must first leave the DAG before it may
   then rejoin at (34) advertising a deeper point.  This allows for the destination
   at node to
   coordinate moving down, freezing its own sub-DAG and poisoning stale
   routes to (45).  Subsequently, if node (34) tears down the DAG, and minimizing routing
   state for the chances of re-attaching destination and node (24) did not hear a no-DAO message
   to its
   own sub-DAG thinking that it has found clean up the original DAG again.  If routing state, a DAO loop may exist. node where allowed (24) will
   forward traffic destined for node (45) to re-attach node (34), who may then
   naively return it into its own sub-DAG a loop would
   most certainly occur, and may (if split horizon is not be broken until a count-to-infinity
   process elapses.  The procedure of detaching before moving down
   eliminates in place).  A
   more complicated DAO loop may result if node (34) instead passes the need
   traffic to count-to-infinity.

   Any neighboring nodes 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 of lesser rank than self are eligible to be
   considered siblings keep choosing amongst
   themselves as alternate DAG parents for forwarding.  But this node
   may only adopt successors such that a parent as new preferred parent if that packet does not
   cause the resulting rank for this node to increase. make forward
   progress.  The goal of a guaranteed consistent and loop free global routing
   solution for an LLN may current draft limits those loops to some degree by
   split horizon (do not be practically achieved given send back to the real
   behavior same sibling) and volatility of parent
   preference (always prefer parents vs. siblings).

   Consider the underlying metrics.  The trade offs to
   achieve DAG Example depicted in Figure 9.  Suppose that Node
   (32) and (34) are reliable neighbors, and thus are siblings.  Then,
   in the case where Nodes (22), (23), and (24) are transiently
   unavailable, and with no other guiding strategy, a stable approximation of global convergence sibling loop may be too
   restrictive with respect to the need of
   exist, e.g. (33->34->32->33) as the LLN to react quickly siblings keep choosing amongst
   each other in
   response to the lossy environment.  Globally the LLN an uncoordinated manner.

3.4.  Local and Temporary Routing Decision

   Although implementation specific, it is worth noting that a node may be able
   decide to
   achieve a weak convergence, in particular implement some local routing decision based on some
   metrics, as link changes are able to
   be handled observed locally and result in minimal changes to global topology.

   RPL does not aim to guarantee loop free path selection, or strong
   global convergence.  In order to reduce control overhead, reported in
   particular the expense RA-DIO message.  For
   example, the routing may reflect a set of mechanisms such as count-to-infinity, RPL
   does try successors (next-hop),
   along with various aggregated metrics used to avoid load balance the creation of loops when undergoing topology
   changes.  Further mechanisms
   traffic according to mitigate the impact of loops, such as
   loop detection when forwarding, some local policy.  Such decisions are under investigation.

3.3.1.1.  Example

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

              -1-                    -2-                    -3-

                         Figure 1: DAG Maintenance

   Consider the example depicted in Figure 1-1.  In this example, Node
   (A) local and
   implementation specific.

   Routing stability is attached to crucial in a LLN: in the presence of unstable
   links, the first option consists of removing the link from the DAG at some rank d.  Node (A) is
   and triggering a DAG parent recomputation across all of
   Nodes (B) and (C).  Node (C) is the nodes affected
   by the removed link.  Such a DAG parent naive approach could unavoidably lead to
   frequent and undesirable changes of Node (D).  There is
   also an undirected sibling link between Nodes (B) the DAG, routing instability, and (C).

   In this example, Node (C) may safely forward
   high-energy consumption.  The alternative approach adopted by RPL
   relies on the ability to Node (A) without
   creating a loop.  Node (C) may temporarily not safely forward to Node (D),
   contained within it's own sub-DAG, without creating use a loop.  Node (C)
   may forward to Node (B) in some cases, e.g. link toward a
   successor marked as valid, with no change on the DAG structure.  If
   the link (C)->(A) is
   temporarily unavailable, but with perceived as non-usable for some chance period of creating time (locally
   configurable), this triggers a loop
   (e.g. if multiple nodes in a set of siblings start forwarding
   `sideways' DAG recomputation, through the DAG
   discovery mechanism further detailed in a cycle) and requiring Section 5.3, after reporting
   the intervention of additional
   mechanisms link failure.  Note that this concept may be extended to detect and break the loop.

   Consider take
   into account other link characteristics: for the case where Node (C) hears a RA-DIO message from a Node
   (Z) at sake of
   illustration, a lesser rank and superior position in the DAG than node (A).
   Node (C) may safely undergo decide to send a fixed number of packets to
   a particular successor (because of limited buffering capability of
   the process successor) before starting to evict send traffic to another successor.

   According to the local policy function, it is possible for the node (A) from its
   to order the DAG parent set and attach directly from `most preferred' to Node (Z) without creating a
   loop, because its rank will decrease.

   Now consider `least
   preferred'.  By constructing such an ordered set, and by appending
   the case where set with siblings, the link (C)->(A) becomes nonviable, and node (C) must move is able to a deeper rank within the DAG:

   o  Node (C) must first detach from construct an ordered list
   of preferred next hops to assist in local and temporary routing
   decisions.  The use of the DAG ordered list by removing Node (A) from
      its DAG parent set, leaving an empty DAG parent set.  Node (C)
      becomes a forwarding engine is
   loosely constrained, and may take into account the root dynamics of its own floating, less preferred, DAG.

   o  Node (D), hearing the
   LLN.  Further, a modified RA-DIO message from Node (C), follows
      Node (C) into the floating DAG.  This is depicted in Figure 1-2.
      In general, any node with no other options in the sub-DAG forwarding engine implementation may decide to
   perform load balancing functions using hash-based mechanisms to avoid
   packet re-ordering.  Note however, that specific details of Node
      (C) will follow Node (C) into the floating DAG, maintaining a
   forwarding engine implementation are beyond the
      structure scope of this
   document.

   These decisions may be local and/or temporary with the sub-DAG.

   o  Node (C) hears a RA-DIO message from Node (B) and determines it is
      able objective to rejoin
   maintain 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 and Node (C) adds Node (B) shape while preserving routing stability.

3.5.  Maintenance of Routing Adjacency

   In order to its DAG parent
      set.  Node (C) has now safely moved deeper within relieve the grounded DAG
      without creating any loops.  Node (D), and any other sub-DAG LLN of
      Node (C), will hear the modified RA-DIO message sourced from Node
      (C) and follow Node (C) overhead of periodic keepalives,
   RPL may employ an as-needed mechanism of NS/NA in a coordinated manner order to reattach verify
   routing adjacencies just prior to forwarding data.  Pending the
      grounded DAG.  The final DAG is depicted
   outcome of verifying the routing adjacency, the packet may either be
   forwarded or an alternate next-hop may be selected.

4.  Constraint Based Routing in Figure 1-3

3.3.2.  DAG Parent Selection, Stability, and Greediness

   If a node LLNs

   This aim of this section is greedy and attempts to move deeper in make a clear distinction between
   routing metrics and constraints and define the DAG, beyond
   its most preferred parent, term constraint based
   routing as used in order this document.

4.1.  Routing Metrics

   Routing metrics are used by the routing protocol to increase compute the size
   shortest path according to one of more defined metrics.  IGPs such as
   IS-IS ([RFC5120]) and OSPF ([RFC4915]) compute the DAG
   parent set, then an instability shortest path
   according to a Link State Data Base (LSDB) using link metrics
   configured by the network administrator.  Such metrics can result.  This represent
   the link bandwidth (in which case the metric is illustrated usually inversely
   proportional to the bandwidth), delay, etc.  Note that in
   Figure 2.

   Suppose some cases
   the metric is a node polynomial function of several metrics defining
   different link characteristics.  The resulting shortest path cost is willing
   equal 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 the sum (or multiplication) of the link metrics along the
   path: such cases a chance exists metrics are said to create a feedback loop, wherein two be additive or multiplicative metrics.

   Some routing protocols support more nodes continue to try and move than one metric: in the DAG in order to
   optimize against each other.  In some cases this will result in an
   instability.  It vast
   majority of the cases, one metric is for this reason that RPL mandates that a node
   never receive and process RA-DIO messages from deeper nodes.  This
   rule creates an `event horizon', whereby used per (sub)topology.  Less
   often, a node cannot second metric may be influenced
   into an instability used as a tie breaker in the presence
   of ECMP (Equal Cost Multiple Paths).  The optimization of multiple
   metrics is known as an NP complete problem and is sometimes supported
   by some centralized path computation engine.

   In the action case of nodes RPL, it is virtually impossible to define *the*
   metric, or even a composite, that will fit it all:

   o  Some information apply when determining routes, other information
      may be apply only when forwarding packets 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 its own
   sub-DAG.

3.3.2.1.  Example

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

              -1-                    -2-                    -3-

                   Figure 2: Greedy DAG Parent Selection

   Consider the example depicted a given scenario and useless in Figure 2.  A DAG another.

   o  Some arguments are scalar, others statistical.

   For that reason, the RPL protocol core is depicted in 3
   different configurations. agnostic to the logic that
   handles metrics.  A usable link between (B) node will be configured with some external logic
   to use and (C) exists
   in all 3 configurations.  In Figure 2-1, Node (A) is a DAG parent prioritize certain metrics for
   Nodes (B) and (C), and (B)--(C) is a sibling link.  In Figure 2-2,
   Node (A) is specific scenario.  As
   new heterogeneous devices are installed to support the evolution of a DAG parent for Nodes (B) and (C), and Node (B) is also
   a DAG parent for Node (C).  In Figure 2-3, Node (A) is
   network, or as networks form in a DAG parent
   for Nodes (B) and (C), totally ad-hoc fashion, it will
   happen that nodes that are programmed with antagonistic logics and Node (C)
   conflicting or orthogonal priorities end up participating in the same
   network.  It is also a DAG thus recommended to use consistent parent for Node
   (B).

   If a selection
   policy, as per Objective Code Points (OCP), to ensure consistent
   optimized paths.

   RPL node is too greedy, designed to survive and still operate, though in that it attempts a somewhat
   degraded fashion, when confronted to optimize such heterogeneity.  The key
   design point is that each node is solely responsible for an
   additional number of parents beyond its preferred parent, then an
   instability can result.  Consider setting the DAG illustrated
   vector of metrics that it sources in Figure 2-1.
   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 DAG, derived in part from
   the metrics sourced from its preferred parent selection causes parent.  As a node to have only one
   parent and no siblings, result, the DAG
   is not broken if another node may decide to insert itself at a
   slightly higher rank makes its decisions in order to have at least one sibling and thus as antagonistic
   fashion, though an alternate forwarding solution.  This does end-to-end path might not deprive other nodes fully achieve any of a forwarding solution and this is considered acceptable
   greediness.

   o  Let Figure 2-1 be the initial condition.

   o  Suppose Node (C) first is able to leave
   optimizations that nodes along the DAG and rejoin at a
      lower rank, taking both Nodes (A) and (B) as DAG parents as
      depicted way expect.  The default operation
   specified in Figure 2-2.  Now Node (C) is deeper than both Nodes
      (A) and (B), and Node (C) OCP 0 clarifies this point.

4.2.  Routing Constraints

   A constraint is a link or a node characteristic that must be
   satisfied to have 2 DAG parents.

   o  Suppose Node (B), in its greediness, is willing to receive by the computed path (using boolean values or lower/upper
   bounds) and
      process is by definition neither additive nor multiplicative.
   Examples of links constraints are "available bandwidth",
   "administrative values (e.g. link coloring)", "protected versus non-
   protected links", "link quality" whereas a RA-DIO message from Node (C) (against node constraint can be the rules
   level of RPL),
      and then Node (B) leaves battery power, CPU processing power, etc.

4.3.  Constraint Based Routing

   The notion of constraint based routing consists of finding the DAG
   shortest path according to some metrics satisfying a set of
   constraints.  A technique consists of first filtering out all links
   and rejoins at nodes that cannot satisfy the constraints (resulting in a lower rank,
      taking both Nodes (A) sub-
   topology) and (C) as DAG parents.  Now then computing the shortest path.

      Example 1:
         Link Metric:     Bandwidth
         Link Constraint: Blue
         Node (B) is
      deeper than both Nodes (A) and (C) and is satisfied Constraint: Mains-powered node

      Objective function 1:
         "Find the shortest path (path with 2 DAG
      parents.

   o  Then Node (C), because it lowest cost where the path
         cost is also greedy, will leave and rejoin
      deeper, to again get 2 parents and have a lower rank then both the sum 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 all link costs (Bandwidth)) along the DAG will oscillate between
      Figure 2-2 path
         such that all links are colored `Blue' and Figure 2-3 until that only traverses
         Mains-powered nodes."

      Example 2:
         Link Metric:     Delay
         Link Constraint: Bandwidth

      Objective function 2:
         "Find the nodes count to infinity and
      restart shortest path (path with lowest cost where the cycle again.

   o  This cycle can be averted through mechanisms in RPL:

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

      *  Nodes (B) other
   information that allows a node to discover a DAG, select its DAG
   parents, and (C) do not process RA-DIO messages from nodes
         deeper than themselves (because such nodes are possibly in
         their own sub-DAGs)

3.3.3.  Merging DAGs identify its siblings while employing loop avoidance
   strategies.

5.1.1.  DAG Information Option (DIO) base option

   The merging of DAGs DAG Information Option is coordinated in a way such container option carried within an
   IPv6 Router Advertisement message as to try and merge
   two DAGs cleanly, preserving as much DAG structure as possible, and defined in the process effecting [RFC4861], which
   might contain a clean merge with minimal likelihood number of
   forming transient DAG loops. suboptions.  The coordinated merge is also intended
   to minimize base option regroups the related control cost.

3.3.3.1.  Example

                                :
                                :
                               (A)       (D)
   minimum information set that is 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |    Length     |G|D|A|  00000  |   Sequence    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | DAGPreference |                BootTimeRandom                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |
                               (B)       (E)   NodePref.   |    DAGRank    |           DAGDelay            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | DIOIntDoubl.  |  DIOIntMin.   |
                               (C)       (F)     DAGObjectiveCodePoint     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           PathDigest                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                            DAGID                              |
       +                                                               +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   sub-option(s)...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 3: Merging DAGs

   Consider 1: DIO Base Option

   Type: 8-bit unsigned identifying the example depicted in Figure 3.  Nodes (A), (B), and (C)
   are part of some larger grounded DAG, where Node (A) DIO base option.  The suggested
         value is at a rank of
   d, Node (B) at d+1, and Node (C) at d+2. 140 to be confirmed by the IANA.

   Length:  8-bit unsigned integer set to 4 when there is no suboption.
         The DAG comprised length of Nodes
   (D), (E), the option (including the type and length fields
         and (F) is a floating, less preferred, DAG, with Node (D)
   as the DAG root.  This floating DAG may have been formed, for
   example, suboptions) in the absence units of a grounded DAG or when Node (D) had to
   detach from a grounded DAG and (E) and (F) followed.  All nodes 8 octets.

   Flag Field:  Three flags are
   using compatible objective code points.

   Nodes (D), (E), and (F) would rather join currently defined:

         Grounded (G):  The Grounded (G) flag is set when the more preferred grounded DAG if they are able than root
               is offering connectivity to remain in an external routed
               infrastructure such as the less preferred floating
   DAG.

   Next, let links (C)--(D) and (A)--(E) become viable. Internet.

         Destination Advertisement Trigger (D):  The following
   sequence of events may then occur in a typical case:

   o  Node (D) will receive and process a RA-DIO message from Node (C)
      on link (C)--(D).  Node Destination
               Advertisement Trigger (D) will consider Node (C) a candidate
      neighbor and process flag is set when the RA-DIO message since Node (C) belongs to
      a different DAG (different DAGID) than Node (D).  Node (D) will
      note that Node (C) is root
               or another node in a grounded DAG at rank d+2, and will
      begin the process successor chain decides to join the grounded DAG at rank d+3.  Node (D)
      will start a DAG Hop timer, logically associated with trigger
               the grounded
      DAG at Node (C), sending of destination advertisements in order to coordinate
               update routing state for the jump. outward direction along the
               DAG, as further detailed in Section 5.9.  Note that the
               use and semantics of this flag are still under
               investigation.

         Destination Advertisement Supported (A) :  The Destination
               Supported (A) bit is set when the DAG Hop timer will
      have a duration proportional root is capable to d+2.

   o  Similarly, Node (E) will receive and process a RA-DIO message from
      Node (A) on link (A)--(E).  Node (E) will consider Node (A) a
      candidate neighbor, will note that Node (A) is in a grounded DAG
      at rank d,
               support the collection of destination advertisement
               related routing state and will begin enables the process operation of the
               destination advertisement mechanism within the DAG.

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

   Sequence Number:  8-bit unsigned integer set by the grounded DAG at
      rank d+1.  Node (E) will start root,
         incremented according to a DAG Hop timer, logically
      associated with the grounded DAG policy provisioned at Node (A), to coordinate the
      jump.  The DAG Hop timer will root,
         and propagated with no change outwards along the DAG.  Each
         increment SHOULD have a duration proportional to d.

   o  Node (F) takes no action, for Node (F) has observed nothing new value of 1 and may cause a wrap back to
      act on.

   o  Node (E)'s DAG Hop timer for the grounded DAG at Node (A) expires
      first.  Node (E), upon
         zero.

   DAGPreference:  8-bit unsigned integer set by the DAG Hop timer expiry, removes Node (D)
      as root to its parent, thus emptying the DAG parent set for the floating
      DAG,
         preference and leaving the floating DAG.  Node (E) then jumps unchanged at propagation.  DAGPreference ranges
         from 0x00 (least preferred) to the
      grounded 0xFF (most preferred).  The
         default is 0 (least preferred).  The DAG by entering Node (A) into preference provides an
         administrative mechanism to engineer the set self-organization of DAG parents for
         the grounded DAG.  Node (E) is now in LLN, for example indicating the grounded most preferred LBR.  If a
         node has the option to join a more preferred DAG at rank
      d+1.  Node (E), by jumping into while still
         meeting other optimization objectives, then the grounded DAG, has created an
      inconsistency by changing its DAGID, and node will begin seek
         to emit RA-DIO
      messages join the more frequently.

   o  Node (F) will receive preferred DAG.

   BootTimeRandom:  A random value computed at boot time and process recomputed
         in case of a RA-DIO message from Node (E).
      Node (F) will observe that Node (E) has changed its DAGID duplication with another node.  The concatenation
         of the NodePreference and will
      directly follow Node (E) into the grounded DAG.  Node (F) BootTimeRandom is now a
      member of the grounded DAG at rank d+2.  Note 32-bit
         extended preference that any additional
      sub-DAG of Node (E) would continue is used to join into resolve collisions.  It is
         set by each node at propagation time.

   NodePreference:  The administrative preference of that LLN Node.
         Default is 0. 255 is the grounded DAG
      in this coordinated manner.

   o highest possible preference.  Set by
         each LLN Node (D) will receive at propagation time.  Forms a RA-DIO message from Node (E).  Since Node
      (E) is now collision
         tiebreaker in a different DAG, Node (D) may process the RA-DIO
      message from Node (E).  Node (D) will observe that, via node (E),
      it could attach to combination with BootTimeRandom.

   DAGRank:  8-bit unsigned integer indicating the grounded DAG at rank d+2.  Node (D) will
      start another DAG Hop timer, logically associated with of the
      grounded DAG at Node (E), with a duration proportional to d+1.
      Node (D) now node
         sending the RA-DIO message.  The DAGRank of the DAG root is running two
         typically 1.  DAGRank is further described in Section 5.3.

   DAGDelay:  16-bit unsigned integer set by the DAG hop timers, one which was started
      with duration proportional root indicating the
         delay before changing the DAG configuration, in TBD-units.  A
         default value is TBD.  It is expected to d+1 and one proportional be an order of
         magnitude smaller than the RA-interval.  It is also expected to d+2.

   o  Generally, Node (D) will expire
         be an order of magnitude longer than the timer associated with typical propagation
         delay inside the jump
      to LLN.

   DIOIntervalDoublings:  8-bit unsigned integer.  Configured on the grounded DAG at node (E) first.  Node (D) may then jump
         root and used to configure the grounded DAG by entering Node (E) into its DAG parent set for trickle timer governing when RA-
         DIO message should be sent within the grounded DAG.  Node (D)
         DIOIntervalDoublings is now in the grounded DAG at rank
      d+2.

   o  In this way RPL has coordinated a merge between number of times that the
         DIOIntervalMin is allowed to be doubled during the trickle
         timer operation.

   DIOIntervalMin:  8-bit unsigned integer.  Configured on the more preferred
      grounded DAG root
         and used to configure the less preferred floating DAG, such that the
      nodes trickle timer governing when RA-DIO
         message should be sent within the two DAGs come together in a generally ordered
      manner, avoiding DAG.  The minimum configured
         interval for the formation of loops RA-DIO trickle timer in the process.

3.4.  Local and Temporary Routing Decision

   Although implementation specific, it units of ms is worth noting that
         2^DIOIntervalMin.  For example, a node may
   decide DIOIntervalMin value of 16ms
         is expressed as 4.

   DAGObjectiveCodePoint:  The DAG Objective Code Point is used to implement some local routing decision based on some
         indicate the cost metrics, as observed locally or reported objective functions, and methods of
         computation and comparison for DAGRank in use in the RA-DIO message.  For
   example, the routing may reflect a DAG.  The
         DAG OCP is set of successors (next-hop),
   along with various aggregated metrics used to load balance by the
   traffic according to some local policy.  Such decisions DAG root.  (Objective Code Points are local and
   implementation specific.

   Routing stability is crucial in a LLN: to
         be further defined in [I-D.ietf-roll-routing-metrics].

   PathDigest:  32-bit unsigned integer CRC, updated by each LLN Node.
         This is the presence result of unstable
   links, a CRC-32c computation on a bit string
         obtained by appending the first option consists of removing received value and the link from ordered set of
         DAG parents at the LLN Node.  DAG
   and triggering roots use a DAG recomputation across all 'previous value'
         of zeroes to initially set the nodes affected
   by the removed link.  Such a naive approach could unavoidably lead PathDigest.  Used to
   frequent and undesirable changes determine
         when something in the set of successor paths has changed.

   DAGID:  128-bit unsigned integer which uniquely identify a DAG.  This
         value is set by the DAG, routing instability, and
   high-energy consumption. DAG root.  The alternative approach adopted by RPL
   relies on global IPv6 address of the ability to temporarily not use a link toward a
   successor marked as valid, with no change on
         DAG root can be used, however. the DAGID MUST be unique per DAG structure.  If
         within the link is perceived as non-usable for some period scope of time (locally
   configurable), this triggers a DAG recomputation, through the LLN.  In the case where a DAG
   discovery mechanism further detailed in Section 5.3, after reporting root is
         rooting multiple DAGs the link failure.  Note that this concept may DAGID MUST be extended to take
   into account other link characteristics: unique for the sake of
   illustration, a node may decide to send a fixed number of packets to each DAG
         rooted at a particular successor (because of limited buffering capability of
   the successor) before starting to send traffic to another successor.

   According to the local policy function, it is possible for the node
   to order the specific DAG parent set from `most preferred' to `least
   preferred'.  By constructing such an ordered set, and by appending
   the set with siblings, the node is able to construct an ordered list
   of preferred next hops to assist in local and temporary routing
   decisions. root.

   The use following values MUST NOT change during the propagation of RA-DIO
   messages outwards along the ordered list by a forwarding engine is
   loosely constrained, DAG: Type, Length, G, DAGPreference,
   DAGDelay and may take into account the dynamics DAGID.  All other fields of 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 of a
   forwarding engine implementation RA-DIO message are beyond the scope
   updated at each hop of this
   document.

   These decisions may be local and/or temporary with the objective propagation.

5.1.1.1.  DAG Information Option (DIO) Suboptions

   In addition to
   maintain the DAG shape while preserving routing stability.

3.5.  Maintenance of Routing Adjacency

   In order to relieve the LLN of the overhead of periodic keepalives,
   RPL may employ an as-needed mechanism of NS/NA minimum options presented in order to verify
   routing adjacencies just prior to forwarding data.  Pending the
   outcome of verifying base option,
   several suboptions are defined for the routing adjacency, 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 the packet may either be
   forwarded or an alternate next-hop may be selected.

4.  Constraint Based Routing in LLNs

   This aim type of this section is to make suboption.  When
         processing a clear distinction between
   routing metrics and constraints and define RA-DIO message containing a suboption for which
         the term constraint based
   routing as used in this document.

4.1.  Routing Metrics

   Routing metrics are used Suboption Type value is not recognized by the routing protocol to compute receiver, the
   shortest path according to one of more defined metrics.  IGPs such as
   IS-IS ([RFC5120]) and OSPF ([RFC4915]) compute
         receiver MUST silently ignore the shortest path
   according unrecognized option, continue
         to a Link State Data Base (LSDB) using link metrics
   configured by process the network administrator.  Such metrics can represent following suboption, correctly handling any
         remaining options in the link bandwidth (in which case message.

   Suboption Length:  8-bit unsigned integer, representing the metric is usually inversely
   proportional to the bandwidth), delay, etc.  Note that length in some cases
   the metric is a polynomial function
         octets of several metrics defining
   different link characteristics.  The resulting shortest path cost is
   equal to the sum (or multiplication) of suboption, not including the link metrics along suboption Type and
         Length fields.

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

   The following subsections specify the RA-DIO message suboptions which
   are said to be additive or multiplicative metrics.

   Some routing protocols support more than one metric: currently defined for use in the vast
   majority of the cases, one metric is used per (sub)topology.  Less
   often, a second metric DAG Information Option.

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

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

5.1.1.1.2.  Pad1

   The Pad1 suboption does not have any alignment requirements.  Its
   format is known as an NP complete problem and is sometimes supported
   by some centralized path computation engine.

   In follows:

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

                              Figure 3: Pad 1

   NOTE! the case format of RPL, it the Pad1 option is virtually impossible to define *the*
   metric, or even a composite, that will fit special case - it all:

   o  Some information apply when determining routes, other information
      may apply only when forwarding packets 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 RPL protocol core has
   neither Option Length nor Option Data fields.

   The Pad1 option 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 used to support the evolution insert one octet of a
   network, or as networks form in a totally ad-hoc fashion, it will
   happen that nodes that are programmed with antagonistic logics and
   conflicting or orthogonal priorities end up participating padding in the same
   network.  It is thus recommended to use consistent parent selection
   policy, as per Objective Code Points (OCP), RA-DIO
   message to ensure consistent
   optimized paths.

   RPL enable suboptions alignment.  If more than one octet of
   padding is designed to survive and still operate, though in a somewhat
   degraded fashion, when confronted to such heterogeneity. required, the PadN option, described next, should be used
   rather than multiple Pad1 options.

5.1.1.1.3.  PadN

   The key
   design point is that each node PadN option does not have any alignment requirements.  Its format
   is solely responsible for setting the
   vector of metrics that it sources in the DAG, derived in part from
   the metrics sourced from its preferred parent.  As a result, the DAG
   is not broken if another node makes its decisions in as antagonistic
   fashion, though an end-to-end path might not fully achieve any of the
   optimizations that nodes along the way expect.  The default operation
   specified in OCP follows:

        0 clarifies this point.

4.2.  Routing Constraints

   A constraint                   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 N

   The PadN option is a link or a node characteristic that must be
   satisfied by the computed path (using boolean values used to insert two or lower/upper
   bounds) and is by definition neither additive nor multiplicative.
   Examples of links constraints are "available bandwidth",
   "administrative values (e.g. link coloring)", "protected versus non-
   protected links", "link quality" whereas a node constraint can be the
   level of battery power, CPU processing power, etc.

4.3.  Constraint Based Routing

   The notion of constraint based routing consists more octets of finding padding in
   the
   shortest path according RA-DIO message to some metrics satisfying a set of
   constraints.  A technique consists enable suboptions alignment.  For N (N > 1)
   octets of first filtering out all links
   and nodes that cannot satisfy padding, the constraints (resulting in a sub-
   topology) Option Length field contains the value N-2,
   and then computing the shortest path.

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

      Objective function 1:
         "Find the shortest path (path with lowest cost where the path
         cost is the sum of all link costs (Bandwidth)) along the path
         such that all links are colored `Blue' 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 Option Data consists of all link costs (Delay)) along N-2 zero-valued octets.  PadN Option
   data MUST be ignored by the path such
         that all links provide at least X Bit/s of reservable
         bandwidth."

5.  RPL Protocol Specification

5.1. receiver.

5.1.1.1.4.  DAG Information Option Metric Container

   The DAG Information Option carries a number of metrics and other
   information that allows a node Metric Container suboption may be aligned as necessary to discover a DAG, select its DAG
   parents, and identify
   support its siblings while employing loop avoidance
   strategies.

5.1.1.  DAG Information Option (DIO) base option

   The DAG Information Option contents.  Its format 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 information set that is mandatory in all cases.

        0                   1                   2                   3 follows:

        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 = 2    |    Length     |G|D|A|  00000  |   Sequence    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | DAGPreference |                BootTimeRandom                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   NodePref.   |    DAGRank    |           DAGDelay            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | DIOIntDoubl.  |  DIOIntMin.   |     DAGObjectiveCodePoint Container Len |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           PathDigest                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                            DAGID                              |
       +                                                               +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   sub-option(s)...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+ DAG Metric Data
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -

                      Figure 4: DIO Base Option

   Type: 8-bit unsigned identifying the DIO base option. 5: DAG Metric Container

   The suggested
         value DAG Metric Container is 140 used to be confirmed by report aggregated path metrics
   along the IANA.

   Length:  8-bit unsigned integer set to 4 when there is no suboption. DAG.  The length DAG Metric Container may contain a number of
   discrete node, link, and aggregate path metrics as chosen by the option (including
   implementer.  The Container Length field contains the type and length fields
         and the suboptions) in units
   octets 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): Metric Data.  The Destination
               Advertisement Trigger (D) flag is set when the DAG root
               or another node in the successor chain decides to trigger
               the sending order, content, and coding of destination advertisements in order to
               update routing state for the outward direction along the
               DAG,
   DAG Metric Container data is as further detailed specified in Section 5.9.  Note that the
               use

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

   The processing and semantics propagation of this flag are still under
               investigation.

         Destination Advertisement Supported (A) :  The Destination
               Supported (A) bit is set when the DAG root Metric Container is capable to
               support the collection
   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: DAG Destination Prefix

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

   The Length is coded as the
               destination advertisement mechanism within length of the DAG.

         Unassigned bits suboption in octets,
   excluding the Type and Length fields.

   The Prefix Length is an 8-bit unsigned integer that indicates the
   number of leading bits in the Flag Field are considered destination prefix.  Prf is the Route
   Preference as reserved.
         They in [RFC4191].  The reserved fields MUST be set to zero
   on transmission and MUST be ignored on receipt.

   Sequence Number:  8-bit

   The Prefix Lifetime is a 32-bit unsigned integer set by representing the DAG root,
         incremented according
   length of time in seconds (relative to a policy provisioned at the DAG root,
         and propagated with no change outwards along time the DAG.  Each
         increment SHOULD have a packet is sent)
   that the Destination Prefix is valid for route determination.  A
   value of 1 and may cause all one bits (0xFFFFFFFF) represents infinity.  A value of
   all zero bits (0x00000000) indicates a wrap back to
         zero.

   DAGPreference:  8-bit unsigned integer set by 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). loss of reachability.

   The DAG preference provides an
         administrative mechanism to engineer the self-organization Destination Prefix contains Prefix Length significant bits of the LLN, for example indicating
   destination prefix.  The remaining bits of the most preferred LBR.  If a
         node has Destination Prefix, as
   required to complete the option trailing octet, are set to join 0.

   In the event that a RA-DIO message may need to specify connectivity
   to more preferred DAG while still
         meeting other optimization objectives, then than one destination, the node will seek
         to join Destination Prefix suboption may be
   repeated.

5.2.  Conceptual Data Structures

   The RPL implementation MUST maintain the more preferred DAG.

   BootTimeRandom:  A random value computed at boot time and recomputed following conceptual data
   structures in case support of DAG discovery:

   o  A set of candidate neighbors

   o  For each DAG:

      *  A set of candidate DAG parents

      *  A set of DAG parents (which are a duplication with another node.  The concatenation subset of the NodePreference candidate DAG
         parents and the BootTimeRandom may be implemented as such)

5.2.1.  Candidate Neighbors Data Structure

   The set of candidate neighbors is a 32-bit
         extended preference that is used to resolve collisions.  It is
         set be populated by each node at propagation time.

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

   DAGRank:  8-bit unsigned integer indicating the DAG rank of the node
         sending neighbor discovery mechanism and further
   qualified as statistically stable as per the RA-DIO message. mechanisms discussed in
   [I-D.ietf-roll-routing-metrics].  The DAGRank of the DAG root is
         typically 1.  DAGRank candidate neighbors, and
   related metrics, should demonstrate stability/reliability beyond a
   certain threshold, and it is further described in Section 5.3.

   DAGDelay:  16-bit unsigned integer set by recommended that a local confidence
   value be maintained with respect to the DAG root indicating neighbor in order to track
   this.  Implementations MAY choose to bound the
         delay before changing maximum size of the DAG configuration,
   candidate neighbor set, in TBD-units.  A
         default which case a local confidence value is TBD.  It is expected will
   assist in ordering neighbors to be an order of
         magnitude smaller than determine which ones should remain in
   the RA-interval.  It is also expected to candidate neighbor set and which should be an order of magnitude evicted.

   If Neighbor Unreachability Detection (NUD) determines that a
   candidate neighbor is no longer than reachable, then it shall be removed
   from the typical propagation
         delay inside candidate neighbor set.  In the LLN.

   DIOIntervalDoublings:  8-bit unsigned integer.  Configured on case that the candidate
   neighbor has associated states in the DAG
         root and used to configure parent set or active DA
   entries, then the trickle timer governing when RA-
         DIO message removal of the candidate neighbor shall be
   coordinated with tearing down these states.  All provisioned routes
   associated with the candidate neighbor should be sent removed.

5.2.2.  Directed Acyclic Graphs (DAGs) Data Structure

   A DAG may be uniquely identified by within the DAG.
         DIOIntervalDoublings LLN by its unique
   DAGID.  When a single device is the number capable to root multiple DAGs in
   support of times that the
         DIOIntervalMin an application need for multiple optimization objectives
   it is allowed expected to be doubled during produce a different and unique DAGID for each of
   the trickle
         timer operation.

   DIOIntervalMin:  8-bit unsigned integer.  Configured on multiple DAGs.

   For each DAG that a node is, or may become, a member of, the
   implementation MUST keep a DAG root
         and used to configure table with the trickle following entries:

   o  DAGID

   o  DAGObjectiveCodePoint

   o  A set of Destination Prefixes offered inwards along the DAG

   o  A set of candidate DAG parents

   o  A timer governing when RA-DIO
         message should be sent within to govern the DAG.  The minimum configured
         interval sending of RA-DIO messages for the RA-DIO trickle timer in units of ms is
         2^DIOIntervalMin.  For example, DAG

   o  DAGSequenceNumber

   When a DIOIntervalMin value of 16ms DAG is expressed as 4.

   DAGObjectiveCodePoint:  The discovered for which no DAG Objective Code Point data structure is used
   instantiated, and the node wants to
         indicate join (i.e. the cost metrics, objective functions, and methods of
         computation and comparison for DAGRank in use neighbor is to
   become a candidate DAG parent in the DAG.  The Held-Up state), then the DAG OCP
   data structure is instantiated.

   When the candidate DAG parent set by is depleted (i.e. the last
   candidate DAG root.  (Objective Code Points are to parent has timed out of the Held-Down state), then the
   DAG data structure SHOULD be further defined in [I-D.ietf-roll-routing-metrics].

   PathDigest:  32-bit unsigned integer CRC, updated by each LLN Node.
         This is suppressed after the result expiration of a CRC-32c computation on a bit string
         obtained by appending an
   implementation-specific local timer.  An implementation SHOULD delay
   before deallocating the received value and DAG data structure in order to observe that
   the ordered set of DAGSequenceNumber has incremented should any new candidate DAG
   parents at appear for the LLN Node. DAG.

5.2.2.1.  Candidate DAG roots use a 'previous value'
         of zeroes to initially set Parents Structure

   When the PathDigest.  Used to determine
         when something in DAG is self-rooted, the set of successor paths has changed.

   DAGID:  128-bit unsigned integer which uniquely identify candidate DAG parents is
   empty.

   In all other cases, for each candidate DAG parent in the set, the
   implementation MUST keep a DAG.  This
         value record of:

   o  a reference to the neighboring device which is set by the DAG root.  The global IPv6 address parent

   o  a record of most recent information taken from the DAG root can be used, however. Information
      Object last processed from the DAGID MUST be unique per candidate DAG
         within parent

   o  a state associated with the scope role of the LLN.  In the case where candidate as a potential
      DAG root is
         rooting multiple DAGs the DAGID MUST be unique for each parent {Current, Held-Up, Held-Down, Collision}, further
      described in Section 5.7

   o  A DAG
         rooted at a specific Hop Timer, if instantiated

   o  A Held-Down Timer, if instantiated

5.2.2.1.1.  DAG root.

   The following values MUST NOT change during Parents

   Note that the propagation subset of RA-DIO
   messages outwards along candidate DAG parents in the DAG: Type, Length, G, DAGPreference,
   DAGDelay and DAGID.  All other fields of `Current' state
   comprises the RA-DIO message are
   updated at each hop set of DAG parents, i.e. the propagation.

5.1.1.1. nodes actively acting as
   parents in the DAG.

   DAG Information Option (DIO) Suboptions

   In addition parents may be ordered, according to the minimum options presented OCP.  When ordering DAG
   parents, in consultation with the base option,
   several suboptions are defined for OCP, 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 5: DIO Suboption Generic Format

   Suboption Type:  8-bit identifier most preferred DAG parent
   may be identified.  All current DAG parents must have a rank less
   than or equal to that of the type of suboption. most preferred DAG parent.

   When
         processing a RA-DIO message containing a suboption for which
         the Suboption Type value is not recognized by the receiver, the
         receiver MUST silently ignore the unrecognized option, continue nodes are added to process the following suboption, correctly handling any
         remaining options in or removed from the message.

   Suboption Length:  8-bit unsigned integer, representing DAG parent set the length most
   preferred DAG parent may have changed and should be reevaluated.  Any
   nodes having a rank greater than self after such a change must be
   placed in
         octets of the suboption, not including the suboption Type Held-Down state and
         Length fields.

   Suboption Data:  A variable length field that contains data specific evicted as per the procedures
   described in Section 5.7

   An implementation may choose to keep these records as an extension of
   the option.

   The following subsections specify Default Router List (DRL).

5.3.  DAG Discovery and Maintenance

   DAG discovery locates the RA-DIO message suboptions nearest sink, as determined according to
   some metrics and constraints, and forms a Directed Acyclic Graph
   towards that sink, by identifying a set of DAG parents.  During this
   process DAG discovery also identifies siblings, which
   are currently defined may be used
   later to provide additional path diversity towards the DAG root.  DAG
   discovery enables nodes to implement different policies for use selecting
   their DAG parents in the DAG Information Option.

   Implementations MUST silently ignore any RA-DIO message suboptions
   options by using implementation specific policy
   functions.  DAG discovery specifies a set of rules to be followed by
   all implementations in order to ensure interoperation.  DAG discovery
   also standardizes the format that they do not understand.

   RA-DIO message suboptions may have alignment requirements.  Following is used to advertise the convention most
   common information that is used in IPv6, order to select DAG parents.

   One of these options are aligned in a packet such
   that multi-octet values within information, the Option Data field of each option
   fall on natural boundaries (i.e., fields of width n octets are placed
   at an integer multiple of n octets from DAG rank, is used by DAG discovery to
   provide loop avoidance even if nodes implement different policies.
   The DAG Rank is computed as specified by the start of Objective Code Point in
   use by the header, DAG, demonstrating the properties described in
   Section 3.2.1.7.  The rank should be computed in such a way so as to
   provide a comparable basis with other nodes which may not use the
   same metric at all.

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

   o  RPL rules for
   n = 1, 2, 4, or 8).

5.1.1.1.2.  Pad1 loop avoidance based on rank

   o  The Pad1 suboption OCP function

   o  The advertised metrics

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

5.3.1.  DAG Discovery Rules

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

   1.   A node that does not have any alignment requirements.  Its
   format DAG parents in a DAG is as follows:

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

                              Figure 6: Pad 1

   NOTE! the format root
        of the Pad1 option its own floating DAG.  It's rank is a special case - 1.  A node will end up in
        that situation when it has
   neither Option Length nor Option Data fields.

   The Pad1 option is used to insert one octet looses all of padding in its current feasible
        parents, i.e. the set of DAG parents becomes depleted.  In that
        case, the node SHOULD remember the DAGID and the sequence
        counter of the last RA-DIO message to enable suboptions alignment.  If more than one octet from the lost parents for a
        period of
   padding time which covers multiple RA-DIO messages.  This is required,
        done so that if the PadN option, described next, should be used
   rather than multiple Pad1 options.

5.1.1.1.3.  PadN

   The PadN option node does not have any alignment requirements.  Its format
   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 7: Pad N

   The PadN option is used encounter another possible
        attachment point to insert two or more octets of padding in the RA-DIO message to enable suboptions alignment.  For N (N > 1)
   octets lost DAGID within a period of padding, time, the Option Length field contains
        node may observe a sequence counter change by comparing the value N-2,
        observed sequence counter to the last observed sequence counter
        and thus verify that the Option Data consists of N-2 zero-valued octets.  PadN Option
   data MUST be ignored by new attachment point is a viable and
        independent alternative to attach back to the receiver.

5.1.1.1.4.  DAG Metric Container

   The DAG Metric Container suboption may be aligned as necessary lost DAGID.

   2.   A node that is attached to an infrastructure that does not
        support RA-DIO messages, is the DAG root of its contents.  Its format own grounded
        DAG.  It's rank is 1.  (For example an LBR 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 a grounded infrastructure at rank 0.  (For example, a
        router that is in communication with an LLN node but not running
        RPL such 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 8: DAG Metric Container a non-LLN public Internet router in communication
        with an LBR)

   4.   The DAG Metric Container is used to report aggregated path metrics
   along root exposes the DAG.  The DAG Metric Container may contain a number of
   discrete node, link, in the RA-DIO message and aggregate path metrics nodes
        propagate the RA-DIO message outwards along the DAG with 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 to increase its own DAG rank,
        as chosen per the rank calculation indicated by the
   implementer.  The Container Length field contains OCP.

   6.   A node MUST NOT move outwards along a DAG that it is attached
        to, causing the length DAG rank to increase, except in
   octets of a special case
        where the DAG Metric Data.  The order, content, and coding of node MAY choose to follow the last DAG Metric Container data is as specified parent in

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

   The processing and propagation the
        set of DAG parents.  In the general case, if a node is required
        to move such that it cannot stay within the DAG Metric Container without a rank
        increase, then it needs to first leave the DAG.  In other words
        a node that is
   governed by implementation specific policy functions.

5.1.1.1.5.  Destination Prefix

   The Destination Prefix suboption has an alignment requirement already part 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 9: a DAG Destination Prefix

   The Destination Prefix suboption is used when the MAY move or follow a DAG root,
        parent at any time and with no delay in order to be closer, or
   another node
        stay as close, to the DAG root of its current DAG as it already
        is, but may not move outwards.  RAs received from other routers
        located inwards along at lesser rank in the same DAG on may be considered as
        coming from candidate parents.  RAs received from other routers
        located at the path to same rank in the same DAG
   root, needs to indicate may be considered as
        coming from siblings.  Nodes MUST ignore RAs that it offers connectivity to destination
   prefixes are received
        from other than routers located at greater rank within the default.  This same DAG.

   7.   A node may be useful in cases where
   more than one LBR jump from its current DAG into any different DAG if
        it is operating within preferred for reasons of connectivity, configured
        preference, free medium time, size, security, bandwidth, DAG
        rank, or whatever metrics the LLN and offering
   connectivity cares to different administrative domains, e.g. a home network use.  A node may jump
        at any time and a utility network.  In such cases, upon observing to whatever rank it reaches in the Destination
   Prefixes offered by a particular new DAG, a node MAY decide but
        it may have to join
   multiple DAGs in support of wait for a particular application.

   The Length is coded as the length of the suboption DAG Hop timer to elapse in octets,
   excluding order to do
        so.  This allows the Type and Length fields.

   The Prefix Length is an 8-bit unsigned integer that indicates new higher parts (closer to the
   number sink) of leading bits in the destination prefix.  Prf is
        the Route
   Preference as in [RFC4191].  The reserved fields MUST be set DAG to zero
   on transmission move first, thus allowing stepped DAG
        reconfigurations and MUST be ignored on receipt.

   The Prefix Lifetime is limiting relative movements.  A node SHOULD
        NOT join a 32-bit unsigned integer representing previous DAG (identified by its DAGID) unless the
   length of time
        sequence number in seconds (relative to the time RA-DIO message has incremented since the packet is sent)
        node left that the Destination Prefix is valid for route determination.  A
   value of all one bits (0xFFFFFFFF) represents infinity. DAG.  A value of
   all zero bits (0x00000000) newer sequence number indicates a loss of reachability.

   The Destination Prefix contains Prefix Length significant bits of the
   destination prefix.  The remaining bits of that the Destination Prefix,
        candidate parents were not attached behind this node, as
   required to complete they
        kept getting subsequent RA-DIO messages with new sequence
        numbers from the trailing octet, are set to 0. same DAG.  In the event that a RA-DIO message may need to specify connectivity
   to old sequence
        numbers (two or more than one destination, behind the Destination Prefix suboption may present value) are encountered
        they are considered stale and the corresponding parent SHOULD be
   repeated.

5.2.  Conceptual Data Structures

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

   o  A set of candidate neighbors

   o  For each DAG:

      *  A set.

   8.   If a node has selected a new set of candidate DAG parents

      *  A set of but has not
        moved yet (because it is waiting for DAG parents (which are Hop timer to elapse),
        the node is unstable MUST NOT send RA-DIOs for that DAG.

   9.   If a subset node receives a RA-DIO from one of candidate its DAG
         parents and may be implemented as such)

5.2.1.  Candidate Neighbors Data Structure

   The set of candidate neighbors is to be populated by neighbors who
   are discovered by the neighbor discovery mechanism parents, and further
   qualified as statistically stable as per if
        the mechanisms discussed in
   [I-D.ietf-roll-routing-metrics].  The candidate neighbors, and
   related metrics, should demonstrate stability/reliability beyond parent contains a
   certain threshold, and it is recommended different DAGID, indicating that a local confidence
   value be maintained with respect to the neighbor in order to track
   this.  Implementations MAY choose to bound
        parent has left the maximum size of DAG, and if the
   candidate neighbor set, in which case a local confidence value will
   assist in ordering neighbors to determine which ones should node can remain in the candidate neighbor set and
        current DAG through an alternate DAG parent, then the node
        SHOULD remove the DAG parent which should be evicted.

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

5.2.2.  Directed Acyclic Graphs (DAGs) Data Structure

   A DAG may be uniquely identified by within node SHOULD follow
        that parent into the LLN by its unique
   DAGID. new DAG.

   10.  When a single device is capable to root multiple DAGs node detects or causes a DAG inconsistency, as described
        in
   support of Section 5.3.4.2, then the node SHOULD send an application need for multiple optimization objectives
   it unsolicited RA-
        DIO message to its one-hop neighbors.  The RA-DIO is expected updated to produce a different and unique DAGID for each of
        propagate the multiple DAGs.

   For each new DAG that a node is, or may become, a member of, the
   implementation information.  Such an event MUST keep a DAG table with the following entries:

   o  DAGID

   o  DAGObjectiveCodePoint

   o  A set of Destination Prefixes offered inwards along also
        cause the DAG

   o  A set of candidate DAG parents

   o  A trickle timer to govern governing the periodic sending of RA-DIO
        messages for the DAG

   o  DAGSequenceNumber

   When to be reset.

   11.  If a DAG is discovered for which no DAG data structure is
   instantiated, and parent increases its rank such that the node wants rank would
        have to join (i.e. change, and if the neighbor is node does not wish to
   become a candidate follow (e.g. it
        has alternate options), then the DAG parent in the Held-Up state), then SHOULD be evicted
        from the DAG
   data structure is instantiated.

   When parent set.  If the candidate DAG parent set is depleted (i.e. the last
   candidate in the
        DAG parent has timed out of the Held-Down state), set, then the
   DAG data structure node SHOULD be suppressed after the expiration chose to follow it.

5.3.2.  Reception and Processing of RA-DIO messages

   When an
   implementation-specific local timer.  An implementation SHOULD delay
   before deallocating RA-DIO message is received from a source device named SRC,
   the DAG data structure in order to observe that receiving node must first determine whether or not the DAGSequenceNumber has incremented RA-DIO
   message should any new candidate DAG
   parents appear be accepted for further processing, and subsequently
   present the DAG.

5.2.2.1.  Candidate DAG Parents Structure

   When RA-DIO message for further processing if eligible.

5.3.2.1.  Determination of Eligibility for DIO Processing

      If the DAG RA-DIO message is self-rooted, malformed, then the set of candidate DAG parents RA-DIO message is
   empty.

   In all other cases, not
      eligible for each candidate DAG parent in the set, the further processing and is silently discarded.  A RPL
      implementation MUST keep a record of:

   o  a reference to MAY log the neighboring device which reception of a malformed RA-DIO
      message.

      If SRC is the DAG parent

   o not a record member of most recent information taken from the DAG Information
      Object last processed from the candidate DAG parent

   o  a state associated with neighbor set, then the role RA-
      DIO is not eligible for further processing.  (Further evaluation/
      confidence of this neighbor is necessary)

      If 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  A Held-Down Timer, if instantiated

5.2.2.1.1. RA-DIO message advertises a DAG Parents

   Note that the subset node is already a
      member of, then:

         If the rank of candidate DAG parents SRC as reported in the `Current' state
   comprises the set RA-DIO message is lesser
         than that of DAG parents, i.e. the nodes actively acting as
   parents in node within the DAG.

   DAG parents may DAG, then the RA-DIO message
         MUST be ordered, according to considered for further processing

         If the OCP.  When ordering DAG
   parents, rank of SRC as reported in consultation with the OCP, RA-DIO message is equal
         to that of the most preferred DAG parent
   may be identified.  All current DAG parents must have node within the DAG, then SRC is marked as a
         sibling and the RA-DIO message is not eligible for further
         processing.

         If the rank less of SRC as reported in the RA-DIO message is higher
         than or equal to that of the most preferred node within the DAG, and SRC is not a DAG parent.

   When nodes are added to or removed from
         parent, then the RA-DIO message MUST NOT be considered for
         further processing

      If SRC is a DAG parent set for any other DAG that the most
   preferred node is attached
      to, then the RA-DIO message MUST be considered for further
      processing (the DAG parent may have changed and should be reevaluated.  Any
   nodes having jumped).

      If the RA-DIO message advertises a rank greater than self after such DAG that offers a change must be
   placed in better (new
      or alternate) solution to an optimization objective desired by the Held-Down state and evicted
      node, then the RA-DIO message MUST 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 DAG if
         necessary

         Place the neighbor in the candidate DAG parent set

         If the node has sent an RA message within the risk window as per
         described in Section 5.7.3 then perform the procedures collision detection
         described in Section 5.7

   An implementation may choose to keep these records as an extension of 5.7.3.  If a collision occurs, place the Default Router List (DRL).

5.3.
         candidate DAG Discovery parent in the collision state and Maintenance

   DAG discovery locates do not process
         the nearest sink, RA-DIO message any further as determined according to
   some metrics and constraints, and forms a Directed Acyclic Graph
   towards that sink, by identifying described in Section 5.7.

         If the SRC node is also a set of DAG parents.  During this
   process parent for another DAG discovery also identifies siblings, which may be used
   later to provide additional path diversity towards that the
         node is a member of, and if the new/alternate DAG root. satisfies an
         equivalent optimization objective as the other DAG, then the
         DAG
   discovery enables nodes parent is known to implement different policies for selecting
   their have jumped.

            Remove SRC as a DAG parents in parent from the other DAG by using implementation specific policy
   functions.  DAG discovery specifies a set of rules to be followed by
   all implementations (place it in order to ensure interoperation.  DAG discovery
   also standardizes
            the format that is used to advertise held-down state)

            If the most
   common information that other DAG is used in order to select DAG parents.

   One now empty of these information, candidate parents, then
            directly follow SRC into the new DAG rank, is used by DAG discovery to
   provide loop avoidance even if nodes implement different policies.
   The DAG Rank is computed adding it as specified by the Objective Code Point a DAG
            parent in
   use by the DAG, demonstrating Current state, else ignore the properties described in
   Section 3.3.1.  The rank should be computed in such a way so as to
   provide a comparable basis with other nodes which may RA-DIO message
            (do not use follow the
   same metric at all.

   The parent).

         If the new/alternate DAG discovery procedures take into account 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. offers a bounded number of candidate
      neighbors).

5.3.1.  DAG Discovery Rules

   In order better solution to organize and maintain loopless structure, the DAG
   discovery implementation in the nodes MUST obey
         optimization objectives, then prepare to jump: copy the following
   rules and definitions:

   1.   A node that does not have any DAG parents in a DAG is DIO
         information into the root
        of its own floating DAG.  It's rank is 1.  A node will end up in
        that situation when it looses all of its current feasible
        parents, i.e. record for the set of candidate DAG parents becomes depleted.  In that
        case, parent, place
         the node SHOULD remember candidate DAG parent into the DAGID Held-Up state, and start the sequence
        counter of
         DAG Hop timer as per Section 5.7.1.

      If the last RA-DIO message from the lost parents is for a
        period of time which covers multiple known/existing DAG:

         Process the RA-DIO messages.  This is
        done so that if message as per the node does encounter another possible
        attachment point rules in Section 5.3

   As candidate parents are identified, they may subsequently be
   promoted to DAG parents by following the lost DAGID within rules of DAG discovery as
   described in Section 5.3.  When a period node adds another node to its set
   of time, candidate parents, the node may observe a sequence counter change by comparing the
        observed sequence counter becomes attached to the last observed sequence counter
        and thus verify that DAG through
   the new attachment point is a viable and
        independent alternative to attach back to parent node.

   In the lost DAGID.

   2.   A node that is attached DAG discovery implementation, the most preferred parent should
   be used to an infrastructure that does not
        support RA-DIO messages, is restrict which other nodes may become DAG parents.  Some
   nodes in the DAG root parent set may be of its own grounded
        DAG.  It's a rank is 1.  (For example less than or equal to
   the most preferred DAG parent.  (This case may occur, for example, if
   an LBR that energy constrained device is in
        communication with a non-LLN router not running RPL).

   3.   A (non-LLN) router sending at a RA messages without DIO is
        considered lesser rank but should be
   avoided as per an optimization objective, resulting in a grounded infrastructure more
   preferred parent at rank 0.  (For example, a
        router greater rank).

5.3.3.  RA-DIO Transmission

   Each node maintains a timer that governs when to multicast RA
   messages.  This timer is in communication with an LLN node but not running
        RPL such implemented as a non-LLN public Internet router trickle timer operating
   over a variable interval.  Trickle timers are further detailed in communication
        with an LBR)

   4.
   Section 5.3.4.  The DAG root exposes governing parameters for the DAG in timer should be
   configured consistently across the RA-DIO message DAG, and nodes
        propagate the RA-DIO message outwards along are provided by the DAG with
   root in the RAs
        that they forward over their RA-DIO message.  In addition to periodic RA messages,
   each LLN links.

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

   6.   A node MUST NOT move outwards along Router Solicitation (RS) messages
   according to [RFC4861].

   o  When a DAG that it node is attached
        to, causing the unstable, because any DAG rank to increase, except Hop timer is running in
      preparation for a special case
        where jump, then the node MAY choose to follow the last DAG parent in the
        set of DAG parents.  In MUST NOT transmit
      unsolicited RA-DIOs (i.e. the general case, if a node is required
        to move such that it cannot stay within will remain silent when the DAG without
      timer expires).

   o  When a rank
        increase, then node detects an inconsistency, it needs to first leave SHOULD reset the DAG.  In other words
        a node that is already part interval
      of the trickle timer to a DAG MAY move or follow a DAG
        parent at any time and with no delay in order minimum value, causing RA messages to be closer, or
        stay
      emitted more frequently as close, part of a strategy to quickly correct
      the DAG root of its current DAG as it already
        is, but inconsistency.  Such inconsistencies may not move outwards.  RAs received from other routers
        located at lesser rank be, for example, an
      update to a key parameter (e.g. sequence number) in the same DAG may be considered as
        coming from candidate parents.  RAs received from other routers RA-DIO
      message or a loop detected when a node located at the same rank in inwards along the same
      DAG may be considered as
        coming from siblings.  Nodes MUST ignore RAs that forwards traffic outwards.  Inconsistencies are received
        from other routers located at greater rank within the same DAG.

   7.   A further
      detailed in Section 5.3.4.2.

   o  When a node may jump enters a mode of consistent operation within a DAG,
      i.e.  RA-DIO messages from its current DAG into any different DAG if
        it is preferred for reasons 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 parents are consistent and to whatever rank it reaches in the new DAG, but no
      other inconsistencies are detected, it may have to wait for a DAG Hop timer to elapse in order to do
        so.  This allows the new higher parts (closer begin to open up the sink)
      interval of the DAG trickle timer towards a maximum value, causing RAs
      to move first, be emitted less frequently, thus allowing stepped DAG
        reconfigurations reducing network maintenance
      overhead and limiting relative movements.  A node SHOULD
        NOT join saving energy consumption (which is of utmost
      importance for battery-operated nodes).

   o  When a previous DAG (identified by its DAGID) unless the
        sequence number in the RA-DIO message has incremented since the node left that DAG.  A newer sequence number indicates that the
        candidate parents were is initialized, it MAY be configured to remain silent
      and not attached behind this node, as they
        kept getting subsequent RA-DIO multicast any RA messages with new sequence
        numbers from the same DAG.  In the event that old sequence
        numbers (two or more behind the present value) are until it has encountered
        they are considered stale and the corresponding parent SHOULD be
        removed from the set.

   8.   If
      joined a node has selected DAG (perhaps initially probing for a new set of nearby DAG parents but has not
        moved yet (because 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 it is waiting
      desired for DAG Hop timer independent nodes to elapse), begin aggregating into scattered
      floating DAGs in the node is unstable MUST NOT send RA-DIOs for that DAG.

   9.   If a node receives absence of a RA-DIO from one grounded node, for example in
      support of its DAG parents, LLN installation and commissioning.

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

5.3.4.  Trickle Timer for RA Transmission

   RPL treats the new DAG from
        its construction of a DAG parent set as a consistency problem, and remain in
   uses a trickle timer [Levis08] to control the original DAG.  If there rate of control
   broadcasts.

   For each DAG that a node is
        no alternate parent for the DAG, then part of, the node SHOULD follow
        that parent into must maintain a single
   trickle timer.  The required state contains the new DAG.

   10.  When following conceptual
   items:

   I:    The current length of the communication interval

   T:    A timer with a node detects or causes duration set to a DAG inconsistency, as described random value in Section 5.3.4.2, then the node SHOULD send an unsolicited RA-
        DIO message to its one-hop neighbors. range
         [I/2, I]

   C:    Redundancy Counter

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

   I_doublings:  The number of times I_min should be doubled before
         maintaining a constant rate, i.e.  I_max = I_min *
         2^I_doublings.  This value is learned from the new DAG information.  Such an event MUST also
        cause RA-DIO message
         as DIOIntervalDoublings.  The default value is
         DEFAULT_DIO_INTERVAL_DOUBLINGS.

5.3.4.1.  Resetting the Trickle Timer

   The trickle timer governing the periodic sending of RA-DIO
        messages to be reset.

   11.  If for a DAG parent increases its rank such that DAGID is reset by:

   1.  Setting I_min and I_doublings to the node rank would
        have values learned from the RA-
       DIO message.

   2.  Setting C to change, and if zero.

   3.  Setting I to I_min.

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

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

   When node does not wish learns about a DAG through a RA-DIO message and makes the
   decision to follow (e.g. join it, it
        has alternate options), then initializes the state of the trickle timer by
   resetting the trickle timer and listening.  Each time it hears a
   consistent RA for this DAG parent SHOULD be evicted from the a DAG parent set.  If parent, it MAY increment C.

   When the DAG parent is timer fires at time T, the last in node compares C to the
        DAG parent set, then redundancy
   constant, DEFAULT_DIO_REDUNDANCY_CONSTANT.  If C is less than that
   value, the node SHOULD chose to follow it.

5.3.2.  Reception generates a new RA and Processing of RA-DIO messages broadcasts it.  When an RA-DIO message is received from a source device named SRC, the receiving
   communication interval I expires, the node must first determine whether or not doubles the RA-DIO
   message should be accepted for further processing, interval I so
   long as it has previously doubled it fewer than I_doubling times,
   resets C, and subsequently
   present the RA-DIO message for further processing if eligible.

5.3.2.1. chooses a new T value.

5.3.4.2.  Determination of Eligibility for DIO Processing

      If the RA-DIO message Inconsistency

   The trickle timer is malformed, then the RA-DIO message reset whenever an inconsistency is not
      eligible detected
   within the DAG, for further processing and is silently discarded.  A RPL
      implementation MAY log the reception of example:

   o  The node joins a malformed RA-DIO
      message.

      If SRC is not new DAGID

   o  The node moves within a member of the candidate neighbor set, then the RA-
      DIO is not eligible for further processing.  (Further evaluation/
      confidence of this neighbor is necessary)

      If the DAGID

   o  The node receives a modified RA-DIO message advertises from a DAG that the node is already parent

   o  A DAG parent forwards a
      member of, then:

         If the rank of SRC as reported packet intended to move inwards,
      indicating an inconsistency and possible loop.

   o  A metric communicated in the RA-DIO message is lesser
         than that of the node within the DAG, then the RA-DIO message
         MUST determined to be considered for further processing

         If the
      inconsistent, as according to a implementation specific path
      metric selection engine.

   o  The rank of SRC as reported in a DAG parent has changed.

5.4.  DAG Heartbeat

   The DAG root makes the RA-DIO message sole determination of when to revise the
   DAGSequenceNumber by incrementing it upwards.  When the
   DAGSequenceNumber is equal increased an inconsistency results, causing RA-
   DIO messages to that of be sent back outwards along the node within DAG to convey the DAG, then SRC
   change.  The degree to which this mechanism is marked relied on may be
   determined by the implementation- on one hand it may serve as a
         sibling
   periodic heartbeat, refreshing the DAG states, and on the RA-DIO message other hand
   it may result in a constant steady-state control cost overhead which
   is not eligible for further
         processing.

         If the rank of SRC desirable.

   Some implementations may provide an administrative interface, such as reported in the RA-DIO message is higher
         than that of the node within the DAG, and SRC is not
   a DAG
         parent, then command line, at the RA-DIO message MUST NOT be considered for
         further processing

      If SRC is a DAG parent for any other DAG that the node is attached
      to, then root whereby the RA-DIO message MUST DAGSequenceNumber may be considered for further
      processing (the DAG parent
   caused to increment in response to some policy outside of the scope
   of RPL.

   Other implementations may have jumped).

      If make use of a periodic timer to
   automatically increment the RA-DIO message advertises DAGSequenceNumber, resulting in a
   periodic DAG that offers Heartbeat at a better (new
      or alternate) solution rate appropriate to an optimization objective desired by the
      node, then the RA-DIO message MUST be considered for further
      processing.

5.3.2.2.  Overview of RA-DIO Message Processing

      If the received RA-DIO message application and
   implementation.

5.5.  DAG Selection

   The DAG selection is for implementation and algorithm dependent.  Nodes
   SHOULD prefer to join DAGs 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 new/alternate DAG:

         Instantiate means to
   filter out a data structure for the new/alternate DAG if
         necessary

         Place the neighbor in the candidate DAG parent set

         If the node has sent an RA message within the risk window whose availability is detected as
         described in Section 5.7.3 then perform the collision detection
         described in Section 5.7.3.  If a collision occurs,
   fluctuating, at least when more stable choices are available.  Nodes
   MAY place the failed candidate DAG parent in a Hold Down mode that
   ensures that the collision state and do not process
         the RA-DIO message any further 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 described much as
   possible into larger DAGs in Section 5.7.

         If order to allow connectivity within the SRC
   LLN.

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

5.6.  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 the node.  For instance, a
   node is that has limited battery should be a member of, and if the new/alternate DAG satisfies an
         equivalent optimization objective as the leaf unless there is no
   other DAG, choice, and may then augment the rank computation specified by
   the OCP in order to expose an exaggerated rank.

5.7.  Candidate DAG parent is known Parent States and Stability

   Candidate DAG parents may or may not be eligible to have jumped.

            Remove SRC act as a DAG
   parents depending on runtime conditions.  The following states are
   defined:

   Current     This candidate parent from the other DAG (place it is in the held-down state)

            If the other DAG is now empty set of DAG parents and
               may be used for forwarding traffic inward along the DAG.
               When a candidate parents, then
            directly follow SRC parent is placed into the new DAG by adding it as a DAG
            parent in the Current state, else ignore the RA-DIO message
            (do not follow the parent).

         If the new/alternate DAG offers a better solution to
               or taken out of the
         optimization objectives, then prepare Current state, it is necessary to jump: copy the DIO
         information into the record for re-
               evaluate which of the candidate remaining DAG parent, place parents is the candidate most
               preferred DAG parent into and its rank.  At that time any
               remaining DAG parents of greater rank than this node must
               be placed in the Held-Up Held-Down state, and start the
         DAG Hop hold-down timer as per Section 5.7.1.

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

         Process the RA-DIO message as per the rules
               started, in Section 5.3

   As candidate parents are identified, they may subsequently be
   promoted order to DAG parents by following the rules of DAG discovery be evicted as
   described in Section 5.3.  When a node adds another node to its set
   of candidate parents, the node becomes attached to the DAG through
   the parent node. parents.  In the DAG discovery implementation, the most preferred
               same fashion, siblings must also be reevaluated.

   Held-Up     This parent should can not be used to restrict which other nodes may become DAG parents.  Some
   nodes in until the DAG hop timer
               elapses.

   Held-Down   This candidate parent set may can not be used till hold down
               timer elapses.  At the end of a rank less than or equal to the most preferred hold-down period, the
               candidate is removed from the candidate DAG parent.  (This case parent set,
               and may occur, for example, be reinserted if
   an energy constrained device is at it appears again with a lesser rank but should be
   avoided as per an optimization objective, resulting in a more
   preferred parent at a greater rank).

5.3.3. RA-DIO Transmission

   Each node maintains a timer that governs when to multicast RA
   messages.
               message.

   Collision   This timer is implemented as a trickle timer operating
   over a variable interval.  Trickle timers are further detailed in
   Section 5.3.4.  The governing parameters for the timer should candidate parent can not be
   configured consistently across the DAG, and are provided by the DAG
   root in the RA-DIO used till its next RA-
               DIO message.  In addition to periodic RA messages,
   each LLN node will respond to Router Solicitation (RS) messages
   according to [RFC4861].

   o  When a node

5.7.1.  Held-Up

   This state is unstable, because any managed by the DAG Hop timer is running in
      preparation for a jump, then the node MUST NOT transmit
      unsolicited RA-DIOs (i.e. the node will remain silent when the
      timer expires).

   o  When a node detects an inconsistency, timer, it SHOULD reset serves 2 purposes:

      Delay the interval reattachment of the trickle timer to a minimum value, causing RA messages sub-DAG that has been forced to be
      emitted more frequently
      detach.  This is not as part safe as the use of a strategy to quickly correct the inconsistency.  Such inconsistencies may be, for example, an
      update to sequence, but still
      covers that when a key parameter (e.g. sequence number) in sub-DAG has detached, the RA-DIO message or a loop detected when a node located inwards along that
      is initiated by the new DAG forwards traffic outwards.  Inconsistencies are further
      detailed in Section 5.3.4.2.

   o  When root has a node enters chance to spread outward
      along the sub-DAG, ideally forming a mode frozen sub-DAG that is aware
      of consistent operation within a DAG,
      i.e. the DAG change, such that two different DAGs have formed prior
      to an attempted reattachment.

      Limit RA-DIO messages message storms (control cost / churn) when two DAGs
      collide/merge.  The idea is that between the nodes from its DAG parents are consistent and no
      other inconsistencies are detected, it may begin A that
      decide to open up the
      interval of move to DAG B, those that see the trickle timer towards a maximum value, causing RAs highest place (closer
      to be emitted less frequently, thus reducing network maintenance
      overhead the DAG root) in DAG B will move first and saving energy consumption (which advertise their new
      locations before other nodes from DAG A actually move.

   A new DAG is of utmost
      importance for battery-operated nodes).

   o  When discovered upon receiving a node is initialized, it MAY be configured to remain silent
      and not multicast any RA messages until it has encountered and
      joined message with or without a
   DIO.  The node joins the DAG (perhaps initially probing for by selecting the source of the RA
   message as a nearby DAG with an
      RS message).  Alternately, it may choose to root its own floating parent (and possibly installing the DAG and begin multicasting RAs using parent as a
   default trickle
      configuration. gateway).  The second case may be advantageous if it node is
      desired for independent nodes to begin aggregating into scattered
      floating DAGs in the absence of then a grounded node, for example in
      support member of LLN installation and commissioning.

   Note that if multiple the DAG roots are participating in and may begin
   to multicast RA-DIO messages containing the same DAG,
   i.e. offering RA-DIO messages with the same DAGID, then they must
   coordinate with each other 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 DIO for the multiple DAG.

   When a new DAG roots issues the RA-DIO message, and
   changes to is discovered, the Sequence number should be issued at candidate parent that advertises
   the same time.
   The specific mechanism of this coordination, e.g. along a non-LLN
   network between new DAG roots, is beyond the scope of this specification.

5.3.4.  Trickle Timer placed in a held up state for RA Transmission

   RPL treats the construction duration of a DAG as a consistency problem, and
   uses a trickle timer [Levis08] to control
   Hop timer.  If the rate resulting new set of control
   broadcasts.

   For each DAG that a node parents is part of, more
   preferable than the current one, or if the node must is intending to
   maintain a single
   trickle timer.  The required state contains membership in the following conceptual
   items:

   I:    The new DAG in addition to its current length of DAG,
   the communication interval

   T: node expects to jump and becomes unstable.

   A node that is unstable may discover other candidate parents from the
   same new DAG during the instability phase.  It needs to start a new
   DAG Hop timer with for all these.  The first timer that elapses for a duration set
   given new DAG clears them all for that DAG, allowing the node to a random value jump
   to the highest position available in the range
         [I/2, I]

   C:    Redundancy Counter

   I_min: new DAG.

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

   I_doublings:  The number DAG Hop timer depends on the DAG Delay 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.  The default value is
         DEFAULT_DIO_INTERVAL_DOUBLINGS.

5.3.4.1.  Resetting new
   DAG and on the Trickle Timer

   The trickle timer for a DAGID rank of candidate parent that triggers it: (candidates
   rank + random) * candidate's DAG_delay (where 0 <= random < 1).  It
   is reset by:

   1.  Setting I_min and I_doublings randomized in order to the values learned limit collisions and synchronizations.

5.7.2.  Held-Down

   When a neighboring node is 'removed' from the RA-
       DIO message.

   2.  Setting C to zero.

   3.  Setting I to I_min.

   4.  Setting T to Default Router List, it
   is actually held down for a random value as described above.

   5.  Restarting the trickle hold down timer period, in order to expire after
   prevent flapping.  This happens when a duration T node disappears (upon
   expiration timer).

   When an LLN learns about a DAG through a RA-DIO message and makes the
   decision to join it, it initializes the state of the trickle hold down timer by
   resetting elapses, the trickle timer and listening.  Each time it hears a
   consistent RA for this DAG node is removed from a DAG parent, it increments C.

   When the timer fires
   candidate DAG parent set.

5.7.3.  Collision

   A race condition occurs if 2 nodes send RA-DIO messages at time T, the node compares C same
   time and then attempt to join each other.  This might happen, for
   example, between nodes which act as DAG root of their own DAGs.  In
   order to detect the redundancy
   constant, DEFAULT_DIO_REDUNDANCY_CONSTANT.  If C is less than that
   value, the node generates a new RA and broadcasts it.  When the
   communication interval I expires, the node doubles situation, LLN Nodes time stamp 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 sending of Inconsistency

   The trickle timer is reset whenever an inconsistency is detected
   RA-DIO message.  Any RA-DIO message received within the DAG, for example:

   o  The node joins a new DAGID

   o  The node moves within short link-
   layer-dependent period introduces a DAGID

   o  The node receives risk.  To resolve the collision,
   a modified 32bits extended preference is constructed from the RA-DIO message from a DAG parent

   o
   by concatenating the NodePreference with the BootTimeRandom.

   A DAG parent forwards node that decides to add a packet intended candidate to move inwards,
      indicating an inconsistency its DAG parents will do so
   between (candidate rank) and possible loop.

   o  A metric communicated in (candidate rank + 1) times the RA-DIO message candidate
   DAG Delay.  But since a node is determined to be
      inconsistent, unstable as according to a implementation specific path
      metric selection engine.

   o  The rank of a DAG parent has changed.

5.4.  DAG Heartbeat

   The DAG root makes soon as it receives the sole determination of when to revise
   RA-DIO message from the
   DAGSequenceNumber by incrementing desired candidate, it upwards.  When will restrain from
   sending a RA-DIO message between 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 time it may serve as a
   periodic heartbeat, refreshing receives the DAG states, RA and on the other hand
   time 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 actually jumps.  So the scope crossing of RPL.

   Other implementations RA may make use of a periodic timer to
   automatically increment only happen during
   the DAGSequenceNumber, resulting in a
   periodic DAG Heartbeat at a rate appropriate to propagation time between the application candidate and
   implementation.

5.5.  DAG Selection

   The the node, plus some
   internal queuing and processing time within each machine.  It is
   expected that one DAG selection delay normally covers that interval, but
   ultimately it is implementation and algorithm dependent.  Nodes
   SHOULD prefer up to join DAGs advertising OCPs and destinations
   compatible with their the 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 the configuration of
   the candidate parent whose availability to define the duration of risk window.

   There is detected as
   fluctuating, at least risk of a collision when more stable choices are available.  Nodes
   MAY place the failed candidate parent in a Hold Down mode that
   ensures that the node receives an RA, for another
   candidate parent will not be reused for a given
   period of time.

   When connection to a fixed network that is not possible or more preferable for
   security or other reasons, scattered DAGs MAY aggregate as much as
   possible into larger DAGs in order to allow connectivity than the current candidate, 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.  Administrative rank

   When risk window.  In 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 face of acceptable rank with that node.  For instance, a potential collision, the node that has limited battery should be a leaf unless there is no
   other choice, and may then augment with
   lowest extended preference processes the rank computation specified by RA-DIO message normally,
   while the OCP router with the highest extended preference places the
   other in order to expose an exaggerated rank.

5.7.  Candidate collision state, does not start the DAG Parent States hop timer, and Stability

   Candidate DAG parents may or may does
   not be eligible to act as DAG
   parents depending on runtime conditions.  The following states are
   defined:

   Current     This candidate parent become instable.  It is in expected that next RAs between the set of DAG parents two
   will not cross anyway.

   For example, consider a case where two nodes are each rooting their
   own transient floating DAGs and
               may be used for forwarding traffic inward along the DAG.
               When multicast RA-DIO messages towards
   each other in a candidate parent close enough interval that the RA-DIO messages
   `cross'.  Then each node may receive the RA-DIO message from the
   other node, and in some scenario decide to join each others DAG.  RPL
   avoids this deadlock scenario via the collision mechanism described
   above - after each node sends the RA-DIO message they will enter the
   risk window.  When the peer RA-DIO message is placed into received in the Current state,
               or taken out of risk
   window, the Current state, nodes will calculate the extended preferences as describe
   above and 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 is necessary prepared to re-
               evaluate which shortly replace a set of the remaining
   DAG parents is in order to jump to a different DAGID.  This happens
   typically when the most node has selected a more preferred DAG candidate
   parent and its rank.  At that time any
               remaining DAG parents of greater rank than this node must
               be placed in the Held-Down state, a different DAG and has to wait for the hold-down DAG hop timer
               started, in order to be evicted as DAG parents.  In
   elapse before adjusting the
               same fashion, siblings must also be reevaluated.

   Held-Up     This DAG parent can not be used until set.  Instability may also
   occur when the entire current DAG hop timer
               elapses.

   Held-Down   This candidate parent can not be used till hold down
               timer elapses.  At the end of the hold-down period, set is lost and the
               candidate next
   best candidates are still held up.  Instability is removed from resolved when the candidate
   DAG parent set,
               and may be reinserted if hop timer of all the candidate(s) causing instability elapse.
   Such candidates then change state to Current or Held- Down.

   Instability is transient (in the order of DAG hop timers).  When a
   node is unstable, it appears again MUST NOT send RAs with a RA-DIO the DIO message.

   Collision  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 parent can
   parents, which do not be used till its next RA-
               DIO message.

5.7.1.  Held-Up

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

      Delay node, so the reattachment node can
   safely attach to them.

5.8.  Guidelines for Objective Code Points

5.8.1.  Objective Function

   An Objective Function (OF) allows for the selection of a sub-DAG DAG to join,
   and a number of peers in that has been forced DAG as parents.  The OF is used to
      detach.  This
   compute an ordered list of parents and provides load balancing
   guidance.  The OF is not as safe as also responsible to compute the use rank of the sequence, but still
      covers that when a sub-DAG has detached,
   device within the DAG.

   The Objective Function is specified in the RA-DIO message that
      is initiated by using an
   objective code point (OCP) and indicates the new DAG root objective function that
   has a chance been used to spread outward
      along the sub-DAG, ideally forming a frozen sub-DAG that is aware
      of compute 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.  The idea is that between the nodes from DAG A that
      decide to move to DAG B, those that see (e.g. "minimize the highest place (closer
      to path cost using
   the DAG root) in DAG B will move first ETX metric and advertise their new
      locations before other nodes from DAG A actually move.

   A new DAG is discovered upon receiving a RA message with or without a
   DIO. avoid `Blue' links").  The node joins the DAG by selecting objective code points
   are specified in [I-D.ietf-roll-routing-metrics].  This document
   specifies the source OCP 0, in support of the RA
   message as a DAG parent (and possibly installing the DAG parent as a default gateway). operation.

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

   o  The node parent selection is then triggered each time an event indicates
      that a member of potential next_hop information is updated.  This might
      happen upon the DAG and may begin
   to multicast reception of a RA-DIO messages containing the DIO for the DAG.

   When message, a new DAG is discovered, the candidate parent timer elapse, or
      a trigger indicating that advertises the new DAG is placed in a held up state for the duration of a DAG
   Hop timer.  If the resulting new set of DAG parents is more
   preferable than the current one, or if candidate neighbor has
      changed.

   o  An OF scans all the node is intending to
   maintain a membership in interfaces on the new DAG device.  Although there may
      typically be only one interface in addition to its current DAG,
   the node expects to jump most application scenarios,
      there might be multiple of them and becomes unstable.

   A node that is unstable may discover other candidate parents from the
   same new DAG during the instability phase.  It needs an interface might be
      configured to start a new
   DAG Hop timer be usable or not for all these.  The first timer RPL operation.  An interface
      can also be configured with a preference or dynamically learned to
      be better than another by some heuristics that elapses for might be link-layer
      dependent and are out of scope.  Finally an interface might or not
      match a
   given new DAG clears them all required criterion for that DAG, allowing the node to jump
   to the highest position available in an Objective Function, for instance
      a degree of security.  As a result some interfaces might be
      completely excluded from the new DAG. computation, while others might be
      more or less preferred.

   o  The duration of OF scans all the DAG Hop timer depends candidate neighbors on the DAG Delay possible
      interfaces to check whether they can act as an attachment router
      for a DAG.  There might be multiple of the new
   DAG them and a candidate
      neighbor might need to pass some validation tests before it can be
      used.  In particular, some link layers require experience on the
      activity with a router to enable the router as a next_hop.

   o  The OF computes self's rank by adding the step of rank to that
      candidate parent to the rank of that triggers it: (candidates candidate.  The step of rank + random) * candidate's DAG_delay (where 0 <= random < 1).  It is randomized in order to limit collisions and synchronizations.

5.7.2.  Held-Down

   When a neighboring node is 'removed'
      estimated as follows:

      *  The step of rank might vary from the Default Router List, it
   is actually held down 1 to 16.

         +  1 indicates a unusually good link, for instance a hold down timer period, link
            between powered devices in order to
   prevent flapping.  This happens when a node disappears (upon
   expiration timer).

   When the hold down timer elapses, the node is removed from the
   candidate DAG parent set.

5.7.3.  Collision

   A race condition occurs if 2 nodes send RA-DIO messages at mostly battery operated
            environment.

         +  4 indicates a `normal'/typical link, as qualified by the same
   time and then attempt
            implementation.

         +  16 indicates a link that can hardly be used to join each other.  This might happen, forward any
            packet, for
   example, between nodes which act as DAG root of their own DAGs.  In
   order instance a radio link with quality indicator or
            expected transmission count that is close to detect the situation, LLN Nodes time stamp acceptable
            threshold.

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

   o  Candidate neighbors that advertise an OF incompatible with the sending set
      of
   RA-DIO message.  Any RA-DIO message received within a short link-
   layer-dependent period introduces a risk.  To resolve OF specified by the collision,
   a 32bits extended preference is constructed from policy functions are ignored.

   o  As it scans all the RA-DIO message
   by concatenating candidate neighbors, the NodePreference OF keeps the current
      best parent and compares its capabilities with the BootTimeRandom.

   A node that decides to add a current
      candidate to its DAG parents will do so neighbor.  The OF defines a number of tests that are
      critical to reach the Objective.  A test between (candidate rank) and (candidate rank + 1) times the candidate
   DAG Delay.  But since a node is unstable as soon as it receives routers
      determines an order relation.

      *  If the
   RA-DIO message from routers are roughly equal for that relation then the desired candidate, it will restrain from
   sending a RA-DIO message
         next test is attempted between the time it receives routers,

      *  Else the RA and best of the
   time it actually jumps.  So 2 becomes the crossing of RA may only happen during current best parent and the propagation time between
         scan continues with the next candidate and neighbor

      *  Some OFs may include a test to compare the node, plus some
   internal queuing and processing time within each machine.  It is
   expected that one DAG delay normally covers ranks that interval, but
   ultimately it is up to would
         result if the implementation and node joined either router

   o  When the configuration of scan is complete, the candidate preferred parent to define the duration of risk window.

   There is risk of a collision when a node receives an RA, for another
   candidate that elected and
      self's rank is more preferable than computed as the current candidate, within preferred parent rank plus the risk window. step
      in rank with that parent.

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

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

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

      *  Candidate neighbors of a potential collision, the node with
   lowest extended preference processes the RA-DIO message normally,
   while better rank than self (non-siblings)
         are preferred

5.8.2.  Objective Code Point 0 (OCP 0)

   Here follows the router with specification for the highest extended preference places default Objective Function
   corresponding to OCP codepoint 0.  This is a very simple reference to
   help design more complex Objective Functions.  In particular, the
   other in collision state,
   Objective Function described here does not start use physical metrics as
   described in [I-D.ietf-roll-routing-metrics], but are only based on
   abstract information from the DAG hop timer, RA-DIO message such as rank and does
   not become instable.  It
   administrative preference.

   This document specifies a default objective metric, called OF0, and
   using the OCP 0.  OF0 is expected that next RAs between the two
   will not cross anyway.

   For example, consider a case where two nodes are each rooting their
   own transient floating DAGs default objective function of RPL, and multicast RA-DIO messages towards
   each other in a close enough interval that
   can be used if allowed by the RA-DIO messages
   `cross'.  Then each policy of the processing node may receive when no
   objective function is included in the RA-DIO message from message, or if the
   other node, and OF
   indicated in some scenario decide to join each others DAG.  RPL
   avoids this deadlock scenario via the collision mechanism described
   above - after each node sends the RA-DIO message they will enter is unknown to the
   risk window.  When node.  If not
   allowed, then the peer RA-DIO message is received in simply ignored and not processed
   by the risk
   window, node.

5.8.2.1.  OCP 0 Objective Function (OF0)

   OF0 favors the nodes will calculate connectivity.  That is, the extended preferences as describe
   above and 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
   designed to shortly replace find the nearest sink into a set 'grounded' topology, and if
   there is none then join any network per order of
   DAG parents administrative
   preference.  The metric in order to jump to a different DAGID.  This happens
   typically when use is the node has selected rank.

   OF0 selects a more preferred candidate parent in a different DAG and has to wait for a backup next_hop if one is
   available.  The backup next_hop might be a parent or a sibling.  All
   the DAG hop timer traffic is routed via the preferred parent.  When the link
   conditions do not let a packet through to
   elapse before adjusting the DAG parent set.  Instability may also
   occur when preferred parent, the entire current DAG parent set
   packet is lost and passed to the next
   best candidates are still held up.  Instability backup next_hop.

   The step of rank is resolved when the
   DAG hop timer 4 for each hop.

5.8.2.2.  Selection of the Preferred Parent

   As it scans all the candidate(s) causing instability elapse.
   Such candidates then change state to Current or Held- Down.

   Instability candidate neighbors, OF0 keeps the parent that is transient
   the best for the following criteria (in order):

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

   2.   A candidate that would cause the order of DAG hop timers).  When a node is unstable, it MUST NOT send RAs with to augment the DIO message.  This
   avoids loops when node rank in the
        current DAG is not considered.

   3.   A decides to attach to node B and node B
   decides to attach to node A. Unless RAs cross (see Collision
   section), router that has been validated as usable, e.g. with a node receives RA-DIO messages from stable candidate
   parents, which do not plan to attach to the node, so the node can
   safely attach to them.

5.8.  Guidelines for Objective Code Points

5.8.1.  Objective Function

   An Objective Function (OF) allows for the selection of local
        confidence that has exceeded some pre-configured threshold, is
        better.

   4.   If none are grounded then a DAG to join,
   and with a number of peers in more preferred
        administrative preference is better.

   5.   A router that offers connectivity to a grounded DAG as parents.  The OF is used to
   compute better.

   6.   A lesser resulting rank is better.

   7.   A DAG for which there is an ordered list of parents and provides load balancing
   guidance.  The OF alternate parent is also responsible to compute the rank of the
   device within better.  This
        check is optional.  It is performed by computing the DAG. backup
        next_hop while assuming that this router won.

   8.   The Objective Function DAG that was in use already is specified preferred.

   9.   The router with a better router preference wins.

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

   11.  A router that has announced a RA-DIO message using an
   objective code point (OCP) more recently is
        preferred.

5.8.2.3.  Selection of the Backup next_hop

   o  The interface must be usable and indicates the objective function that
   has been used to compute the DAG (e.g. "minimize the path cost using
   the ETX metric and avoid `Blue' links"). administrative preference (if
      any) applies first.

   o  The objective code points preferred parent is ignored.

   o  Candidate neighbors that are specified not in [I-D.ietf-roll-routing-metrics].  This document
   specifies the OCP 0, in support same DAG are ignored.

   o  Candidate neighbors with a higher rank are ignored.

   o  Candidate neighbors of default operation.

   Most Objective Functions a better rank than self (non-siblings) are expected to follow the same abstract
   behavior:
      preferred.

   o  The parent selection is triggered each time an event indicates  A router that has been validated as usable, e.g. with a local
      confidence that has exceeded some pre-configured threshold, is
      better.

   o  The router with a potential better router preference wins.

   o  The backup next_hop information that was in use already is updated.  This might
      happen upon better.

5.9.  Establishing Routing State Outward Along the reception DAG

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

   As a RA-DIO message, result of destination advertisement operation:

   o  DAG discovery establishes a timer elapse, or DAG oriented toward a trigger indicating that DAG root using
      extended Neighbor Discovery RS/RA flows, along which inward routes
      toward the state of a candidate neighbor has
      changed. DAG root are set up.

   o  An OF scans all the interfaces on the device.  Although there may
      typically be only one interface  Destination advertisement extends Neighbor Discovery in most application scenarios,
      there might be multiple of them and an interface might be
      configured order to be usable or not for RPL operation.  An interface
      can also be configured with a preference or dynamically learned to
      be better than another by some heuristics that might be link-layer
      dependent and are out
      establish outward routes along the DAG.  Such paths consist of:
      *  Hop-By-Hop routing state within islands of scope.  Finally an interface might or `stateful' nodes.
      *  Source Routing `bridges' across nodes who do not
      match a required criterion for an Objective Function, for instance
      a degree of security.  As a result some interfaces might be
      completely excluded from retain state.

   Destinations disseminated with the computation, while others might destination advertisement
   mechanism may be
      more prefixes, individual hosts, or less preferred.

   o multicast listeners.
   The OF scans all the candidate neighbors on the possible
      interfaces to check whether they can act as an attachment router
      for a DAG.  There might be multiple mechanism supports nodes of them and a candidate
      neighbor might need to pass some validation tests before it can be
      used.  In particular, some link layers require experience on the
      activity with a router to enable the router varying capabilities as a next_hop. follows:

   o  The OF computes self's rank  When nodes are capable of storing routing state, they may inspect
      destination advertisements and learn hop-by-hop routing state
      toward destinations by adding populating their routing tables with the step of rank to that
      candidate
      routes learned from nodes in their sub-DAG.  In this process they
      may also learn necessary piecewise source routes to the rank traverse
      regions of the LLN that candidate.  The step of rank is
      estimated as follows:

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

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

         +  4 indicates a `normal'/typical link, do not maintain routing state.  They may
      perform route aggregation on known destinations before emitting
      Destination Advertisements.

   o  When nodes are incapable of storing routing state, they may
      forward destination advertisements, recording the reverse route as qualified by
      the
            implementation.

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

      *  Candidate neighbors construction of piecewise source
      routes.

   Nodes that would cause self's rank to increase are ignored

   o  Candidate neighbors that advertise an OF incompatible with the set capable of OF specified by storing routing state, and finally the policy functions DAG
   roots, are ignored.

   o  As it scans all the candidate neighbors, able to learn which destinations are contained in the OF keeps sub-
   DAG below the current
      best parent node, and compares its capabilities with the current
      candidate neighbor. via which next-hop neighbors.  The OF defines a number
   dissemination and installation of tests that are
      critical to reach the Objective.  A test between the routers
      determines an order relation.

      *  If the routers are roughly equal this routing state into nodes
   allows for that relation then Hop-By-Hop routing from the
         next test is attempted between DAG root outwards along the routers,

      *  Else
   DAG.  The mechanism is further enhance by supporting the best construction
   of source routes across stateless `gaps' in the 2 becomes the current best parent and DAG, where nodes are
   incapable of storing additional routing state.  An adaptation of this
   mechanism allows for the
         scan continues with implementation of loose-source routing.

   A special case, the next candidate neighbor

      *  Some OFs may include reception of a test destination advertisement
   addressed to compare the ranks that would
         result if the a link-local multicast address, allows for a node joined either router

   o  When the scan to
   learn destinations directly available from its one-hop neighbors.

   A design choice behind advertising routes via destination
   advertisements is complete, not to synchronize the preferred parent is elected and
      self's rank is computed as the preferred parent rank plus children
   databases along the step
      in rank with that parent.

   o  Other rounds of scans might be necessary DAG, but instead to elect alternate
      parents and siblings.  In update them regularly to
   recover from the next rounds:

      *  Candidate neighbors loss of packets.  The rationale for that are not choice is
   time variations in connectivity across unreliable links.  If the same DAG are ignored

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

      *  Candidate neighbors
   topology can be expected to change frequently, synchronization might
   be an excessive goal in terms of exchanges and protocol complexity.
   The approach used here results in a better rank than self (non-siblings)
         are preferred

5.8.2.  Objective Code Point 0 (OCP 0)

   Here follows the specification simple protocol with no real
   peering.  The destination advertisement mechanism hence provides for
   periodic updates of the default Objective Function
   corresponding to OCP codepoint 0.  This is a very simple reference routing state, as cued by occasional RAs and
   other mechanisms, similarly to
   help design more complex Objective Functions.  In particular, the
   Objective Function described here does not use physical metrics as
   described in [I-D.ietf-roll-routing-metrics], but are only based on
   abstract information from the RA-DIO message other protocols such as rank RIP [RFC2453].

5.9.1.  Destination Advertisement Message Formats

5.9.1.1.  DAO Option

   RPL extends Neighbor Discovery [RFC4861] and
   administrative preference.

   This document specifies RFC4191 [RFC4191] to
   allow a default objective metric, called OF0, and
   using the OCP 0.  OF0 is the default objective function of RPL, and
   can be used if allowed by the policy of the processing node when no
   objective function is included in the RA-DIO message, or if the OF
   indicated in the RA-DIO message is unknown to include a destination advertisement option, which
   includes prefix information, in the node.  If not
   allowed, then the RA-DIO message Neighbor Advertisement (NA)
   messages.  A prefix option is simply ignored and not processed
   by the node.

5.8.2.1.  OCP 0 Objective Function (OF0)

   OF0 favors the connectivity.  That is, normally present in RA messages only,
   but the Objective Function NA is
   designed augmented with this option in order to find propagate
   destination information inwards along the nearest sink into a 'grounded' topology, and if
   there is none then join any network per order of administrative
   preference. DAG.  The metric in use option is named
   the rank.

   OF0 selects a preferred parent Destination Advertisement Option (DAO), and a backup next_hop if one is
   available.  The backup next_hop might an NA message
   containing this option may be referred to as a parent destination
   advertisement, or a sibling.  All NA-DAO.  The RPL use of destination advertisements
   allows the traffic is routed via nodes in the preferred parent.  When the link
   conditions do not let a packet through to the preferred parent, the
   packet is passed DAG to build up routing state for nodes
   contained in the backup next_hop.

   The step sub-DAG in support of rank is traffic flowing outward along
   the DAG.

        0                   1                   2                   3
        0 1 2 3 4 for each hop.

5.8.2.2.  Selection of 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    |         DAO Sequence          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Prefix (Variable Length)                    |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Reverse Route Stack (Variable Length)             |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 7: The Destination Advertisement Option (DAO)

   Type: 8-bit unsigned identifying the Preferred Parent

   As it scans all Destination Advertisement
         option.  IANA had defined the candidate neighbors, OF0 keeps IPv6 Neighbor Discovery Option
         Formats registry.  The suggested type value for the parent that Destination
         Advertisement Option carried within a NA message is 141, to be
         confirmed by IANA.

   Length:  8-bit unsigned integer.  The length of the best for option (including
         the following criteria (in order):

   1.   The interface must be usable Type and Length fields) in units of 8 octets.

   Prefix Length:  Number of valid leading bits in the administrative preference
        (if any) applies first.

   2.   A candidate that would cause the node IPv6 Prefix.

   RRCount:  8-bit unsigned integer.  This counter is used to augment count the rank
         number of entries in the
        current DAG is not considered.

   3. Reverse Route Stack.  A router that has been validated as usable, e.g. with a local
        confidence value of `0'
         indicates that has exceeded some pre-configured threshold, no Reverse Route Stack is
        better.

   4.   If none are grounded then a DAG with a more preferred
        administrative preference present.

   DAO Lifetime:  32-bit unsigned integer.  The length of time in
         seconds (relative to the time the packet is better.

   5.   A router sent) that offers connectivity to a grounded DAG the
         prefix is better.

   6. valid for route determination.  A lesser resulting rank is better.

   7. value of all one
         bits (0xFFFFFFFF) represents infinity.  A DAG for which there is an alternate parent is better.  This
        check is optional.  It is performed by computing the backup
        next_hop while assuming that this router won.

   8. value of all zero
         bits (0x00000000) indicates a loss of reachability.

   Route Tag:  32-bit unsigned integer.  The DAG Route Tag may be used to
         give a priority to prefixes that was should be stored.  This may be
         useful in use already is preferred.

   9.   The router with cases where intermediate nodes are capable of storing
         a better router preference wins.

   10. limited amount of routing state.  The preferred parent that was in further specification
         of this field and its use already is better.

   11.  A router under investigation.

   DAO Depth:  Set to 0 by the node that has announced a RA-DIO message more recently is
        preferred.

5.8.2.3.  Selection of owns the Backup next_hop

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

   o  The preferred parent is ignored.

   o  Candidate neighbors NA-DAO message.  Incremented by all LLN nodes that are not in
         propagate the same DAG are ignored.

   o  Candidate neighbors with a higher rank are ignored.

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

   o  A router NA-DAO message.

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

   DAO Sequence:  Incremented by the node that has been validated as usable, e.g. with a local
      confidence owns the prefix for each
         new NA-DAO message for that has exceeded some pre-configured threshold, is
      better.

   o  The router with prefix.

   Prefix:  Variable-length field containing an IPv6 address or a better router preference wins.

   o prefix
         of an IPv6 address.  The backup next_hop that was Prefix Length field contains the
         number of valid leading bits in use already is better.

5.9.  Establishing Routing State Outward Along the DAG prefix.  The destination advertisement mechanism supports bits in the dissemination
         prefix after the prefix length (if any) are reserved and MUST
         be set to zero on transmission and MUST be ignored on receipt.

   Reverse Route Stack:  Variable-length field containing a sequence of
   routing state required
         RRCount (possibly compressed) IPv6 addresses.  A node who adds
         on to support traffic flows outward along the
   DAG, from Reverse Route Stack will append to the DAG root toward nodes.

   As list and
         increment the RRCount.

5.9.2.  Destination Advertisement Operation

5.9.2.1.  Overview

   According to implementation specific policy, a result subset or all of the
   feasible parents in the DAG may be selected to receive prefix
   information from the destination advertisement operation:

   o  DAG discovery establishes a DAG oriented toward a mechanism.  This
   subset of DAG root using
      extended Neighbor Discovery RS/RA flows, along which inward routes
      toward parents shall be designated the DAG root are set up.

   o  Destination advertisement extends Neighbor Discovery in order to
      establish outward routes along the DAG.  Such paths consist of:
      *  Hop-By-Hop routing state within islands of `stateful' nodes.
      *  Source Routing `bridges' across nodes who do not retain state.

   Destinations disseminated with DA parents.

   As NA-DAO messages for particular destinations move inwards along the destination advertisement
   mechanism may be prefixes, individual hosts, or multicast listeners.
   DAG, a sequence counter is used to guarantee their freshness.  The mechanism supports nodes of varying capabilities as follows:

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

   o  When nodes are incapable of storing routing state, they may
      forward destination advertisements, recording the reverse route as owns the go in order to support prefix, or learned the construction of piecewise source
      routes. prefix via some other
   means), each time it issues a NA-DAO message for its prefix.  Nodes that are capable of storing routing state, and finally the DAG
   roots, are able to learn which destinations are contained in
   who receive the sub-
   DAG below NA-DAO message and, if scope allows, will be
   forwarding a NA-DAO message for the node, and via which next-hop neighbors.  The
   dissemination unmodified destination inwards
   along the DAG, will leave the sequence number unchanged.
   Intermediate nodes will check the sequence counter before processing
   a NA-DAO message, and installation if the DAO is unchanged (the sequence counter
   has not changed), then the NA-DAO message will be discarded without
   additional processing.  Further, if the NA-DAO message appears to be
   out of this routing synch (the sequence counter is 2 or more behind the present
   value) then the DAO state into nodes
   allows for Hop-By-Hop routing from is considered to be stale and may be
   purged, and the DAG root outwards along NA-DAO message is discarded.  A depth is also added
   for tracking purposes; the
   DAG.  The mechanism depth is further enhance by supporting incremented at each hop as the construction
   of source routes across stateless `gaps' in
   NA-DAO message is propagated up the DAG, where nodes DAG.  Nodes who are
   incapable of storing additional
   routing state.  An adaptation of this
   mechanism allows for the implementation of loose-source routing.

   A special case, state may use the reception of a destination advertisement
   addressed depth to a link-local multicast address, allows determine which possible next-hops
   for a the destination are more optimal.

   If destination advertisements are activated in the RA-DIO message as
   indicated by the `D' bit, the node sends unicast destination
   advertisements to
   learn destinations directly available from its one-hop neighbors.

   A design choice behind advertising routes via DA parents, and only accepts unicast
   destination advertisements is not to synchronize from any nodes but those contained in the
   DA parent and children
   databases along the DAG, but instead to update them regularly to
   recover from the loss of packets.  The rationale for that choice is
   time variations in connectivity across unreliable links.  If the
   topology can be expected subset.

   Every NA to change frequently, synchronization might
   be an excessive goal in terms of exchanges and protocol complexity.
   The approach used here results in a simple protocol DA parent MAY contain one or more DAOs.  Receiving a
   RA-DIO message with no real
   peering.  The the `D' destination advertisement mechanism hence provides for
   periodic updates of the routing state, as cued by occasional RAs and
   other mechanisms, similarly to other protocols such as RIP [RFC2453].

5.9.1.  Destination Advertisement Message Formats

5.9.1.1.  DAO Option

   RPL extends Neighbor Discovery [RFC4861] and RFC4191 [RFC4191] to
   allow bit set from a node to include
   DAG parent stimulates the sending of a delayed destination
   advertisement option, which
   includes prefix information, in back, with the Neighbor Advertisement (NA)
   messages.  A prefix option collection of all known prefixes (that
   is normally present in RA messages only,
   but the NA is augmented with this option in order to propagate prefixes learned via destination information inwards along advertisements for nodes
   lower in the DAG.  The option is named DAG, and any connected prefixes).  If the Destination
   Advertisement Option (DAO), and an NA Supported (A) bit is set in the RA-DIO message
   containing this option may be referred to as for the
   DAG, then a destination
   advertisement, or NA-DAO.  The RPL use of destination advertisements
   allows the nodes in the advertisement is also sent to a DAG parent
   once it has been added to build up routing state for nodes
   contained in the sub-DAG in support of traffic flowing outward along DA parent set after a movement, or when
   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    |         DAO Sequence          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Prefix (Variable Length)                    |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Reverse Route Stack (Variable Length)             |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 10: The list of advertised prefixes has changed.  Destination Advertisement Option (DAO)

   Type: 8-bit unsigned identifying
   advertisements may also be scheduled for sending when the Destination Advertisement
         option.  IANA had defined PathDigest
   of the IPv6 Neighbor Discovery Option
         Formats registry.  The suggested type value for RA-DIO message has changed, indicating that some aspect of the
   inwards paths along the DAG has been modified.

   Destination
         Advertisement Option carried within a NA message advertisements may advertise positive (prefix is 141, to be
         confirmed present)
   or negative (removed) NA-DAO messages, termed as no-DAOs.  A no-DAO
   is stimulated 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 disappearance of valid leading bits in the IPv6 Prefix.

   RRCount:  8-bit unsigned integer. a prefix below.  This counter is used to count the
         number of entries in the Reverse Route Stack.  A value of `0'
         indicates that no Reverse Route Stack
   discovered by timing out after a request (a RA-DIO message) or by
   receiving a no-DAO.  A no-DAO is present. a conveyed as a NA-DAO message with
   a DAO Lifetime:  32-bit unsigned integer.  The length Lifetime of time in
         seconds (relative to 0.

   A node who is capable of recording the time state information conveyed in
   a unicast NA-DAO message will do so upon receiving and processing the packet is sent) that
   NA-DAO message, thus building up routing state concerning
   destinations below it in the
         prefix is valid for route determination.  A value of all one
         bits (0xFFFFFFFF) represents infinity.  A value of all zero
         bits (0x00000000) indicates DAG.  If a loss node capable of reachability.

   Route Tag:  32-bit unsigned integer.  The Route Tag may be used to
         give recording
   state information receives a priority to prefixes NA-DAO message containing a Reverse
   Route Stack, then the node knows that should be stored.  This may be
         useful in cases where intermediate the NA-DAO message has
   traversed one or more nodes are capable of storing
         a limited amount of that did not retain any routing state.  The further specification
         of this field and its use is under investigation. state as
   it traversed the path from the DAO Depth:  Set source to 0 by the node.  The node that owns may
   then extract the prefix Reverse Route Stack and first
         issues retain the NA-DAO message.  Incremented by all LLN nodes that
         propagate included state in
   order to specify Source Routing instructions along the NA-DAO message.

   Reserved:  8-bit unused field. return path
   towards the destination.  The reserved field node MUST be set the RRCount back to zero on transmission
   and MUST be ignored on receipt.

   DAO Sequence:  Incremented by clear the node that owns Reverse Route Stack prior to passing the prefix for each
         new NA-DAO message for that prefix.

   Prefix:  Variable-length field containing an IPv6 address or a prefix
         of an IPv6 address.  The Prefix Length field contains the
         number of valid leading bits in
   information on.

   A node who is unable to record the prefix.  The bits state information conveyed in the
         prefix after
   NA-DAO message will append the prefix length (if any) are reserved and MUST
         be set next-hop address to zero on transmission and MUST be ignored on receipt. the Reverse Route Stack:  Variable-length field containing a sequence of
         RRCount (possibly compressed) IPv6 addresses.  A node who adds
   Stack, increment the RRCount, and then pass the destination
   advertisement on to without recording any additional state.  In this way
   the Reverse Route Stack will append to contain a vector of next hops that must
   be traversed along the list and
         increment reverse path that the RRCount.

5.9.2.  Destination Advertisement Operation

5.9.2.1.  Overview

   According NA-DAO message has
   traveled.  The vector will be ordered such that the node closest to implementation specific policy, a subset or all of
   the
   feasible parents destination will appear first in the DAG list.  In such cases, if it
   is useful to the implementation to try and build up redundant paths,
   the node may be selected choose to receive prefix
   information from convey the destination advertisement mechanism.  This
   subset of to one or
   more DAG parents shall be designated the set in order of DA parents.

   As NA-DAO messages for particular destinations move inwards preference as guided by an
   implementation specific policy.

   In some cases (called hybrid cases), some nodes along the
   DAG, path a sequence counter is used to guarantee their freshness.  The
   sequence counter is incremented by the source of the NA-DAO message
   (the node that owns the prefix, or learned
   destination advertisement follows inward along the prefix via DAG may store
   state and some other
   means), each time it issues a NA-DAO message may not.  The destination advertisement mechanism
   allows for its prefix.  Nodes
   who receive the NA-DAO message and, if scope allows, will be
   forwarding provisioning of routing state such that when a NA-DAO message for the unmodified destination inwards packet
   is traversing outwards along the DAG, will leave the sequence number unchanged.
   Intermediate some nodes will check may be able to
   directly forward to the sequence counter before processing
   a NA-DAO message, next hop, and if the DAO is unchanged (the sequence counter
   has not changed), then the NA-DAO message will other nodes may be discarded without
   additional processing.  Further, if the NA-DAO message appears able to be
   out
   specify a piecewise source route in order to bridge spans of synch (the sequence counter is 2 or more behind
   stateless nodes within the present
   value) then path on the DAO state is considered way to be stale and may be
   purged, and the NA-DAO message is discarded.  A depth is also added
   for tracking purposes; desired
   destination.

   In the depth case where no node is incremented at each hop able to store any routing state as
   destination advertisements pass by, and the
   NA-DAO message is propagated DAG root ends up with NA-
   DAO messages that contain a completely specified route back to the DAG.  Nodes who are storing
   routing state may use
   originating node in the depth to determine which possible next-hops
   for form of the destination are more optimal.

   If inverted Reverse Route Stack.  A
   DAG root should not request (Destination Advertisement Trigger) nor
   indicate support (Destination Advertisement Supported) for
   destination advertisements are activated if it is not able to store the Reverse
   Route Stack information in this case.

   The destination advertisement mechanism requires stateful nodes to
   maintain lists of known prefixes.  A prefix entry contains the RA-DIO message as
   indicated by
   following abstract information:

   o  A reference to the `D' bit, ND entry that was created for the node sends unicast destination
   advertisements to its DA parents, advertising
      neighbor.

   o  The IPv6 address and only accepts unicast
   destination advertisements from any nodes but those contained in interface for the
   DA parent subset.

   Every NA to a DA parent MAY contain one or more DAOs.  Receiving a
   RA-DIO message with advertising neighbor.

   o  The logical equivalent of the `D' full destination advertisement bit set from a
   DAG parent stimulates
      information (including the sending 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 a delayed destination
   advertisement back, with the collection DA parents.

   o  A counter of all known prefixes (that
   is retries to count how many RA-DIO messages were sent
      on the prefixes learned via destination advertisements interface to the advertising neighbor without reachability
      confirmation for nodes
   lower in the DAG, and any connected prefixes).  If prefix.

   Note that nodes may receive multiple information from different
   neighbors for a specific destination, as different paths through the Destination
   Advertisement Supported (A) bit is set in
   DAG may be propagating information inwards along the RA-DIO message DAG for the
   DAG, then a destination advertisement is also sent to a DAG parent
   once it has been added to the DA parent set after a movement, or when
   the list of advertised prefixes has changed.  Destination
   advertisements may also be scheduled for sending when the PathDigest
   of the RA-DIO message has changed, indicating that some aspect of the
   inwards paths along the DAG has been modified.

   Destination advertisements may advertise positive (prefix is present)
   or negative (removed) NA-DAO messages, termed as no-DAOs.  A no-DAO
   is stimulated by the disappearance of a prefix below.  This is
   discovered by timing out after a request (a RA-DIO message) or by
   receiving a no-DAO.  A no-DAO is a conveyed as a NA-DAO message with
   a DAO Lifetime of 0. same
   destination.  A node who is capable of recording the routing state information conveyed in
   a unicast NA-DAO message will do so upon receiving and processing keep track
   of the
   NA-DAO message, thus building up routing state concerning
   destinations below information from each neighbor independently, and when it in
   comes time to propagate the DAG.  If a node capable of recording
   state information receives a NA-DAO message containing for a Reverse
   Route Stack, then the node knows that particular prefix to
   the NA-DAO message has
   traversed one or more nodes that did not retain any routing state as
   it traversed DA parents, then the path DAO information will be selected from among
   the DAO source advertising neighbors who offer the least depth to the node.
   destination.

   The node may
   then extract destination advertisement mechanism stores the Reverse Route Stack prefix entries in
   one of 3 abstract lists; the Connected, the Reachable and retain the included state in
   order to specify Source Routing instructions along the return path
   towards the destination.
   Unreachable lists.

   The node MUST set the RRCount back Connected list corresponds to zero the prefixes owned and clear managed by
   the Reverse Route Stack prior to passing local node.

   The Reachable list contains prefixes for which the NA-DAO message
   information on.

   A node who is unable to record the state information conveyed keeps
   receiving NA-DAO messages, and for those prefixes which have not yet
   timed out.

   The Unreachable list keeps track of prefixes which are no longer
   valid and in the process of being deleted, in order to send NA-DAO message will append the next-hop address
   messages with zero lifetime (also called no-DAO) to the Reverse Route
   Stack, increment DA parents.

5.9.2.1.1.  Destination Advertisement Timers

   The destination advertisement mechanism requires 2 timers; the RRCount,
   DelayNA timer and then pass the RemoveTimer.

   o  The DelayNA timer is armed upon a stimulation to send a
      destination advertisement on without recording any additional state.  In this way
   the Reverse Route Stack will contain (such as a vector of next hops that must
   be traversed along RA-DIO message from a DA
      parent).  When the reverse path that timer is armed, all entries in the NA-DAO message has
   traveled.  The vector will Reachable
      list as well as all entries for Connected list are set to not be ordered such
      reported yet for that particular DA parent.

   o  The DelayNA timer has a duration that is DEF_NA_LATENCY divided by
      a multiple of the node closest to DAG rank of the destination will appear first node.  The intention is that
      nodes located deeper in the list.  In such cases, if it
   is useful DAG should have a shorter DelayNA
      timer, allowing NA-DAO messages a chance to be reported from
      deeper in the implementation to try DAG and build up redundant paths,
   the node may choose potentially aggregated along sub-DAGs before
      propagating further inwards.

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

      *  When a RA-DIO message is sent that is requesting destination advertisement to one or
   more DAG parents
         advertisements, a flag is set for all DAO entries in order of preference as guided by an
   implementation specific policy.

   In some cases (called hybrid cases), some nodes along the path a
   destination advertisement follows inward along
         routing table.

      *  If the DAG may store
   state and some may not.  The destination advertisement mechanism
   allows flag has already been set for a DAO entry, the provisioning of routing state such that when retry
         count is incremented.

      *  If a packet NA-DAO message is traversing outwards along the DAG, some nodes may be able to
   directly forward received to confirm the next hop, entry, the entry
         is refreshed and other nodes the flag and count may be able to
   specify cleared.

      *  If at least one entry has reached a piecewise source route in order to bridge spans of
   stateless nodes within the path on the way to threshold value and the desired
   destination.

   In
         RemoveTimer is not running, the case where no node entry is able considered to store any routing state as
   destination advertisements pass by, be
         probably gone and the DAG root ends up with NA-
   DAO RemoveTimer is started.

      *  When the RemoveTimer elapse, NA-DAO messages that contain a completely specified route back with lifetime 0,
         i.e. no-DAOs, are sent to explicitly inform DA parents that the
   originating node in
         entries who have reached the form of threshold are no longer available,
         and the inverted Reverse Route Stack.  A
   DAG root should not request (Destination Advertisement Trigger) nor
   indicate support (Destination related routing states may be propagated and cleaned
         up.

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

5.9.2.2.  Multicast Destination Advertisement Supported) for
   destination advertisements if it messages

   It is not able also possible for a node to multicast a NA-DAO message to store the Reverse
   Route Stack information
   link-local scope all-nodes multicast address FF02::1.  This message
   will be received by all node listening in this case. range of the emitting node.
   The destination advertisement mechanism requires stateful nodes objective is to
   maintain lists of known prefixes.  A prefix entry contains enable direct P2P communication, between
   destinations directly supported by neighboring nodes, without needing
   the
   following abstract information:

   o RPL routing structure to relay the packets.

   A reference multicast NA-DAO message MUST be used only to advertise information
   about self, i.e. prefixes in the ND entry Connected list or addresses owned by
   this node.  This would typically be a multicast group that was created for the advertising
      neighbor.

   o  The IPv6 this node
   is listening to or a global address and interface owned by this node, though it can
   be used to advertise any prefix owned by this node as well.  A
   multicast NA-DAO message is not used for routing and does not presume
   any DAG relationship between the advertising neighbor.

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

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

   o  A counter of retries to count how many RA-DIO messages were sent
      on the interface receiver; it MUST
   NOT be used to relay information learned (e.g. information in the advertising neighbor without reachability
      confirmation for the prefix.

   Note that nodes may receive multiple
   Reachable list) from another node; information obtained from different
   neighbors for a specific destination, as different paths through the
   DAG may
   multicast NA-DAO MAY be propagating information inwards along the DAG for installed in the same
   destination.  A node who is recording routing state will keep track
   of the information from each neighbor independently, table and when it
   comes time MAY be
   propagated by a router in unicast NA-DAOs.

   A node receiving a multicast NA-DAO message addressed to propagate FF02::1 MAY
   install prefixes contained in the NA-DAO message in the routing table
   for local use.  Such a particular prefix to node MUST NOT perform any other processing on
   the NA-DAO message (i.e. such a node does not presume it is a DA parents, then the DAO information will be selected
   parent).

5.9.2.3.  Unicast Destination Advertisement messages from among
   the advertising neighbors who offer the least depth child to the
   destination.

   The
          parent

   When sending a destination advertisement mechanism stores to a DA parent, a node
   includes the DAOs for prefix entries in
   one of 3 abstract lists; not already reported (since the Connected,
   last DA Trigger from an RA-DIO message) in the Reachable and the
   Unreachable lists.

   The
   Connected list corresponds to lists, as well as no-DAOs for all the prefixes owned and managed by entries in the local node.

   The Reachable list contains prefixes for which
   Unreachable list.  Depending on its policy and ability to retain
   routing state, the node keeps receiving NA-DAO messages, and for those prefixes which have not yet
   timed out.

   The Unreachable list keeps track node SHOULD keep a record of prefixes which are no longer
   valid the
   reported NA-DAO message.  If the NA-DAO message offers the best route
   to the prefix as determined by policy and other prefix records, the
   node SHOULD install a route to the prefix reported in the process NA-DAO
   message via the link local address of being deleted, the reporting neighbor and it
   SHOULD further propagate the information in order to send a NA-DAO
   messages with zero lifetime (also called no-DAO) message.

   The RA-DIO message from the DAG root is used to synchronize the DA parents.

5.9.2.1.1.  Destination Advertisement Timers

   The destination advertisement mechanism requires 2 timers; whole
   DAG, including the
   DelayNA timer and periodic reporting of destination advertisements
   back up the RemoveTimer.

   o  The DelayNA timer DAG.  Its period is armed upon a stimulation expected to send vary, depending on the
   configuration of the trickle timer that governs the RAs.

   When a
      destination advertisement (such as node receives a RA-DIO message over an LLN interface from a DA
      parent).  When
   parent, the timer DelayNA is armed, all entries in armed to force a full update.

   When the Reachable
      list as well as node broadcasts a RA-DIO message on an LLN interface, for
   all entries for Connected list are set to not be
      reported yet for on that particular DA parent. interface:

   o  The DelayNA timer has a duration that  If the entry is DEF_NA_LATENCY divided by
      a multiple of the DAG rank of the node.  The intention is that
      nodes located deeper in CONFIRMED, it goes PENDING with the DAG should have a shorter DelayNA
      timer, allowing NA-DAO messages a chance retry count
      set to be reported from
      deeper in the DAG and potentially aggregated along sub-DAGs before
      propagating further inwards. 0.

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

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

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

      *  If it
      reaches a NA-DAO message is received to confirm the entry, maximum threshold, the entry
         is refreshed and the flag and count may be cleared.

      * goes ELAPSED If at least
      one entry is ELAPSED at the end of the process: if the Destroy
      timer is not running then it is armed with a jitter.

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

   Once the RemoveTimer is not running, elapsed, the prefix entry is considered scheduled to be
         probably gone
   removed and moved to the RemoveTimer is started.

      *  When the RemoveTimer elapse, NA-DAO messages with lifetime 0,
         i.e. no-DAOs, Unreachable list if there are sent to explicitly inform any DA parents
   that need to be informed of the
         entries who have reached change in status for the threshold are no longer available,
         and prefix,
   otherwise the related routing states may be propagated and prefix entry is cleaned
         up.

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

5.9.2.2.  Multicast Destination Advertisement messages

   It prefix
   entry is also possible for a node to multicast a NA-DAO message to removed from the
   link-local scope all-nodes multicast address FF02::1. Unreachable list when no more DA parents
   need to be informed.  This message
   will condition may be received by satisfied when a no-DAO
   is sent to all node listening current DA parents indicating the loss of the prefix,
   and noting that in range some cases parents may have been removed from the
   set of DA parents.

5.9.2.4.  Other events

   Finally, the emitting node.
   The objective is destination advertisement mechanism responds to enable direct P2P communication, between
   destinations directly supported by neighboring nodes, without needing a series
   of events, such as:

   o  Destination advertisement operation stopped: All entries in the RPL routing structure to relay
      abstract lists are freed.  All the packets.

   A multicast routes learned from NA-DAO message MUST be used only to advertise information
   about self, i.e. prefixes
      messages are removed.

   o  Interface going down: for all entries in the Connected Reachable list or addresses owned by
   this node.  This would typically be a multicast group on
      that this node interface, the associated route is listening removed, and the entry is
      scheduled to or a global address owned by this node, though it can be used to advertise any prefix owned by this node as well.  A
   multicast NA-DAO message is not used for removed.

   o  Loss of routing and does not presume
   any DAG relationship between the emitter and adjacency: When the receiver; it MUST
   NOT be used to relay information learned (e.g. information routing adjacency for a
      neighbor is lost, as per the procedures described in Section 5.11,
      and if the associated entries are in the Reachable list) from another node; information obtained from a
   multicast NA-DAO MAY list, the
      associated routes are removed, and the entries are scheduled to be installed
      destroyed.

   o  Changes to DA parent set: all entries in the routing table Reachable list are
      set to not 'reported' and MAY be
   propagated DelayNA is armed.

5.9.2.5.  Aggregation of prefixes by a router in unicast NA-DAOs.

   A node receiving

   There may be number of cases where a multicast NA-DAO message addressed to FF02::1 MAY
   install prefixes contained in the NA-DAO message in the routing table
   for local use.  Such aggregation may be shared within
   a node MUST NOT perform any other processing on
   the NA-DAO message (i.e. group of nodes.  In such a node does not presume case, it is a DA
   parent).

5.9.2.3.  Unicast Destination Advertisement messages from child possible to
          parent

   When sending a use aggregation
   techniques with destination advertisement to a DA parent, a node
   includes the DAOs advertisements and improve scalability.

   Other cases might occur for prefix entries not already reported (since which additional support is required:

   1.  The aggregating node is attached within the
   last DA Trigger from an RA-DIO message) in sub-DAG of the Reachable and
   Connected lists, as well as no-DAOs nodes
       it is aggregating for.

   2.  A node that is to be aggregated for all is located somewhere else
       within the entries DAG, not in the
   Unreachable list.  Depending on its policy and ability to retain
   routing state, the receiving node SHOULD keep a record sub-DAG of the
   reported NA-DAO message.  If the NA-DAO message offers the best route aggregating node.

   3.  A node that is to be aggregated for is located somewhere else in
       the prefix as determined by policy and other prefix records, the LLN.

   Consider a node SHOULD install M who is performing an aggregation, and a route node N who
   is to the prefix reported in the NA-DAO
   message via the link local address be a member of the reporting neighbor and it
   SHOULD further propagate aggregation group.  A node Z situated above
   the information node M in a NA-DAO message.

   The RA-DIO message from the DAG root is used to synchronize the whole DAG, including but not above node N, will see the periodic reporting of destination
   advertisements
   back up the DAG.  Its period is expected to vary, depending on the
   configuration of for the trickle timer aggregation owned by M but not that governs of the RAs.

   When
   individual prefix for N. Such a node receives a RA-DIO message over an LLN interface from a DA
   parent, Z will route all the DelayNA is armed packets for
   node N towards node M, but node M will have no route to force a full update.

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

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

   o  If the entry is PENDING, the retry count is incremented.  If it
      reaches a maximum threshold, the entry goes ELAPSED If at least
      one entry is ELAPSED at forward.

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

5.9.2.6.  Default Values

   DEF_NA_LATENCY = To Be Determined

   MAX_DESTROY_INTERVAL = To Be Determined

5.10.  Multicast Operation

   This section describes further the process: if the Destroy
      timer is not running then it 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 armed with traditional, a jitter.

   Since the DelayNA timer has listener uses a duration that decreases protocol such as MLD with the depth,
   it is expected a
   router to receive all NA-DAO messages from all children
   before register to a multicast group.

   Along the timer elapses path between the router and the full update is sent to root of the DA
   parents.

   Once DAG, MLD
   requests are mapped and transported as NA-DAO messages within the RemoveTimer is elapsed, RPL
   protocol; each hop coalesces the prefix entry is scheduled multiple requests for a same group
   as a single NA-DAO message to be
   removed the parent(s), in a fashion similar to
   proxy IGMP, but recursively between child router and moved parent up to the Unreachable list if there are any DA parents
   that need
   root.

   A router might select to be informed of the change pass a listener registration NA-DAO message
   to its preferred parent only, in status which case multicast packets coming
   back might be lost for all of its sub-DAG if the prefix,
   otherwise transmission fails
   over that link.  Alternatively the prefix entry is cleaned up right away.  The prefix
   entry is removed from the Unreachable list when no more DA 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 that
   the router will need to be informed.  This condition may be satisfied when prune.

   As a no-DAO
   is sent to all current DA parents indicating the loss of the prefix,
   and noting that result, multicast routing states are installed in some cases parents may have been removed from each router on
   the
   set of DA parents.

5.9.2.4.  Other events

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

   o  Destination advertisement operation stopped: All entries in the
      abstract lists are freed.  All root, enabling the routes learned from root to copy a
   multicast packet to all its children routers that had issued a NA-DAO
      messages are removed.

   o  Interface going down:
   message including a DAO for that multicast group, as well as all entries in the Reachable list on
   attached nodes that interface, the associated route registered over MLD.

   For unicast traffic, it is removed, and expected that the entry is
      scheduled to be removed.

   o  Loss grounded root of routing adjacency: When an RPL
   DAG terminates RPL and MAY redistribute the RPL routes over the
   external infrastructure using whatever routing adjacency for a
      neighbor protocol is lost, as per used
   there.  For multicast traffic, the procedures described in Section 5.11,
      and root MAY proxy MLD for all the
   nodes attached to the RPL routers (this would be needed if the associated entries are
   multicast source is located in the Reachable list, the
      associated routes are removed, and external infrastructure).  For
   such a source, the entries are scheduled to packet will be
      destroyed.

   o  Changes to DA parent set: all replicated as it flows outwards
   along the DAG based on the multicast routing table entries in installed
   from the Reachable list are
      set to not 'reported' and DelayNA is armed.

5.9.2.5.  Aggregation of prefixes by a node

   There may be number of cases where a aggregation may be shared within
   a group of nodes.  In such NA-DAO message.

   For a case, it source inside the DAG, the packet is possible passed to use aggregation
   techniques with destination advertisements the preferred
   parents, and improve scalability.

   Other cases might occur for which additional support is required:

   1. if that fails then to the alternates in the DAG.  The aggregating node
   packet is attached within also copied to all the sub-DAG of registered children, except for the nodes
       it is aggregating for.

   2.  A node
   one that passed the packet.  Finally, if there is a listener in the
   external infrastructure then the DAG root has to be aggregated further propagate
   the packet into the external infrastructure.

   As a result, the DAG Root acts as an automatic proxy Rendez-vous
   Point for is located somewhere else
       within the DAG, not RPL network, and as source towards the Internet for all
   multicast flows started in the sub-DAG RPL LLN.  So regardless of whether the aggregating node.

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

   Consider a node M who is performing an aggregation, Internet, and a node N who
   is to be a member regardless of whether
   the aggregation group.  A node Z situated above DAG is grounded or floating, the node M in root can serve inner multicast
   streams at all times.

5.11.  Maintenance of Routing Adjacency

   The selection of successors, along the default paths inward along the
   DAG, but not above node N, will see or along the paths learned from destination advertisements for
   outward along the aggregation owned by M but not that DAG, leads to the formation of routing adjacencies
   that require maintenance.

   In IGPs such as OSPF [RFC4915] or IS-IS [RFC5120], the
   individual prefix for N. Such maintenance of
   a node Z will route all the packets for
   node N towards node M, but node M will have no route to the node N
   and will fail to forward.

   Additional protocols may be applied beyond routing adjacency involves the scope use of this
   specification to dynamically elect/provision 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 aggregating node approach is not desirable in constrained
   environments such as LLN and
   groups of nodes eligible would lead to be aggregated excessive control traffic
   in order to provide route
   summarization for light of the data traffic with 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 negative impact on both link
   loads and nodes resources.  Overhead to maintain the multicast routing operations over
   an IPv6 RPL network, and specifically how unicast NA-DAOs can
   adjacency should be used minimized.  Furthermore, it is not always
   possible to relay group registrations inwards.  Wherever rely on the following text
   mentions MLD, one can read MLDv2 link or v3.

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

   Along the path between the router and the root provide
   information of the DAG, MLD
   requests are mapped and transported as NA-DAO messages within the associated link state.  The network layer needs to
   fall back on its own mechanism.

   Thus RPL
   protocol; each hop coalesces makes use of a different approach consisting of probing the multiple requests for
   neighbor using a same group
   as Neighbor Solicitation message (see [RFC4861]).  The
   reception of a single NA-DAO Neighbor Advertisement (NA) message to with the parent(s), in a fashion similar to
   proxy IGMP, but recursively between child router and parent up
   "Solicited Flag" set is used to verify the
   root.

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

   As sending a result, multicast routing states are installed in each router on data
   packet.  This allows for detecting whether or not the way from routing
   adjacency is still valid, and should it not be the listeners case, select
   another feasible successor to forward the root, enabling the root to copy packet.

5.12.  Packet Forwarding

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

   For unicast traffic, it destination, precedence is expected that the grounded root given to
   selection of an RPL a next-hop successor as follows:

   1.  It is preferred to select a successor from a DAG terminates RPL who is
       supporting an OCP and MAY redistribute related optimization that maps to an
       objective marked in the RPL routes over IPv6 header of the
   external infrastructure using whatever 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 used
   there.  For multicast traffic, the root MAY proxy MLD for all the
   nodes attached to an entry in the RPL routers (this would be needed if routing table matching the
       destination that has been learned from a multicast source is located in destination
       advertisement (e.g. the external infrastructure).  For
   such destination is a source, the packet will be replicated as it flows outwards
   along the DAG based on one-hop neighbor), then
       use that successor.

   4.  If there is an entry in the multicast routing table entries installed
   from matching the NA-DAO message.

   For
       destination that has been learned from a source inside the DAG, unicast destination
       advertisement (e.g. the packet destination is passed to located outwards along the preferred
   parents, and if that fails
       sub-DAG), then to the alternates in the DAG.  The
   packet is also copied to all the registered children, except for the
   one use that passed the packet.  Finally, if successor.

   5.  If there is a listener in the
   external infrastructure then the DAG root has offering a route to further propagate
   the packet into the external infrastructure.

   As a result, prefix matching the
       destination, then select one of those DAG Root acts parents as an automatic proxy Rendez-vous
   Point for the RPL network, and a successor.

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

   7.  If there is a DAG offering a route to a prefix matching the Internet for
       destination, but all
   multicast flows started in DAG parents have been tried and are
       temporarily unavailable (as determined by the RPL LLN.  So regardless of whether forwarding
       procedure), then select a DAG sibling as a successor.

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

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

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

5.11.  Maintenance of Routing Adjacency

   The selection neighbor who was the
   predecessor of successors, along the default paths inward along packet (split horizon), except in the
   DAG, or along case where
   it is intended for the paths learned packet to change from destination advertisements
   outward along the DAG, leads an inward to the formation of routing adjacencies
   that require maintenance.

   In IGPs an outward
   flow, such as OSPF [RFC4915] or IS-IS [RFC5120], the maintenance of
   a routing adjacency involves the use of Keepalive mechanisms (Hellos)
   or other protocols such switching from DIO routes to DAO routes as BFD ([I-D.ietf-bfd-base]) and MANET
   Neighborhood Discovery Protocol (NHDP [I-D.ietf-manet-nhdp]).
   Unfortunately, such an approach the
   destination is not desirable neared.

6.  RPL Variables

   DIO Timer  One instance per DAG that a node is a member of.  Expiry
         triggers RA-DIO message transmission.  Trickle timer with
         variable interval in constrained
   environments such as LLN and would lead [0,
         DIOIntervalMin..2^DIOIntervalDoublings].  See Section 5.3.4

   DAG Hop Timer  Up to excessive control traffic one instance per candidate DAG parent in light of the data traffic with
         `Held-Up' state per DAG that a negative impact on both link
   loads and nodes resources.  Overhead to maintain the routing
   adjacency should be minimized.  Furthermore, it node is not always
   possible going to rely on the link or transport layer jump to.
         Expiry triggers candidate DAG parent to provide
   information of become a DAG parent in
         the `Current' state, as well as cancellation of any other DAG
         Hop timers associated link state.  The network layer needs to
   fall back with other DAG parents for that DAG.
         Duration is computed based on its own mechanism.

   Thus RPL makes use of a different approach consisting of probing the
   neighbor using a Neighbor Solicitation message (see [RFC4861]).  The
   reception rank of a Neighbor Advertisement (NA) message with the
   "Solicited Flag" set is used 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 verify one instance per candidate DAG parent in the validity
         `Held-Down' state per DAG.  Expiry triggers the eviction of the routing
   adjacency.  Such mechanism MAY
         candidate DAG parent from the candidate DAG parent set.  The
         interval should be used prior chosen as appropriate to sending a data
   packet.  This allows for detecting whether or not prevent flapping.
         See Section 5.7.

   DAG Heartbeat Timer  Up to one instance per DAG that the routing
   adjacency node is still valid, and should it
         acting as DAG root of.  May not be the case, select
   another feasible successor to forward the packet.

5.12.  Packet Forwarding

   When forwarding supported in all
         implementations.  Expiry triggers revision of
         DAGSequenceNumber, causing a packet new series of updated RA-DIO
         message to a destination, precedence is given be sent.  Interval should be chosen appropriate to
   selection
         propagation time of a next-hop successor DAG and as follows:

   1.  It is preferred appropriate to select a successor from a 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 who is
       supporting an OCP and related optimization that maps
         parents chosen to an
       objective marked in the IPv6 header receive destination advertisements) per DAG.
         Expiry triggers sending of NA-DAO message to 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 DA parent.
         The interval is an entry in the routing table matching the
       destination to be proportional to DEF_NA_LATENCY/(node
         rank), such that has been learned from a multicast destination
       advertisement (e.g. nodes of greater rank (further outward along
         the destination is DAG) expire first, coordinating the sending of NA-DAO
         messages to allow for a one-hop neighbor), then
       use that successor.

   4.  If there is an chance of aggregation.  See
         Section 5.9.2.1.1

   DestroyTimer  Up to one instance per DA entry in the routing table matching the
       destination that has been learned from per neighbor (i.e.
         those neighbors who have given NA-DAO messages to this node as
         a unicast destination
       advertisement (e.g. DAG parent) Expiry triggers a change in state for the destination is located outwards along DA
         entry, setting up to do unreachable (No-DAO) advertisements or
         immediately deallocating the
       sub-DAG), then use that successor.

   5.  If DA entry if there are no DA
         parents.  The interval is min(MAX_DESTROY_INTERVAL,
         RA_INTERVAL).  See Section 5.9.2.1.1

7.  Manageability Considerations

   The aim of this section is a DAG offering a route to a prefix matching give consideration to the
       destination, then select one manageability
   of RPL, and how RPL will be operated in LLN beyond the use of those DAG parents as a successor.

   6.  If there MIB
   module.  The scope of this section is a DAG offering a default route with a compatible OCP,
       then select one to consider the following
   aspects of those DAG parents as manageability: fault management, configuration, accounting
   and performance.

7.1.  Control of Function and Policy

7.1.1.  Initialization Mode

   When a successor.

   7.  If there node 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 offering a route DAG,
   or to immediately root a prefix matching the
       destination, but all transient DAG parents have been tried and are
       temporarily unavailable (as determined by start sending multicast
   RA-DIO messages.  A RPL implementation SHOULD allow configuring
   whether the forwarding
       procedure), then select 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 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 sibling Root behavior:  In some cases, a node may not want to permanently
         act as a successor.

   8.  Finally, if no DAG siblings are available, the packet is dropped.
       ICMP Destination Unreachable root if it cannot join a grounded DAG.  For
         example a battery-operated node may be invoked.  An inconsistency is
       detected.

   TTL MUST be decremented when forwarding.  If the packet is being
   forwarded via not want to act as a sibling, then the TTL DAG
         root for a long period of time.  Thus a RPL implementation MAY be decremented more
   aggressively (by more than one) to limit
         support the impact ability to configure whether or not a node could
         act as a DAG root for a configured period of possible
   loops.

   Note that the chosen successor time.

   DAG Hop Timer:  A RPL implementation MUST NOT be provide the neighbor who was ability to
         configure the
   predecessor value of the packet (split horizon), except DAG Hop Timer, expressed in ms.

   DAG Table Entry Suppression  A RPL implementation SHOULD provide the case where
   it is intended for the packet to change from an inward to an outward
   flow, such as switching from DIO routes
         ability to DAO routes as the
   destination is neared.

5.12.1.  Loop Taxonomy

   The following is configure a summary of timer after the sort expiration of loops which the
         DAG table that may occur within
   RPL.  This is provided in part as contains all the records about a basis for discussion of loop
   detection at forwarding.

5.12.1.1.  DAG Loops

   A DAG loop may occur when a node detaches from is
         suppressed, to be invoked if the DAG and reattaches parent set becomes empty.

7.1.3.  Trickle Timers

   A RPL implementation makes use of trickle timer to a device in its prior sub-DAG that has missed the whole detachment
   sequence and kept advertising govern the original DAG.  This may happen in
   particular when sending
   of RA-DIO messages are missed.  Use message.  Such an algorithm is determined a by a set of
   configurable parameters that are then advertised by the DAG sequence
   number can eliminate this type of loop.  If root
   along the DAG sequence number
   is not in use, RA-DIO messages.

   For each DAG, a RPL implementation MUST allow for the protection is limited (it depends on propagation monitoring of RA-DIO messages during DAG hop timer), and temporary loops might
   occur.
   the following parameters, further described in Section 5.3.4:

   I

   T

   C

   I_min

   I_doublings:

   A RPL will move to eliminate such a loop as soon as a RA-DIO
   message is received from implementation SHOULD provide a parent command (for example via API,
   CLI, or SNMP MIB) whereby any procedure that appears to be going down, as detects an inconsistency
   may cause the child has trickle timer to detach from it immediately.  (The alternate choice
   of staying attached and following reset.

7.1.4.  DAG Heartbeat

   A RPL implementation may allow by configuration at the parent in its fall would have
   counted to infinity and led DAG root to detach as well).

   Consider node (24) in
   refresh the DAG Example depicted in Figure 12, and its
   sub-DAG nodes (34), (44), and (45).  An example of a states by updating the DAGSequenceNumber.  A RPL
   implementation SHOULD allow configuring whether or not periodic or
   event triggered mechanism are used by the DAG loop would
   be if node (24) were root to detach from control
   DAGSequenceNumber change.

7.1.5.  The Destination Advertisement Option

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

   o  The DelayNA timer

   o  The Remove timer

7.1.6.  Policy Control

   DAG rooted at (LBR), and discovery enables nodes (34) and (45) were to miss the detachment sequence.
   Subsequently, if the link (24)--(45) were to become viable and node
   (24) heard node (45) advertising the implement different policies for
   selecting their DAG rooted at (LBR), parents.

   A RPL implementation SHOULD allow configuring the set of acceptable
   or preferred Objective Functions (OF) referenced by their Objective
   Codepoints (OCPs) for a DAG loop
   (45->34->24->45) may form if node (24) attaches to node (45).

5.12.1.2.  DAO Loops

   A DAO loop may occur when the parent has join a route installed upon
   receiving DAG, and processing a NA-DAO message from what action should be
   taken if none of 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 not explicitly handled by the current specification.  Split
   horizon, not forwarding a packet back to node's candidate neighbors advertise one of the
   configured allowable Objective Functions.

   A node it came from, may
   mitigate the DAO loop in some cases, but does not eliminate it.

   Consider node (24) an LLN may learn routing information from different routing
   protocols including RPL.  It is in this case desirable to control via
   administrative preference which route should be favored.  An
   implementation SHOULD allow for specifying an administrative
   preference for the DAG Example depicted in Figure 12.  Suppose
   node (24) has received a DA routing protocol from node (34) advertising a destination
   at node (45).  Subsequently, if node (34) tears down which the routing
   state route was learned.

   A RPL implementation SHOULD allow for the destination and node (24) did not hear a no-DAO message
   to clean up configuration of the routing state, "Route
   Tag" field of the NA-DAO messages according to a DAO loop set of rules defined
   by policy.

7.1.7.  Data Structures

   Some RPL implementation may exist. node (24) will
   forward traffic destined for node (45) limit the size of the candidate neighbor
   list in order to node (34), who bound the memory usage, in which case some otherwise
   viable candidate neighbors may then
   naively return it into a loop (if split horizon is not in place). be considered and simply dropped
   from the candidate neighbor list.

   A
   more complicated DAO loop may result if node (34) instead passes RPL implementation MAY provide an indicator on the
   traffic to it's sibling, node (33), potentially resulting in a
   (24->34->33->23->13->24) loop.

5.12.1.3.  Sibling Loops

   Sibling loops occur when a group size of siblings keep choosing amongst
   themselves as successors such that a packet does not make forward
   progress.  The current draft limits those loops to some degree by
   split horizon (do not send back to the same sibling) and parent
   preference (always prefer parents vs. siblings).

   Consider the DAG Example depicted in Figure 12.  Suppose that Node
   (32)
   candidate neighbor list.

7.2.  Information and (34) are reliable neighbors, Data Models

   The information and thus are siblings.  Then, data models necessary for the operation of RPL
   will be defined in a separate document specifying the case where Nodes (22), (23), and (24) are transiently
   unavailable, RPL SNMP MIB.

7.3.  Liveness Detection and with no other guiding strategy, a sibling loop may
   exist, e.g. (33->34->32->33) as Monitoring

   The aim of this section is to describe the siblings keep choosing amongst
   each other various RPL mechanisms
   specified to monitor the protocol.

   As specified in Section 5.2, an uncoordinated manner.

6.  RPL Variables

   DIO Timer  One instance per DAG that a node is implementation must maintain a member of.  Expiry
         triggers RA-DIO message transmission.  Trickle timer with
         variable interval set of
   data structures in [0,
         DIOIntervalMin..2^DIOIntervalDoublings].  See Section 5.3.4 support of DAG Hop Timer  Up to one instance per discovery:

   o  The candidate neighbors data structure

   o  For each DAG:

      *  A set of candidate DAG parent in the
         `Held-Up' state per parents

      *  A set of DAG that parents (which are a node is going to jump to.
         Expiry triggers subset of candidate DAG parent to become a DAG parent
         parents and may be implemented as such)

7.3.1.  Candidate Neighbor Data Structure

   A node in the `Current' state, as well as cancellation of any other DAG
         Hop timers associated with other DAG parents for that DAG.
         Duration candidate neighbor list is computed based on the rank of a node discovered by the candidate DAG
         parent
   some means and DAG delay, as (candidates rank + random) *
         candidate's DAG_delay (where 0 <= random < 1).  See
         Section 5.7.1.

   Hold-Down Timer  Up qualified to one instance per potentially become of neighbor or a
   sibling (with high enough local confidence).  A RPL implementation
   SHOULD provide a way monitor the candidate DAG parent in neighbors list with some
   metric reflecting local confidence (the degree of stability of the
         `Held-Down' state per DAG.  Expiry triggers
   neighbors) measured by some metrics.

   A RPL implementation MAY provide a counter reporting the eviction 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 parent from table values:

   o  DAGID

   o  DAGObjectiveCodePoint

   o  A set of Destination Prefixes offered inwards along the DAG

   o  A set of candidate DAG parent set.  The
         interval should be chosen as appropriate Parents
   o  timer to prevent flapping.
         See Section 5.7. govern the sending of RA-DIO messages for the DAG Heartbeat Timer  Up to one instance per

   o  DAGSequenceNumber

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

   o  A reference to the neighboring device which is
         acting as the DAG root of.  May not be supported in all
         implementations.  Expiry triggers revision of
         DAGSequenceNumber, causing a new series of updated RA-DIO
         message to be sent.  Interval should be chosen appropriate to
         propagation time parent

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

   o  a state associated with the role of the candidate as appropriate to application
         requirements (e.g. response time vs. overhead).  See a potential
      DAG Parent {Current, Held-Up, Held-Down, Collision}, further
      described in Section 5.4

   DelayNA Timer  Up to one instance per DA parent (the subset of 5.7

   o  A DAG
         parents chosen to receive destination advertisements) per DAG.
         Expiry triggers sending of NA-DAO message to Hop Timer, if instantiated

   o  A Held-Down Timer, if instantiated

   o  A flag reporting if the DA parent.
         The interval Parent is to a DA Parent as described in
      Section 5.9

7.3.3.  Routing Table

   To be proportional to DEF_NA_LATENCY/(node
         rank), such that nodes of greater rank (further outward along
         the DAG) expire first, coordinating completed.

7.3.4.  Other RPL Monitoring Parameters

   A RPL implementation SHOULD provide a counter reporting the sending number of NA-DAO
         messages
   a times the node has detected an inconsistency with respect to allow for a chance DAG
   parent, e.g. if the DAGID has changed.

   A RPL implementation MAY log the reception of aggregation.  See
         Section 5.9.2.1.1

   DestroyTimer  Up to one instance per DA entry per a malformed RA-DIO
   message along with the neighbor (i.e.
         those neighbors who have given NA-DAO messages to this node as identification if avialable.

7.3.5.  RPL Trickle Timers

   A RPL implementation operating on a DAG parent) Expiry triggers a change in state root MUST allow for the DA
         entry, setting up to do unreachable (No-DAO) advertisements or
         immediately deallocating
   configuration of the DA entry if there are no DA
         parents. following trickle parameters:

   o  The interval is min(MAX_DESTROY_INTERVAL,
         RA_INTERVAL).  See Section 5.9.2.1.1

7.  Manageability Considerations DIOIntervalMin expressed in ms

   o  The aim DIOIntervalDoublings

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

7.4.  Verifying Correct Operation

   This section is to give consideration has to the manageability
   of RPL, and how RPL will be operated completed in LLN beyond the use of a MIB
   module.  The scope further revision of this section is document
   to consider the following
   aspects of manageability: fault management, configuration, accounting list potential Operations and performance.

7.1.  Control Management (OAM) tools that could be
   used for verifying the correct operation of Function and Policy

7.1.1.  Initialization Mode

   When a node is first powered up, it may either choose to stay silent
   and not send any multicast RA-DIO message until it has joined a DAG,
   or to immediately root a transient DAG RPL.

7.5.  Requirements on Other Protocols and start sending multicast
   RA-DIO messages.  A Functional Components

   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 does not have any impact on the node should start sending an RS message as an initial
   probe for nearby DAGs, or should simply wait until it received RA
   messages from other nodes that are part operation of existing DAGs.

7.1.2.  DIO Base option

   RPL specifies a number of protocol parameters.

   A protocols.

7.6.  Impact on Network Operation

   To be completed.

8.  Security Considerations

   Security Considerations for RPL implementation SHOULD allow configuring the following routing
   protocol parameters:

   DAGPreference:  8-bit unsigned integer set by the DAG root to its
         preference and unchanged at propagation.

   NodePreference:  The administrative preference of that LLN Node.

   DAGDelay:  16-bit unsigned integer set by the DAG root indicating the
         delay before changing the DAG configuration,

   DIOIntervalDoublings:  8-bit unsigned integer.  Configured on the DAG
         root and used are to configure the trickle timer governing when RA-
         DIO messages should be sent within the DAG.

   DIOIntervalMin:  8-bit unsigned integer.  Configured on the developed in accordance
   with recommendations laid out in, for example,
   [I-D.tsao-roll-security-framework].

9.  IANA Considerations

9.1.  DAG root
         and used to configure the trickle timer governing when RA-DIO
         messages should be sent within the DAG. Information Option (DIO) Base Option

   The minimum configured
         interval for the RA-DIO trickle timer in units of ms DAG Information Option is
         2^DIOIntervalMin (e.g. a DIOIntervalMin value of 16ms is
         expressed container option carried within an
   IPv6 Router Advertisement message as 4).

   DAGObjectiveCodePoint defined in [RFC4861], which
   might contain a number of suboptions.  The DAG Objective Code Point is used to
         indicate base option regroups the cost metrics, objective functions, and methods of
         computation and comparison for DAGRank in use
   minimum information set that is mandatory in all cases.

   IANA had defined the DAG. IPv6 Neighbor Discovery Option Formats registry.
   The
         DAG OCP is set by suggested type value for the DAG root.

   PathDigest:  32-bit unsigned integer CRC, updated by each LLN Node.
         This Information Option (DIO) Base
   Option is 140, to be confirmed by IANA.

9.2.  New Registry for the result Flag Field of a CRC-32c computation on a bit string
         obtained by appending the received value and the ordered set of
         DAG parents at the LLN Node.  DAG roots use DIO Base Option

   IANA is requested to create a 'previous value' registry for the Flag field of zeroes to initially set the PathDigest.

   DAGID:  128-bit unsigned integer which uniquely identify a DAG.  This
         value is set DIO
   Base Option.

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

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

   o  Capability description
   o  Defining RFC

   Three flags are currently defined:

       +-----+-------------------------------------+---------------+
       | Bit | Description                         | Reference     |
       +-----+-------------------------------------+---------------+
       |  0  | Grounded DAG root.  The global IPv6 address of                        | This document |
       |  1  | Destination Advertisement Trigger   | This document |
       |  2  | Destination Advertisement Supported | This document |
       +-----+-------------------------------------+---------------+

                           DIO Base Option Flags

9.3.  DAG Information Option (DIO) Suboption

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

         +-------+------------------------------+---------------+
         | Value | Meaning                      | Reference     |
         +-------+------------------------------+---------------+
         |   0   | Pad1 - DIO Padding           | This document |
         |   1   | PadN - DIO suboption padding | This document |
         |   2   | DAG root can be used. Metric Container         | This Document |
         |   3   | Destination Prefixes  List of advertised destinations Prefix           | This Document |
         +-------+------------------------------+---------------+

            DAG Root behavior:  In some cases, Information Option (DIO) Base Option Suboptions

9.4.  Destination Advertisement Option (DAO) Option

   The RPL protocol extends Neighbor Discovery [RFC4861] and [RFC4191]
   to allow a node may not want to permanently
         act as include a DAG root if it cannot join a grounded DAG.  For
         example a battery-operated node may not want to act as a DAG
         root for a long period of time.  Thus a RPL implementation MAY
         support the ability to configure whether or not a node could
         act as a DAG root for a configured period of time.

   DAG Hop Timer:  A RPL implementation MUST provide the ability to
         configure the value of the DAG Hop Timer, expressed Destination Advertisement Option, which
   includes prefix information in ms.

   DAG Table Entry Suppression  A RPL implementation SHOULD provide the
         ability to configure a timer after Neighbor Advertisements messages.
   The Neighbor Advertisement messages are augmented with the expiration of which
   Destination Advertisement Option (DAO).

   IANA had defined the
         DAG table that contains all IPv6 Neighbor Discovery Option Formats registry.
   The suggested type value for the records about Destination Advertisement Option
   carried within a DAG Neighbor Advertisement message is
         suppressed, 141, to be invoked if the DAG parent set becomes empty.

7.1.3.  Trickle Timers

   A RPL implementation makes use of trickle timer to govern the sending
   of RA-DIO message.  Such an algorithm is determined a
   confirmed by a set of
   configurable parameters IANA.

9.5.  Objective Code Point

   This specification requests that are then advertised by the DAG root
   along the DAG an Objective Code Point registry, as
   to be specified in RA-DIO messages.

   For each DAG, a RPL implementation MUST allow for [I-D.ietf-roll-routing-metrics], reserve the monitoring of
   Objective Code Point value 0x0000, for the following parameters:

   I: purposes designated as OCP
   0 in this document.

10.  Acknowledgements

   The current length of ROLL Design Team would like to acknowledge the communication interval

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

   C:    Redundancy Counter

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

   I_doublings:  The number of times I_min should be doubled before
         maintaining a constant rate, i.e.  I_max = I_min *
         2^I_doublings.  This value is learned review, feedback,
   and comments from the RA-DIO message
         as DIOIntervalDoublings. Dominique Barthel, Yusuf Bashir, Mathilde Durvy,
   Manhar Goindi, Mukul Goyal, Quentin Lampin, Philip Levis, Jerry
   Martocci, Alexandru Petrescu, and Don Sturek.

   The default value is
         DEFAULT_DIO_INTERVAL_DOUBLINGS.

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

7.1.4.  DAG Heartbeat

   A RPL implementation may allow by configuration at the DAG root ROLL Design Team would like to
   refresh the DAG states by updating acknowledge the DAGSequenceNumber.  A RPL
   implementation SHOULD allow configuring whether or not periodic or
   event triggered mechanism are used guidance and input
   provided by the DAG root to control
   DAGSequenceNumber change.

7.1.5.  The Destination Advertisement Option

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

   o  The DelayNA timer

   o ROLL Chairs, David Culler and JP Vasseur.

   The Remove timer

7.1.6.  Policy Control

   DAG discovery enables nodes ROLL Design Team would like to implement different policies for
   selecting their DAG parents.

   A RPL implementation SHOULD allow configuring the set acknowledge prior contributions of acceptable
   or preferred Objective Functions (OF) referenced by their Objective
   Codepoints (OCPs) for a node
   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 join a DAG, 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 what action should be
   taken if none 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 node's candidate neighbors advertise one Networking Primitive in Wireless
              Sensor Networks", Communications of the
   configured allowable Objective Functions.

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

   A RPL implementation SHOULD allow 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 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 of the candidate neighbor
   list in order IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4875]  Aggarwal, R., Papadimitriou, D., and S. Yasukawa,
              "Extensions to bound the memory usage, in which case some otherwise
   viable candidate neighbors may not be considered 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 simply dropped
   from the candidate neighbor list.

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

7.2.  Information P.
              Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
              RFC 4915, June 2007.

   [RFC5120]  Przygienda, T., Shen, N., and Data Models

   The information 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 data models necessary D. Barthel,
              "Routing Requirements for the operation of RPL
   will be defined in a separate document specifying the RPL SNMP MIB.

7.3.  Liveness Detection Urban Low-Power and Monitoring

   The aim of this section is to describe the various Lossy
              Networks", RFC 5548, May 2009.

Appendix A.  Deferred Requirements

   NOTE: RPL mechanisms
   specified to monitor the protocol.

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

   o  The candidate neighbors data structure

   o  For each DAG:

      *  A set of candidate DAG parents

      *  A set of DAG parents (which progress.  At this time there remain
   several unsatisfied application requirements, but these are a subset of candidate DAG
         parents and may to be implemented
   addressed as such)

7.3.1.  Candidate Neighbor Data Structure

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

   A RPL implementation MAY provide a counter reporting the number of
   times a candidate neighbor has been ignored, should is further specified.

Appendix B.  Examples

   Consider the number of
   candidate neighbors exceeds example LLN physical topology in Figure 8.  In this
   example the maximum authorized value.

7.3.2.  Directed Acyclic Graph (DAG) Table

   For each DAG, a RPL implementation MUST keep track of links depicted are all usable L2 links.  Suppose that all
   links are equally usable, and that the following
   DAG table values:

   o  DAGID

   o  DAGObjectiveCodePoint

   o  A set of Destination Prefixes offered inwards along implementation specific policy
   function is simply to minimize hops.  This LLN physical topology then
   yields the DAG

   o  A set of candidate DAG Parents

   o  timer to govern depicted in Figure 9, where the sending of RA-DIO messages for links depicted are the
   edges toward DAG

   o  DAGSequenceNumber parents.  This topology includes one DAG, rooted by
   an LBR node (LBR) at rank 1.  The set of candidate DAG parents structure is itself LBR node will issue RAs containing
   DIO, as governed by a table with the
   following entries:

   o  A reference trickle timer.  Nodes (11), (12), (13), have
   selected (LBR) as their only parent, attached to the neighboring device which is the DAG parent

   o  A record of most recent information taken from the DAG Information
      Object last processed from the candidate at rank 2,
   and periodically advertise RA-DIO multicasts.  Node (22) has selected
   (11) and (12) in its DAG Parent

   o parent set, and advertises itself at rank 3.
   Node (22) thus has a state associated with the role 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 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  A Held-Down Timer, if instantiated

   o  A flag reporting if links depicted represent the Parent is a DA Parent as described usable L2 connectivity
   available in
      Section 5.9

7.3.3.  Routing Table

   To be completed.

7.3.4.  Other RPL Monitoring Parameters

   A RPL implementation SHOULD provide a counter reporting the number of
   a times the node has detected an inconsistency LLN.  For example, Node (31) can communicate
   directly with respect to a DAG
   parent, its neighbors, Nodes (21), (22), (32), and (41).  Node
   (31) cannot communicate directly with any other nodes, e.g. if the DAGID has changed.

   A RPL implementation MAY log (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 reception of a malformed RA-DIO
   message along with links depicted represent directed links in the neighbor identification if avialable.

7.3.5.  RPL Trickle Timers

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

   o  The DIOIntervalMin expressed physical topology depicted in ms

   o  The DIOIntervalDoublings

   A RPL implementation MAY provide a counter reporting Figure 8.  As
   such, the number of
   times an inconsistency (and thus depicted edges represent the trickle timer has been reset).

7.4.  Verifying Correct Operation

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

7.5.  Requirements on Other Protocols
   their DAG parents, wherein all depicted edges are directed and Functional Components

   RPL does not have any impact
   oriented `up' on the operation of existing protocols.

7.6.  Impact on Network Operation

   To be completed.

8.  Security Considerations

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

9.  IANA Considerations

9.1. page toward the DAG Information Option (DIO) Base Option root (LBR).  The DAG Information Option is a container option carried may
   provide default routes within an
   IPv6 Router Advertisement message the LLN, and serves as defined in [RFC4861], the foundation
   on which
   might contain RPL builds further routing structure, e.g. through the
   destination advertisement mechanism.

                           Figure 9: Example DAG

B.1.  Moving Down a number DAG

   Consider node (56) in the example of suboptions.  The base option regroups Figure 8.  In the
   minimum information set that unmodified
   example, node (56) is mandatory in all cases.

   IANA had defined the IPv6 Neighbor Discovery Option Formats registry.
   The suggested type value 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 Information Option (DIO) Base
   Option with {(43), (55)} as it's DAG parents.  When
   node (56) detaches from the DAG, it is 140, able to be confirmed by IANA.

9.2.  New Registry for act as the Flag Field 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 DIO Base Option

   IANA
   original DAG, that its sequence number is requested able to create a registry increment, and
   deduce that Node (55) is safely not behind Node (56).  There is then
   little change for the Flag field of the DIO
   Base Option.

   New bit numbers a loop, and Node (56) may be allocated only by an IETF Consensus action.
   Each bit should be tracked with safely reattach to the following qualities:

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

   o  Capability description example of Figure 8 when link (13)-(24) goes down.

   o  Defining RFC

   Three flags are currently defined:

       +-----+-------------------------------------+---------------+
       | Bit | Description                         | Reference     |
       +-----+-------------------------------------+---------------+
       |  0  | Grounded  Node (24) will detach and become the root of its own floating DAG                        | This document |
       |  1  | Destination Advertisement Trigger   | This document |
       |  2  | Destination Advertisement Supported | This document |
       +-----+-------------------------------------+---------------+

                           DIO Base Option Flags

9.3.

   o  Node (34) will learn that its DAG Information Option (DIO) Suboption

   IANA parent is requested to create now part of its own
      floating DAG, will consider that it can remain a registry for part of 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 |
         +-------+------------------------------+---------------+
      rooted at node (LBR) via node (33), and will initiate procedures
      to detach from DAG Information Option (DIO) Base Option Suboptions

9.4.  Destination Advertisement Option (DAO) Option

   The RPL protocol extends Neighbor Discovery [RFC4861] (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 [RFC4191] 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 allow a node attach closer to include a Destination Advertisement Option, which
   includes prefix information in the Neighbor Advertisements messages.
   The 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 root of the Destination Advertisement Option
   carried within DAG.

   o  Node (45) will cancel plans to detach/reattach, keep node (34) as
      a Neighbor Advertisement message is 141, DAG parent, and update its dependent rank accordingly.

   o  Node (45) may now anyway add node (44) to be
   confirmed by IANA.

9.5.  Objective Code Point

   This specification requests that an Objective Code Point registry, 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 be specified in [I-D.ietf-roll-routing-metrics], reserve the
   Objective Code Point value 0x0000, for the purposes designated DAG rooted at
      node (LBR) by selecting node (34) as OCP
   0 its DAG parent, thus
      reversing the relationship that existed in this document.

10.  Acknowledgements

   The ROLL Design Team would like to acknowledge 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 to acknowledge initial state.

B.3.  Link Added

   Consider the guidance and input
   provided by example of Figure 8 when link (12)-(42) appears.

   o  Node (42) will see a chance to get closer to the ROLL Chairs, David Culler and JP Vasseur.

   The ROLL Design Team would like LBR by adding
      (12) to acknowledge prior contributions its set 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 DAG parents, {(32), (12)}

   o  Node (42) may be content 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 leave its advertised rank at 5,
      reflecting 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 a reason for use in RFCs Node (42) to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

12.2.  Informative References

   [I-D.ietf-bfd-base]
              Katz, D. evict
      Node (32) from its set 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 D. Ward, "Bidirectional Forwarding
              Detection", draft-ietf-bfd-base-09 (work in progress),
              February 2009.

   [I-D.ietf-manet-nhdp]
              Clausen, T., Dearlove, C., advertise
      themselves at rank 3.

B.4.  Node Removed

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

   o  Node (51) 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., (52) will now have empty DAG parent sets and W. Vermeylen,
              "Building Automation Routing Requirements be
      detached from the DAG rooted by (LBR), advertising themselves as
      the root of their own floating DAGs.

   o  Node (52) would observe a chance to reattach to the DAG rooted at
      (LBR) by adding Node (53) to its set of DAG parents, after an
      appropriate delay to avoid creating loops.  Node (52) will then
      advertise itself in Low Power the DAG rooted at (LBR) at rank 7.

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

B.5.  New LBR Added

   Consider the example of Figure 8 when a new LBR, (LBR2) appears, with
   connectivity (LBR2)-(52), (LBR2)-(53).

   o  Nodes (52) and Node (53) will see a chance to join 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 of forming loops when
      joining a DAG that has never before been encountered.  Note that
      the nodes may choose to join the new DAG rooted at (LBR2) if and G. Porcu, "Home Automation
              Routing Requirements
      only if (LBR2) offers more optimum properties in Low Power line with the
      implementation specific local policy.

   o  Nodes (52) and Lossy Networks",
              draft-ietf-roll-home-routing-reqs-08 (work (53) begin to send RA-DIO messages advertising
      themselves at rank 2 in progress),
              September 2009.

   [I-D.ietf-roll-indus-routing-reqs]
              Networks, D., Thubert, P., Dwars, S., the DAGID (LBR2).

   o  Nodes (51), (41), (42), and T. Phinney,
              "Industrial Routing Requirements (54) may then choose to join the new
      DAG at rank 3, possibly to get closer to the DAG root.  Note that
      in Low Power and Lossy
              Networks", draft-ietf-roll-indus-routing-reqs-06 (work a more advanced case, these nodes also remain members of the
      DAG rooted at (LBR), for example in
              progress), June 2009.

   [I-D.ietf-roll-routing-metrics]
              Vasseur, J. and D. Networks, "Routing Metrics used support of different
      constraints for
              Path Calculation different types of traffic.

   o  Node (55) may then join the new DAG at rank 4, possibly to get
      closer to the DAG root.

   o  The remaining nodes may choose to remain in Low Power their current
      positions within the DAG rooted at node (LBR), since there is no
      clear advantage to be gained by moving to DAG (LBR2).

B.6.  Destination Advertisement

   Consider the example DAG depicted in Figure 9.  Suppose that Nodes
   (22) and (32) are unable to record routing state.  Suppose that Node
   (42) is able to perform prefix aggregation on behalf of Nodes (53),
   (54), and (55).

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

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

   o  Node (42) would collect and A.
              Lozano, "A Security Framework store the routing state 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.,
      destinations (53), (54), and A.
              Woo, "The Emergence (55).

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

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

   o  Node (32) does not want to maintain any routing state, so it adds
      onto to the Reverse Route Stack in 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. NA-DAO message and IAB, "Writing Protocol Models", RFC 4101,
              June 2005.

   [RFC4191]  Draves, R. 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 the Reverse Route Stack in the NA-DAO message and D. Thaler, "Default Router Preferences passes it
      on to Node (12) as (42'):[(42), (32)].  It also relays the NA-DAO
      message containing destination (32) to Node 12 as (32):[(32)], and
              More-Specific Routes", RFC 4191, November 2005.

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

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., itself indicating
      destination (22).

   o  Node (12) is capable to maintain routing state again, and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC4875]  Aggarwal, R., Papadimitriou, D., 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) and S. Yasukawa,
              "Extensions the piecewise
         source route 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., (32)
      *  Destination (42') is available via Node (22) and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System the piecewise
         source route to
              Intermediate Systems (IS-ISs)", RFC 5120, February 2008.

   [RFC5548]  Dohler, M., Watteyne, T., Winter, T., (32), (42').

   o  Node (12) sends NA-DAO messages to (LBR), allowing (LBR) to learn
      routes to the destinations (12), (22), (32), and D. Barthel,
              "Routing Requirements for Urban Low-Power (42'). (42),
      (53), (54), and Lossy
              Networks", RFC 5548, May 2009.

Appendix A.  Deferred Requirements

   NOTE: RPL (55) are available via the aggregation (42').  It
      is still not necessary for Node (12) to propagate the piecewise source
      routes to (LBR).

B.7.  Example: DAG Parent Selection

   For example, suppose that a work in progress.  At this time there remain
   several unsatisfied application requirements, but these are node (N) is not attached to be
   addressed as RPL any DAG, and
   that it is further specified.

Appendix B.  Examples

   Consider the example LLN physical topology in Figure 11.  In this
   example the links depicted are range of nodes (A), (B), (C), (D), and (E).  Let all usable L2 links.  Suppose
   nodes be configured to use an OCP which defines a policy such that all
   links are equally usable,
   ETX is to be minimized and that paths with the implementation specific policy
   function is attribute `Blue' should be
   avoided.  Let the rank computation indicated by the OCP simply to minimize hops.  This LLN physical topology then
   yields
   reflect the DAG depicted in Figure 12, where ETX aggregated along the links depicted are path.  Let the edges toward DAG parents.  This topology includes one DAG, rooted
   by links between
   node (N) and its neighbors (A-E) all have an LBR ETX of 1 (which is
   learned by node (LBR) at rank 1.  The LBR (N) through some implementation specific method).
   Let node will issue RAs
   containing DIO, as governed by a trickle timer.  Nodes (11), (12),
   (13), have selected (LBR) as their only parent, attached (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 the DAG RA-DIO message, and
      learns that Node (B) is a member of DAGID 1 at rank 2, 4, and periodically advertise RA-DIO multicasts. not
      `Blue'.  Node (22)
   has selected (11) and (12) in its DAG parent set, (N) takes note of this, but is not yet confident.

   o  Similarly, Node (N) hears from Node (A) at rank 9, Node (C) at
      rank 5, and advertises
   itself Node (E) at rank 3. 4.

   o  Node (22) thus (D) responds.  Node (D) has a set RA-DIO message that indicates
      that it is a member of DAG parents {(11),
   (12)} DAGID 1 at rank 2, but it carries the
      attribute `Blue'.  Node (N)'s policy function rejects Node (D),
      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 no further consideration is given.

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

   o  Node (N) adds Node (E) (rank 4) to its set of DAG parents for
      DAGID 1.  Following the mechanisms specified by the OCP, and given
      that the links depicted represent ETX is 1 for the usable L2 connectivity
   available link between (N) and (E), Node (N) is
      now at rank 5 in the LLN.  For example, DAGID 1.

   o  Node (31) can communicate
   directly with (N) adds Node (B) (rank 4) to its neighbors, Nodes (21), (22), (32), and (41). set of DAG parents for
      DAGID 1.

   o  Node
   (31) cannot communicate directly with any other nodes, e.g. (33),
   (23), (42). (N) is a sibling of Node (C), both are at rank 5.

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

                      Figure 11: Example LLN Topology
                                     (LBR)
                                     / | \
                                .---`  |  `----.
                               /       |        \
                            (11)      (12)      (13)
                             | \       | \       | \
                             |  `----. |  `----. |  `----.
                             |        \|        \|        \
                            (21)      (22)      (23)      (24)
                             |        /|        /|         |
                             |  .----` |  .----` |         |
      have a viable parent, it should never send the packet back to Node
      (N) (to avoid a 2-node loop).

B.8.  Example: DAG Maintenance

          :                      :                      :
          :                      :                      :
         (A)                    (A)                    (A)
          |\                     | /                      | /
          | `-----.              |
                            (31)      (32)      (33)      (34)                      |        /| \
          |        \             | \                      |  .----`
         (B)       (C)          (B)       (C)          (B)
                    |  `----.                      |  `----.             \
                    |  `----.                      | /              `-----.
                    |        \|        \|        \
                   .--------(41)      (42)      (43)      (44)      (45)
                  /         /         /| \                      |                     \
            .----`    .----`    .----`
                   (D)                    (D)                    (C)
                                                                  |  `----.
                                                                  |  `----.
           /         /         /
                                                                  |        \|        \
        (51)      (52)      (53)      (54)      (55)      (56)

   Note that the links depicted represent directed links in the
                                                                 (D)

              -1-                    -2-                    -3-

                        Figure 10: DAG
   overlaid on top of Maintenance

   Consider the physical topology example depicted in Figure 11.  As
   such, the depicted edges represent the relationship between nodes and
   their 10-1.  In this example, Node
   (A) is attached to a DAG parents, wherein all depicted edges are directed and
   oriented `up' on the page toward the at some rank d.  Node (A) is a DAG root (LBR).  The parent of
   Nodes (B) and (C).  Node (C) is a DAG parent of Node (D).  There is
   also an undirected sibling link between Nodes (B) and (C).

   In this example, Node (C) may
   provide default routes safely forward to Node (A) without
   creating a loop.  Node (C) may not safely forward to Node (D),
   contained within the LLN, and serves as the foundation
   on which RPL builds further routing structure, it's own sub-DAG, without creating a loop.  Node (C)
   may forward to Node (B) in some cases, e.g. through the
   destination advertisement mechanism.

                          Figure 12: Example DAG

B.1.  Moving Down link (C)->(A) is
   temporarily unavailable, but with some chance of creating a DAG

   Consider node (56) loop
   (e.g. if multiple nodes in a set of siblings start forwarding
   `sideways' in a cycle) and requiring the intervention of additional
   mechanisms to detect and break the example of Figure 11.  In loop.

   Consider the unmodified
   example, node (56) is case where Node (C) hears a RA-DIO message from a Node
   (Z) at a lesser rank 6 with one DAG parent, {(43)}, and one
   sibling (55).  Suppose, for example, that superior position in the DAG than node (56) wished (A).
   Node (C) may safely undergo the process to expand evict node (A) from its
   DAG parent set and attach directly to contain node (55), as {(43), (55)}.  Such Node (Z) without creating a
   change would require
   loop, because its rank will decrease.

   Now consider the case where the link (C)->(A) becomes nonviable, and
   node (56) (C) must move to a deeper rank within the DAG:

   o  Node (C) must first 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 by removing Node (A) from
      its DAG parents.  When
   node (56) detaches parent set, leaving an empty DAG parent set.  Node (C)
      becomes the root 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 is depicted in Figure 10-2.
      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 RA-DIO message from Node (B) and determines it is
      able to act as rejoin the root of its
   own floating 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 and establish its frozen sub-DAG (which is empty). Node (56) can then observe that (C) adds Node (55) is still attached (B) to the
   original DAG, that its sequence number is able to increment, and
   deduce that DAG parent
      set.  Node (55) is (C) has now safely not behind moved deeper within the grounded DAG
      without creating any loops.  Node (56).  There is then
   little change for a loop, (D), and any other sub-DAG of
      Node (56) may safely (C), will hear the modified RA-DIO message sourced from Node
      (C) and follow Node (C) in a coordinated manner to 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
      grounded DAG.  The final DAG
   parent (node (55) at rank 6), rank 7.

B.2.  Link Removed is depicted in Figure 10-3

B.9.  Example: Greedy Parent Selection and Instability

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

              -1-                    -2-                    -3-

                  Figure 11: Greedy DAG Parent Selection

   Consider the example of depicted in Figure 11 when 11.  A DAG is depicted in 3
   different configurations.  A usable link (13)-(24) goes down.

   o  Node (24) will detach between (B) and become the root of its own floating (C) exists
   in all 3 configurations.  In Figure 11-1, Node (A) is a DAG

   o parent
   for Nodes (B) and (C), and (B)--(C) is a sibling link.  In
   Figure 11-2, Node (34) will learn that its (A) is a DAG parent for Nodes (B) and (C), and Node
   (B) is also a DAG parent for Node (C).  In Figure 11-3, Node (A) 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), parent for Nodes (B) and (C), 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 (C) is also a DAG rooted at (LBR) by detaching from parent
   for Node (34) and re-
      attaching at (B).

   If a lower rank to RPL node (44).

   o  Node (34) will complete re-attachment to Node (33) first, since it is able to attach closer too greedy, in that it attempts to the root optimize for an
   additional number of parents beyond its preferred parent, then an
   instability can result.  Consider the DAG.

   o DAG illustrated in Figure 11-1.
   In this example, Nodes (B) and (C) may most prefer Node (45) will cancel plans to detach/reattach, keep node (34) (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 update its dependent rank accordingly.

   o  Node (45) may now anyway add no siblings, the node (44) may decide to its set of DAG parents,
      as such insert itself at a
   slightly higher rank in order to have at least one sibling and thus
   an addition alternate forwarding solution.  This does not require any modification to its own
      rank. deprive other nodes
   of a forwarding solution and this is considered acceptable
   greediness.

   o  Let Figure 11-1 be the initial condition.

   o  Suppose Node (24) will observe that it may reattach (C) first is able to leave the DAG rooted and rejoin at
      node (LBR) by selecting node (34) a
      lower rank, taking both Nodes (A) and (B) as its DAG parent, thus
      reversing the relationship that existed parents as
      depicted in the initial state.

B.3.  Link Added

   Consider the example of Figure 11 when link (12)-(42) appears.

   o 11-2.  Now Node (42) will see a chance to get closer to the LBR by adding
      (12) (C) is deeper than both Nodes
      (A) and (B), and Node (C) is satisfied to its set of have 2 DAG parents, {(32), (12)} parents.

   o  Suppose Node (42) may be content to leave (B), in its advertised rank greediness, is willing to receive and
      process a RA-DIO message from Node (C) (against the rules of RPL),
      and then Node (B) leaves the DAG and rejoins at 5,
      reflecting a rank lower rank,
      taking both Nodes (A) and (C) as DAG parents.  Now Node (B) is
      deeper than its deepest parent (32). both Nodes (A) and (C) and is satisfied with 2 DAG
      parents.

   o  Then Node (42) may now choose to remain where (C), because it is, with two is also greedy, will leave and rejoin
      deeper, to again get 2 parents
      {(12), (32)}.  Should there be and have a reason for Node (42) to evict
      Node (32) from its set lower rank then both of DAG parents,
      them.

   o  Next Node (42) would then
      advertise itself at rank 2, thus moving up the DAG.  In this case, (B) will again leave and rejoin deeper, to again get 2
      parents

   o  And again Node (53), (54), (C) leaves and (55) may similarly follow rejoins deeper...

   o  The process will repeat, and advertise
      themselves at rank 3.

B.4.  Node Removed

   Consider the example of DAG will oscillate between
      Figure 11 when node (41) disappears. 11-2 and Figure 11-3 until the nodes count to infinity and
      restart the cycle again.

   o  Node (51)  This cycle can be averted through mechanisms in RPL:

      *  Nodes (B) and (52) will now have empty DAG (C) stay at a rank sufficient to attach to their
         most preferred parent sets (A) and be
      detached don't go for any deeper (worse)
         alternate parents (Nodes are not greedy)

      *  Nodes (B) and (C) do not process RA-DIO messages from the DAG rooted by (LBR), advertising nodes
         deeper than themselves as
      the root of (because such nodes are possibly in
         their own floating DAGs.

   o sub-DAGs)

B.10.  Example: DAG Merge

                                :
                                :
                               (A)       (D)
                                |         |
                                |         |
                                |         |
                               (B)       (E)
                                |         |
                                |         |
                                |         |
                               (C)       (F)

                          Figure 12: Merging DAGs

   Consider the example depicted in Figure 12.  Nodes (A), (B), and (C)
   are part of some larger grounded DAG, where Node (52) would observe (A) is at a chance to reattach to the DAG rooted rank of
   d, Node (B) at
      (LBR) by adding d+1, and Node (53) to its set of (C) at d+2.  The DAG parents, after an
      appropriate delay to avoid creating loops. comprised of Nodes
   (D), (E), and (F) is a floating, less preferred, DAG, with Node (52) will then
      advertise itself in (D)
   as the DAG rooted at (LBR) at rank 7.

   o  Node (51) will then be able to reattach to root.  This floating DAG may have been formed, for
   example, in the absence of a grounded DAG rooted at (LBR)
      by adding or when Node (52) (D) had to its set of
   detach from a grounded DAG parents and advertising
      itself at rank 8.

B.5.  New LBR Added

   Consider (E) and (F) followed.  All nodes are
   using compatible objective code points.

   Nodes (D), (E), and (F) would rather join the example 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 Figure 11 when events may then occur in a new LBR, (LBR2) appears,
   with connectivity (LBR2)-(52), (LBR2)-(53). typical case:

   o  Nodes (52)  Node (D) will receive and process a RA-DIO message from Node (53) (C)
      on link (C)--(D).  Node (D) will see consider Node (C) a chance candidate
      neighbor and process the RA-DIO message since Node (C) belongs to join
      a new different DAG
      rooted at (LBR2) with a rank of 2. (different DAGID) than Node (52) and (53) may take
      this chance immediately, as there (D).  Node (D) will
      note that Node (C) is no risk of forming loops when
      joining in a grounded DAG that has never before been encountered.  Note that at rank d+2, and will
      begin the nodes may choose process to join the new grounded DAG rooted at (LBR2) if and
      only if (LBR2) offers more optimum properties in line 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 to join d+3.  Node (D)
      will start a DAG Hop timer, logically associated with the new grounded
      DAG at rank 3, possibly to get closer Node (C), to coordinate the jump.  The DAG root.  Note Hop timer will
      have a duration proportional to d+2.

   o  Similarly, Node (E) will receive and process a RA-DIO message from
      Node (A) on link (A)--(E).  Node (E) will consider Node (A) a
      candidate neighbor, will note that Node (A) is in a more advanced case, these nodes also remain members of the grounded DAG rooted
      at (LBR), for example in support of different
      constraints for different types of traffic.

   o  Node (55) may then rank d, and will begin the process to join the new grounded DAG at
      rank 4, possibly to get
      closer to the d+1.  Node (E) will start a DAG root.

   o  The remaining nodes may choose to remain in their current
      positions within Hop timer, logically
      associated with the grounded DAG rooted at node (LBR), since there is no
      clear advantage to be gained by moving Node (A), to DAG (LBR2).

B.6.  Destination Advertisement

   Consider coordinate the example
      jump.  The DAG depicted in Figure 12.  Suppose that Nodes
   (22) and (32) are unable to record routing state.  Suppose that Node
   (42) is able Hop timer will have a duration proportional to perform prefix aggregation on behalf of Nodes (53),
   (54), and (55). d.

   o  Node (53) would send a NA-DAO message to (F) takes no action, for Node (42), indicating the
      availability of destination (53). (F) has observed nothing new to
      act on.

   o  Node (54) and (E)'s DAG Hop timer for the grounded DAG at Node (55) would similarly send NA-DAO messages to (A) expires
      first.  Node (42) indicating their own destinations.

   o (E), upon the DAG Hop timer expiry, removes Node (42) would collect and store (D)
      as its parent, thus emptying the routing state DAG parent set for
      destinations (53), (54), the floating
      DAG, and (55).

   o  In this example, leaving the floating DAG.  Node (42) may (E) then be capable jumps to the
      grounded DAG by entering Node (A) into the set of representing
      destinations (42), (53), (54), and (55) DAG parents for
      the grounded DAG.  Node (E) is now in the aggregation (42'). grounded DAG at rank
      d+1.  Node (E), by jumping into the grounded DAG, has created an
      inconsistency by changing its DAGID, and will begin to emit RA-DIO
      messages more frequently.

   o  Node (42) sends (F) will receive and process a NA-DAO RA-DIO message advertising destination (42') to from Node 32.

   o (E).
      Node (32) does not want to maintain any routing state, so it adds
      onto to the Reverse Route Stack in the NA-DAO message (F) will observe that Node (E) has changed its DAGID and passes
      it on to will
      directly follow Node (22) as (42'):[(42)].  It may send a separate NA-DAO
      message to indicate destination (32).

   o (E) into the grounded DAG.  Node (22) does not want to maintain (F) is now a
      member of the grounded DAG at rank d+2.  Note that any routing state, so it adds
      on additional
      sub-DAG of Node (E) would continue to join into the Reverse Route Stack grounded DAG
      in the NA-DAO message and passes it
      on to Node (12) as (42'):[(42), (32)].  It also relays the NA-DAO
      message containing destination (32) to this coordinated manner.

   o  Node 12 as (32):[(32)], and
      finally may send (D) will receive a NA-DAO RA-DIO message for itself indicating
      destination (22).

   o from Node (12) (E).  Since Node
      (E) is capable to maintain routing state again, and receives now in a different DAG, Node (D) may process the NA-DAO messages RA-DIO
      message from Node (22).  Node (12) then learns:
      *  Destination (22) is available via (E).  Node (22)
      *  Destination (32) is available (D) will observe that, via node (E),
      it could attach to the grounded DAG at rank d+2.  Node (22) and (D) will
      start another DAG Hop timer, logically associated with the piecewise
         source route
      grounded DAG at Node (E), with a duration proportional to (32)
      *  Destination (42') is available via d+1.
      Node (22) (D) now is running two DAG hop timers, one which was started
      with duration proportional to d+1 and the piecewise
         source route one proportional to (32), (42'). d+2.

   o  Generally, Node (12) sends NA-DAO messages to (LBR), allowing (LBR) to learn
      routes (D) will expire the timer associated with the jump
      to the destinations (12), (22), (32), and (42'). (42),
      (53), (54), and (55) are available via grounded DAG at node (E) first.  Node (D) may then jump to
      the aggregation (42').  It
      is not necessary grounded DAG by entering Node (E) into its DAG parent set for
      the grounded DAG.  Node (12) to propagate (D) is now in the piecewise source
      routes to (LBR). 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 a generally ordered
      manner, avoiding the formation of loops in the process.

Appendix C.  Additional Examples

   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 the number of hops, and that both LBRs are configured to
   root different DAGIDs.  We may now walk through the formation of 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)
                    Figure 15: 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: DAG Construction Step 3
                                     (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 17: 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)
                    Figure 18: DAG Construction Step 5

Appendix D.  Outstanding Issues

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

D.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.  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.  Destination Advertisement / DAO Fan-out

   When NA-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
   messages conveying information about the same destination flow inward
   along the DAG.  It is desirable to bound/limit the multiplication/
   fan-out of NA-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.  Source Routing

   In support of nodes who 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.  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