Networking Working Group                                  T. Winter, Ed.
Internet-Draft
Intended status: Standards Track                        ROLL Design Team                         P. Thubert, Ed.
Expires: March 18, 27, 2010                                    Cisco Systems
                                                        ROLL Design Team
                                                            IETF ROLL WG
                                                      September 14, 23, 2009

         RPL: Routing Protocol for Low Power and Lossy Networks
                         draft-ietf-roll-rpl-01
                         draft-ietf-roll-rpl-02

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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), in accordance with the requirements described in

   [I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs],
   [I-D.ietf-roll-indus-routing-reqs], and [RFC5548]. (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 . . . . . . . . . . . . . . . . . . . . . . . . .  4  6
     1.1.  Design Principles  . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . .  6
     1.2.  Expectations of Link Layer Behavior  . . . . . . . . . . .  4
   3.  Protocol Model  7
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Problem .  7
   3.  Protocol Model . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  9
     3.1.  Protocol Properties Overview . . . . . . . . . . . . . . .  7
       3.2.1.  9
       3.1.1.  IPv6 Architecture  . . . . . . . . . . . . . . . . . .  7
       3.2.2.  Path Properties for  9
       3.1.2.  Typical LLN Traffic Flows  . . . . . . . .  7
       3.2.3.  Constraint Based Routing . . Patterns . . . . . . . . . . . . .  8
       3.2.4.  Autonomous Operation . . 10
       3.1.3.  Constraint Based Routing . . . . . . . . . . . . . . .  8
     3.3. 10
     3.2.  Protocol Operation . . . . . . . . . . . . . . . . . . . .  8
       3.3.1. 10
       3.2.1.  DAG Construction . . . . . . . . . . . . . . . . . . .  9
       3.3.2.  Source Routing . . . . . . . . . . . . . . . . . . . . 19
       3.3.3. 11
       3.2.2.  Destination Advertisement  . . . . . . . . . . . . . . 19
     3.4. 21
     3.3.  Other Considerations . . . . . . . . . . . . . . . . . . . 21
       3.4.1. 23
       3.3.1.  DAG Rank and Loop Avoidance  . . . . . . . . . . . . . 21
       3.4.2. 23
       3.3.2.  DAG Parent Selection, Stability, and Greediness  . . . 25
       3.4.3. 27
       3.3.3.  Merging DAGs . . . . . . . . . . . . . . . . . . . . . 27
       3.4.4. 29
     3.4.  Local and Temporary Routing Decision . . . . . . . . . 29
       3.4.5.  Scalability . . 32
     3.5.  Maintenance of Routing Adjacency . . . . . . . . . . . . . . . . . . . 30
       3.4.6.  Maintenance of Routing Adjacency . . . . . . . . . . . 30 32
   4.  Constraint Based Routing in LLNs . . . . . . . . . . . . . . . 30 33
     4.1.  Routing Metrics  . . . . . . . . . . . . . . . . . . . . . 30 33
     4.2.  Routing Constraints  . . . . . . . . . . . . . . . . . . . 32 34
     4.3.  Constraint Based Routing . . . . . . . . . . . . . . . . . 32 34
   5.  Specification of Core  RPL Protocol Specification . . . . . . . . . . . . . . . . 32 . . 35
     5.1.  DAG Information Option . . . . . . . . . . . . . . . . . . 33 35
       5.1.1.  DIO  DAG Information Option (DIO) base option . . . . . . . . . . . . . . . . . . . 33 35
     5.2.  Conceptual Data Structures . . . . . . . . . . . . . . . . 39 42
       5.2.1.  Candidate Neighbors Data Structure . . . . . . . . . . . . . . . . . 39 42
       5.2.2.  DAGs . . . . . . . . . . . . . . . . . . . . .  Directed Acyclic Graphs (DAGs) Data Structure  . . . . 40 43
     5.3.  Initialization  DAG Discovery and Configuration Maintenance  . . . . . . . . . . . . . 41
     5.4. . 44
       5.3.1.  DAG Discovery Rules  . . . . . . . . . . . . . . . . . . . . . . 42
       5.4.1.  RA-DIO 45
       5.3.2.  Reception and Processing of RA-DIO messages  . . . . . . . . . . . . . . . . . . . 45
       5.4.2. 47
       5.3.3.  RA-DIO Transmission  . . . . . . . . . . . . . . . . . 47
       5.4.3. 49
       5.3.4.  Trickle Timer for RA Transmission  . . . . . . . . . . 48
     5.5. 50
     5.4.  DAG Heartbeat  . . . . . . . . . . . . . . . . . . . . . . 49
     5.6. 52
     5.5.  DAG Selection  . . . . . . . . . . . . . . . . . . . . . . 50
     5.7. 52
     5.6.  Administrative rank  . . . . . . . . . . . . . . . . . . . 50
     5.8. 53
     5.7.  Candidate DAG Parent States and Stability  . . . . . . . . 51
       5.8.1. 53
       5.7.1.  Held-Up  . . . . . . . . . . . . . . . . . . . . . . . 51
       5.8.2. 53
       5.7.2.  Held-Down  . . . . . . . . . . . . . . . . . . . . . . 52
       5.8.3. 54
       5.7.3.  Collision  . . . . . . . . . . . . . . . . . . . . . . 52
       5.8.4. 54
       5.7.4.  Instability  . . . . . . . . . . . . . . . . . . . . . 53
     5.9. 55
     5.8.  Guidelines for Objective Code Points . . . . . . . . . . . 53
       5.9.1. 56
       5.8.1.  Objective Function . . . . . . . . . . . . . . . . . . 53
       5.9.2. 56
       5.8.2.  Objective Code Point 0 (OCP 0) . . . . . . . . . . . . 55
     5.10. 58
     5.9.  Establishing Routing State Outward Along the DAG . . . . . 57
       5.10.1. 60
       5.9.1.  Destination Advertisement Message Formats  . . . . . . 58
       5.10.2. 61
       5.9.2.  Destination Advertisement Operation  . . . . . . . . . 60 63
     5.10. Multicast Operation  . . . . . . . . . . . . . . . . . . . 70
     5.11. Maintenance of Routing Adjacency . . . . . . . . . . . . . 67 71
     5.12. Packet Forwarding  . . . . . . . . . . . . . . . . . . . . 67 72
       5.12.1. Loop Taxonomy  . . . . . . . . . . . . . . . . . . . . 68
     5.13. Expectations of Link Layer Behavior 73
   6.  RPL Variables  . . . . . . . . . . . 70
   6.  Summary of RPL Timers . . . . . . . . . . . . . 74
   7.  Manageability Considerations . . . . . . . 70
   7.  Protocol Extensions . . . . . . . . . . 75
     7.1.  Control of Function and Policy . . . . . . . . . . . 71
   8.  Manageability Considerations . . . 75
       7.1.1.  Initialization Mode  . . . . . . . . . . . . . . 71
   9.  Security Considerations . . . 75
       7.1.2.  DIO Base option  . . . . . . . . . . . . . . . . 71
   10. IANA Considerations . . . 76
       7.1.3.  Trickle Timers . . . . . . . . . . . . . . . . . . 72
     10.1. DAG Information Option . . 77
       7.1.4.  DAG Heartbeat  . . . . . . . . . . . . . . . . 72
     10.2. Objective Code Point . . . . 77
       7.1.5.  The Destination Advertisement Option . . . . . . . . . 78
       7.1.6.  Policy Control . . . . . . 72
     10.3. Destination Advertisement Option . . . . . . . . . . . . . 72
   11. Acknowledgements . 78
       7.1.7.  Data Structures  . . . . . . . . . . . . . . . . . . . 78
     7.2.  Information and Data Models  . . . 72
   12. Contributors . . . . . . . . . . . . 78
     7.3.  Liveness Detection and Monitoring  . . . . . . . . . . . . 79
       7.3.1.  Candidate Neighbor Data Structure  . 72
   13. References . . . . . . . . . 79
       7.3.2.  Directed Acyclic Graph (DAG) Table . . . . . . . . . . 79
       7.3.3.  Routing Table  . . . . . . . 74
     13.1. Normative References . . . . . . . . . . . . . 80
       7.3.4.  Other RPL Monitoring Parameters  . . . . . . . . 74
     13.2. Informative References . . . 80
       7.3.5.  RPL Trickle Timers . . . . . . . . . . . . . . . 74
   Appendix A.  Deferred . . . 80
     7.4.  Verifying Correct Operation  . . . . . . . . . . . . . . . 80
     7.5.  Requirements on Other Protocols and Functional
           Components . . . . . . . . . . . . . . . . 76
   Appendix B.  Examples . . . . . . . . 81
     7.6.  Impact on Network Operation  . . . . . . . . . . . . . . 76
     B.1.  Moving Down a DAG . 81
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 78
     B.2.  Link Removed 81
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 81
     9.1.  DAG Information Option (DIO) Base Option . . . . . 79
     B.3.  Link Added . . . . 81
     9.2.  New Registry for the Flag Field of the DIO Base Option . . 81
     9.3.  DAG Information Option (DIO) Suboption . . . . . . . . . . 82
     9.4.  Destination Advertisement Option (DAO) Option  . . . . . . 82
     9.5.  Objective Code Point . . . . . . . . 79
     B.4.  Node Removed . . . . . . . . . . . 82
   10. Acknowledgements . . . . . . . . . . . . 80
     B.5.  New LBR Added . . . . . . . . . . . 83
   11. Contributors . . . . . . . . . . . 80
     B.6.  Destination Advertisement . . . . . . . . . . . . . . 83
   12. References . . . . 81
   Appendix C.  Additional Examples . . . . . . . . . . . . . . . . . 82
   Appendix D.  Outstanding Issues . . . . . 84
     12.1. Normative References . . . . . . . . . . . . 86
     D.1.  Additional Support for P2P Routing . . . . . . . 84
     12.2. Informative References . . . . . . 86
     D.2.  Loop Detection . . . . . . . . . . . . 84
   Appendix A.  Deferred Requirements . . . . . . . . . . . . . . . . 86
     D.3.  DAO Fan-out
   Appendix B.  Examples  . . . . . . . . . . . . . . . . . . . . . . 87
     B.1.  Moving Down a DAG  . 86
     D.4.  Source Routing . . . . . . . . . . . . . . . . . . . 88
     B.2.  Link Removed . . . 86
     D.5.  Address / Header Compression . . . . . . . . . . . . . . . 86
   Authors' Addresses . . . . . 89
     B.3.  Link Added . . . . . . . . . . . . . . . . . . . 87

1.  Introduction

   The defining characteristics of Low Power and Lossy Networks (LLNs)
   offer unique challenges to a routing solution.  The IETF ROLL . . . . . 89
     B.4.  Node Removed . . . . . . . . . . . . . . . . . . . . . . . 90
     B.5.  New LBR Added  . . . . . . . . . . . . . . . . . . . . . . 90
     B.6.  Destination Advertisement  . . . . . . . . . . . . . . . . 91
   Appendix C.  Additional Examples . . . . . . . . . . . . . . . . . 92
   Appendix D.  Outstanding Issues  . . . . . . . . . . . . . . . . . 96
     D.1.  Additional Support for P2P Routing . . . . . . . . . . . . 96
     D.2.  Loop Detection . . . . . . . . . . . . . . . . . . . . . . 96
     D.3.  Destination Advertisement / DAO Fan-out  . . . . . . . . . 96
     D.4.  Source Routing . . . . . . . . . . . . . . . . . . . . . . 96
     D.5.  Address / Header Compression . . . . . . . . . . . . . . . 97
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 97

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
   [I-D.ietf-roll-building-routing-reqs]
   [I-D.ietf-roll-home-routing-reqs] [I-D.ietf-roll-indus-routing-reqs] 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].  RPL is a new routing protocol designed to meet these
   requirements.  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:  Refers to the  The ability of a routing protocol to independently
         function without requiring relying on any external influence or guidance.
         Includes self-organization capabilities.

   DAG:  Directed Acyclic Graph- 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 LBR). Low Power and Lossy Network Border Router (LBR)).

   DAGID:  DAG Identifier- Identifier.  A globally unique identifier for a DAG.  All
         nodes who are members 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 maintenance while avoiding loops.

   DAG Parent: 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. parents.  If a node is a member
         of multiple DAGs then it must conceptually maintain a set of
         DAG Parents parents for each DAGID.

   DAG Sibling: 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. siblings.  If a node is a member of multiple DAGs
         then it must conceptually maintain a set of DAG Siblings siblings for
         each DAG.

   DAG Root: root:  A DAG root is a sink within the DAG graph. 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 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 default routing routes
         towards DAG roots is therefore a prominent functionality for
         RPL.

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

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

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

   Outward:  In the context of RPL, 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 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].  An architectural protocol overview (the big picture of
   the protocol) is provided in this section.  Protocol details can be found in further sections.

3.1.  Problem

   Some well-defined LLN application-specific scenarios are Building
   Automation, Home Automation, Industrial, and Urban; for which the
   unique routing requirements have been detailed respectively in
   [I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs],
   [I-D.ietf-roll-indus-routing-reqs], and [RFC5548].  Within these
   application-specific scenarios there are some common elements
   required of routing.  RPL intends to address the requirements of
   these application-specific scenarios, and it is further intended to
   be flexible enough to extend to other application scenarios.

3.2.  Protocol Properties Overview

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

3.2.1.

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 should be able to
   operate operates over lossy
   links (usually low bandwidth with low packet delivery success rate).

3.2.2.  Path Properties for

3.1.2.  Typical LLN Traffic Flows Patterns

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

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

3.2.3.

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.

3.2.4.  Autonomous Operation

   Nodes running RPL are able to independently and autonomously discover
   a network topology and compute and install routes, without requiring
   further administrative interaction.

3.3.

3.2.  Protocol Operation

   LLN nodes running RPL will construct Directed Acyclic Graphs (DAGs)
   rooted at designated nodes that generally have some application
   significance, such as providing a default route connectivity to an external routed
   infrastructure.  The term "external" is used top refer to the public
   Internet or a core private (non-LLN) IP network.  The DAG is
   sufficient to support inward MP2P traffic flows, flowing inward along
   the LLN towards a sink (DAG Root), root), which is one of the dominant
   traffic flows described in the requirements documents
   ([I-D.ietf-roll-building-routing-reqs],

   [I-D.ietf-roll-home-routing-reqs],
   [I-D.ietf-roll-indus-routing-reqs], and [RFC5548]).

   By utilizing a DAG for dominant MP2P flows, RPL allows each 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 as a tree, 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 will be described in more detail later in this
   specification.

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

   Arbitrary P2P traffic MAY may flow inward along the DAG until a common
   parent is reached who has stored an entry for the destination in its
   routing state table and is capable of directing the traffic outward along
   the correct outward path.  In 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.
   (Note that in some application scenarios it may be important to
   support arbitrary P2P traffic along more optimal paths `across' the
   DAG).  This functionality is to be investigated further in a future
   revision.

   This section further describes the high level operation more optimal routes into LLN nodes in support of RPL.

3.3.1.
   arbitrary P2P traffic according to some routing metric.

3.2.1.  DAG Construction

3.3.1.1.

3.2.1.1.  Overview of a Typical Case

   RPL constructs one or more base routing topologies, in the form of DAGs, over gradients defined by optimizing
   cost metrics along paths rooted at designated nodes.

   DAGs may be grounded, in which case the DAG Root root (e.g. an LBR) is
   offering a default route connectivity to an external routed infrastructure such as
   the Internet. public Internet or a private core (non-LLN) IP network.  A
   typical goal for a node participating in DAG
   Construction construction may be to
   find and join a grounded DAG.  Any DAG which is not grounded is
   floating, and default routes may still be provisioned toward the DAG root
   although with no expectations of reaching an external infrastructure.

   In the context of a particular LLN application one or more nodes will
   be capable of, e.g. serving as an LBR or acting as a data collection
   point, and thus be provisioned to act as the most preferred DAG
   roots.  These nodes will begin initiate and continue the process of
   constructing a DAG by occasionally emitting IPv6 Router Advertisements Advertisement
   (RA) messages containing the necessary information for neighboring
   nodes to evaluate the DAG Root root as a potential DAG parent.  This
   information will include at least a DAGID, a
   DAGPreference, an administrative
   preference, and an Objective Code Point (OCP).  The DAGID is an
   identifier unique to the DAG.  The DAGPreference administrative preference offers a
   way to engineer the formation of the DAG in support of the
   application, by providing a mechanism by which the DAG may look more
   or less attractive for other nodes to join.  The OCP provides
   information as to which metrics and optimization goals are being
   employed across the DAG.  Note that a
   single DAG Root may conceptually root different DAGs with different
   OCPs as required to support different sets of routing constraints.
   In this case the DAG Root must provision each different DAG with a
   different DAGID.  Note that if multiple nodes acting as DAG roots are
   rooting the same DAG, i.e. presenting the same DAGID, then they must
   have some means of coordinating with each other when emitting Router
   Advertisements (This may be the case, for example, when the DAG is
   provisioned with a `virtual root' through some backbone mechanism).
   This is described further below.

   Nodes who hear Router Advertisements, 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 the implementation specific routing constraints understood
   by the node.  In particular, a node will be seeking to find a least
   cost path satisfying some objective function as indicated by the OCP
   according to some routing metrics defined 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 minimum DAGPreference most desirable administrative preference when selecting a DAG,
   all else being equal.  Based on the evaluation of such criteria, a
   node may determine if the node who emitted the Router Advertisement RA message should be
   considered as a potential DAG parent.  If so, then the node may add
   the advertising node to its set of candidate DAG parents for the
   advertised DAGID, and after waiting for a designated delay, the node
   may follow the procedures to activate the advertising node as a DAG
   parent and may then be considered to have joined the DAG designated
   by DAGID.

   When a node adds the first DAG parent to the set of DAG parents for a
   particular DAGID, the node is said to have joined, or attached to,
   the DAG designated by DAGID.  Adding additional DAG parents beyond
   the first simply increases path diversity inwards toward the DAG
   root.  When a node removes the last DAG Parent parent from the set of DAG
   parents for a particular DAGID, the node is said to have left, or
   detached from, the DAG designated by DAGID.  RPL will coordinate the
   joining, leaving, and movement of nodes within a DAGID in such a way
   so as to avoid the formation of loops, as described further below.

   As nodes join the DAG they are able advertise the fact by beginning
   to multicast
   multicasting the DAG information in Router Advertisements RA messages (to neighbors with a
   link-local scope).  In this way, nodes are able to join the DAG at
   ever-increasing rank outward from the DAG root.  As nodes continue to
   receive DAG multicasts they may continue to expand their set of DAG
   parents, while employing loop avoidance strategies as describe described
   below, in order to build path diversity inwards toward the DAG root.

   Using the information conveyed in the metrics of its most preferred
   DAG parent, its own metrics, and the conventions and functions
   indicated by the OCP, a node is able to compute a rank value within
   the DAG which it will use to coordinate its DAG maintenance.

   Once a preferred parent is selected, the node can compute its own
   rank in the DAG and determine alternate parents.  Any node inwards
   from this node, that is with a lower rank than this node, can be used
   as an alternate parent for forwarding.

   In addition to identifying DAG parents, a node also may hear the
   Router Advertisements RA
   messages of other neighboring nodes at the same rank within the DAG.
   In this way a node can discover DAG Siblings. siblings.  As it selects its
   initial position within a DAG, 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 DAG parents according to some
   implementation specific preference. preference, and it SHOULD install a DAG
   parent as a default gateway.  To this list the node may also append a
   similarly ordered set of DAG siblings.  By forwarding traffic
   intended for the default destination towards the DAG parents, the
   node is able to support the main Multipoint-to-point (MP2P) traffic
   flows required by a typical LLN application.  By using the ordered
   set of DAG parents and DAG siblings the node is able to take
   advantage of path diversity.  For example, preferring to forward
   traffic towards parents guarantees to get the traffic inwards, closer
   to the DAG root, by definition, regardless of which parent is
   selected.  In this example, if forwarding towards parents is not
   possible, perhaps due to a transient phenomena, then a node may then
   choose to forward traffic towards siblings, moving across the 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 in order to avoid a 2-node loop.  In a further
   example, a forwarding implementation may choose to decrease the hop
   limit more quickly when forwarding along sibling paths than along
   parent paths.  A forwarding engine may behave in a manner similar to
   these examples, however the specific implementation of a forwarding
   engine and related path diversity strategies is beyond the scope of
   this specification.  Various related techniques are currently under
   investigation to be added in a later revision of this specification.

   Note that the further interaction of the routing solution and the
   forwarding engine, in particular how they utilize and react to
   changes in metrics, and how the forwarding engine may use the
   constrained set of successors provided by the routing engine based on
   L2 triggers and metrics, is under investigation.

   By employing this procedure, the LLN is able to set up a path-
   constrained DAG, rooted at designated nodes, with other nodes
   organized along paths leading inward toward the DAG root.  MP2P
   traffic intended for the destinations available to or through the DAG
   root, e.g. the default destination or other advertised prefixes,
   flows inward along the DAG towards the root, and nodes forwarding
   traffic are able to leverage the path diversity of the DAG as
   necessary.

   The DAG is then used by RPL as a reference topology, constraining

   Further mechanisms described below will populate the
   LLN routing problem, on which to build additional routing mechanisms.

3.3.1.2. tables
   along the DAG in support of P2MP and P2P traffic.

3.2.1.2.  Further Operation

   The sub-DAG of a node is the set of other nodes of greater rank in
   the DAG that might use a path towards the DAG root that contains this
   node.  Rank in the DAG is defined such that nodes contained in the
   sub-DAG of a specific node should have a greater rank than the node.
   This is an important property that is leveraged for loop avoidance-
   if a node has lesser rank then it is NOT not in the sub-DAG.  (An
   arbitrary node with greater rank may or may not be contained in the
   sub-DAG).  Paths through siblings are not contained in this set.

   As a further illustration, consider the DAG examples in Appendix B.
   Consider Node (24) in the DAG Example depicted in Figure 12.  In this
   example, the sub-DAG of Node (24) is comprised of Nodes (34), (44),
   and (45).

   A DAG may also be floating.  Floating DAGs may be encountered, for
   example, during coordinated reconfigurations of the network topology
   wherein a node and its sub-DAG breaks off the DAG, temporarily
   becomes a floating DAG, and reattaches to a grounded DAG at a
   different (more optimal) location.  (Such coordination endeavors to
   avoid the construction of transient loops in the LLN).  A DAG, or a
   sub-DAG, may also become floating because of a network element
   failure.  Note that in the case where a floating DAG exists as a
   consequence of DAG repair, the floating DAG is also intended to be
   transient and carries a marking to make it less attractive.  Some
   specific application scenarios may employ permanent floating DAGS, DAGs,
   e.g.  DAGs without connectivity to an external routed infrastructure,
   as a matter of normal operation.  In such cases the floating DAG is
   likely to have been provisioned by the application with a marking to an
   administrative preference which will make it more attractive.  DAGPreference, a configurable property that
   may be used to engineer the attractiveness of a DAG, is further
   described below.

   A node will generally join at least one DAG, typically (but not
   necessarily) to or through a grounded DAG rooted at an LBR.  In some
   cases, as suitable to the application scenario, a DAG may still
   provision the default route toward DAG Parents parents as default gateways and not be connected to
   a backbone network or non-LLN infrastructure such as the Internet. public Internet or a private IP
   network.

   This specification does not preclude a node from joining multiple
   DAGs.  In one such case, a particular application may require the
   node to maintain membership in multiple DAGs in order to satisfy
   competing constraints, for example to support different types of
   traffic, similar to the concept of MTR (Multi-topology routing) as
   supported by other routing protocols such as IS-IS [RFC5120] 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 the RPL implementation must independently maintain
   requisite information and state for each DAG in parallel.  In cases
   where a competing constraints must be satisfied toward the same DAG
   root, the OCP should differ by definition and may serve to coordinate
   the maintenance of the multiple DAGs.  Further, additional
   recommendations for the operation of loop avoidance/loop detection
   mechanisms in the presence of multiple DAGs are under investigation.

   An administered preference (DAGPreference) administrative preference, the DAG preference, shall be associated
   with each DAG.  In cases where a RPL node has a choice of joining
   more than one DAG to satisfy a particular constraint, and all else
   being equal, the node will seek to join the most preferred DAG with as
   indicated by the lowest preference
   value. administrative preference.  In practice this
   mechanism may be assist in engineering the construction of a DAG as
   appropriate to an application.  For example, nodes that are to become
   DAG roots in support of a particular application role, e.g. as a data
   sink or a controller, may be provisioned with a low DAG preference, e.g. 0x00. such that they have are more
   preferred.  Nodes who are serving as the DAG root of a transient DAG,
   e.g. for DAG repair, may take on a high DAG preference, e.g. 0xFF. less desirable preference value.
   Nodes will then be able to yield their transient DAGs to join the
   DAGs with lower DAGPreference.

3.3.1.3. 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 in order to build and maintain a DAG.

   The IPv6 Router Advertisement RA message is augmented with a DAG Information Option (DIO) (DIO),
   forming an RA-DIO message, in order to facilitate the formation and
   maintenance of DAGs.  The information conveyed in the DIO RA-DIO message
   includes the following:

   o  A DAGID used to identify the DAG as sourced from the DAG Root. root.
      Typically the (potentially compressed) IPv6 address of the DAG
      Root.  May be tested for equality.
      root.  The DAGID MUST must be unique to a single DAG in the scope of
      the LLN.  If the DAG Root root is rooting multiple DAGs, each DAG must
      be provisioned with their own IPv6 address and thus derive unique
      DAGIDs.

   o  Objective Code Point (OCP) as described below.

   o  Rank information used by nodes to determine their relationships in
      the DAG relative to each other, i.e. parents, siblings, or
      children.  This is not a metric, although its derivation is
      typically closely related to one or more metrics as specified by
      the OCP.  Used  The rank information is used to support loop avoidance
      strategies and in support of ordering alternate successors when
      engaged in path maintenance.

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

   o  Indications for 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 be cumulative,
      may report a maximum, minimum, or average scalar value, or a link
      property.

   o  List of additional destination prefixes reachable via the DAG
      root.

   The Router Advertisements RA messages are issued whenever a change is detected to the DAG
   such that a node is able to determine that a region of the DAG has
   become inconsistent.  As the DAG stabilizes the period at which Router Advertisements RA
   messages occur is configured to taper off, reducing the steady-state
   overhead of DAG maintenance.  The periodic issue of Router Advertisements, RA messages,
   along with the triggered Router
   Advertisements RA messages in response to inconsistency, is
   one feature that enables RPL to operate in the presence of unreliable
   links.

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

3.3.1.4.

3.2.1.4.  Objective Code Point (OCP)

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

   o  The set of metrics used within the DAG
   o  The objective functions 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 DIO RA-DIO message

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

   By using defined OCPs that are understood by all nodes in a
   particular implementation, and by conveying them in the DIO, RA-DIO
   message, RPL nodes may work to build optimized LLN using a variety of
   application and implementation specific 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 the case where a
   node encounters a DIO RA-DIO message containing a code point that it does
   not support.

3.3.1.5.

3.2.1.5.  Selection of Feasible DAG Parents

   The decision for 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.  For example, a node may be
   configured for one goal to optimize a bandwidth metric (OCP-1), and
   with a parallel goal to optimize for a reliability metric (OCP-2).
   Thus two DAGs, with two unique DAGIDs, may be constructed and
   maintained in the LLN: DAG-1 would be optimized according to OCP-1,
   whereas DAG-2 would be optimized according to OCP-2.  A node may then
   maintain two parallel sets of DAG parents and related data
   structures.  Note that in such a case traffic may directed along the
   appropriate constrained DAG based on traffic marking within the IPv6
   header.

   As a node hears RAs RA messages from its neighbors it may process their DIOs.
   attached DIO messages.  At this time the node may be able to take
   into consideration, for example, the following:

   o  Is the neighboring node heard reliably enough, and are the metrics
      stable enough, that a local degree of confidence may be
      established with respect to the neighboring node?  Should the
      neighboring node be considered in the set of candidate neighbors?

   o  In consultation with implementation specific policy (OCP goal), is
      the neighboring node a feasible parent from a constrained-path
      perspective?

   o  According to the implementation specific policy (OCP), does the
      neighboring node offer a better optimized position into the DAG?

   o  Does the neighboring node offer a DAG with a better DAGPreference more desirable
      administrative preference for an otherwise currently satisfied
      optimization objective, all else being equal?

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

   Based on such considerations, the node may incorporate the
   neighboring
   neighboring node into the set of DAG parents.  When the node inserts
   the first DAG parent into the empty DAG parent set, it is able to
   join the DAG.  As the DAG parent set is updated, the node will
   consult a rank computation function indicated by the OCP for 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 used to select a parent (the
   detailed mode of operation for the selection of the candidate DAG
   parent(s) is described in Section 5.3.  First, it is important to
   note that the rank of the node is not directly used as a selection
   criteria.  The metric of choice as indicated by the OCP advertised by
   the candidate parents is used to select the parent, although the use
   of a cumulative metric to reflect the rank is not precluded.

   Consider an example where 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 rank but smaller metric).  Once the
   parent, B, is selected, the node into the set of DAG parents computes its own rank according to
   implementation specific algorithms that are outside
   the scope of this
   document.

   When OCP.

   If the node inserts the first DAG parent into the empty DAG parent
   set, receives other RA messages it is able cannot attach to join the DAG.  After the DAG other
   parents if choosing that parent set is
   updated, would cause the node will consult a nodes own rank computation function indicated
   by the OCP for to
   increase.  Back to the DAG in order previous example, suppose that a node C
   appears with a (rank, metric) equal to determine its (5,1).  By selecting C as the
   new parent, N would have a rank value, which it
   will subsequently advertise when it emits its own DIOs.  A general
   property of 6 (making the rank value presented by assumption that the node
   rank is that it should be
   greater than that presented increased by any a value of its DAG parents.  A node must
   maintain its DAG Parent set such that its most preferred parent from
   the OCP goals also has 1 according to the greatest rank value in OCP).  Although
   the path metric would be lower, this may lead to a DAG parent set. Loop should C
   belong to the sub-DAG of N as further discussed in Section 3.3.1.

   All reliable neighboring nodes of a lesser rank than the node may
   then be
   considered as potential DAG parents (Note that that, as in the above
   example, as a consequence of satisfying a particular OCP goal, the
   most preferred parent may not necessarily be the potential parent of
   least rank, for example a potential parent of lesser rank may also be
   an energy constrained device that is to generally be avoided and thus
   not the most preferred).  No nodes of greater rank than the most preferred
   parent self may be
   in the DAG Parent 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 in the DAG).  All neighboring
   nodes of equal rank may be considered as siblings within the DAG
   (even though they may not have parents in common, they may still
   provide path diversity towards the DAG root).

   The computation of rank, and related properties, are further
   described in Section 3.4.1.

3.3.1.5.1. 3.3.1.

3.2.1.5.1.  Example

   For example, suppose that a node (N) is not attached to any DAG, and
   that it is in range of nodes (A), (B), (C), (D), and (E).  Let all
   nodes be configured to use an OCP which defines a policy such that
   ETX is to be minimized and paths with the attribute `Blue' should be
   avoided.  Let the rank computation indicated by the OCP simply
   reflect the ETX aggregated along the path.  Let the links between
   node (N) and its neighbors (A-E) all have an ETX of 1 (which is
   learned by node (N) through some implementation specific method).
   Let node (N) be configured to send IPv6 Router Solicitations Solicitation (RS)
   messages to probe for nearby DAGs.

   o  Node (N) transmits a Router Solicitation.

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

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

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

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

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

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

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

   o  Node (N) may now forward traffic intended for the default
      destination inward along DAGID 1 via nodes (B) and (E).  In some
      cases, e.g. if nodes (B) and (E) are tried and fail, node (N) may
      also choose to forward traffic to its sibling node (C), without
      making inward progress but with the intention that node (C) or a
      following successor can make inward progress.  Should Node (C) not
      have a viable parent, it should never send the packet back to Node
      (N) (to avoid a 2-node loop).

3.3.1.6.

3.2.1.6.  DAG Maintenance

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

   A jump in the context of a from one DAG to another DAG is attaching to a new DAGID, 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 a node in a DAG follows a DAG parent, it means that the DAG
   parent has changed its DAGID (e.g. by joining a new DAG) and that the
   node updates its own DAGID in order to keep the DAG parent.

   A frozen sub-DAG is a subset of nodes in the sub-DAG of a node who
   have been informed of a change to the node, and choose to follow the
   node in a manner consistent with the change, for example in
   preparation for a coordinated move.  Nodes in the sub-DAG who hear of
   a change and have other options than to follow the node do not have
   to become part of the frozen sub-DAG, for example such a node may be
   able to remain attached to the original DAG through a different DAG
   parent.  A further example may be found in Section 3.4.1.1. 3.3.1.1.

   When the node encounters new candidate neighbors that offer higher
   positions in the DAG, it may incorporate them directly into its set
   of DAG parents.  In this case the node may update its choice of most
   preferred parent, possibly causing its own advertised rank 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 the DAG parent set of the node becomes completely depleted, the
   node will have detached from the DAG, and may, if so configured,
   become the root of its own transient floating DAG with a high
   DAGPreference (0xFF) less
   desirable administrative preference (thus beginning the process of
   establishing the frozen sub-DAG), and then may reattach to the
   original DAG at a lower point if it is able (after hearing RA-DIOs RA-DIO
   messages from alternate attachment points).

   When the node encounters candidate parents that are in a different
   DAG, and decides to leave the current DAG in order to join the
   different DAG, DAG (thus doing a jump), it may do so safely without regard
   to loop avoidance.  However, it may not return immediately to the
   current DAG as such movement may result in the creation of loops. the creation of a DAG
   Loop, in particular if it reattaches back into its own former sub-DAG
   in an uncoordinated manner.

   When a node, and perhaps a related frozen sub-DAG, jumps to a
   different DAG, the move is coordinated by a DAG Hop timer.  The DAG
   Hop timer allows the nodes who will attach closer to the sink of the
   new DAG to `jump' first, and then drag dependent nodes behind them,
   thus endeavoring to efficiently coordinate the attachment of the
   frozen sub-DAG into the new DAG.  A further illustration of this
   mechanism may be found in Section 3.4.3.

   Section 5 contains more detail on the processes and rules used for
   DAG discovery and maintenance. 3.3.3.

   Appendix B provides additional examples of DAG discovery and
   maintenance.

3.3.2.  Source Routing

   A Source Routing mechanism for RPL is currently under investigation.

3.3.3.

3.2.2.  Destination Advertisement

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

3.3.3.1.

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

   A
          (NA-DAO)

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

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

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

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

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

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

3.3.3.2.

3.2.2.2.  Destination Advertisement Operation

   As the DAG is constructed and maintained, nodes are capable to emit
   NA-DAO messages containing Destination Advertisement Options to a subset subset, or all, of their DAG Parents. parents.  The
   selection of this subset is according to an implementation specific
   policy.

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

   When a (unicast) DAO NA-DAO message reaches a node capable of storing
   routing state, the node extracts information from the DAO NA-DAO message
   and updates its local database with a record of the DAO NA-DAO message
   and who it was received from.  When the node later propagates DAOs, NA-DAO
   messages, it selects the best (least depth) information for each
   destination and conveys this information again in the form of DAOs 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 DAOs. NA-DAO messages.

   When a (unicast) DAO NA-DAO message reaches a node incapable of storing
   additional state, the node MUST must append the next-hop address (from
   which neighbor the DAO NA-DAO message was received) to a Reverse Route
   Stack carried within the
   DAO. NA-DAO message.  The node then passes the DAO
   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) DAO NA-DAO message with a Reverse Route Stack that has been
   populated, the node knows that the DAO 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 DAO. 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 Hop-
   By-Hop routing to reach the destination.

   In this way the Destination Advertisement 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 DAOs 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 destination advertisement
   mechanism is available in Appendix B.6

3.4.

3.3.  Other Considerations

3.4.1.

3.3.1.  DAG Rank and Loop Avoidance

   When nodes select DAG Parents, parents, they should select the most preferred
   parent according to their implementation specific objectives, using
   the cost metrics conveyed in the DIOs RA-DIO messages along 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 be able to
   compute a value for its rank 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 to select DAG parents and take up a position in the DAG.  In
   other words, routing metrics and OCP (not rank directly) are used to
   determine the DAG structure and consequently the path cost.  The only
   aim of the rank is to inform loop avoidance as explained hereafter.
   The computation of the DAG Rank MUST be done in such a way so as to
   maintain the following properties for any nodes M and N who are
   neighbors in the LLN:

      For a node N, and its most preferred parent M, DAGRank(N) >
      DAGRank(M) must hold.  Further, all parents in the DAG parent set
      must be of a rank less than or equal to DAGRank(M). self's DAGRank(N).  In other words,
      the rank presented by a node N MUST be greater (deeper) than that
      presented by any of its parents.  (This mechanism serves to avoid
      loops in the case where an alternate parent is used- if no
      alternate parent is deeper than the preferred parent then loops
      are avoided.  The risk of loops occurs if there is a chance for an
      alternate parent to forward traffic to a deeper successor, which
      may be in the sub-DAG, and traffic then makes backwards progress
      and comes back to the node again).

      If DAGRank(M) < DAGRank(N), then M is probably located in a more
      optimum position than N in the DAG with respect to the metrics and
      optimizations defined by the objective code point.  In any
      fashion, Node M may safely be a DAG Parent parent for Node N without risk
      of creating a loop.  For example, a Node M of rank 3 is located in
      a more optimum position than a Node N of rank 5.  A packet
      directed inwards and forwarded from Node N to Node M will always
      make forward progress with respect to the 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
      a sufficient condition for a loop to occur).

      If DAGRank(M) == DAGRank(N), then M and N are located positions of
      relatively the same optimality within the DAG.  In some cases,
      Node M may be used as a successor by Node N, but with related
      chance of creating a loop that must be detected and broken by some
      other means.  If Node M is at rank 3 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 may
      then be some chance for 3 or more way loops, which is the risk of
      sibling forwarding.

      If DAGRank(M) > DAGRank(N), then node M is located in a less
      optimum position than N in the DAG with respect to the metrics and
      optimizations defined by the objective code point.  Further, Node
      (M) may in fact be in Node (N)'s sub-DAG.  There is no advantage
      to Node (N) selecting Node (M) as a DAG Parent, parent, and such a
      selection may create a 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 in
      fact be in Node M's own sub-DAG, and forwarding a packet directed
      inwards towards the DAG root from M to N will result in backwards
      progress and possibly a loop.

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

   The DAG rank is subsequently used to restrict the options a node has
   for movement within the DAG and to coordinate movements in order to
   avoid the creation of loops.

   A node may safely move `up' in 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 the DAG in all cases, as
   defined by the objective code point.  In the case where a node looses
   connectivity to the DAG, it must first leave the DAG before it may
   then rejoin at a deeper point.  This allows for the node to
   coordinate moving down, freezing its own sub-DAG and poisoning stale
   routes to the DAG, and minimizing the chances of re-attaching to its
   own sub-DAG thinking that it has found the original DAG again.  If a
   node where allowed to re-attach into its own sub-DAG a loop would
   most certainly occur, and may not be broken until a count-to-infinity
   process elapses.  The procedure of detaching before moving down
   eliminates the need to count-to-infinity.

   Any neighboring nodes of lesser or equal rank to the current most
   preferred DAG parent than self are eligible to be
   considered as alternate DAG
   parents. parents for forwarding.  But this node
   may only adopt such a parent as new preferred parent if that does not
   cause the resulting rank for this node to increase.

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

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

3.4.1.1.

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) is attached to a DAG at some rank d.  Node (A) is a DAG Parent parent of
   Nodes (B) and (C).  Node (C) is a DAG Parent parent of Node (D).  There is
   also an undirected sibling link between Nodes (B) and (C).

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

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

   Consider

   Now consider the case where the link (C)->(A) becomes nonviable, and
   node (C) must move to a deeper rank within the DAG:

   o  Node (C) must first detach from the DAG by removing Node (A) from
      its DAG Parent parent set, leaving an empty DAG Parent 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 1-2.
      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 rejoin the grounded DAG by reattaching at a deeper rank to
      Node (B).  Node (C) starts a DAG Hop timer to coordinate this
      move.

   o  The timer expires and Node (C) adds Node (B) to its DAG Parent parent
      set.  Node (C) has now safely moved deeper within the grounded DAG
      without creating any loops.  Node (D), and any other sub-DAG of
      Node (C), will hear the modified RA-DIO message sourced from Node
      (C) and follow Node (C) in a coordinated manner to reattach to the
      grounded DAG.  The final DAG is depicted in Figure 1-3

3.4.2.

3.3.2.  DAG Parent Selection, Stability, and Greediness

   If a node is greedy and attempts to move deeper in the DAG, beyond
   its most preferred parent, in order to increase the size of the DAG
   Parent
   parent set, then an instability can result.  This is illustrated in
   Figure 2.

   Suppose a node is willing to receive and process a RA-DIOs RA-DIO messages
   from a node in its own sub-DAG, and in general a node deeper than it.
   In such cases a chance exists to create a feedback loop, wherein two
   or more nodes continue to try and move in the DAG in order to
   optimize against each other.  In some cases this will result in an
   instability.  It is for this reason that RPL mandates that a node
   MUST NOT
   never receive and process RA-DIOs RA-DIO messages from deeper 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.

3.4.2.1.

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

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

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

   o  Let Figure 2-1 be the initial condition.

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

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

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

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

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

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

   o  This cycle can be averted through mechanisms in RPL:

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

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

3.4.3.

3.3.3.  Merging DAGs

   The merging of DAGs is coordinated in a way such as to try and merge
   two DAGs cleanly, preserving as much DAG structure as possible, and
   in the process effecting a clean merge with minimal likelihood of
   forming transient loops

3.4.3.1. DAG loops.  The coordinated merge is also intended
   to minimize the related control cost.

3.3.3.1.  Example

                                :
                                :
                               (A)       (D)
                                |         |
                                |         |
                                |         |
                               (B)       (E)
                                |         |
                                |         |
                                |         |
                               (C)       (F)

                          Figure 3: Merging DAGs

   Consider the example depicted in Figure 3.  Nodes (A), (B), and (C)
   are part of some larger grounded DAG, where Node (A) is at a rank of
   d, Node (B) at d+1, and Node (C) at d+2.  The DAG comprised of Nodes
   (D), (E), and (F) is a floating, less preferred, DAG, with Node (D)
   as the DAG root.  This floating DAG may have been formed, for
   example, in the absence of a grounded DAG or when Node (D) had to
   detach from a grounded DAG and (E) and (F) followed.  All nodes are
   using compatible objective code points.

   Nodes (D), (E), and (F) would rather join the more preferred grounded
   DAG if they are able than to remain in the less preferred floating
   DAG.

   Next, let links (C)--(D) and (A)--(E) become viable.  The following
   sequence of events may then occur in a typical case:

   o  Node (D) will receive and process a RA-DIO message from Node (C)
      on link (C)--(D).  Node (D) will consider Node (C) a candidate neighbor,
      neighbor and process 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 in a grounded DAG at rank d+2, and will
      begin the process to join the grounded DAG at rank d+3.  Node (D)
      will start a DAG Hop timer, logically associated with the grounded
      DAG at Node (C), to coordinate the jump.  The DAG 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 grounded DAG
      at rank d, and will begin the process to join the grounded DAG at
      rank d+1.  Node (E) will start a DAG Hop timer, logically
      associated with the grounded DAG at Node (A), to coordinate the
      jump.  The DAG Hop timer will have a duration proportional to d.

   o  Node (F) takes no action, for Node (F) has observed nothing new to
      act on.

   o  Node (E)'s DAG Hop timer for the grounded DAG at Node (A) expires
      first.  Node (E), upon the DAG Hop timer expiry, is removes Node
      (D), (D)
      as its parent, thus emptying the DAG parent set for the floating DAG
      DAG, and leaving the floating DAG.  Node (E) then jumps to the
      grounded DAG by entering Node (A) into the set of DAG Parents parents for
      the grounded DAG.  Node (E) is now in the 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-DIOs RA-DIO
      messages more frequently.

   o  Node (F) will receive and process a RA-DIO message from Node (E).
      Node (F) will observe that Node (E) has changed its DAGID and will
      directly follow Node (E) into the grounded DAG.  Node (F) is now a
      member of the grounded DAG at rank d+2.  Note that any additional
      sub-DAG of Node (E) would continue to join into the grounded DAG
      in this coordinated manner.

   o  Node (D) will receive a RA-DIO message from Node (E).  Since Node
      (E) is now 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 the grounded DAG at rank d+2.  Node (D) will
      start another DAG Hop timer, logically associated with the
      grounded DAG at Node (E), with a duration proportional to d+1.
      Node (D) now is running two DAG hop timers, one which was started
      with duration proportional to d+1 and one proportional to d+2.

   o  Generally, Node (D) will expire the timer associated with the jump
      to the grounded DAG at node (E) first.  Node (D) may then jump to
      the grounded DAG by entering Node (E) into its DAG Parent parent set for
      the grounded DAG.  Node (D) is now in the grounded DAG at rank
      d+2.

   o  In this way RPL has coordinated a merge between the more preferred
      grounded DAG and the less preferred floating DAG, such that the
      nodes within the two DAGs come together in a generally ordered
      manner, avoiding the formation of loops in the process.

3.4.4.

3.4.  Local and Temporary Routing Decision

   Although implementation specific, it is worth noting that a node may
   decide to implement some local routing decision based on some
   metrics, as observed locally or reported in the DIO. RA-DIO message.  For
   example, the routing may reflect a set of successors (next-hop),
   along with various aggregated metrics used to load balance the
   traffic according to some local policy.  Such decisions are local and
   implementation specific.

   Routing stability is crucial in a LLN: in the presence of unstable
   links, the first option consists of removing the link from the DAG
   and triggering a DAG recomputation across all of the nodes affected
   by the removed link.  Such a naive approach could unavoidably lead to
   frequent and undesirable changes of the DAG, routing instability, and
   high-energy consumption.  The alternative approach adopted by RPL
   relies on the ability to temporarily not use a link toward a
   successor marked as valid, with no change on the DAG structure.  If
   the link is perceived as non-usable for some period of time (locally
   configurable), this triggers a DAG recomputation, through the DAG
   Discovery
   discovery mechanism further detailed in Section 5.4, 5.3, after reporting
   the link failure.  Note that this concept may be extended to take
   into account other link characteristics: for the sake of
   illustration, a node may decide to send a fixed number of packets to
   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 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.  The use of the ordered list by a forwarding engine is
   loosely constrained, and may take into account the dynamics 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 are beyond the scope of this
   document.

   These decisions may be local and/or temporary with the objective to
   maintain the DAG shape while preserving routing stability.

3.4.5.  Scalability

   As each node selects DAG Parents according to implementation specific
   objectives, RPL is able to dynamically partition an LLN network into
   different regions, each anchored by a DAG root.  Multiple DAG roots
   may be deployed in accordance be local and/or temporary with an implementation specific policy
   designed the objective to limit
   maintain the size of a partition, e.g. for performance or
   other reasons.

   A further example is illustrated in Appendix C.

3.4.6. 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 may employ an as-needed mechanism of NS/NA in order to verify
   routing adjacencies just prior to forwarding data.  Pending the
   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 LLNs

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

4.1.  Routing Metrics

   Routing metrics are used by the routing protocol to compute the
   shortest path according to one of more defined metrics.  IGPs such as
   IS-IS ([RFC5120]) and OSPF ([RFC4915]) compute the shortest path
   according to a Link State Data Base (LSDB) using link metrics
   configured by the network administrator.  Such metrics can represent
   the link bandwidth (in which case the metric is usually inversely
   proportional to the bandwidth), delay, etc.  Note that in some cases
   the metric is a polynomial function of several metrics defining
   different link characteristics.  The resulting shortest path cost is
   equal to the sum (or multiplication) of the link metrics along the
   path: such metrics are said to be additive or multiplicative metrics.

   Some routing protocols support more than one metric: in the vast
   majority of the cases, one metric is used per (sub)topology.  Less
   often, a second metric may be used as a tie breaker in the presence
   of 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 case of RPL, it is virtually impossible to define *the*
   metric, or even a composite, that will fit it all:

   o  Some information apply to path setup time, when determining routes, other information
      may apply to packet only when forwarding time. 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 is agnostic to the logic that
   handles metrics.  A node will be configured with some external logic
   to use and prioritize certain metrics for a specific scenario.  As
   new heterogeneous devices are installed to support the evolution of a
   network, or as networks form in a totally ad-hoc fashion, it will
   happen that nodes that are programmed with antagonistic logics and
   conflicting or orthogonal priorities end up participating in the same
   network.  It is thus RECOMMENDED recommended to use consistent parent selection
   policy, as per Objective Code Points (OCP), to ensure consistent
   optimized paths.

   RPL is designed to survive and still operate, though in a somewhat
   degraded fashion, when confronted to such heterogeneity.  The key
   design point is that each node 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 0 clarifies this point.

4.2.  Routing Constraints

   A constraint is a link or a node characteristic that must be
   satisfied by the computed path (using boolean values or lower/upper
   bounds) and is by definition neither additive nor multiplicative.
   Examples of links constraints 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 of finding the
   shortest path according to some metrics satisfying a set of
   constraints.  A technique consists of first filtering out all links
   and nodes that cannot satisfy the constraints (resulting in a sub-
   topology) and then computing the shortest path.

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

      Objective function 1:
         "Find 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 of all link costs (Delay)) along the path such
         that all links provide at least X Bit/s of reservable
         bandwidth."

5.  Specification of Core  RPL Protocol Specification

5.1.  DAG Information Option

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

5.1.1.  DIO  DAG Information Option (DIO) base option

   The DAG Information Option is a container option, 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
        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|  Rsvd  00000  |   Sequence    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | DAGPreference |                BootTimeRandom                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   NodePref.   |    DAGRank    |           DAGDelay            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | DIOIntDoubl.  |  DIOIntMin.   |     DAGObjectiveCodePoint     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           PathDigest                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                            DAGID                              |
       +                                                               +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   sub-option(s)...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 4: DIO Base Option

   Type: 8-bit unsigned identifying the DIO base option.  The suggested
         value is 140 to be assigned confirmed by the IANA.

   Length:  8-bit unsigned integer set to 4 when there is no suboption.
         The length of the option (including the type and length fields
         and the suboptions) in units of 8 octets.

   Flag Field:  Three flags are currently defined:

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

         Destination Advertisement Trigger (D):  The Destination
               Advertisement Trigger (D) flag is set when the DAG root
               or another node in the successor chain decides to trigger
               the sending of
         Destination Advertisements destination advertisements in order to
               update routing state for the outward direction along the
               DAG, as further detailed in Section 5.10. 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 root is capable to
               support the collection of Destination Advertisement destination advertisement
               related routing state and enables the Destination Advertisement operation of the
               destination advertisement mechanism within the DAG.

   Reserved:  5-bit unsigned integer

         Unassigned bits of the Flag Field are considered as reserved.
         They MUST be set to 0 by the DAG root zero on transmission and left
         unchanged by nodes propagating the DIO. MUST be ignored on
         receipt.

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

   DAGPreference:  8-bit unsigned integer set by the DAG root to its
         preference and unchanged at propagation.  Default  DAGPreference ranges
         from 0x00 (least preferred) to 0xFF (most preferred).  The
         default is 0 (lowest
         preference). (least preferred).  The DAG preference provides an
         administrative mechanism to engineer the self-organization of
         the LLN, for example indicating the most preferred LBR.  If a
         node has the option to join a more preferred DAG of lower preference while still
         meeting other optimization objectives, then the node will seek
         to join the
         minimum available preference. more preferred DAG.

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

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

   DAGRank:  8-bit unsigned integer.  The integer indicating the DAG rank of the DAG root is 0.
         The DAG Rank of a node attached to
         sending the DAG should be greater
         than rank RA-DIO message.  The DAGRank of its deepest DAG parent, as computed by an
         implementation specific routine.  All nodes in the DAG
         advertise their DAG rank in the DAG Information Options that
         they append to the RA messages over their LLN interfaces as
         part of the propagation process. root is
         typically 1.  DAGRank is further described in Section 5.3.

   DAGDelay:  16-bit unsigned integer set by the DAG root indicating the
         delay before changing the DAG configuration, in TBD-units.  A
         default value is TBD.  It is expected to be an order of
         magnitude smaller than the RA-interval.  It is also expected to
         be an order of magnitude longer than the typical propagation
         delay inside the LLN.

   DIOIntervalDoublings:  8-bit unsigned integer.  Used  Configured on the DAG
         root and used to configure the trickle timer governing when RA-DIO RA-
         DIO message should be sent within the DAG.
         DIOIntervalDoublings is the number of times that the
         DIOIntervalMin is allowed to be doubled during the trickle
         timer operation, i.e.  DIOIntervalMax = DIOIntervalMin *
         2^(DIOIntervalDoublings). operation.

   DIOIntervalMin:  8-bit unsigned integer.  Used  Configured on the DAG root
         and used to configure the trickle timer governing when RA-DIO
         message should be sent within the DAG.  The minimum configured
         interval for the RA-DIO trickle timer in units of ms is
         2^DIOIntervalMin.  For example, a DIOIntervalMin value of 16ms
         is expressed as 4.

   DAGObjectiveCodePoint:  The DAG Objective Code Point is used to
         indicate the cost metrics, objective functions, and methods of
         computation and comparison for DAGRank in use in the DAG.  The
         DAG OCP is set by the DAG Root. root.  (Objective Code Points are 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 result 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 a 'previous value'
         of zeroes to initially set the PathDigest.  Used to 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 root.  The global IPv6 address of the
         DAG root can be used. used, however. the DAGID MUST be unique per DAG
         within the scope of the LLN.  In the case where a DAG root is
         rooting multiple DAGs the DAGID MUST be unique for each DAG
         rooted at a specific DAG root.

   The following values MUST NOT change during the propagation of the
   DIO RA-DIO
   messages outwards along the DAG: Type, Length, G, DAGPreference,
   DAGDelay and DAGID.  All other fields of the DIO RA-DIO message are
   updated at each hop of the propagation.

5.1.1.1.  DIO  DAG Information Option (DIO) Suboptions

   In addition to the minimum options presented in the base option, a
   number of
   several suboptions are defined for the DIO: 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 of the type of suboption.  When
         processing a DIO RA-DIO message containing a suboption for which
         the Suboption Type value is not recognized by the receiver, the
         receiver MUST silently ignore and skip over the unrecognized option, continue
         to process the following suboption, correctly handling any
         remaining options in the message.

   Suboption Length:  8-bit unsigned integer, representing the length in
         octets of the suboption, not including the suboption Type and
         Length fields.

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

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

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

   DIO

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

5.1.1.1.2.  Pad1

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

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

                              Figure 6: Pad 1

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

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

5.1.1.1.3.  PadN

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

5.1.1.1.4.  DAG Metric Container

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

        0                   1
        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

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

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

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

5.1.1.1.5.  Destination Prefix

   The Destination Prefix suboption has an alignment requirement of
   4n+1.  Its format is as follows:

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

                     Figure 9: DAG Destination Prefix

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

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

   The Prefix Length is an 8-bit unsigned integer that indicates the
   number of leading bits in the destination prefix.  Prf is the Route
   Preference as in [RFC4191].  The Destination Prefix contains Prefix Length significant bits of the
   destination prefix.  The remaining bits of the Destination Prefix, as
   required to complete the trailing octet, are reserved fields MUST be set to 0. zero
   on transmission and MUST be ignored on receipt.

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

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

   In the event that a DAG root RA-DIO message may need to specify that it offers connectivity
   to more than one destination, the Destination Prefix suboption may be
   repeated.

5.2.  Conceptual Data Structures

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

   o  A set of Candidate Neighbors candidate neighbors

   o  For each DAG:

      *  A set of Candidate candidate DAG Parents parents

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

5.2.1.  Candidate Neighbors Data Structure

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

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

5.2.2.  DAGs  Directed Acyclic Graphs (DAGs) Data Structure

   A DAG may be uniquely identified by within the LLN by its unique
   DAGID.  When a single device is capable to root multiple DAGs in
   support of an application need for multiple optimization objectives
   it is expected to produce a different and unique DAGID for each of
   the multiple DAGs.

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

   o  DAGID

   o  DAGObjectiveCodePoint

   o  A set of Destination Prefixes offered by inwards along the DAG root

   o  A set of candidate DAG Parents parents

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

   o  DAGSequenceNumber

   When a DAG is discovered for which no DAG data structure is
   instantiated, and the node wants to join (i.e. the neighbor is to
   become a Candidate candidate DAG Parent parent in the Held-Up state), then the DAG
   data structure is instantiated.

   When the Candidate candidate DAG Parent parent set is depleted (i.e. the last
   Candidate
   candidate DAG Parent parent has timed out of the Held-Down state), then the
   DAG data structure may SHOULD be deallocated. suppressed after the expiration of an
   implementation-specific local timer.  An implementation should SHOULD delay
   before deallocating the DAG data structure in order to observe that
   the DAGSequenceNumber has incremented should any new candidate DAG Parents
   parents appear for the DAG.

5.2.2.1.  Candidate DAG Parents Structure

   When the DAG is self-rooted, the set of candidate DAG Parents parents is
   empty.

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

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

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

   o  a state associated with the role of the candidate as a potential
      DAG Parent parent {Current, Held-Up, Held-Down, Collision}, further
      described in Section 5.8 5.7

   o  A DAG Hop Timer, if instantiated

   o  A Held-Down Timer, if instantiated

5.2.2.1.1.  DAG Parents

   Note that the subset of candidate DAG Parents parents in the `Current' state
   comprises the set of DAG Parents, parents, i.e. the nodes actively acting as
   parents in the DAG.

   DAG Parents parents may be ordered, according to the OCP.  When ordering DAG
   Parents,
   parents, in consultation with the OCP, the most preferred DAG Parent parent
   may be identified.  All current DAG Parents parents must have a rank less
   than or equal to that of the most preferred DAG Parent. parent.

   When nodes are added to or removed from the DAG Parent parent set the most
   preferred DAG Parent parent may have changed and should be reevaluated.  Any
   nodes having a rank greater than the most preferred parent self after such a change must be
   placed in the Held-Down state and evicted as per the procedures
   described in Section 5.8 5.7

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

5.3.  Initialization  DAG Discovery and Configuration

   An Maintenance

   DAG discovery locates the 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 may be used
   later to provide additional path diversity towards the DAG root.  DAG
   discovery enables nodes to implement different policies for selecting
   their DAG parents in the DAG by using implementation must specific policy
   functions.  DAG discovery specifies a set of rules to be followed by
   all implementations in order to ensure interoperation.  DAG discovery
   also standardizes the format that is used to advertise the most
   common information that is used in order to select DAG parents.

   One of these information, the DAG rank, is used by DAG discovery to
   provide loop avoidance even if nodes implement different policies.
   The DAG Rank is computed as specified by the Objective Code Point in
   use by the DAG, demonstrating the properties described in
   Section 3.3.1.  The rank should be computed in such a way so as to
   provide a means, e.g. comparable basis with other nodes which may not use the
   same metric at all.

   The DAG discovery procedures take into account a set number of APIs, to allow
   the node to initialize/configure the factors,
   including:

   o  RPL implementation. rules for loop avoidance based on rank

   o  The RPL 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 on in the node must be provisioned nodes MUST obey to know:

      Is the following
   rules and definitions:

   1.   A node serving that does not have any DAG parents in a role DAG is the root
        of its own floating DAG.  It's rank is 1.  A node will end up in an application scenario whereby
        that situation when it
      should permanently act as a looses all of its current feasible
        parents, i.e. the set of DAG root?  (For example, parents becomes depleted.  In that
        case, the node may
      act as an LBR, provide Internet access, serve as an application
      specific data-collection point, or provide application control to SHOULD remember the LLN.)  If so,

         What is DAGID and the DAGPreference value sequence
        counter of the last RA-DIO message from the lost parents for a
        period of time which covers multiple RA-DIO messages.  This is
        done so that if the self-rooted DAG (likely
         0)?

         What OCP are supported?
         Is connectivity node does encounter another possible
        attachment point to external infrastructure provided (is the DAG
         grounded?)

         What destination prefixes are offered?

         What is lost DAGID within a period of time, the DAGDelay?

         Is
        node may observe a sequence counter change by comparing the Destination Advertisement mechanism in effect?

         What are
        observed sequence counter to the values for DIOIntervalDoublings, DIOIntervalMin?

         Is last observed sequence counter
        and thus verify that the node new attachment point is a viable and
        independent alternative to periodically emit DIOs (e.g. revise the DAG
         Sequence Number upwards) in order attach back to provide a heartbeat for
         the DAG?  If so, with what period?

      If the lost DAGID.

   2.   A node that is attached to an infrastructure that does not permanently act as a
        support RA-DIO messages, is the DAG root, should it
      actively root a (floating, DAGPreference 0xFF) DAG when no other
      DAG of its own grounded
        DAG.  It's rank is available? 1.  (For example, a battery powered node may not
      wish expend energy to do this, but will instead passively listen
      for other options).

      For each DAG example an LBR that the node may root, what is the DAGID?

      What are the supported OCP (optimization goals)?

      What, if any, destination prefixes are being sought, associated in
        communication with supported OCP?

   When a node non-LLN router not running RPL).

   3.   A (non-LLN) router sending a RA messages without DIO is provisioned with
        considered a set of optimization goals,
   effectively indicating targeted OCPs for given destinations (possibly
   including the default destination), it may conceptually organize
   these into grounded infrastructure at rank 0.  (For example, a table where each row indicates an optimization goal.  As
   DAGs are joined
        router that is in order to satisfy optimization objectives,
   references to the DAG supporting the objective may be entered into
   each row.  In this way a communication with an LLN node may track which objectives are
   satisfied by which DAGs, as well but not running
        RPL such as which objectives are unsatisfied
   by any DAG.  This will help to inform a nodes decision to join a new
   DAG, or perhaps leave an existing DAG in order to join a better
   alternate DAG, non-LLN public Internet router in order to meet specific optimization objectives.

5.4. communication
        with an LBR)

   4.   The DAG Discovery root exposes the DAG Discovery locates in the nearest sink RA-DIO message and forms a Directed Acyclic
   Graph towards nodes
        propagate the RA-DIO message outwards along the DAG with the RAs
        that sink, 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 per the rank calculation indicated by identifying the OCP.

   6.   A node MUST NOT move outwards along a set of DAG parents.  During
   this process that it is attached
        to, causing the DAG Discovery also identifies siblings, which may be
   used later rank to provide additional path diversity towards increase, except in a special case
        where the DAG root.
   DAG Discovery enables nodes node MAY choose to implement different policies for
   selecting their follow the last DAG parents parent in the DAG by using implementation
   specific policy functions.  DAG Discovery specifies a
        set of rules to
   be followed by all implementations in order to ensure interoperation. DAG Discovery also standardizes parents.  In the format that general case, if a node is used required
        to advertise
   the most common information move such that is used in order to select DAG
   parents.

   One of these information, it cannot stay within the DAG rank, is used by DAG Discovery without a rank
        increase, then it needs to
   provide loop avoidance even if nodes implement different policies.
   The DAG Rank is computed as specified by the Objective Code Point in
   use by the DAG, demonstrating first leave the properties described DAG.  In other words
        a node that is already part of a DAG MAY move or follow a DAG
        parent at any time and with no delay in
   Section 3.4.1.  The rank should order to be computed in such a way so closer, or
        stay as close, to
   provide a comparable basis with other nodes which the DAG root of its current DAG as it already
        is, but may not use move outwards.  RAs received from other routers
        located at lesser rank in the same metric DAG may be considered as
        coming from candidate parents.  RAs received from other routers
        located at all.

   In order to organize and maintain loopless structure, the DAG
   Discovery implementation same rank in the nodes same DAG may be considered as
        coming from siblings.  Nodes MUST obey to the following
   rules and definitions:

   1.   A node ignore RAs that does not have any DAG parents in a DAG is are received
        from other routers located at greater rank within the root
        of its own floating same DAG.  It's rank is 1.

   7.   A node will end up in
        that situation when it looses all of may jump from its current feasible
        parents, i.e. the set DAG into any different DAG if
        it is preferred for reasons of connectivity, configured
        preference, free medium time, size, security, bandwidth, DAG parents becomes depleted.  In that
        case,
        rank, or whatever metrics the LLN cares to use.  A node SHOULD remember the DAGID may jump
        at any time and the sequence
        counter to whatever rank it reaches in the DIO of the lost parents new DAG, but
        it may have to wait for a period of time
        which covers multiple DIO.

   2.   A LLN Node that is attached DAG Hop timer to elapse in order to do
        so.  This allows the new higher parts (closer to an infrastructure that does not
        support DIO, is the DAG root sink) of its own grounded DAG.  It's rank
        is 1.

   3.
        the DAG to move first, thus allowing stepped DAG
        reconfigurations and limiting relative movements.  A router sending a RA 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 SHOULD
        NOT join a
        backbone router in communication with an LBR)

   4.   The previous DAG root exposes (identified by its DAGID) unless the DAG
        sequence number in the RA-DIO and nodes propagate message has incremented since the DIO outwards along
        node left that DAG.  A newer sequence number indicates that the DAG
        candidate parents were not attached behind this node, as they
        kept getting subsequent RA-DIO messages with new sequence
        numbers from the RAs same DAG.  In the event that old sequence
        numbers (two or more behind the present value) are encountered
        they forward
        over their LLN links.

   5.   A node MAY move at any time, with no delay, within its DAG as
        long as such a move does not increase its own DAG rank, as per are considered stale and the rank calculation indicated by corresponding parent SHOULD be
        removed from the OCP. set.

   8.   If a node is
        required to move such that it cannot stay within the DAG without has selected a rank increase, then new set of DAG parents but has not
        moved yet (because it needs is waiting for DAG Hop timer to first leave elapse),
        the DAG.  In other
        words a node that is already part of unstable MUST NOT send RA-DIOs for that DAG.

   9.   If a DAG MAY move or follow node receives a RA-DIO from one of its DAG parents, and if
        the parent at any time contains a different DAGID, indicating that the
        parent has left the DAG, and with no delay if the node can remain in order to be closer,
        or stay as close, to the DAG root of its
        current DAG as it
        already is.  But a node MUST NOT move outwards along the through an alternate DAG
        that it is attached, except in parent, then the special case when choosing to
        follow node
        SHOULD remove the last DAG parent in which has joined the set of new DAG parents.  RAs
        received from other routers located higher
        its DAG parent set and remain in the same DAG may
        be considered as coming from candidate parents.  RAs received
        from other routers located at original DAG.  If there is
        no alternate parent for the same rank in DAG, then the same DAG may
        be considered as coming from siblings.  Nodes MUST ignore RAs node SHOULD follow
        that are received from other routers located deeper within parent into the
        same new DAG.

   6.   A

   10.  When a node may jump 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, detects or whatever metrics causes a DAG inconsistency, as described
        in Section 5.3.4.2, then the LLN cares to use.  A node may jump
        at any time and SHOULD send an unsolicited RA-
        DIO message to whatever rank it reaches in its one-hop neighbors.  The RA-DIO is updated to
        propagate the new DAG, but
        it may have DAG information.  Such an event MUST also
        cause the trickle timer governing the periodic sending of RA-DIO
        messages to wait for be reset.

   11.  If a DAG Hop timer to elapse in order parent increases its rank such that the node rank would
        have to do
        so.  This allows change, and if the new higher parts (closer node does not wish to follow (e.g. it
        has alternate options), then the sink) of DAG parent SHOULD be evicted
        from the DAG to move first, thus allowing stepped parent set.  If the DAG
        reconfigurations and limiting relative movements.  A parent is the last in the
        DAG parent set, then the node SHOULD
        NOT join chose to follow it.

5.3.2.  Reception and Processing of RA-DIO messages

   When an RA-DIO message is received from a previous DAG (identified by its DAGID) unless source device named SRC,
   the
        sequence number in receiving node must first determine whether or not the RA-DIO
   message should be accepted for further processing, and subsequently
   present the RA-DIO message for further processing if eligible.

5.3.2.1.  Determination of Eligibility for DIO has incremented since Processing

      If the node left
        that DAG. RA-DIO message is malformed, then the RA-DIO message is not
      eligible for further processing and is silently discarded.  A newer sequence number indicates that RPL
      implementation MAY log the reception of a malformed RA-DIO
      message.

      If SRC is not a member of the candidate
        parents were neighbor set, then the RA-
      DIO is not attached behind eligible for further processing.  (Further evaluation/
      confidence of this node, as they kept getting
        subsequent DIOs with new sequence numbers from neighbor is necessary)

      If the same DAG.  In RA-DIO message advertises a DAG that the node is already a
      member of, then:

         If the rank of SRC as reported in the event RA-DIO message is lesser
         than that old sequence numbers (two or more behind of the
        present value) are encountered they are considered stale and node within the
        corresponding parent SHOULD be removed from DAG, then the set.

   7. RA-DIO message
         MUST be considered for further processing

         If a node has selected a new set the rank of DAG parents but has not
        moved yet (because it SRC as reported in the RA-DIO message is waiting for DAG Hop timer equal
         to elapse), that of the node within the DAG, then SRC is unstable marked as a
         sibling and refrains from sending RA-DIOs the RA-DIO message is not eligible for that
        DAG.

   8. further
         processing.

         If a node receives a RA-DIO from one the rank of its DAG parents, and if SRC as reported in the parent contains a different DAGID, indicating RA-DIO message is higher
         than that of the
        parent has left node within the DAG, and if SRC is not a DAG
         parent, then the RA-DIO message MUST NOT be considered for
         further processing

      If SRC is a DAG parent for any other DAG that the node can remain in is attached
      to, then the
        current RA-DIO message MUST be considered for further
      processing (the DAG through an alternate parent may have jumped).

      If the RA-DIO message advertises a DAG parent, that offers a better (new
      or alternate) solution to an optimization objective desired by the
      node, then the node
        should remove 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 parent which has joined if
         necessary

         Place the neighbor in the new DAG from
        its candidate DAG parent set and remain in the original DAG.

         If the node
        was the last DAG parent then has sent an RA message within the node SHOULD follow that parent.

   9.   When a node detects or causes a DAG inconsistency, risk window as
         described in Section 5.4.3.2, 5.7.3 then perform the node sends an unsolicited RA-DIO
        message to its one-hop neighbors.  The RA contains an updated
        DIO to propagate collision detection
         described in Section 5.7.3.  If a collision occurs, place the new
         candidate DAG information.  Such an event will
        also cause parent in the trickle timer governing collision state and do not process
         the periodic RAs to be
        reset.

   10. RA-DIO message any further as described in Section 5.7.

         If the SRC node is also a DAG parent increases its rank such for another DAG that the
         node rank would
        have to change, is a member of, and if the node does not wish to follow (e.g. it
        has alternate options), then the new/alternate DAG parent should be evicted
        from satisfies an
         equivalent optimization objective as the DAG parent set.  If other DAG, then the
         DAG parent is the last in the known to have jumped.

            Remove SRC as a DAG parent set, then the node may chose to follow it.

5.4.1.  RA-DIO Reception

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

5.4.1.1.  Determination of Eligibility for DIO Processing held-down state)

            If the DIO other DAG is malformed, now empty of candidate parents, then
            directly follow SRC into the new DAG by adding it as a DAG
            parent in the Current state, else ignore the DIO is RA-DIO message
            (do not eligible for further
      processing. follow the parent).

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

      If the DIO advertises a candidate DAG that parent, place
         the node candidate DAG parent into the Held-Up state, and start the
         DAG Hop timer as per Section 5.7.1.

      If the RA-DIO message is already for a member of,
      then:

         If known/existing DAG:

         Process the rank of SRC RA-DIO message as reported per the rules in Section 5.3

   As candidate parents are identified, they may subsequently be
   promoted to DAG parents by following the DIO is less then that rules of DAG discovery as
   described in Section 5.3.  When a node adds another node to its set
   of candidate parents, the node within becomes attached to the DAG, then DAG through
   the DIO MUST be considered for
         further processing

         If parent node.

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

5.3.3.  RA-DIO Transmission

   Each node within the DAG, then SRC maintains a timer that governs when to multicast RA
   messages.  This timer is marked implemented as a sibling and
         the DIO is not eligible for trickle timer operating
   over a variable interval.  Trickle timers are further processing.

         If the rank of SRC as reported detailed in
   Section 5.3.4.  The governing parameters for the DIO is lesser than that
         of the node within timer should be
   configured consistently across the DAG, and SRC is not a are provided by the DAG Parent, then
   root in the DIO is not eligible for further processing

      If SRC is 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 DAG Parent for node is unstable, because any other DAG that the node Hop timer is attached
      to, running in
      preparation for a jump, then the DIO node MUST be considered for further processing (the
      DAG Parent may have jumped).

      If NOT transmit
      unsolicited RA-DIOs (i.e. the DIO advertises a DAG that offers node will remain silent when the
      timer expires).

   o  When a better (new or
      alternate) solution to node detects an optimization objective desired by the
      node, then inconsistency, it SHOULD reset the DIO MUST be considered for further processing.

5.4.1.2.  Overview interval
      of DIO Processing

      If the DIO is for a new/alternate DAG:

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

         Place the neighbor in the Candidate DAG Parent set

         Has the node sent an minimum value, causing RA within the risk window messages to be
      emitted more frequently as described in
         Section 5.8.3?  If so, perform part of a strategy to quickly correct
      the collision detection
         described inconsistency.  Such inconsistencies may be, for example, an
      update to a key parameter (e.g. sequence number) in Section 5.8.3.  If the RA-DIO
      message or a collision occurs, place loop detected when a node located inwards along the
         Candidate
      DAG Parent in the collision state and do not process
         the DIO any forwards traffic outwards.  Inconsistencies are further as described
      detailed in Section 5.8.

         If 5.3.4.2.

   o  When a node enters a mode of consistent operation within a DAG,
      i.e.  RA-DIO messages from its DAG parents are consistent and no
      other inconsistencies are detected, it may begin to open up the SRC
      interval of the trickle timer towards a maximum value, causing RAs
      to be emitted less frequently, thus reducing network maintenance
      overhead and saving energy consumption (which is of utmost
      importance for battery-operated nodes).

   o  When a node is also initialized, it MAY be configured to remain silent
      and not multicast any RA messages until it has encountered and
      joined a DAG Parent (perhaps initially probing for another a nearby DAG that the
         node 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
      desired for independent nodes to begin aggregating into scattered
      floating DAGs in the absence of a member of, grounded node, for example in
      support of LLN installation and commissioning.

   Note that if the new/alternate multiple DAG satisfies an
         equivalent optimization objective as roots are participating in the other same DAG, then
   i.e. offering RA-DIO messages with the
         DAG Parent is known same DAGID, then they must
   coordinate with each other to have jumped.

            Remove SRC as a DAG Parent ensure that their RA-DIO messages are
   consistent when they emit RA-DIO messages.  In particular the
   Sequence number must be identical from each DAG root, regardless of
   which of the other multiple DAG (place it in roots issues the held-down state)

            If RA-DIO message, and
   changes to the other Sequence number should be issued at the same time.
   The specific mechanism of this coordination, e.g. along a non-LLN
   network between DAG roots, is now empty beyond the scope of candidate Parents, then
            directly follow SRC into this specification.

5.3.4.  Trickle Timer for RA Transmission

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

   For each DAG
            Parent in that a node is part of, the Current node must maintain a single
   trickle timer.  The required state

            Else ignore the DIO (do not follow contains the parent).

         If following conceptual
   items:

   I:    The current length of the new/alternate DAG offers communication interval

   T:    A timer with a better solution to the
         optimization objectives, then prepare duration set to jump: copy the DIO
         information into a random value in the record for range
         [I/2, I]

   C:    Redundancy Counter

   I_min:  The smallest communication interval in milliseconds.  This
         value is learned from the Candidate DAG Parent, place 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 from the Candidate DAG Parent into RA-DIO message
         as DIOIntervalDoublings.  The default value is
         DEFAULT_DIO_INTERVAL_DOUBLINGS.

5.3.4.1.  Resetting the Held-Up state, Trickle Timer

   The trickle timer for a DAGID is reset by:

   1.  Setting I_min and start I_doublings to the
         DAG Hop timer as per Section 5.8.1.

      If values learned from the RA-
       DIO is for message.

   2.  Setting C to zero.

   3.  Setting I to I_min.

   4.  Setting T to a known/existing DAG:

         Process the DIO random value as per described above.

   5.  Restarting the rules in Section 5.4

   As candidate parents are identified, they may subsequently be
   promoted trickle timer to DAG parents by following the rules of DAG Discovery as
   described in Section 5.4. expire after a duration T

   When an LLN learns about a node adds another node to its set
   of candidate parents, DAG through a RA-DIO message and makes the node becomes attached
   decision to join it, it initializes the DAG through state of the parent node.

   In trickle timer by
   resetting the trickle timer and listening.  Each time it hears a
   consistent RA for this DAG Discovery implementation, from a DAG parent, it increments C.

   When the most preferred parent should
   be used timer fires at time T, the node compares C to restrict which other nodes may become DAG parents.  All
   nodes in the DAG Parent set should be of a rank redundancy
   constant, DEFAULT_DIO_REDUNDANCY_CONSTANT.  If C is less than or equal to that
   value, the most preferred DAG parent.  (This case may occur, for example, if
   an energy constrained device is at node generates a lesser rank but should be
   avoided new RA and broadcasts it.  When the
   communication interval I expires, the node doubles the interval I so
   long as per it has previously doubled it fewer than I_doubling times,
   resets C, and chooses a new T value.

5.3.4.2.  Determination of Inconsistency

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

   o  The node joins a more
   preferred parent at new DAGID

   o  The node moves within a greater rank).

5.4.2.  RA-DIO Transmission

   Each DAGID

   o  The node maintains receives a timer that governs when to multicast RAs.  This
   timer is implemented as modified RA-DIO message from a trickle timer operating over DAG parent

   o  A DAG parent forwards a variable
   interval.  Trickle timers are further detailed in Section 5.4.3.  The
   governing parameters for the timer should be configured consistently
   across the DAG, packet intended to move inwards,
      indicating an inconsistency and are provided by the DAG root possible loop.

   o  A metric communicated in the DIO.  In
   addition to periodic RAs, each LLN node will respond RA-DIO message is determined to Router
   Solicitation messages be
      inconsistent, as according to [RFC4861]. a implementation specific path
      metric selection engine.

   o  When  The rank of a node is unstable, because any DAG Hop timer is running in
      preparation for a jump, then the node must not transmit
      unsolicited RA-DIOs (i.e. parent has changed.

5.4.  DAG Heartbeat

   The DAG root makes the node will remain silent sole determination of when to revise the
      timer expires).

   o  When a node detects an inconsistency,
   DAGSequenceNumber by incrementing it may reset the interval of upwards.  When the trickle timer to a minimum value,
   DAGSequenceNumber is increased an inconsistency results, causing RAs RA-
   DIO messages to be emitted
      more frequently as part of a strategy sent back outwards along the DAG to quickly correct convey the
      inconsistency.  Such inconsistencies may be, for example, an
      update
   change.  The degree to a key parameter (e.g. sequence number) in which this mechanism is relied on may be
   determined by the DIO or a
      point-to-point loop detected when implementation- on one hand it may serve as a node located inwards along
   periodic heartbeat, refreshing the DAG forwards traffic intended for states, and on the default destination.
      Inconsistencies are further detailed other hand
   it may result in Section 5.4.3.2.

   o  When a node enters a mode of consistent operation within constant steady-state control cost overhead which
   is not desirable.

   Some implementations may provide an administrative interface, such as
   a DAG,
      i.e.  DIOs from its command line, at the DAG Parents are consistent and no other
      inconsistencies are detected, it root whereby the DAGSequenceNumber may begin be
   caused to open up the interval increment in response to some policy outside of the trickle scope
   of RPL.

   Other implementations may make use of a periodic timer towards to
   automatically increment the DAGSequenceNumber, resulting in a maximum value, causing RAs
   periodic DAG Heartbeat at a rate appropriate to be
      emitted less frequently, thus reducing network maintenance
      overhead the application and saving energy consumption (which is of utmost
      importance for battery-operated nodes).

   o  When a node
   implementation.

5.5.  DAG Selection

   The DAG selection is initialized, it may be configured implementation and algorithm dependent.  Nodes
   SHOULD prefer to remain silent join DAGs advertising OCPs and not multicast any RAs until it has encountered destinations
   compatible with their implementation specific objectives.  In order
   to limit erratic movements, and joined all metrics being equal, nodes SHOULD
   keep their previous selection.  Also, nodes SHOULD provide a
      DAG (perhaps initially probing means to
   filter out a candidate parent whose availability is detected as
   fluctuating, at least when more stable choices are available.  Nodes
   MAY place the failed candidate parent in a Hold Down mode that
   ensures that the candidate parent will not be reused for a nearby DAG with an RS).
      Alternately, it may choose given
   period of time.

   When connection to root its own floating DAG and begin
      multicasting RAs using a default trickle configuration.  The
      second case may be advantageous if it fixed network is desired not possible or preferable for independent
      nodes to begin aggregating into
   security or other reasons, scattered floating DAGs MAY aggregate as much as
   possible into larger DAGs in order to allow connectivity within the
      absence of a grounded node, for example in support of LLN
      installation and commissioning.

   Note
   LLN.

   A node SHOULD verify that if multiple DAG roots are participating in the same DAG,
   i.e. offering DIOs with the same DAGID, then they must coordinate bidirectional connectivity and adequate
   link quality is available with each other to ensure a candidate neighbor before it
   considers that their DIOs are consistent when they
   emit RA-DIOs.  In particular the Sequence number must be identical
   from each candidate as a DAG root, regardless of which of parent.

5.6.  Administrative rank

   When the multiple DAG roots
   issues the DIO, and changes is formed under a common administration, or when a node
   performs a certain role within a community, it might be beneficial to the Sequence number
   associate a range of acceptable rank with that node.  For instance, a
   node that has limited battery should be issued
   at the same time.  The specific mechanism of this coordination, e.g.
   along a backbone between leaf unless there is no
   other choice, and may then augment the rank computation specified by
   the OCP in order to expose an exaggerated rank.

5.7.  Candidate DAG Parent States and Stability

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

   Current     This candidate parent is beyond the scope of this
   specification.

5.4.3.  Trickle Timer for RA Transmission

   RPL treats in the construction set of a DAG as a consistency problem, parents and
   uses
               may be used for forwarding traffic inward along the DAG.
               When a trickle timer [Levis08] to control candidate parent is placed into the rate of control
   broadcasts.  The operation Current state,
               or taken out of this timer the Current state, it is in support necessary to re-
               evaluate which of the
   procedures further discussed in Section 5.4

   For each remaining DAG that a node parents is part of, the most
               preferred DAG parent and its rank.  At that time any
               remaining DAG parents of greater rank than this node must maintain a single
   trickle timer.  The required state contains
               be placed in the following conceptual
   items:

   I:    The current length of Held-Down state, and the communication interval

   T:    A hold-down timer with a duration set to a random value
               started, in order to be evicted as DAG parents.  In the range
         [I/2, I]

   C:    Redundancy Counter

   I_min:  The smallest communication interval in milliseconds.
               same fashion, siblings must also be reevaluated.

   Held-Up     This
         value is learned from the DIO as (2^DIOIntervalMin)ms.  The
         default value is DEFAULT_DIO_INTERVAL_MIN.

   I_doublings:  The number of times I_min should parent can not be doubled before
         maintaining a constant rate, i.e.  I_max = I_min *
         2^I_doublings. used until the DAG hop timer
               elapses.

   Held-Down   This value candidate parent can not be used till hold down
               timer elapses.  At the end of the hold-down period, the
               candidate is learned removed from the candidate DAG parent set,
               and may be reinserted if it appears again with a RA-DIO
               message.

   Collision   This candidate parent can not be used till its next RA-
               DIO as
         DIOIntervalDoublings.  The default value message.

5.7.1.  Held-Up

   This state is
         DEFAULT_DIO_INTERVAL_DOUBLINGS.

5.4.3.1.  Resetting managed by the Trickle Timer

   The trickle timer for DAG Hop timer, it serves 2 purposes:

      Delay the reattachment of a DAGID is reset by:

   1.  Setting I_min and I_doublings sub-DAG that has been forced to
      detach.  This is not as safe as the values learned from use of the RA-
       DIO.

   2.  Setting C to zero.

   3.  Setting I to I_min.

   4.  Setting T to sequence, but still
      covers that when a random value as described above.

   5.  Restarting sub-DAG has detached, the trickle timer to expire after a duration T

   When an LLN learns about a RA-DIO message that
      is initiated by the new DAG through root has a RA and makes the decision chance to
   join it, it initializes spread outward
      along the state sub-DAG, ideally forming a frozen sub-DAG that is aware
      of the trickle timer by resetting
   the trickle timer and listening.  Each time it hears a consistent RA
   for this DAG from a DAG Parent, it increments C.

   When the timer fires at time T, the node compares C change, such that two different DAGs have formed prior
      to the redundancy
   constant, DEFAULT_DIO_REDUNDANCY_CONSTANT.  If C an attempted reattachment.

      Limit RA-DIO message storms (control cost / churn) when two DAGs
      collide/merge.  The idea is less than that
   value, the node generates a new RA and broadcasts it.  When between the
   communication interval I expires, nodes from DAG A that
      decide to move to DAG B, those that see the node doubles highest place (closer
      to the interval I so
   long as it has previously doubled it fewer then I_doubling times,
   resets C, DAG root) in DAG B will move first and chooses a advertise their new T value.

5.4.3.2.  Determination of Inconsistency

   The trickle timer is reset whenever an inconsistency
      locations before other nodes from DAG A actually move.

   A new DAG is detected
   within the DAG, for example:

   o  The node joins discovered upon receiving a new DAGID

   o  The node moves within RA message with or without a DAGID

   o
   DIO.  The node receives a modified DIO from joins the DAG by selecting the source of the RA
   message as a DAG parent

   o  A (and possibly installing the DAG parent forwards as a packet intended for the
   default route,
      indicating an inconsistency and possible loop.

   o  A metric communicated in the DIO gateway).  The node is determined to be inconsistent,
      as according to then a implementation specific path metric selection
      engine.

   o  The rank member of the DAG and may begin
   to multicast RA-DIO messages containing the DIO for the DAG.

   When a new DAG is discovered, the candidate parent has changed.

   The implementation SHOULD provide an API whereby any procedure that
   detects an inconsistency may cause advertises
   the trickle timer to reset.

5.5. new DAG Heartbeat

   The is placed in a held up state for the duration of a DAG Root makes
   Hop timer.  If the sole determination resulting new set of when to revise DAG parents is more
   preferable than the
   DAGSequenceNumber by incrementing it upwards.  When current one, or if the
   DAGSequenceNumber node is increased an inconsistency results, causing RA-
   DIOs intending to be sent back outwards along
   maintain a membership in the new DAG in addition to convey its current DAG,
   the change.
   The degree node expects to which this mechanism jump and becomes unstable.

   A node that is relied on may be determined by
   the implementation- on one hand it unstable may serve as a periodic heartbeat,
   refreshing discover other candidate parents from the
   same new DAG states, and on during the other hand it may result in instability phase.  It needs to start a
   constant steady-state control cost overhead which is not desirable.

   Some implementations may provide an administrative API at the new
   DAG
   Root whereby Hop timer for all these.  The first timer that elapses for a
   given new DAG clears them all for that DAG, allowing the DAGSequenceNumber may be caused node to increment in
   response jump
   to some policy outside of the scope of RPL.

   Other implementations may make use highest position available in the new DAG.

   The duration of a periodic timer to
   automatically increment the DAGSequenceNumber, resulting in a
   periodic DAG Heartbeat at a rate appropriate to Hop timer depends on the application and
   implementation.

5.6. DAG Selection

   The Delay of the new
   DAG selection is implementation and algorithm dependent.  Nodes
   SHOULD prefer to join DAGs advertising OCPs and destinations
   compatible with their implementation specific objectives.  In on the rank of candidate parent that triggers it: (candidates
   rank + random) * candidate's DAG_delay (where 0 <= random < 1).  It
   is randomized in order to limit erratic movements, collisions and all metrics being equal, nodes SHOULD
   keep their previous selection.  Also, nodes SHOULD provide synchronizations.

5.7.2.  Held-Down

   When a means neighboring node is 'removed' from the Default Router List, it
   is actually held down for a hold down timer period, in order to
   filter out
   prevent flapping.  This happens when a candidate parent whose availability node disappears (upon
   expiration timer).

   When the hold down timer elapses, the node is detected as
   fluctuating, at least when more stable choices are available.  Nodes
   MAY place removed from the failed
   candidate DAG parent in a Hold Down mode that
   ensures that set.

5.7.3.  Collision

   A race condition occurs if 2 nodes send RA-DIO messages at the candidate parent will not be reused for a given
   period of time.

   When connection same
   time and then attempt to a fixed network is not possible or preferable join each other.  This might happen, for
   security or other reasons, scattered DAGs MAY aggregate as much
   example, between nodes which act as
   possible into larger DAGs in DAG root of their own DAGs.  In
   order to allow connectivity detect the situation, LLN Nodes time stamp the sending of
   RA-DIO message.  Any RA-DIO message received within a short link-
   layer-dependent period introduces a risk.  To resolve the
   LLN.  How to balance these DAGs is implementation dependent, and MAY
   use collision,
   a specific visitor-counter suboption in 32bits extended preference is constructed from the DIO. RA-DIO message
   by concatenating the NodePreference with the BootTimeRandom.

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

5.7.  Administrative parents will do so
   between (candidate rank) and (candidate rank

   When + 1) times the DAG is formed under a common administration, or when a node
   performs a certain role within a community, it might be beneficial to
   associate a range of acceptable rank with that node.  For instance, candidate
   DAG Delay.  But since a node that has limited battery should be a leaf unless there is no
   other choice, unstable as soon as it receives the
   RA-DIO message from the desired candidate, it will restrain from
   sending a RA-DIO message between the time it receives the RA and the
   time it actually jumps.  So the crossing of RA may then augment only happen during
   the rank computation specified by propagation time between the OCP in order to expose an exaggerated rank.

5.8.  Candidate DAG Parent States candidate and Stability

   Candidate DAG Parents may or may not be eligible to act as the node, plus some
   internal queuing and processing time within each machine.  It is
   expected that one DAG
   Parents depending on runtime conditions.  The following states are
   defined:

   Current     This candidate parent delay normally covers that interval, but
   ultimately it is in up to the set of DAG parents implementation and
               may be used for forwarding traffic inward along the DAG.
               When a configuration of
   the candidate parent is placed into to define the Current state,
               or taken out duration of the Current state, it risk window.

   There is necessary to re-
               evaluate which risk of the remaining DAG Parents a collision when a node receives an RA, for another
   candidate that is more preferable than the most
               preferred DAG Parent and its rank.  At that time any
               remaining DAG Parents current candidate, within
   the risk window.  In the face of greater rank than a potential collision, the most
               preferred DAG parent must be placed in node with
   lowest extended preference processes the Held-Down
               state, and RA-DIO message normally,
   while the router with the highest extended preference places the hold-down timer started,
   other in order to be
               evicted as DAG Parents.

   Held-Up     This parent can collision state, does not be used until start the DAG hop timer
               elapses.

   Held-Down   This candidate parent can timer, and does
   not be used till hold down
               timer elapses.  At become instable.  It is expected that next RAs between the end of two
   will not cross anyway.

   For example, consider a case where two nodes are each rooting their
   own transient floating DAGs and multicast RA-DIO messages towards
   each other in a close enough interval that the hold-down period, RA-DIO messages
   `cross'.  Then each node may receive the
               candidate is removed RA-DIO message from the Candidate DAG Parent set,
   other node, and may be reinserted if it appears again with a RA.

   Collision   This candidate parent can not be used till its next RA.

5.8.1.  Held-Up

   This state is managed by in some scenario decide to join each others DAG.  RPL
   avoids this deadlock scenario via the DAG Hop timer, it serves 2 purposes:

      Delay collision mechanism described
   above - after each node sends the reattachment of a sub-DAG that has been forced to
      detach.  This RA-DIO message they will enter the
   risk window.  When the peer RA-DIO message is not as safe as received in the use of risk
   window, the sequence, but still
      covers that when a sub-DAG has detached, nodes will calculate the Router Advertisement
      - DAG Information Option that is initiated by extended preferences as describe
   above and the new DAG root has
      a chance node with the lowest extended preference will proceed
   to spread outward along process the sub-DAG so that two different
      DAGs have formed.

      Limit RA-DIO storms when two DAGs collide/merge.  The idea is that
      between message, while the nodes from DAG other node will defer,
   avoiding the deadlock scenario.

5.7.4.  Instability

   A that decide to move node is instable when it is prepared to shortly replace a set of
   DAG B, those
      that see the highest place (closer parents in order to jump to a different DAGID.  This happens
   typically when the DAG root) node has selected a more preferred candidate
   parent in a different DAG B will
      move first and advertise their new locations has to wait for the DAG hop timer to
   elapse before other nodes
      from adjusting the DAG A actually move.

   A new parent set.  Instability may also
   occur when the entire current DAG parent set is discovered upon a router advertisement message with or
   without a RA-DIO.  The node joins lost and the DAG by selecting next
   best candidates are still held up.  Instability is resolved when the source
   DAG hop timer of all the RA message as a DAG parent (and possible default gateway) and
   propagating candidate(s) causing instability elapse.
   Such candidates then change state to Current or Held- Down.

   Instability is transient (in the DIO accordingly. order of DAG hop timers).  When a new DAG
   node is discovered, unstable, it MUST NOT send RAs with the DIO message.  This
   avoids loops when node A decides to attach to node B and node B
   decides to attach to node A. Unless RAs cross (see Collision
   section), a node receives RA-DIO messages from stable candidate parent that advertises
   parents, which do not plan to attach to the new DAG is placed in a held up state 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 duration selection of a DAG
   Hop timer.  If the resulting new set to join,
   and a number of peers in that DAG as parents.  The OF is used to
   compute an ordered list of parents and provides load balancing
   guidance.  The OF is more
   preferable than also responsible to compute the current one, or if rank of the node
   device within the DAG.

   The Objective Function is intending to
   maintain a membership specified in the new RA-DIO message using an
   objective code point (OCP) 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").  The objective code points
   are specified in addition to its current DAG, [I-D.ietf-roll-routing-metrics].  This document
   specifies the node expects OCP 0, in support of default operation.

   Most Objective Functions are expected to jump and becomes unstable.

   A node follow the same abstract
   behavior:

   o  The parent selection is triggered each time an event indicates
      that a potential next_hop information is unstable may discover other updated.  This might
      happen upon the reception of a RA-DIO message, a timer elapse, or
      a trigger indicating that the state of a candidate parents from neighbor has
      changed.

   o  An OF scans all the
   same new DAG during interfaces on the instability phase.  It needs device.  Although there may
      typically be only one interface in most application scenarios,
      there might be multiple of them and an interface might be
      configured to start be usable or not for RPL operation.  An interface
      can also be configured with a preference or dynamically learned to
      be better than another by some heuristics that might be link-layer
      dependent and are out of scope.  Finally an interface might or not
      match a new
   DAG Hop timer required criterion for all these.  The first timer that elapses an Objective Function, for instance
      a
   given new DAG clears them all for that DAG, allowing the node to jump
   to the highest position available in 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 on the rank of a candidate parent that triggers it: (candidates
   rank + random) * candidate's DAG_delay (where 0 <= random < 1).  It
   is randomized in order
      neighbor might need to limit collisions and synchronizations.

5.8.2.  Held-Down

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

   When the hold down timer elapses, the node is removed from the
   Candidate DAG Parent set.

5.8.3.  Collision

   A race condition occurs if 2 nodes send RA-DIO at enable the same time and
   then attempt to join each other.  This might happen, for example,
   between nodes which act router as DAG root a next_hop.

   o  The OF computes self's rank by adding the step of their own DAGs.  In order rank to that
      candidate to
   detect the situation, LLN Nodes time stamp the sending rank of RA-DIO.
   Any RA-DIO received within that candidate.  The step of rank is
      estimated as follows:

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

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

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

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

   A node acceptable
            threshold.

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

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

   o  As it scans all the candidate to its DAG parents will do so
   between (candidate rank) neighbors, the OF keeps the current
      best parent and (candidate rank + 1) times compares its capabilities with the current
      candidate
   DAG Delay.  But since neighbor.  The OF defines a node is unstable as soon as it receives number of tests that are
      critical to reach the
   RA-DIO from Objective.  A test between the desired candidate, it will restrain from sending a
   RA-DIO routers
      determines an order relation.

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

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

      *  Some OFs may only happen during include a test to compare the propagation
   time between ranks that would
         result if the candidate and node joined either router

   o  When the node, plus some internal queuing
   and processing time within each machine.  It is expected that one DAG
   delay normally covers that interval, but ultimately it scan is up to complete, the
   implementation preferred parent is elected and
      self's rank is computed as the configuration of the candidate preferred parent rank plus the step
      in rank with that parent.

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

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

      *  Candidate neighbors that are of risk window.

   There is risk worse rank than self are
         ignored

      *  Candidate neighbors of a collision when a node receives an RA, better rank than self (non-siblings)
         are preferred

5.8.2.  Objective Code Point 0 (OCP 0)

   Here follows the specification for another
   candidate that the default Objective Function
   corresponding to OCP codepoint 0.  This is a very simple reference to
   help design more preferable than complex Objective Functions.  In particular, the current candidate, within
   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 risk window.  In RA-DIO message such as rank and
   administrative preference.

   This document specifies 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 face policy of a potential collision, the processing node with
   lowest extended preference processes when no
   objective function is included in the RA-DIO normally, while message, or if the
   router with OF
   indicated in the highest extended preference places RA-DIO message is unknown to the other in
   collision state, does node.  If not start
   allowed, then the DAG hop timer, RA-DIO message is simply ignored and does not
   become instable.  It is expected that next RAs between processed
   by the two will
   not cross anyway.

5.8.4.  Instability

   A node is instable when it node.

5.8.2.1.  OCP 0 Objective Function (OF0)

   OF0 favors the connectivity.  That is, the 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 a backup next_hop if one is
   available.  The backup next_hop might be a parent or a sibling.  All
   the traffic is routed via the preferred parent.  When the link
   conditions do not let a packet through to wait the preferred parent, the
   packet is passed to the backup next_hop.

   The step of rank is 4 for each hop.

5.8.2.2.  Selection of the DAG hop timer Preferred Parent

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

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

   2.   A candidate that would cause the node to
   elapse before adjusting augment the DAG parent set.  Instability may also
   occur when rank in the entire
        current DAG parent set is lost and the next
   best candidates are still held up.  Instability not considered.

   3.   A router that has been validated as usable, e.g. with a local
        confidence that has exceeded some pre-configured threshold, is resolved when the
   DAG hop timer of all the candidate(s) causing instability elapse.
   Such candidates
        better.

   4.   If none are grounded then change state to Current or Held- Down.

   Instability is transient (in the order of a DAG hop timers).  When with a
   node more preferred
        administrative preference is unstable, it MUST NOT send RAs with DIO.  This avoids loops
   when node better.

   5.   A decides to attach to node B and node B decides to attach router that offers connectivity to node A. Unless RAs cross (see Collision section), a node receives
   DIO from stable candidate parents, grounded DAG is better.

   6.   A lesser resulting rank is better.

   7.   A DAG for which do not plan to attach to the
   node, so there is an alternate parent is better.  This
        check is optional.  It is performed by computing the node can safely attach to them.

5.9.  Guidelines for Objective Code Points

5.9.1.  Objective Function

   An objective function (OF) selects a DAG to join, and a number of
   peers in backup
        next_hop while assuming that this router won.

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

   9.   The OF computes an ordered list of
   parents and provides load balancing guidance. router with a better router preference wins.

   10.  The OF preferred parent that was in use already is also
   responsible to compute the rank better.

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

5.8.2.3.  Selection of the device within Backup next_hop

   o  The interface must be usable and the DAG.

   An Objective Function administrative preference (if
      any) applies first.

   o  The preferred parent is indicated ignored.

   o  Candidate neighbors that are not in the DIO using an objective code
   point (OCP).  The objective code point same DAG are administered by IANA that
   might delegate some ranges to other organizations.  This
   specification reserves OCP 0, in support 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 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, 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 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 a Candidate Neighbor has changed.

   o  An OF scans all the interfaces on `stateful' nodes.
      *  Source Routing `bridges' across nodes who do not retain state.

   Destinations disseminated with the device.  Although there destination advertisement
   mechanism may
      typically be only one interface in most application scenarios,
      there might be multiple prefixes, individual hosts, or multicast listeners.
   The mechanism supports nodes of them varying capabilities as follows:

   o  When nodes are capable of storing routing state, they may inspect
      destination advertisements and an interface might be
      configured to be usable or not for RPL operation.  An interface
      can also be configured learn hop-by-hop routing state
      toward destinations by populating their routing tables with a preference or dynamically the
      routes learned from nodes in their sub-DAG.  In this process they
      may also learn necessary piecewise source routes to
      be better than another by some heuristics that might be link-layer
      dependent and are out of scope.  An interface might not be ready
      for IPv6 operation with a usable link-local address.  Finally an
      interface might or not match a required criterion for an Objective
      Function, for instance a degree traverse
      regions of security.  As a result some
      interfaces might be completely excluded from the computation,
      while others might be more or less preferred.

   o  The OF scans all the Candidate Neighbors LLN that do not maintain routing state.  They may
      perform route aggregation on the possible
      interfaces to check whether known destinations before emitting
      Destination Advertisements.

   o  When nodes are incapable of storing routing state, they can act as an attachment router
      for a DAG.  There might be multiple may
      forward destination advertisements, recording the reverse route as
      the go in order to support the construction of them piecewise source
      routes.

   Nodes that are capable of storing routing state, and a Candidate
      Neighbor might need to pass some validation tests before it can be
      used.  In particular, some link layers require experience on finally the
      activity with a router DAG
   roots, are able to enable learn which destinations are contained in the sub-
   DAG below the node, and via which next-hop neighbors.  The
   dissemination and raise installation of this routing state into nodes
   allows for Hop-By-Hop routing from the router value as a
      next_hop.

   o DAG root outwards along the
   DAG.  The OF computes self's rank mechanism is further enhance by adding supporting the step construction
   of rank to that
      candidate to source routes across stateless `gaps' in the rank DAG, where nodes are
   incapable of that candidate.  The step storing additional routing state.  An adaptation of rank is
      estimated as follows:

      *  When a router has reached a value that's qualified as normal, this
   mechanism allows for the step implementation of rank for that hop is 4.

      *  The step loose-source routing.

   A special case, the reception of rank might vary from 1 a destination advertisement
   addressed to 16.

         +  1 indicates a unusually good link, link-local multicast address, allows for instance a link
            between powered devices in a mostly battery operated
            environment.

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

      *  Candidate Neighbors that would cause self's rank
   learn destinations directly available from its one-hop neighbors.

   A design choice behind advertising routes via destination
   advertisements is not to increase
         are ignored

   o  As it scans all the Candidate Neighbors, the OF keeps synchronize the current
      best parent and compares its capabilities with children
   databases along the current
      Candidate Neighbor.  The OF defines a number of tests that are
      critical DAG, but instead to reach the Objective.  A test between the routers
      determines an order relation.

      *  If update them regularly to
   recover from the routers are roughly equal loss of packets.  The rationale for that relation then the
         next test choice is attempted between the routers,

      *  Else
   time variations in connectivity across unreliable links.  If the best
   topology can be expected to change frequently, synchronization might
   be an excessive goal in terms of the 2 becomes the current best parent exchanges and the
         scan continues protocol complexity.
   The approach used here results in a simple protocol with no real
   peering.  The destination advertisement mechanism hence provides for
   periodic updates of the next Candidate 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

      *  One of these tests might Discovery [RFC4861] and RFC4191 [RFC4191] to
   allow a node to include comparing the resulting ranks
         but it isn't necessarily so

   o  When a destination advertisement option, which
   includes prefix information, in the scan Neighbor Advertisement (NA)
   messages.  A prefix option is complete, normally present in RA messages only,
   but the preferred parent is elected and
      self's rank NA is computed as the preferred parent rank plus the step
      in rank augmented with that parent.

   o  Other rounds of scans might be necessary this option in order to elect alternate
      parents and siblings.  Self's rank propagate
   destination information inwards along the DAG.  The option is now determined by named
   the new
      preferred parent if it has changed.  In Destination Advertisement Option (DAO), and an NA message
   containing this option may be referred to as a destination
   advertisement, or NA-DAO.  The RPL use of destination advertisements
   allows the next rounds:

      *  Candidate Neighbors that are not nodes in the same DAG are ignored

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

      *  Candidate Neighbors of a better rank than self (non-siblings)
         are preferred

5.9.2.  Objective Code Point 0 (OCP 0)

   Here follows the specification for the Objective Function build up routing state for OCP 0.
   This is a very simple references to help design more complex
   Objective Functions.  In particular, nodes
   contained in the Objective Function described
   here does not use physical metrics as described sub-DAG in
   [I-D.ietf-roll-routing-metrics], but are only based on abstract
   information from support of traffic flowing outward along
   the DAG.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     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 Destination Advertisement Option (DAO)

   Type: 8-bit unsigned identifying the DIO such as rank and administrative preference.

   OCP 0 is as a default fall back behavior when a node joins a DAG but
   does not support Destination Advertisement
         option.  IANA had defined the OF that's preferred IPv6 Neighbor Discovery Option
         Formats registry.  The suggested type value for this DAG.

5.9.2.1.  OCP 0 Objective Function (OF0)

   OF0 favors the connectivity.  That is, the Objective Function Destination
         Advertisement Option carried within a NA message is
   designed 141, to find be
         confirmed by IANA.

   Length:  8-bit unsigned integer.  The length of the nearest sink into a 'grounded' topology, option (including
         the Type and if
   there's none then join any network per order Length fields) in units of administrative
   preference.

   OF0 selects a preferred parent and a backup next_hop if that's
   available.  The backup next_hop might be a parent or a sibling.  All 8 octets.

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

   RRCount:  8-bit unsigned integer.  This counter is routed via used to count the preferred parent.  When
         number of entries in the link
   conditions do not let a packet through Reverse Route Stack.  A value of `0'
         indicates that no Reverse Route Stack is present.

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

   Route Tag:  32-bit unsigned integer.  The Route Tag may be used to
         give a priority to prefixes that should be stored.  This may be
         useful in cases where intermediate nodes are capable of storing
         a limited amount of routing state.  The step further specification
         of rank this field and its use is 4 for each hop.

5.9.2.2.  Selection of under investigation.

   DAO Depth:  Set to 0 by the Preferred Parent

   As it scans all node that owns the Candidate Neighbors, OF0 keeps prefix and first
         issues the parent NA-DAO message.  Incremented by all LLN nodes that is
   the best for
         propagate the following criteria (in order):

   1. NA-DAO message.

   Reserved:  8-bit unused field.  The interface must reserved field MUST be usable set to
         zero on transmission and the administrative preference
        (if any) applies first.

   2.   A candidate that would cause MUST be ignored on receipt.

   DAO Sequence:  Incremented by the node to augment the rank in the
        current DAG is not considered.

   3.   A router that is validated as usable is better.

   4.   If none are grounded then a DAG with a better DAG preference
        wins.

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

   6.   A lesser resulting rank is better.

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

   8.   The DAG prefix for each
         new NA-DAO message for that was in use already is preferred.

   9.   The router with prefix.

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

   10. prefix
         of an IPv6 address.  The preferred parent that was in use already is better.

   11.  A router that is fresher (most recent RA) is better.

5.9.2.3.  Selection Prefix Length field contains the
         number of valid leading bits in the Backup next_hop

   o prefix.  The interface must be usable and bits in the administrative preference
         prefix after the prefix length (if any) applies first.

   o are reserved and MUST
         be set to zero on transmission and MUST be ignored on receipt.

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

5.9.2.  Destination Advertisement Operation

5.9.2.1.  Overview

   According to implementation specific policy, a subset or all of the
   feasible parents in the
      current DAG is not considered.

   o  The preferred parent is ignored

   o  Candidate Neighbors that are not in may be selected to receive prefix
   information from the same destination advertisement mechanism.  This
   subset of DAG are ignored

   o  Candidate Neighbors that would cause self's rank (from that
      determined by parents shall be designated the preferred parent) to increase are ignored

   o  Candidate Neighbors set of DA parents.

   As NA-DAO messages for particular destinations move inwards along the
   DAG, a better rank than self (non-siblings) are
      preferred

   o  A router that is validated as usable sequence counter is better

   o  The router with a better router preference wins

   o used to guarantee their freshness.  The backup next_hop that was in use already
   sequence counter is better.

5.10.  Establishing Routing State Outward Along the DAG

   The Destination Advertisement mechanism supports incremented by the dissemination source of
   routing state required to support traffic flows outward along the
   DAG, from the DAG root toward nodes.

   Note NA-DAO message
   (the node that some aspects of owns the prefix, or learned the Destination Advertisement mechanism are
   still under investigation.

   As a result of Destination Advertisement operation:

   o  DAG Discovery establishes prefix via some other
   means), each time it issues a DAG oriented toward NA-DAO message for its prefix.  Nodes
   who receive the NA-DAO message and, if scope allows, will be
   forwarding a DAG root using
      extended Neighbor Discovery RS/RA flows, along which inward routes
      toward NA-DAO message for the DAG root are set up.

   o  Destination Advertisement extends Neighbor Discovery in order to
      establish outward routes unmodified destination inwards
   along the DAG, along paths containing DA
      parents.  Such paths consist of:
      *  Hop-By-Hop routing state within islands of `stateful' nodes.
      *  Source Routing `bridges' across will leave the sequence number unchanged.
   Intermediate nodes who do will check the sequence counter before processing
   a NA-DAO message, and if the DAO is unchanged (the sequence counter
   has not retain state.

   Destinations disseminated with changed), then the Destination Advertisement
   mechanism may NA-DAO message will be prefixes, individual hosts, or multicast listeners.
   The mechanism supports nodes of varying capabilities as follows:

   o  When nodes are capable of storing routing state, they may inspect
      Destination Advertisements and learn hop-by-hop routing state
      toward destinations.  In this process they may also learn
      necessary piecewise source routes discarded without
   additional processing.  Further, if the NA-DAO message appears to traverse regions be
   out of synch (the sequence counter is 2 or more behind the LLN
      that do not maintain routing state.  They may perform route
      aggregation on known destinations before emitting Destination
      Advertisements.

   o  When nodes are incapable of storing routing state, they present
   value) then the DAO state is considered to be stale and may
      forward Destination Advertisements, recording be
   purged, and the reverse route NA-DAO message is discarded.  A depth is also added
   for tracking purposes; the depth is incremented at each hop as the go in order to support
   NA-DAO message is propagated up the construction of piecewise source
      routes. DAG.  Nodes that who are capable of storing
   routing state, and finally state may use the DAG
   roots, are able depth to learn determine which destinations possible next-hops
   for the destination are contained more optimal.

   If destination advertisements are activated in the sub-
   DAG below RA-DIO message as
   indicated by the node, and via which next-hop neighbors.  The
   dissemination `D' bit, the node sends unicast destination
   advertisements to its DA parents, and installation of this routing state into nodes
   allows for Hop-By-Hop routing only accepts unicast
   destination advertisements from any nodes but those contained in the
   DA parent subset.

   Every NA to a DA parent MAY contain one or more DAOs.  Receiving a
   RA-DIO message with the `D' destination advertisement bit set from a
   DAG root outwards along parent stimulates the
   DAG.  The mechanism is further enhance by supporting sending of a delayed destination
   advertisement back, with the construction collection of source routes across stateless `gaps' all known prefixes (that
   is the prefixes learned via destination advertisements for nodes
   lower in the DAG, where nodes are
   incapable of storing additional routing state.  An adaptation and any connected prefixes).  If the Destination
   Advertisement Supported (A) bit is set in the RA-DIO message for the
   DAG, then a destination advertisement is also sent to a DAG parent
   once it has been added to the DA parent set after a movement, or when
   the list of this
   mechanism allows advertised prefixes has changed.  Destination
   advertisements may also be scheduled for sending when the implementation PathDigest
   of loose-source 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 landmark
   (waypoint) routing. negative (removed) NA-DAO messages, termed as no-DAOs.  A special case, no-DAO
   is stimulated by the reception disappearance of a Destination Advertisement
   addressed to prefix below.  This is
   discovered by timing out after a link-local multicast address, allows for request (a RA-DIO message) or by
   receiving a node to
   learn destination prefixes directly available from its one-hop
   neighbors.

   The design choice behind this no-DAO.  A no-DAO is not to synchronize the parent and
   children databases along the DAG, but instead to update them
   regularly to cover from the loss a conveyed as a NA-DAO message with
   a DAO Lifetime of packets.  The rationale for that
   choice 0.

   A node who is time variations in connectivity across unreliable links.
   If capable of recording the topology can be expected to change frequently, synchronization
   might be an excessive goal state information conveyed in terms of exchanges
   a unicast NA-DAO message will do so upon receiving and protocol
   complexity.  The approach used here results processing the
   NA-DAO message, thus building up routing state concerning
   destinations below it in the DAG.  If a simple protocol with
   no real peering.  The Destination Advertisement mechanism hence
   provides for periodic updates node capable of recording
   state information receives a NA-DAO message containing a Reverse
   Route Stack, then the derivative routing state, as
   cued by occasional RAs and other mechanisms, similarly to other
   protocols such node knows that the NA-DAO message has
   traversed one or more nodes that did not retain any routing state as RIP [RFC2453].

5.10.1.  Destination Advertisement Message Formats

5.10.1.1.
   it traversed the path from the DAO Option

   RPL extends Neighbor Discovery [RFC4861] and RFC4191 [RFC4191] source to
   allow a the node.  The node to include a Destination Advertisement option, which
   includes prefix information, in may
   then extract the Neighbor Advertisements (NAs).  A
   prefix option is normally present in Router Advertisements (RAs)
   only, but Reverse Route Stack and retain the NA is augmented with this option included state in
   order to propagate
   destination information inwards specify Source Routing instructions along the DAG. return path
   towards the destination.  The option is named node MUST set the Destination Advertisement Option (DAO), and an NA containing this
   option may be referred RRCount back to as a Destination Advertisement.  The RPL
   use of Destination Advertisements allows zero
   and clear the nodes in Reverse Route Stack prior to passing the DAG NA-DAO message
   information on.

   A node who is unable to
   build up routing record the state for nodes contained information conveyed in the sub-DAG in support
   of traffic flowing outward along
   NA-DAO message will append 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                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | next-hop address to the Reverse Route Tag                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   DAO Depth   |   Reserved    |         DAO Sequence          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Prefix (Variable Length)                    |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |
   Stack, increment the RRCount, and then pass the destination
   advertisement on without recording any additional state.  In this way
   the Reverse Route Stack (Variable Length)             |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

             Figure 10: Destination Advertisement Option (DAO)

   Type: 8-bit unsigned identifying will contain a vector of next hops that must
   be traversed along the Destination Advertisement
         option. reverse path that the NA-DAO message has
   traveled.  The value vector will be ordered such that the node closest to
   the destination will appear first in the list.  In such cases, if it
   is useful to be assigned the implementation to try and build up redundant paths,
   the node may choose to convey the destination advertisement to one or
   more DAG parents in order of preference as guided by an
   implementation specific policy.

   In some cases (called hybrid cases), some nodes along the IANA.

   Length:  8-bit unsigned integer. path a
   destination advertisement follows inward along the DAG may store
   state and some may not.  The length destination advertisement mechanism
   allows for the provisioning of routing state such that when a packet
   is traversing outwards along the option (including DAG, some nodes may be able to
   directly forward to the Type next hop, and Length fields) other nodes may be able to
   specify a piecewise source route in units of 8 octets.

   Prefix Length:  Number order to bridge spans of valid leading bits in
   stateless nodes within the IPv6 Prefix.

   RRCount:  8-bit unsigned integer.  This counter path on the way to the desired
   destination.

   In the case where no node is used able to store any routing state as
   destination advertisements pass by, and the DAG root ends up with NA-
   DAO messages that contain a completely specified route back to count the
         number of entries
   originating node in the form of the inverted Reverse Route Stack.  A value of `0'
         indicates that no
   DAG root should not request (Destination Advertisement Trigger) nor
   indicate support (Destination Advertisement Supported) for
   destination advertisements if it is not able to store the Reverse
   Route Stack is present.

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

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

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

   o  The IPv6 address and interface for route determination.  A value the advertising neighbor.

   o  The logical equivalent of all one
         bits (0xFFFFFFFF) represents infinity. the full destination advertisement
      information (including the prefixes, depth, and Reverse Route
      Stack, if any).

   o  A value 'reported' Boolean to keep track whether this prefix was
      reported already, and to which of all zero
         bits (0x00000000) indicates a loss the DA parents.

   o  A counter of reachability.

   Route Tag:  32-bit unsigned integer.  The Route Tag may be used retries to
         give a priority count how many RA-DIO messages were sent
      on the interface to prefixes the advertising neighbor without reachability
      confirmation for the prefix.

   Note that should be stored.  This may be
         useful in cases where intermediate nodes are capable of storing may receive multiple information from different
   neighbors for a limited amount of routing state.  The further specification
         of this field and its use is under investigation.

   DAO Depth:  Set to 0 by specific destination, as different paths through the
   DAG may be propagating information inwards along the DAG for the same
   destination.  A node that owns who is recording routing state will keep track
   of the prefix information from each neighbor independently, and first
         issues the DAO.  Incremented by all LLN nodes that when it
   comes time to propagate the DAO.

   Reserved:  8-bit unused field.  It MUST be initialized NA-DAO message for a particular prefix to zero by
   the
         sender and MUST be ignored by DA parents, then the receiver. DAO Sequence:  Incremented by the node that owns information will be selected from among
   the prefix for each
         new DAO for that prefix.

   Prefix:  Variable-length field containing an IPv6 address or a prefix
         of an IPv6 address.  The Prefix Length field contains advertising neighbors who offer the
         number of valid leading bits in least depth to the prefix.
   destination.

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

   The Connected list corresponds to zero by the sender prefixes owned and ignored managed by
   the
         receiver.

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

   The Reachable list contains prefixes for which the node 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
         increment in the RRCount.

5.10.2.  Destination Advertisement Operation

5.10.2.1.  Overview

   Note that some aspects process of being deleted, in order to send NA-DAO
   messages with zero lifetime (also called no-DAO) to the DA parents.

5.9.2.1.1.  Destination Advertisement mechanism are
   still under investigation

   According to implementation specific policy, a subset or all of Advertisement Timers

   The destination advertisement mechanism requires 2 timers; the
   feasible parents in
   DelayNA timer and the DAG may be selected RemoveTimer.

   o  The DelayNA timer is armed upon a stimulation to receive prefix
   information send a
      destination advertisement (such as a RA-DIO message from a DA
      parent).  When the Destination Advertisement mechanism.  This
   subset of DAG parents shall be designated timer is armed, all entries in the Reachable
      list as well as all entries for Connected list are set of to not be
      reported yet for that particular DA parents.

   RPL takes advantage of the DAG structure and allows parent.

   o  The DelayNA timer has a node capable duration that is DEF_NA_LATENCY divided by
      a multiple of
   storing sufficient routing state to autonomously discover the
   destinations below itself through the operation DAG rank of the Destination
   Advertisement mechanism.  This allows participating node.  The intention is that
      nodes to build up
   routing state to support traffic flowing outwards along located deeper in the DAG.
   Destination Advertisement DAG should have a shorter DelayNA
      timer, allowing NA-DAO messages convey the necessary information a chance to learn be reported from
      deeper in the destinations.

   As Destination Advertisements for particular destinations move
   inwards DAG and potentially aggregated along the DAG, a sequence counter sub-DAGs before
      propagating further inwards.

   o  The RemoveTimer is used to guarantee their
   freshness.  The sequence counter is incremented by the source of clean up entries for which NA-DAO
      messages are no longer being received from the
   DAO (the node sub-DAG.

      *  When a RA-DIO message is sent that owns the prefix), each time it issues is requesting destination
         advertisements, a DAO flag is set for
   its prefix.  Nodes who receive all DAO entries in the
         routing table.

      *  If the DAO and, if scope allows, will be
   forwarding flag has already been set for a DAO for entry, the unmodified destination inwards along retry
         count is incremented.

      *  If a NA-DAO message is received to confirm the
   DAG, will leave entry, the sequence number unchanged.  Intermediate nodes
   will check entry
         is refreshed and the sequence counter before processing flag and count may be cleared.

      *  If at least one entry has reached a DAO, threshold value and if the
   DAO
         RemoveTimer is unchanged (the sequence counter has not changed), then running, the DAO
   will entry is considered to be discarded without additional processing.  Further, if
         probably gone and the DAO
   appears RemoveTimer is started.

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

   o  The RemoveTimer has a duration of synch (the sequence counter min (MAX_DESTROY_INTERVAL,
      RA_INTERVAL).

5.9.2.2.  Multicast Destination Advertisement messages

   It is 2 or more behind also possible for a node to multicast a NA-DAO message to the present value) then
   link-local scope all-nodes multicast address FF02::1.  This message
   will be received by all node listening in range of the DAO state emitting node.
   The objective is considered to enable direct P2P communication, between
   destinations directly supported by neighboring nodes, without needing
   the RPL routing structure to relay the packets.

   A multicast NA-DAO message MUST be stale and
   may used only to advertise information
   about self, i.e. prefixes in the Connected list or addresses owned by
   this node.  This would typically be a multicast group that this node
   is listening to or a global address owned by this node, though it can
   be purged, and the DAO is discarded. used to advertise any prefix owned by this node as well.  A depth
   multicast NA-DAO message is also added not used for
   tracking purposes; the depth is incremented at each hop as the DAO is
   propagated up the DAG.  Nodes who are storing routing state may use and does not presume
   any DAG relationship between the depth emitter and the receiver; it MUST
   NOT be used to determine which possible next-hops for relay information learned (e.g. information in the destination
   are more optimal.

   If Destination Advertisements are activated
   Reachable list) from another node; information obtained from a
   multicast NA-DAO MAY be installed in the DIO as indicated routing table and MAY be
   propagated by the `D' bit, the node sends a router in unicast Destination Advertisements NA-DAOs.

   A node receiving a multicast NA-DAO message addressed to
   its DA parents, and only accepts unicast Destination Advertisements
   from any nodes BUT those FF02::1 MAY
   install prefixes contained in the DA parent subset.

   Every NA to a DA parent MAY contain one or more DAOs.  Receiving NA-DAO message in the routing table
   for local use.  Such a
   DAG Discovery RA-DIO with node MUST NOT perform any other processing on
   the `D' NA-DAO message (i.e. such a node does not presume it is a DA
   parent).

5.9.2.3.  Unicast Destination Advertisement bit set messages from a DAG child to
          parent stimulates the

   When sending of a delayed Destination
   Advertisement back, with the collection of all known prefixes (that
   is destination advertisement to a DA parent, a node
   includes the prefixes learned via Destination Advertisements DAOs for nodes
   lower prefix entries not already reported (since the
   last DA Trigger from an RA-DIO message) in the DAG, Reachable and any connected prefixes).  If
   Connected lists, as well as no-DAOs for all the Destination
   Advertisement Supported (A) bit is set entries in the DIO for
   Unreachable list.  Depending on its policy and ability to retain
   routing state, the DAG, then receiving node SHOULD keep a
   Destination Advertisement is also sent record of 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 DAG parent once it has
   been added route to the DA parent set after a movement, or when prefix reported in the list of
   advertised prefixes has changed.  Destination Advertisements may also
   be scheduled for sending when NA-DAO
   message via the PathDigest link local address of the DIO has changed,
   indicating that some aspect of reporting neighbor and it
   SHOULD further propagate the inwards paths along information in a NA-DAO message.

   The RA-DIO message from the DAG has
   been modified.

   Destination Advertisements may advertise positive (prefix root is present)
   or negative (removed) DAOs.  A no-DAO used to synchronize the whole
   DAG, including the periodic reporting of destination advertisements
   back up the DAG.  Its period is stimulated by expected to vary, depending on the
   disappearance
   configuration of the trickle timer that governs the RAs.

   When a prefix below.  This is discovered by timing out
   after node receives a request (a RA-DIO) or by receiving RA-DIO message over an LLN interface from a no-DAO.  A no-DAO DA
   parent, the DelayNA is armed to force a
   conveyed as full update.

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

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

   A node who is capable of recording

   o  If the state information conveyed in
   a unicast DAO will do so upon receiving and processing entry is PENDING, the DAO, thus
   building up routing state concerning destinations below retry count is incremented.  If it in
      reaches a maximum threshold, the
   DAG. entry goes ELAPSED If a node capable at least
      one entry is ELAPSED at the end of recording state information receives a DAO
   containing a Reverse Route Stack, then the node knows that process: if the DAO
   has traversed one or more nodes that did Destroy
      timer is not retain any routing state
   as running then it traversed is armed with a jitter.

   Since the path from DelayNA timer has a duration that decreases with the DAO source depth,
   it is expected to receive all NA-DAO messages from all children
   before the node.  The node
   may then extract the Reverse Route Stack timer elapses and retain the included
   state in order full update is sent to specify Source Routing instructions along the
   return path towards DA
   parents.

   Once the destination.  The node MUST set RemoveTimer is elapsed, the RRCount
   back prefix entry is scheduled to zero be
   removed and clear the Reverse Route Stack prior moved to passing the
   DAO information on.

   A node who is unable Unreachable list if there are any DA parents
   that need to record be informed of the state information conveyed change in status for the
   DAO will append prefix,
   otherwise the next-hop address prefix entry is cleaned up right away.  The prefix
   entry is removed from the Unreachable list when no more DA parents
   need to be informed.  This condition may be satisfied when a no-DAO
   is sent to all current DA parents indicating the Reverse Route Stack,
   increment loss of the RRCount, prefix,
   and then pass noting that in some cases parents may have been removed from the Destination Advertisement on
   without recording any additional state.  In this way
   set of DA parents.

5.9.2.4.  Other events

   Finally, the Reverse
   Route Stack will come destination advertisement mechanism responds to contain a vector series
   of next hops that must be
   traversed along the reverse path that the DAO has traveled.  The
   vector will be ordered events, such that as:

   o  Destination advertisement operation stopped: All entries in the node closest to
      abstract lists are freed.  All the destination
   will appear first routes learned from NA-DAO
      messages are removed.

   o  Interface going down: for all entries in the list.  In such cases Reachable list on
      that interface, the node may choose to
   convey associated route is removed, and the Destination Advertisement entry is
      scheduled to one or more DAG Parents in
   order be removed.

   o  Loss of preference as guided by an implementation specific policy.

   In hybrid cases, some nodes along routing adjacency: When the path routing adjacency for a Destination
   Advertisement follows inward along
      neighbor is lost, as per the DAG may store state procedures described in Section 5.11,
      and some
   may not.  The Destination Advertisement mechanism allows for if the
   provisioning associated entries are in the Reachable list, the
      associated routes are removed, and the entries are scheduled to be
      destroyed.

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

5.9.2.5.  Aggregation of routing state such that when prefixes by a packet is traversing
   outwards along the DAG, some nodes node

   There may be able to directly forward to
   the next hop, and other nodes number of cases where a aggregation may be able to specify shared within
   a piecewise
   source route in order to bridge spans group of stateless nodes within the
   path on the way to the desired destination. nodes.  In the degenerate such a case, no node it is able possible to store any routing state as
   Destination Advertisements pass by, use aggregation
   techniques with destination advertisements and improve scalability.

   Other cases might occur for which additional support is required:

   1.  The aggregating node is attached within the DAG Root ends up with
   DAOs sub-DAG of the nodes
       it is aggregating for.

   2.  A node that contain a completely specified route back is to be aggregated for is located somewhere else
       within the
   originating node DAG, not in the form sub-DAG of the inverted Reverse Route Stack. aggregating node.

   3.  A
   DAG Root should not request nor indicate support for Destination
   Advertisements if it node that is not able to store the Reverse Route Stack
   information in the degenerate case.

   Information learned through Destination Advertisements can be
   redistributed aggregated for is located somewhere else in a routing protocol, MANET or IGP.  But the MANET or
       the IGP SHOULD NOT be redistributed into Destination Advertisements.
   This creates LLN.

   Consider a hierarchy of routing protocols where DA routes stand
   somewhere between connected node M who is performing an aggregation, and IGP routes.

   The Destination Advertisement mechanism requires stateful nodes a node N who
   is to
   maintain lists be a member of known prefixes. the aggregation group.  A prefix entry contains node Z situated above
   the node M in the DAG, but not above node N, will see the
   following abstract information:

   o  A reference to
   advertisements for the ND entry aggregation owned by M but not that was created for of the advertising
      neighbor.

   o  The IPv6 address and interface
   individual prefix for N. Such a node Z will route all the advertising neighbor.

   o  The logical equivalent of the full Destination Advertisement
      information (including packets for
   node N towards node M, but node M will have no route to the prefixes, depth, node N
   and Reverse Route
      Stack, if any).

   o  A 'reported' Boolean will fail to keep track whether forward.

   Additional protocols may be applied beyond the scope of this prefix was
      reported already, and
   specification to which of the DA parents.

   o  A counter dynamically elect/provision an aggregating node and
   groups of retries nodes eligible to count how many RA-DIOs were sent on the
      interface be aggregated in order to the advertising neighbor without reachability
      confirmation 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 prefix.

   Note that nodes may receive multiple information from different
   neighbors for multicast routing operations over
   an IPv6 RPL network, and specifically how unicast NA-DAOs can be used
   to relay group registrations inwards.  Wherever the following text
   mentions MLD, one can read MLDv2 or v3.

   As is traditional, a specific destination, listener uses a protocol such as different paths through MLD with a
   router to register to a multicast group.

   Along the
   DAG may be propagating information inwards along path between the DAG for router and the same
   destination.  A node who is recording routing state will keep track root of the information from each neighbor independently, and when it
   comes time to propagate DAG, MLD
   requests are mapped and transported as NA-DAO messages within the DAO RPL
   protocol; each hop coalesces the multiple requests for a particular prefix same group
   as a single NA-DAO message to the DA
   parents, then parent(s), in a fashion similar to
   proxy IGMP, but recursively between child router and parent up to the DAO information will
   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 selected from among lost for all of its sub-DAG if the
   advertising neighbors who offer transmission fails
   over that link.  Alternatively the least depth router might select to the destination.

   The Destination Advertisement mechanism stores the prefix entries copy
   additional parents as it would do for NA-DAO messages advertising
   unicast destinations, in
   one of 3 abstract lists; which case there might be duplicates that
   the Connected, router will need to prune.

   As a result, multicast routing states are installed in each router on
   the Reachable and way from the
   Unreachable lists.

   The Connected list corresponds listeners to the prefixes owned and managed by root, enabling the local node.

   The Reachable list contains prefixes root to copy a
   multicast packet to all its children routers that had issued a NA-DAO
   message including a DAO for which that multicast group, as well as all the node keeps
   receiving DAOs, and for those prefixes which have not yet timed out.

   The Unreachable list keeps track
   attached nodes that registered over MLD.

   For unicast traffic, it is expected that the grounded root of prefixes which are no longer
   valid an RPL
   DAG terminates RPL and in MAY redistribute the process of being destroyed, in order to send no-DAOs
   to RPL routes over the
   external infrastructure using whatever routing protocol is used
   there.  For multicast traffic, the root MAY proxy MLD for all the DA parents.

5.10.2.1.1.  Destination Advertisement Timers

   The Destination Advertisement mechanism requires 2 timers;
   nodes attached to the
   DelayNA timer and RPL routers (this would be needed if the DestroyTimer.

   o  The DelayNA timer
   multicast source is armed upon a stimulation to send located in the external infrastructure).  For
   such a
      Destination Advertisement (such source, the packet will be replicated as a DIO it flows outwards
   along the DAG based on the multicast routing table entries installed
   from the NA-DAO message.

   For a DA parent).  When source inside the timer DAG, the packet is armed, all entries passed to the preferred
   parents, and if that fails then to the alternates in the Reachable list as well as
      all entries for Connected list are set DAG.  The
   packet is also copied to not reported yet all the registered children, except for the
   one that particular DA parent.

   o  The DelayNA timer has a duration that passed the packet.  Finally, if there is DEF_NA_LATENCY divided by a multiple of listener in the
   external infrastructure then the DAG rank of root has to further propagate
   the node.  The intention is that
      nodes located deeper in packet into the DAG should have a shorter DelayNA
      timer, allowing DAOs external infrastructure.

   As a chance to be reported from deeper in result, the DAG Root acts as an automatic proxy Rendez-vous
   Point for the RPL network, and potentially aggregated along sub-DAGs before propagating
      further inwards.

   o  The DestroyTimer as source towards the Internet for all
   multicast flows started in the RPL LLN.  So regardless of whether the
   root is armed when actually attached to the Internet, and regardless of whether
   the DAG is grounded or floating, the root can serve inner multicast
   streams at least one entry has exhausted
      its retries, which means that a number all times.

5.11.  Maintenance of RA-DIO were sent toward
      the reporting neighbor but that Routing Adjacency

   The selection of successors, along the entry was not confirmed with a
      DAO.  When default paths inward along the destroy timer elapses, for all exhausted entries,
   DAG, or along the associated route is removed, and paths learned from destination advertisements
   outward along the entry is scheduled DAG, leads to be
      destroyed.

   o  The Destroy timer has the formation of routing adjacencies
   that require maintenance.

   In IGPs such as OSPF [RFC4915] or IS-IS [RFC5120], the maintenance of
   a duration routing adjacency involves the use of min (MAX_DESTROY_INTERVAL,
      RA_INTERVAL).

5.10.2.2.  Multicast Destination Advertisement messages

   It Keepalive mechanisms (Hellos)
   or other protocols such as BFD ([I-D.ietf-bfd-base]) and MANET
   Neighborhood Discovery Protocol (NHDP [I-D.ietf-manet-nhdp]).
   Unfortunately, such an approach is also possible for a node to multicast a DAO not desirable in constrained
   environments such as LLN and would lead to the link-local
   scope all-nodes multicast address FF02::1.  This message will be
   received by all node listening excessive control traffic
   in range light of the emitting node.  The
   objective is data traffic with a negative impact on both link
   loads and nodes resources.  Overhead to enable direct P2P communication, between destination
   prefixes directly supported by neighboring nodes, without needing maintain the
   RPL routing structure
   adjacency should be minimized.  Furthermore, it is not always
   possible to relay rely on the packets.

   A multicast DAO MUST be used only link or transport layer to advertise provide
   information about
   self, i.e. prefixes in of the Connected list.  This would typically be associated link state.  The network layer needs to
   fall back on its own mechanism.

   Thus RPL makes use of a different approach consisting of probing the
   neighbor using a
   multicast group that this node Neighbor Solicitation message (see [RFC4861]).  The
   reception of a Neighbor Advertisement (NA) message with the
   "Solicited Flag" set is listening used to or a global address
   owned by this node, though it can verify the validity of the routing
   adjacency.  Such mechanism MAY be used prior to advertise any prefix
   owned by this node as well.  A multicast DAO is not used sending a data
   packet.  This allows for routing
   and does detecting whether or not presume any DAG relationship between the emitter routing
   adjacency is still valid, and the
   receiver; should it MUST NOT not be used to relay information learned (e.g.
   information in the Reachable list) from case, select
   another node.

   A node receiving a multicast DAO addressed feasible successor to FF02::1 MAY install
   prefixes contained in the DAO in forward the routing table for local use.
   Such packet.

5.12.  Packet Forwarding

   When forwarding a node MUST NOT perform any other processing on the DAO (i.e.
   such packet to a node does not presume it destination, precedence is a DA parent).

5.10.2.3.  Unicast Destination Advertisement messages from child given to
           parent

   When sending
   selection of a Destination Advertisement next-hop successor as follows:

   1.  It is preferred to select a DA parent, successor from a LLN Node
   includes the DAOs about not already reported prefix entries DAG who is
       supporting an OCP and related optimization that maps to an
       objective marked in the
   Reachable and Connected lists, as well as no-DAOs for all IPv6 header of the entries packet being
       forwarded.

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

   3.  If there is an entry in the Unreachable list.  Depending on its policy and ability to
   retain routing state, table matching the receiving node SHOULD keep
       destination that has been learned from a record of multicast destination
       advertisement (e.g. the
   reported DAO. destination is a one-hop neighbor), then
       use that successor.

   4.  If there is an entry in the DAO offers routing table matching the best route to
       destination that has been learned from a unicast destination
       advertisement (e.g. the prefix as
   determined by policy and other prefix records, destination is located outwards along the node SHOULD
   install
       sub-DAG), then use that successor.

   5.  If there is a DAG offering a route to the a prefix in the DAO via matching the link local address
       destination, then select one of the reporting neighbor and it SHOULD further propagate the
   information, either those DAG parents as a DAO or by means of redistribution into successor.

   6.  If there is a
   routing protocol.

   The RA-DIO from the DAG root offering a default route with a compatible OCP,
       then select one of those DAG parents as a successor.

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

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

   TTL MUST be decremented when forwarding.  If the DAG.  Its period packet is expected being
   forwarded via a sibling, then the TTL MAY be decremented more
   aggressively (by more than one) to vary, depending on limit the
   configuration impact of the trickle timer possible
   loops.

   Note that governs the RAs.

   When a node receives a RA-DIO over an LLN interface from a DA parent, chosen successor MUST NOT be the DelayNA is armed to force a full update.

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

   o  If
   predecessor of the entry is CONFIRMED, packet (split horizon), except in the case where
   it goes PENDING with is intended for the retry count
      set packet to 0.

   o  If change from an inward to an outward
   flow, such as switching from DIO routes to DAO routes as the entry
   destination is PENDING, the retry count neared.

5.12.1.  Loop Taxonomy

   The following is incremented.  If it
      reaches a maximum threshold, summary of the entry goes ELAPSED If at least
      one entry sort of loops that may occur within
   RPL.  This is ELAPSED provided in part as 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 the end DAG and reattaches
   to a device in its prior sub-DAG that has missed the whole detachment
   sequence and kept advertising the original DAG.  This may happen in
   particular when RA-DIO messages are missed.  Use of the process: if DAG sequence
   number can eliminate this type of loop.  If the Destroy
      timer DAG sequence number
   is not running then it in use, the protection is armed with limited (it depends on propagation
   of RA-DIO messages during DAG hop timer), and temporary loops might
   occur.  RPL will move to eliminate such a loop as soon as a jitter.

   Since the DelayNA has RA-DIO
   message is received from a duration parent that decreases with appears to be going down, as
   the depth, it is
   expected child has to receive all DAOs detach from all children before it immediately.  (The alternate choice
   of staying attached and following the timer
   elapses parent in its fall would have
   counted to infinity and led to detach as well).

   Consider node (24) in the full update is sent DAG Example depicted in Figure 12, and its
   sub-DAG nodes (34), (44), and (45).  An example of a DAG loop would
   be if node (24) were to detach from the DA parents.

   Once DAG rooted at (LBR), and
   nodes (34) and (45) were to miss the Destroy timer is elapsed, detachment sequence.
   Subsequently, if the prefix entry is scheduled link (24)--(45) were to
   be destroyed become viable and moved to node
   (24) heard node (45) advertising the Unreachable list DAG rooted at (LBR), a DAG loop
   (45->34->24->45) may form if there are any DA
   parents that need node (24) attaches to be informed of the change in status for node (45).

5.12.1.2.  DAO Loops

   A DAO loop may occur when the
   prefix, otherwise parent has a route installed upon
   receiving and processing a NA-DAO message from a child, but the prefix entry is child
   has subsequently cleaned up right away.  The
   prefix entry is removed from the Unreachable list when no more DA
   parents need to be informed. state.  This condition may be satisfied loop happens when a
   no-DAO no-
   DAO was missed till a heartbeat cleans up all states.  The DAO loop
   is sent not explicitly handled by the current specification.  Split
   horizon, not forwarding a packet back to all current DA parents indicating the loss of node it came from, may
   mitigate the
   prefix, and noting that DAO loop in some cases parents may have been removed
   from cases, but does not eliminate it.

   Consider node (24) in the set of DAG Example depicted in Figure 12.  Suppose
   node (24) has received a DA parents.

5.10.2.4.  Other events

   Finally, the Destination Advertisement mechanism responds to from node (34) advertising a series
   of events, such as:

   o  Destination Advertisement operation stopped: All entries in the
      abstract lists are freed.  All destination
   at node (45).  Subsequently, if node (34) tears down the routes learned from DAOs are
      destroyed.

   o  Interface going down: routing
   state for all entries in the Reachable list on
      that interface, the associated route is removed, destination and the entry is
      scheduled node (24) did not hear a no-DAO message
   to be destroyed.

   o  Loss of routing adjacency: When clean up the routing adjacency state, a DAO loop may exist. node (24) will
   forward traffic destined for node (45) to node (34), who may then
   naively return it into a
      neighbor loop (if split horizon is lost, as per not in place).  A
   more complicated DAO loop may result if node (34) instead passes the procedures described
   traffic to it's sibling, node (33), potentially resulting in Section 5.11, a
   (24->34->33->23->13->24) loop.

5.12.1.3.  Sibling Loops

   Sibling loops occur when a group 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 if parent
   preference (always prefer parents vs. siblings).

   Consider the associated entries are DAG Example depicted in the Reachable list, the
      associated routes Figure 12.  Suppose that Node
   (32) and (34) are removed, reliable neighbors, and the entries thus are scheduled to be
      destroyed.

   o  Changes to DA parent set: All entries siblings.  Then,
   in the Reachable list case where Nodes (22), (23), and (24) are
      set to not 'reported' transiently
   unavailable, and DelayNA is armed.

5.10.2.5.  Aggregation of prefixes by with no other guiding strategy, a node

   There sibling loop may be number of cases where
   exist, e.g. (33->34->32->33) as the siblings keep choosing amongst
   each other in an uncoordinated manner.

6.  RPL Variables

   DIO Timer  One instance per DAG that a aggregation may be shared within node is a platoon of nodes.  In such member of.  Expiry
         triggers RA-DIO message transmission.  Trickle timer with
         variable interval in [0,
         DIOIntervalMin..2^DIOIntervalDoublings].  See Section 5.3.4

   DAG Hop Timer  Up to one instance per candidate DAG parent in the
         `Held-Up' state per DAG that a case, it node is possible going to use
   aggregation techniques with Destination Advertisements and improve
   scalability.  For example, consider jump to.
         Expiry triggers candidate DAG parent to become a platoon formed by firefighters
   and their commander.  Specifically, DAG parent in
         the commander may be configured `Current' state, as the Destination Advertisement aggregator well as cancellation of any other DAG
         Hop timers associated with other DAG parents for a group prefix.  At
   run time, the commander absorbs that DAG.
         Duration is computed based on the individual DAO information
   received from rank of the platoon members down its sub-DAG candidate DAG
         parent and only reports
   the aggregation up DAG delay, as (candidates rank + random) *
         candidate's DAG_delay (where 0 <= random < 1).  See
         Section 5.7.1.

   Hold-Down Timer  Up to one instance per candidate DAG parent in the
         `Held-Down' state per DAG.  This works fine when  Expiry triggers the whole platoon
   is attached within eviction of the commander's sub-DAG.

   Other cases might occur for which additional support is required:

   1.  The commander is attached within
         candidate DAG parent from the sub-DAG of candidate DAG parent set.  The
         interval should be chosen as appropriate to prevent flapping.
         See Section 5.7.

   DAG Heartbeat Timer  Up to one of its
       platoon members.

   2.  A platoon member is somewhere else within instance per DAG that the DAG.

   3.  A platoon member node is somewhere else
         acting as DAG root of.  May not be supported in the LLN.

   In all those cases,
         implementations.  Expiry triggers revision of
         DAGSequenceNumber, causing a node situated above the commander in the new series of updated RA-DIO
         message to be sent.  Interval should be chosen appropriate to
         propagation time of DAG
   but not above the platoon member will see the advertisements for the
   aggregation owned by and as appropriate to application
         requirements (e.g. response time vs. overhead).  See
         Section 5.4

   DelayNA Timer  Up to one instance per DA parent (the subset of DAG
         parents chosen to receive destination advertisements) per DAG.
         Expiry triggers sending of NA-DAO message to the commander but not DA parent.
         The interval is to be proportional to DEF_NA_LATENCY/(node
         rank), such that nodes of greater rank (further outward along
         the individual
   platoon member prefix.  So it will route all DAG) expire first, coordinating the packets sending of NA-DAO
         messages to allow for a chance of aggregation.  See
         Section 5.9.2.1.1

   DestroyTimer  Up to one instance per DA entry per neighbor (i.e.
         those neighbors who have given NA-DAO messages to this node as
         a DAG parent) Expiry triggers a change in state for the
   platoon member towards the commander, but DA
         entry, setting up to do unreachable (No-DAO) advertisements or
         immediately deallocating the commander will have DA entry if there are no
   route 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 to give consideration to the individual platoon member manageability
   of RPL, and how RPL will fail to forward.

   Additional protocols may be applied operated in LLN beyond the scope use of this
   specification to dynamically elect/provision a commander and platoon
   in order to provide route summarization for a sub-DAG.

5.10.2.6.  Default Values

   DEF_NA_LATENCY = To Be Determined

   MAX_DESTROY_INTERVAL = To Be Determined

5.11.  Maintenance of Routing Adjacency MIB
   module.  The selection scope of successors, along the default paths inward along the
   DAG, or along the paths learned from Destination Advertisements
   outward along the DAG, leads this section is to consider the formation of routing adjacencies
   that require maintenance.

   In IGPs such as OSPF [RFC4915] or IS-IS [RFC5120], the maintenance following
   aspects of
   a routing adjacency involves the use manageability: fault management, configuration, accounting
   and performance.

7.1.  Control of Keepalive mechanisms (Hellos)
   or other protocols such as BFD ([I-D.ietf-bfd-base]) Function and MANET
   Neighborhood Discovery Protocol (NHDP [I-D.ietf-manet-nhdp]).
   Unfortunately, such an approach Policy

7.1.1.  Initialization Mode

   When a node is not desirable in constrained
   environments such as LLN first powered up, it may either choose to stay silent
   and would lead not send any multicast RA-DIO message until it has joined a DAG,
   or to excessive control traffic
   in light of the data traffic with immediately root a negative impact on both link
   loads transient DAG and nodes resources.  Overhead to maintain start sending multicast
   RA-DIO messages.  A RPL implementation SHOULD allow configuring
   whether the routing
   adjacency node should stay silent or should be minimized. start advertising RA-
   DIO messages.

   Furthermore, it is not always
   possible the implementation SHOULD to rely on allow configuring whether
   or not the link node should start sending an RS message as an initial
   probe for nearby DAGs, or transport layer to provide
   information should simply wait until it received RA
   messages from other nodes that are part of the associated link state.  The network layer needs to
   fall back on its own mechanism.

   Thus existing DAGs.

7.1.2.  DIO Base option

   RPL makes use of specifies a different approach consisting number of probing protocol parameters.

   A RPL implementation SHOULD allow configuring the
   neighbor using a Neighbor Solicitation message (see [RFC4861]). following routing
   protocol parameters:

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

   NodePreference:  The
   reception administrative preference of a Neighbor Advertisement (NA) message with the
   "Solicited Flag" that LLN Node.

   DAGDelay:  16-bit unsigned integer set is used to verify by the validity of DAG root indicating the routing
   adjacency.  Such mechanism MAY be
         delay before changing the DAG configuration,

   DIOIntervalDoublings:  8-bit unsigned integer.  Configured on the DAG
         root and used prior to sending a data
   packet.  This allows for detecting whether or not configure the routing
   adjacency is still valid, and trickle timer governing when RA-
         DIO messages should it not be sent within the case, select
   another feasible successor to forward the packet.

5.12.  Packet Forwarding

   When forwarding a packet to a destination, precedence is given to
   selection of a next-hop successor, with consideration given to
   selecting a DAG/OCP to follow as per marking in the IPv6 header, as
   follows:

   1.  If the packet header contains any source routing directives (TBD)
       then DAG.

   DIOIntervalMin:  8-bit unsigned integer.  Configured on the highest precedence DAG root
         and used to configure the trickle timer governing when RA-DIO
         messages should be given to follow them.

   2.  If there is an entry in sent within the routing table matching DAG.  The minimum configured
         interval for the
       destination that has been provisioned outside RA-DIO trickle timer in units of ms is
         2^DIOIntervalMin (e.g. a DIOIntervalMin value of 16ms is
         expressed as 4).

   DAGObjectiveCodePoint  The DAG Objective Code Point is used to
         indicate the context cost metrics, objective functions, and methods of
       RPL, e.g. through an application intervention or a co-hosted
       (P2P) routing protocol, then
         computation and comparison for DAGRank in use that successor.

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

   4.  If there DAG.  The
         DAG OCP is an entry in set by the routing table matching DAG root.

   PathDigest:  32-bit unsigned integer CRC, updated by each LLN Node.
         This is the
       destination that has been learned from result of a unicast Destination
       Advertisement (e.g. CRC-32c computation on a bit string
         obtained by appending the destination is located outwards along received value and the
       sub-DAG), then use that successor.

   5.  If there is a ordered set of
         DAG offering parents at the LLN Node.  DAG roots use a route 'previous value'
         of zeroes to initially set the PathDigest.

   DAGID:  128-bit unsigned integer which uniquely identify a prefix matching DAG.  This
         value is set by the
       destination, then select one DAG root.  The global IPv6 address of those the
         DAG root can be used.

   Destination Prefixes  List of advertised destinations

   DAG Parents as Root behavior:  In some cases, a successor.

   6.  If there is node may not want to permanently
         act as a DAG offering root if it cannot join a default route with grounded DAG.  For
         example a compatible OCP,
       then select one of those DAG Parents battery-operated node may not want to act as a successor.

   7.  If there is a DAG offering
         root for a route to long period of time.  Thus a prefix matching the
       destination, but all DAG Parents have been tried and are
       temporarily unavailable (as determined by RPL implementation MAY
         support the forwarding
       procedure), then select ability to configure whether or not a DAG sibling node could
         act as a successor.

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

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

   Note that unless overridden by a source routing directive or root for a route
   that has been provisioned outside configured period of RPL, the chosen successor time.

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

5.12.1.  Loop Taxonomy

   The following is a summary value of the sort of loops that may occur within
   RPL.  This is provided DAG Hop Timer, expressed in part as a basis for discussion of loop
   detection at forwarding.

5.12.1.1. ms.

   DAG Loops Table Entry Suppression  A DAG loop may occur when a node detaches from the DAG and reattaches RPL implementation SHOULD provide the
         ability to configure a device in its prior sub-DAG that has missed the whole detachment
   sequence and kept advertising timer after the original DAG.  This may happen in
   particular when RA-DIOs are missed.  Use expiration of which the
         DAG sequence number
   can eliminate this type of loop.  If table that contains all the records about a DAG sequence number is not
   in use,
         suppressed, to be invoked if the protection is limited (it depends on propagation of DIOs
   during DAG hop timer), and temporary loops might occur. parent set becomes empty.

7.1.3.  Trickle Timers

   A RPL will
   move implementation makes use of trickle timer to eliminate such a loop as soon as a DIO govern the sending
   of RA-DIO message.  Such an algorithm is received from determined a
   parent by a set of
   configurable parameters that appears to be going down, as are then advertised by the child has to detach from
   it immediately.  (The alternate choice DAG root
   along the DAG in RA-DIO messages.

   For each DAG, a RPL implementation MUST allow for the monitoring of staying attached and
   the following parameters:

   I:    The current length of the parent in its fall would have counted to infinity and
   led communication interval

   T:    A timer with a duration set to detach as well).

   Consider Node (24) a random value in the DAG Example depicted range
         [I/2, I]

   C:    Redundancy Counter

   I_min:  The smallest communication interval in Figure 12, and its
   sub-DAG Nodes (34), (44), and (45).  An example 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 a DAG loop would times I_min should be if Node (24) were to detach doubled before
         maintaining a constant rate, i.e.  I_max = I_min *
         2^I_doublings.  This value is learned from the DAG rooted at (LBR), and Node
   (45) were to miss the detachment sequence.  Subsequently, if RA-DIO message
         as DIOIntervalDoublings.  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 link
   (24)--(45) were trickle timer to become viable and Node (24) heard Node (45)
   advertising the reset.

7.1.4.  DAG rooted Heartbeat

   A RPL implementation may allow by configuration at (LBR), a the DAG loop (45->34->24->45) may
   form if Node (24) attaches root to Node (45).

5.12.1.2.  DAO Loops

   A DAO loop may occur when
   refresh the parent has a route installed DAG states by a DAO
   via a child, but the child has cleaned up updating the state.  This loop
   happens when a no-DAO was missed till a heartbeat cleans up all
   states.  The DAO loop is DAGSequenceNumber.  A RPL
   implementation SHOULD allow configuring whether or not explicitly handled periodic or
   event triggered mechanism are used by the current
   specification.  Split horizon, not forwarding a packet back DAG root to control
   DAGSequenceNumber change.

7.1.5.  The Destination Advertisement Option

   The following set of parameters of the
   node it came from, may mitigate the DAO loop in some cases, but does
   not eliminate it.

   Consider Node (24) in the NA-DAO messages SHOULD be
   configurable:

   o  The DelayNA timer

   o  The Remove timer

7.1.6.  Policy Control

   DAG Example depicted in Figure 12.  Suppose
   Node (24) has received a DA from Node (34) advertising a destination
   at Node (45).  Subsequently, if Node (34) tears down discovery enables nodes to implement different policies for
   selecting their DAG parents.

   A RPL implementation SHOULD allow configuring the DA state set of acceptable
   or preferred Objective Functions (OF) referenced by their Objective
   Codepoints (OCPs) for
   the destination and Node (24) did not hear a no-DAO node to clean up the
   state, join a DAO loop may exist.  Node (24) will forward traffic destined
   for Node (45) to Node (34), who may then naively return it into DAG, and what action should be
   taken if none of a
   loop (if split horizon node's candidate neighbors advertise one of the
   configured allowable Objective Functions.

   A node in an LLN may learn routing information from different routing
   protocols including RPL.  It is not in place). this case desirable to control via
   administrative preference which route should be favored.  An
   implementation SHOULD allow for specifying an administrative
   preference for the routing protocol from which the route was learned.

   A more complicated DAO loop
   may result if Node (34) instead passes RPL implementation SHOULD allow for the traffic configuration of the "Route
   Tag" field of the NA-DAO messages according 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 set 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 rules defined
   by
   split horizon (do not send back to policy.

7.1.7.  Data Structures

   Some RPL implementation may limit the same sibling) and parent
   preference (always prefer parents vs. siblings).  Further approaches size of the candidate neighbor
   list in order to mitigate sibling loops bound the memory usage, in which case some otherwise
   viable candidate neighbors may include:

   o  aggressively dropping not be considered and simply dropped
   from the candidate neighbor list.

   A RPL implementation MAY provide an indicator on the TTL to limit size of the impact
   candidate neighbor list.

7.2.  Information and Data Models

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

   o  randomizing RPL SNMP MIB.

7.3.  Liveness Detection and Monitoring

   The aim of this section is to describe the next hop various RPL mechanisms
   specified to try and exit monitor the loop if there is one
      one protocol.

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

   o  maintaining per packet states  The candidate neighbors data structure

   o  tunneling or source routing (path vector)

   Consider the  For each DAG:

      *  A set of candidate DAG Example depicted in Figure 12.  Suppose that Node
   (32) and (34) parents

      *  A set of DAG parents (which are reliable neighbors, a subset of candidate DAG
         parents and thus are siblings.  Then, may be implemented as such)

7.3.1.  Candidate Neighbor Data Structure

   A node in the case where Nodes (22), (23), and (24) are transiently
   unavailable, candidate neighbor list is a node discovered by the
   some means and with no other guiding strategy, qualified to potentially become of neighbor or a
   sibling loop may
   exist, e.g. (33->34->32->33) as (with high enough local confidence).  A RPL implementation
   SHOULD provide a way monitor the siblings keep choosing amongst
   each other in an uncoordinated manner.

5.13.  Expectations candidate neighbors list with some
   metric reflecting local confidence (the degree of Link Layer Behavior

   This specification does not rely on any particular features stability of a
   specific link layer technologies.  It is anticipated that an
   implementer should be able to operate the
   neighbors) measured by some metrics.

   A RPL over implementation MAY provide a variety counter reporting the number of different
   low power wireless or PLC (Power Line Communication) link layer
   technologies.

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

6.  Summary implementation MUST keep track of RPL Timers

   DIO Timer  One instance per the following
   DAG that table values:

   o  DAGID

   o  DAGObjectiveCodePoint

   o  A set of Destination Prefixes offered inwards along the DAG

   o  A set of candidate DAG Parents

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

   o  DAGSequenceNumber

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

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

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

   o  a member of.  Expiry
         triggers RA-DIO transmission.  Trickle timer state associated with variable
         interval the role of the candidate as a potential
      DAG Parent {Current, Held-Up, Held-Down, Collision}, further
      described in [0, DIOIntervalMin..2^DIOIntervalDoublings].  See Section 5.4.3 5.7

   o  A DAG Hop Timer  Up to one instance per candidate DAG Timer, if instantiated

   o  A Held-Down Timer, if instantiated

   o  A flag reporting if the Parent is a DA Parent as described 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
         `Held-Up' state per DAG that number of
   a times the node is going to jump to.
         Expiry triggers candidate DAG Parent has detected an inconsistency with respect to become a DAG Parent in
   parent, e.g. if the `Current' state, as well as cancellation DAGID has changed.

   A RPL implementation MAY log the reception of any other DAG
         Hop timers associated a malformed RA-DIO
   message along with other the neighbor identification if avialable.

7.3.5.  RPL Trickle Timers

   A RPL implementation operating on a DAG Parents root MUST allow for that DAG.
         Duration is computed based on the rank
   configuration of the candidate DAG
         parent and DAG delay, as (candidates rank + random) *
         candidate's DAG_delay (where 0 <= random < 1).  See
         Section 5.8.1.

   Hold-Down Timer  Up to one instance per candidate DAG Parent following trickle parameters:

   o  The DIOIntervalMin expressed in ms

   o  The DIOIntervalDoublings

   A RPL implementation MAY provide a counter reporting the
         `Held-Down' state per DAG.  Expiry triggers number of
   times an inconsistency (and thus the eviction trickle timer has been reset).

7.4.  Verifying Correct Operation

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

7.5.  Requirements on Other Protocols and Functional Components

   RPL does not have any impact on the candidate DAG Parent set.  The
         interval should operation of existing protocols.

7.6.  Impact on Network Operation

   To be chosen as appropriate completed.

8.  Security Considerations

   Security Considerations for RPL are to prevent flapping.
         See Section 5.8 be developed in accordance
   with recommendations laid out in, for example,
   [I-D.tsao-roll-security-framework].

9.  IANA Considerations

9.1.  DAG Heartbeat Timer  Up to one instance per Information Option (DIO) Base Option

   The DAG Information Option is a container option carried within an
   IPv6 Router Advertisement message as defined in [RFC4861], which
   might contain a number of suboptions.  The base option regroups the
   minimum information set that the node is
         acting as DAG Root of.  May not be supported mandatory in all
         implementations.  Expiry triggers revision cases.

   IANA had defined the IPv6 Neighbor Discovery Option Formats registry.
   The suggested type value for the DAG Information Option (DIO) Base
   Option is 140, to be confirmed by IANA.

9.2.  New Registry for the Flag Field of
         DAGSequenceNumber, causing the DIO Base Option

   IANA is requested to create a new series registry for the Flag field of updated RA-DIO to the DIO
   Base Option.

   New bit numbers may be
         sent.  Interval allocated only by an IETF Consensus action.
   Each bit should be chosen appropriate to propagation
         time of DAG and tracked with the following qualities:

   o  Bit number (counting from bit 0 as appropriate to application requirements
         (e.g. response time vs. overhead).  See Section 5.5

   DelayNA Timer  Up to one instance per DA Parent (the subset of the most significant bit)

   o  Capability description

   o  Defining RFC

   Three flags are currently defined:

       +-----+-------------------------------------+---------------+
       | Bit | Description                         | Reference     |
       +-----+-------------------------------------+---------------+
       |  0  | Grounded DAG
         Parents chosen to receive                        | This document |
       |  1  | Destination Advertisements) per DAG.
         Expiry triggers sending of NA-DAO to the DA Parent.  The
         interval 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 be proportional to DEF_NA_LATENCY/(node rank),
         such that nodes of greater rank (further outward along the DAG)
         expire first, coordinating create a registry for the sending of DAOs 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 |
         +-------+------------------------------+---------------+

            DAG Information Option (DIO) Base Option Suboptions

9.4.  Destination Advertisement Option (DAO) Option

   The RPL protocol extends Neighbor Discovery [RFC4861] and [RFC4191]
   to allow for a
         chance of aggregation.  See Section 5.10.2.1.1

   DestroyTimer  Up to one instance per DA entry per neighbor (i.e.
         those neighbors who have given DAO to this node as a DAG
         Parent) Expiry triggers to include a change Destination Advertisement Option, which
   includes prefix information in state for the DA entry,
         setting up to do unreachable (No-DAO) advertisements or
         immediately deallocating the DA entry if there are no DA
         Parents. Neighbor Advertisements messages.
   The interval is min(MAX_DESTROY_INTERVAL,
         RA_INTERVAL).  See Section 5.10.2.1.1

7.  Protocol Extensions

8.  Manageability Considerations

9.  Security Considerations

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

10.  IANA Considerations

10.1.  DAG Information the
   Destination Advertisement Option (DAO).

   IANA is requested to allocate a new Neighbor Discovery Option Type
   from had defined the IPv6 Neighbor Discovery Option Formats Registry in order to
   represent registry.
   The suggested type value for the DAG Information Destination Advertisement Option as described in Section 5.1

10.2.
   carried within a Neighbor Advertisement message is 141, to be
   confirmed by IANA.

9.5.  Objective Code Point

   This specification requests that an Objective Code Point registry, as
   to be specified in [I-D.ietf-roll-routing-metrics], reserve the
   Objective Code Point value 0x0000, for the purposes designated as OCP
   0 in this document.

10.3.  Destination Advertisement Option

   IANA is requested to allocate a new Neighbor Discovery Option Type
   from the IPv6 Neighbor Discovery Option Formats Registry in order to
   represent the Destination Advertisement Option as described in
   Section 5.10.1.1

11.

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, Richard Kelsey, Quentin Lampin, Philip Levis, Jerry
   Martocci, Alexandru Petrescu, and Don Sturek.

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

   The ROLL Design Team would like to acknowledge prior contributions of
   Richard Kelsey,
   Robert Assimiti, Mischa Dohler, Julien Abeille, Ryuji Wakikawa, Teco
   Boot, Patrick Wetterwald, Bryan Mclaughlin, Carlos J. Bernardos,
   Thomas Watteyne, Zach Shelby, Dominique Barthel, Caroline Bontoux, Marco Molteni, Billy
   Moon, and Arsalan Tavakoli, in addition
   to contributions from [I-D.thubert-roll-fundamentals] and
   [I-D.tavakoli-hydro] which have provided useful design
   considerations to RPL.

12.

11.  Contributors

   ROLL Design Team in alphabetical order:

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

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

   Email: abr@zen-sys.com 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

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

   Email: jhui@archrock.com

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

   Email: kpister@dustnetworks.com

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

   Phone: +33 497 23

   Anders Brandt
   Zensys, Inc.
   Emdrupvej 26 34
   Copenhagen, DK-2100
   Denmark

   Email: pthubert@cisco.com

   Tim Winter (editor)

   wintert@acm.org

13. abr@zen-sys.com

12.  References

13.1.

12.1.  Normative References

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

13.2.

12.2.  Informative References

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

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

   [I-D.ietf-roll-building-routing-reqs]
              Martocci, J., Riou, N., Mil, P., and W. Vermeylen,
              "Building Automation Routing Requirements in Low Power and
              Lossy Networks", draft-ietf-roll-building-routing-reqs-06 draft-ietf-roll-building-routing-reqs-07
              (work in progress), August September 2009.

   [I-D.ietf-roll-home-routing-reqs]
              Brandt, A., Buron, J., and G. Porcu, G., "Home Automation
              Routing Requirements in Low Power and Lossy Networks",
              draft-ietf-roll-home-routing-reqs-06
              draft-ietf-roll-home-routing-reqs-08 (work in progress),
              November 2008.
              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.tavakoli-hydro]
              Tavakoli, A., Dawson-Haggerty, S., Hui, J., and D. Culler,
              "HYDRO: A Hybrid Routing Protocol for Lossy and Low Power
              Networks", draft-tavakoli-hydro-01 (work in progress),
              March 2009.

   [I-D.thubert-roll-fundamentals]
              Thubert, P., Watteyne, T., Shelby, Z., and D. Barthel,
              "LLN Routing Fundamentals",
              draft-thubert-roll-fundamentals-01 (work in progress),
              April 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-00 draft-tsao-roll-security-framework-01
              (work in progress), February September 2009.

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

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

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

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

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

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

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

   [RFC4875]  Aggarwal, R., Papadimitriou, D., and S. Yasukawa,
              "Extensions to Resource Reservation Protocol - Traffic
              Engineering (RSVP-TE) for Point-to-Multipoint TE Label
              Switched Paths (LSPs)", RFC 4875, May 2007.

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

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

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

Appendix A.  Deferred Requirements

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

Appendix B.  Examples

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

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

   Note that the links depicted represent the usable L2 connectivity
   available in the LLN.  For example, Node (31) can communicate
   directly with its neighbors, Nodes (21), (22), (32), and (41).  Node
   (31) cannot communicate directly with any other nodes, e.g. (33),
   (23), (42).  In this example these links offer bidirectional
   communication, and `bad' links are not depicted.

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

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

                          Figure 12: Example DAG

B.1.  Moving Down a DAG

   Consider node (56) in the example of Figure 11.  In the unmodified
   example, node (56) is at depth rank 6 with one DAG parent, {(43)}, and one
   sibling (55).  Suppose, for example, that node (56) wished to expand
   its DAG parent set to contain node (55), as {(43), (55)}.  Such a
   change would require node (56) to detach from the DAG, to defer
   reattachment until a loop avoidance algorithm has completed, and to
   then reattach to the DAG with {(43), (55)} as it's DAG parents.  When
   node (56) detaches from the DAG, it is able to act as the root of its
   own floating DAG and establish its frozen sub-DAG (which is empty).
   Node (56) can then observe that Node (55) is still attached to the
   original DAG, that its sequence number is able to increment, and
   deduce that Node (55) is safely not behind Node (56).  There is then
   little change for a loop, and Node (56) may safely reattach to the
   DAG, with parents {(43), (55)}.  At reattachment time, node (56)
   would present itself with a depth rank deeper than that of its deepest DAG
   parent (node (55) at depth rank 6), depth rank 7.

B.2.  Link Removed

   Consider the example of Figure 11 when link (13)-(24) goes down.

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

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

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

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

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

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

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

B.3.  Link Added

   Consider the example of Figure 11 when link (12)-(42) appears.

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

   o  Node (42) may be content to leave its advertised depth rank at 5,
      reflecting a depth 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 Node (42) to evict
      Node (32) from its set of DAG parents, Node (42) would then
      advertise itself at depth rank 2, thus moving up the DAG.  In this case,
      Node (53), (54), and (55) may similarly follow and advertise
      themselves at depth rank 3.

B.4.  Node Removed

   Consider the example of Figure 11 when node (41) disappears.

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

   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 the DAG rooted at (LBR) at depth 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 advertising
      itself at depth rank 8.

B.5.  New LBR Added

   Consider the example of Figure 11 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 depth 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
      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
      depth rank 2 in the DAGID (LBR2).

   o  Nodes (51), (41), (42), and (54) may then choose to join the new
      DAG at depth rank 3, possibly to get closer to the DAG root.  Note that
      in a more advanced case, these nodes also remain members of the
      DAG rooted at (LBR), for example in support of different
      constraints for different types of traffic.

   o  Node (55) may then join the new DAG at depth rank 4, possibly to get
      closer to the DAG root.

   o  The remaining nodes may choose to remain in 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 12.  Suppose that Nodes
   (22) and (32) are unable to record routing state.  Suppose that Node
   (42) is able to perform prefix aggregation on behalf of Nodes (53),
   (54), and (55).

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

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

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

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

   o  Node (42) sends a DAO 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 the DAO NA-DAO message and passes
      it on to Node (22) as (42'):[(42)].  It may send a separate DAO 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 DAO NA-DAO message and passes it
      on to Node (12) as (42'):[(42), (32)].  It also relays the DAO NA-DAO
      message containing destination (32) to Node 12 as (32):[(32)], and
      finally may send a
      DAO NA-DAO message for itself indicating
      destination (22).

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

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

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 DAOs NA-DAO messages are relayed to more than one DAG Parent, parent, in some
   cases a situation may be created where a large number of DAOs NA-DAO
   messages conveying information about the same destination flow inward
   along the DAG.  It is desirable to bound/limit the multiplication/fan-out multiplication/
   fan-out of DAOs 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 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