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

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


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

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Abstract

   Low Power and Lossy Networks (LLNs) are made largely of constrained
   nodes (with limited processing power, memory, and sometimes energy
   when they are battery operated).  These routers are interconnected by
   lossy links, most of the time supporting only low data rates, that
   are usually fairly unstable with relatively low packet delivery
   rates.  Another characteristic of such networks is that the traffic
   patterns are not simply unicast, but in many cases point-to-
   multipoint or multipoint-to-point.  Furthermore such networks may
   potentially comprise a large number of nodes, up to several dozens or
   hundreds or more nodes in the network.  These characteristics offer
   unique challenges to a routing solution: the IETF ROLL Working Group
   has defined application-specific routing requirements for a Low Power
   and Lossy Network (LLN) routing protocol.  This document specifies
   the Routing Protocol for Low Power and Lossy Networks (RPL).

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].





























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Table of Contents

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



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



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     B.8.  Example: DAG Maintenance . . . . . . . . . . . . . . . . . 83
     B.9.  Example: Greedy Parent Selection and Instability . . . . . 84
     B.10. Example: DAG Merge . . . . . . . . . . . . . . . . . . . . 86
   Appendix C.  Additional Examples . . . . . . . . . . . . . . . . . 88
   Appendix D.  Outstanding Issues  . . . . . . . . . . . . . . . . . 92
     D.1.  Additional Support for P2P Routing . . . . . . . . . . . . 92
     D.2.  Loop Detection . . . . . . . . . . . . . . . . . . . . . . 92
     D.3.  Destination Advertisement / DAO Fan-out  . . . . . . . . . 92
     D.4.  Source Routing . . . . . . . . . . . . . . . . . . . . . . 92
     D.5.  Address / Header Compression . . . . . . . . . . . . . . . 93
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 93








































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1.  Introduction

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

1.1.  Design Principles

   RPL was designed with the objective to meet the requirements spelled
   out in [I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs],
   [I-D.ietf-roll-indus-routing-reqs], and [RFC5548].  Because those
   requirements are heterogeneous and sometimes incompatible in nature,
   the approach is first taken to design a protocol capable of
   supporting a core set of functionalities corresponding to the
   intersection of the requirements.  (Note: it is intended that as this
   design evolves optional features may be added to address some
   application specific requirements).  This is a key protocol design
   decision providing a granular approach in order to restrict the core
   of the protocol to a minimal set of functionalities, and to allow
   each instantiation of the protocol to be optimized in terms of
   required code space.  It must be noted that RPL is not restricted to
   the aforementioned applications and is expected to be used in other
   environments.  All "MUST" application requirements that cannot be
   satisfied by RPL will be specifically listed in the Appendix A,
   accompanied by a justification.

   The core set of functionalities is to be capable of operating in the
   most severely constrained environments, with minimal requirements for
   memory, energy, processing, communication, and other consumption of
   limited resources from nodes.  Trade-offs inherent in the
   provisioning of protocol features will be exposed to the implementer
   in the form of configurable parameters, such that the implementer can



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   further tweak and optimize the operation of RPL as appropriate to a
   specific application and implementation.  Finally, RPL is designed to
   consult implementation specific policies to determine, for example,
   the evaluation of routing metrics.

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

1.2.  Expectations of Link Layer Behavior

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

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


2.  Terminology

   The terminology used in this document is consistent with and
   incorporates that described in `Terminology in Low power And Lossy
   Networks' [I-D.ietf-roll-terminology].  The terminology is extended
   in this document as follows:

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

   DAG:  Directed Acyclic Graph.  A directed graph having the property
         that all edges are oriented in such a way that no cycles exist.
         In the RPL context, all edges are contained in paths oriented
         toward and terminating at a root node (a DAG root, or sink-
         typically a Low Power and Lossy Network Border Router (LBR)).

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







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   DAG parent:  A parent of a node within a DAG is one of the immediate
         successors of the node on a path towards the DAG root.  For
         each DAGID that a node is a member of, the node will maintain a
         set containing one or more DAG parents.  If a node is a member
         of multiple DAGs then it must conceptually maintain a set of
         DAG parents for each DAGID.

   DAG sibling:  A sibling of a node within a DAG is defined in this
         specification to be any neighboring node which is located at
         the same rank (depth) within a DAG.  Note that siblings defined
         in this manner do not necessarily share a common parent.  For
         each DAG that a node is a member of, the node will maintain a
         set of DAG siblings.  If a node is a member of multiple DAGs
         then it must conceptually maintain a set of DAG siblings for
         each DAG.

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

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

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

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

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

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







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   P2MP: Point-to-Multipoint.  This refers to traffic between one node
         and a set of nodes.  This is similar to the P2MP concept in
         Multicast or MPLS Traffic Engineering ([RFC4461] and
         [RFC4875]).  A common RPL use case involves P2MP flows from or
         through a DAG root outward towards other nodes contained in the
         DAG.

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

   OCP:  Objective Code Point.  In RPL, the Objective Code Point (OCP)
         indicates which routing metrics, optimization objectives, and
         related functions are in use in a DAG.  Instances of the
         Objective Code Point are further described in
         [I-D.ietf-roll-routing-metrics].

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


3.  Protocol Model

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

3.1.  Protocol Properties Overview

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

3.1.1.  IPv6 Architecture

   RPL is strictly compliant with layered IPv6 architecture.

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






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3.1.2.  Typical LLN Traffic Patterns

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

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

3.1.3.  Constraint Based Routing

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

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

3.2.  Protocol Operation

   A LLN deployment will consist of a number of nodes and a number of
   edges (links) between them, whose characteristics will depend on
   implementation and link layer (L2) specifics.  Due to the nature of
   the LLN environment the L2 links are expected to demonstrate a large
   degree of variance as to their availability, quality, and other
   related parameters.  Certain links, demonstrating a viability above a
   confidence threshold for particular node and link metrics, as based
   on guidelines from [I-D.ietf-roll-routing-metrics], will be extracted
   from the L2 graph, and the resulting graph will be used as the basis



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   on which to operate the routing protocol.  Note that as the
   characteristics of the L2 topology vary over time the set of viable
   links is to be updated and the routing protocol thus continues to
   evaluate the LLN.  In RPL this process happens in a distributed
   manner, and from the perspective of a single node running RPL this
   process results in a set of candidate neighbors, with associated node
   and link metrics as well as confidence values.

   Many of the dominant traffic flows in support of the LLN application
   scenarios are MP2P flows ([I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs],
   [I-D.ietf-roll-indus-routing-reqs], and [RFC5548]).  These flows are
   rooted at designated nodes that have some application significance,
   such as providing connectivity to an external routed infrastructure.
   The term "external" is used top refer to the public Internet or a
   core private (non-LLN) IP network.  In support of this dominant flow
   RPL constructs Directed Acyclic Graphs (DAGs) on top of the viable
   LLN topology, selecting and orienting links among candidate neighbors
   toward DAG roots which root the MP2P flows.

   LLN nodes running RPL will construct Directed Acyclic Graphs (DAGs)
   rooted at designated nodes that generally have some application
   significance, such as providing connectivity to an external routed
   infrastructure.  The term "external" is used top refer to the public
   Internet or a core private (non-LLN) IP network.  This structure
   provides the routing solution for the dominant MP2P traffic flows.
   The DAG structure further provides each node potentially multiple
   successors for MP2P flows, which may be used for, e.g., local route
   repair or load balancing.

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

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



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   the outward routing state.

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

3.2.1.  DAG Construction

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

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

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

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

   The IPv6 RA message is augmented with a DAG Information Option (DIO),
   forming an RA-DIO message, to convey information about the DAG
   including:

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

   o  Objective Code Point (OCP) as described below.

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

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




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

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

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

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

3.2.1.2.  DAG Identification

   Each DAG is identified by a particular identifier (DAGID) as well as
   its supported optimization objectives and available destination
   prefixes.  The optimization objectives are conveyed as an Objective
   Code Point (OCP) as described further below.  Available destination
   prefixes, which may include destinations available beyond the DAG
   root, multicast destinations, or IPv6 node addresses, are advertised
   outwards along the DAG and recipient nodes may then provision routing
   tables with entries inwards towards the destinations.  The RPL
   implementation at each node will be provisioned by the application
   with sufficient information to determine which objectives and
   destinations are required, and thus the RPL implementation may
   determine which DAG to join.

   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 independent sets of DAG parents and related data structures
   for each DAG.  Note that in such a case traffic may directed along
   the appropriate constrained DAG based on traffic marking within the
   IPv6 header.  This specification will focus on the case where the
   node only joins one DAG; further elaboration on the proper operation
   of RPL in the presence of multiple DAGs, including traffic marking
   and related rules, are to be specified further in future revisions of



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   this or companion specifications.

3.2.1.3.  Grounded and Floating DAGs

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

3.2.1.4.  Administrative Preference

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

3.2.1.5.  Objective Code Point (OCP)

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

   o  The set of metrics used within the DAG

   o  The objective functions used for least cost path determination.

   o  The function used to compute DAG Rank

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

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




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

3.2.1.6.  Distributed Algorithm Operation

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

   o  Nodes will maintain a data structure containing their candidate
      (viable) neighbors, as based on guidelines in
      [I-D.ietf-roll-routing-metrics] This data structure will also
      track DAG information as learned from and associated with each
      neighbor.

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

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

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

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



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      more frequently.  When the DAG structure becomes consistent, RA-
      DIO messages taper off.

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

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

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

3.2.1.7.  DAG Rank

   When nodes select DAG parents, they will select the most preferred
   parent according to their implementation specific objectives, using
   the cost metrics conveyed in the 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 OCP, a node will be able to compute a value
   for its rank as a consequence of selecting a most preferred DAG
   parent.

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

   It is important to note that the DAG Rank is not itself a metric,
   although its value is derived from and influenced by the use of
   metrics to select DAG parents and take up a position in the DAG.  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:




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      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 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.

      If DAGRank(M) < DAGRank(N), then M is probably located in a more
      preferred position than N in the DAG with respect to the metrics
      and optimizations defined by the objective code point.  In any
      fashion, Node M may safely be a DAG parent for Node N without risk
      of creating a loop.

         For example, a Node M of rank 3 is likely 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
      preferred position than N in the DAG with respect to the metrics
      and optimizations defined by the objective code point.  Further,
      Node (M) may in fact be in Node (N)'s sub-DAG.  There is no
      advantage to Node (N) selecting Node (M) as a DAG 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.




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

   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.

3.2.1.8.  Sub-DAG

   The sub-DAG of a node is the set of other nodes of greater rank in
   the DAG, and thus might use a path towards the DAG root that contains
   this node.  This is an important property that is leveraged for loop
   avoidance- if a node has lesser rank then it is 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 9.  In this
   example, the sub-DAG of Node (24) is comprised of Nodes (34), (44),
   and (45).

   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 Appendix B.8.

3.2.1.9.  Moving up in a DAG

   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.

3.2.1.10.  Moving down in a DAG

   A node may not consider to move `down' the DAG, causing its DAG rank
   to increase and moving further from the DAG root.  In the case where
   a node looses connectivity to the DAG, it must first leave the DAG
   before it may then rejoin at 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



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   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.

3.2.1.11.  DAG Jumps

   A jump 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.

3.2.1.12.  Floating DAGs for DAG Repair

   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.  (Such
   coordination endeavors to avoid the construction of transient loops
   in the LLN).

   A DAG, or a sub-DAG temporarily promoted to a DAG, may also become
   floating because of a network element failure.  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 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-DIO messages from alternate
   attachment points).

3.2.2.  Destination Advertisement

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



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   indicate such in the RA-DIO message.  A DAG root that supports using
   the destination advertisement mechanism must be capable of allocating
   enough state to store the routing state received from the LLN.

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

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

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

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

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

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

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

3.2.2.2.  Destination Advertisement Operation

   As the DAG is constructed and maintained, nodes are capable to emit
   NA-DAO messages to a subset, or all, of their DAG 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 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 may use it
   to provision the one-hop route only, and not engage in any additional
   processing (so as not to engage the mechanisms used by a DAG parent).

   When a (unicast) NA-DAO message reaches a node capable of storing
   routing state, the node extracts information from the NA-DAO message
   and updates its local database with a record of the NA-DAO message
   and who it was received from.  When the node later propagates NA-DAO



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   messages, it selects the best (least depth) information for each
   destination and conveys this information again in the form of NA-DAO
   messages to a subset of its own DAG parents.  At this time the node
   may perform route aggregation if it is able, thus reducing the
   overall number of NA-DAO messages.

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

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

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

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

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

3.3.  Loop Avoidance and Stability

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

   RPL does not aim to guarantee loop free path selection, or strong
   global convergence.  In order to reduce control overhead, in



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   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.

3.3.1.  Greediness and Rank-based Instabilities

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

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

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

3.3.2.  Merging DAGs

   The merging of DAGs is 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 DAG loops.  The coordinated merge is also intended
   to minimize the related control cost.

   When a node, and perhaps a related frozen sub-DAG, jumps to a
   different DAG, the move is coordinated by a set of timers (DAG Hop
   timers).  The DAG Hop timers allow 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 example of a DAG Merge operation may be found in
   Appendix B.10







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3.3.3.  DAG Loops

   A DAG loop may occur when a node detaches from the 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 DAG sequence
   number can eliminate this type of loop.  If the DAG sequence number
   is not in use, the protection is limited (it depends on propagation
   of RA-DIO messages during DAG hop timer), and temporary loops might
   occur.  RPL will move to eliminate such a loop as soon as a RA-DIO
   message is received from a parent that appears to be going down, as
   the child has to detach from it immediately.  (The alternate choice
   of staying attached and following the parent in its fall would have
   counted to infinity and led to detach as well).

   Consider node (24) in the DAG Example depicted in Figure 9, 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 DAG rooted at (LBR), and
   nodes (34) and (45) were to miss the detachment sequence.
   Subsequently, if the link (24)--(45) were to become viable and node
   (24) heard node (45) advertising the DAG rooted at (LBR), a DAG loop
   (45->34->24->45) may form if node (24) attaches to node (45).

3.3.4.  DAO Loops

   A DAO loop may occur when the parent has a route installed upon
   receiving and processing a NA-DAO message from a child, but the child
   has subsequently cleaned up the state.  This loop happens when a no-
   DAO was missed till a heartbeat cleans up all states.  The DAO loop
   is not explicitly handled by the current specification.  Split
   horizon, not forwarding a packet back to the node it came from, may
   mitigate the DAO loop in some cases, but does not eliminate it.

   Consider node (24) in the DAG Example depicted in Figure 9.  Suppose
   node (24) has received a DA from node (34) advertising a destination
   at node (45).  Subsequently, if node (34) tears down the routing
   state for the destination and node (24) did not hear a no-DAO message
   to clean up the routing 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 loop (if split horizon is not in place).  A
   more complicated DAO loop may result if node (34) instead passes the
   traffic to it's sibling, node (33), potentially resulting in a
   (24->34->33->23->13->24) loop.

3.3.5.  Sibling Loops

   Sibling loops occur when a group of siblings keep choosing amongst
   themselves as successors such that a packet does not make forward



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   progress.  The current draft limits those loops to some degree by
   split horizon (do not send back to the same sibling) and parent
   preference (always prefer parents vs. siblings).

   Consider the DAG Example depicted in Figure 9.  Suppose that Node
   (32) and (34) are reliable neighbors, and thus are siblings.  Then,
   in the case where Nodes (22), (23), and (24) are transiently
   unavailable, and with no other guiding strategy, a sibling loop may
   exist, e.g. (33->34->32->33) as the siblings keep choosing amongst
   each other in an uncoordinated manner.

3.4.  Local and Temporary Routing Decision

   Although implementation specific, it is worth noting that a node may
   decide to implement some local routing decision based on some
   metrics, as observed locally or reported in the 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 mechanism further detailed in Section 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



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   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.

3.5.  Maintenance of Routing Adjacency

   In order to relieve the LLN of the overhead of periodic keepalives,
   RPL may employ an as-needed mechanism of NS/NA 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 when determining routes, other information
      may apply only when forwarding packets along provisioned routes.




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   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 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.




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      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.  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.  DAG Information Option (DIO) base option

   The DAG Information Option is a container option carried within an
   IPv6 Router Advertisement message as defined in [RFC4861], which
   might contain a number of suboptions.  The base option regroups the
   minimum information set that is mandatory in all cases.












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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |    Length     |G|D|A|  00000  |   Sequence    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | DAGPreference |                BootTimeRandom                 |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   NodePref.   |    DAGRank    |           DAGDelay            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | DIOIntDoubl.  |  DIOIntMin.   |     DAGObjectiveCodePoint     |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           PathDigest                          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                            DAGID                              |
       +                                                               +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   sub-option(s)...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 1: DIO Base Option

   Type: 8-bit unsigned identifying the DIO base option.  The suggested
         value is 140 to be 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 in order to
               update routing state for the outward direction along the
               DAG, as further detailed in Section 5.9.  Note that the
               use and semantics of this flag are still under
               investigation.




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

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

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

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

   BootTimeRandom:  A random value computed at boot time and recomputed
         in case of a duplication with another node.  The concatenation
         of the NodePreference and 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 indicating the DAG rank of the node
         sending the RA-DIO message.  The DAGRank of the DAG 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.




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   DIOIntervalDoublings:  8-bit unsigned integer.  Configured on the DAG
         root and used to configure the trickle timer governing when 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.

   DIOIntervalMin:  8-bit unsigned integer.  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.  (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, 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 RA-DIO
   messages outwards along the DAG: Type, Length, G, DAGPreference,
   DAGDelay and DAGID.  All other fields of the RA-DIO message are
   updated at each hop of the propagation.

5.1.1.1.  DAG Information Option (DIO) Suboptions

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

5.1.1.1.1.  Format






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

                  Figure 2: DIO Suboption Generic Format

   Suboption Type:  8-bit identifier of the type of suboption.  When
         processing a RA-DIO message containing a suboption for which
         the Suboption Type value is not recognized by the receiver, the
         receiver MUST silently ignore the unrecognized option, continue
         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 RA-DIO message suboptions which
   are currently defined for use in the DAG Information Option.

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

   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 3: Pad 1



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

   The Pad1 option is used to insert one octet of padding in the 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 4: Pad N

   The PadN option is used to insert two or more octets of padding in
   the 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 5: DAG Metric Container

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



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

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

5.1.1.1.5.  Destination Prefix

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



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


                     Figure 6: DAG Destination Prefix

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

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

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

   The Prefix Lifetime is a 32-bit unsigned integer representing the
   length of time in seconds (relative to the time the packet is sent)
   that the Destination Prefix is valid for route determination.  A



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   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 RA-DIO message may need to specify connectivity
   to more than one destination, the Destination Prefix suboption may be
   repeated.

5.2.  Conceptual Data Structures

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

   o  A set of candidate neighbors

   o  For each DAG:

      *  A set of candidate DAG parents

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

5.2.1.  Candidate Neighbors Data Structure

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

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





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5.2.2.  Directed Acyclic Graphs (DAGs) Data Structure

   A DAG may be uniquely identified by within the LLN by its unique
   DAGID.  When a single device is capable 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 DAG table with the following entries:

   o  DAGID

   o  DAGObjectiveCodePoint

   o  A set of Destination Prefixes offered inwards along the DAG

   o  A set of candidate DAG parents

   o  A timer to govern the sending of 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 DAG parent in the Held-Up state), then the DAG
   data structure is instantiated.

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

   In all other cases, for each candidate DAG 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



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

   o  A DAG Hop Timer, if instantiated

   o  A Held-Down Timer, if instantiated

5.2.2.1.1.  DAG Parents

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

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

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

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

5.3.  DAG Discovery and 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 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 3.2.1.7.  The rank should be computed in such a way so as to
   provide a comparable basis with other nodes which may not use the



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   same metric at all.

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

   o  RPL rules for loop avoidance based on rank

   o  The OCP function

   o  The advertised metrics

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

5.3.1.  DAG Discovery Rules

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

   1.   A node that does not have any DAG parents in a DAG is the root
        of its own floating DAG.  It's rank is 1.  A node will end up in
        that situation when it looses all of its current feasible
        parents, i.e. the set of DAG parents becomes depleted.  In that
        case, the node SHOULD remember the DAGID and the sequence
        counter of the last RA-DIO message from the lost parents for a
        period of time which covers multiple RA-DIO messages.  This is
        done so that if the node does encounter another possible
        attachment point to the lost DAGID within a period of time, the
        node may observe a sequence counter change by comparing the
        observed sequence counter to the last observed sequence counter
        and thus verify that the new attachment point is a viable and
        independent alternative to attach back to the lost DAGID.

   2.   A node that is attached to an infrastructure that does not
        support RA-DIO messages, is the DAG root of its own grounded
        DAG.  It's rank is 1.  (For example an LBR that is in
        communication with a non-LLN router not running RPL).

   3.   A (non-LLN) router sending a RA messages without DIO is
        considered a grounded infrastructure at rank 0.  (For example, a
        router that is in communication with an LLN node but not running
        RPL such as a non-LLN public Internet router in communication
        with an LBR)

   4.   The DAG root exposes the DAG in the RA-DIO message and nodes
        propagate the RA-DIO message outwards along the DAG with the RAs
        that they forward over their LLN links.



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   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 the OCP.

   6.   A node MUST NOT move outwards along a DAG that it is attached
        to, causing the DAG rank to increase, except in a special case
        where the node MAY choose to follow the last DAG parent in the
        set of DAG parents.  In the general case, if a node is required
        to move such that it cannot stay within the DAG without a rank
        increase, then it needs to first leave the DAG.  In other words
        a node that is already part of a DAG MAY move or follow a DAG
        parent at any time and with no delay in order to be closer, or
        stay as close, to the DAG root of its current DAG as it already
        is, but may not move outwards.  RAs received from other routers
        located at lesser rank in the same DAG may be considered as
        coming from candidate parents.  RAs received from other routers
        located at the same rank in the same DAG may be considered as
        coming from siblings.  Nodes MUST ignore RAs that are received
        from other routers located at greater rank within the same DAG.

   7.   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, or whatever metrics the LLN cares to use.  A node may jump
        at any time and to whatever rank it reaches in the new DAG, but
        it may have to wait for a DAG Hop timer to elapse in order to do
        so.  This allows the new higher parts (closer to the sink) of
        the DAG to move first, thus allowing stepped DAG
        reconfigurations and limiting relative movements.  A node SHOULD
        NOT join a previous DAG (identified by its DAGID) unless the
        sequence number in the RA-DIO message has incremented since the
        node left that DAG.  A newer sequence number indicates that the
        candidate parents were not attached behind this node, as they
        kept getting subsequent RA-DIO messages with new sequence
        numbers from the same DAG.  In the event that old sequence
        numbers (two or more behind the present value) are encountered
        they are considered stale and the corresponding parent SHOULD be
        removed from the set.

   8.   If a node has selected a new set of DAG parents but has not
        moved yet (because it is waiting for DAG Hop timer to elapse),
        the node is unstable MUST NOT send RA-DIOs for that DAG.

   9.   If a node receives a RA-DIO from one of its DAG parents, and if
        the parent contains a different DAGID, indicating that the
        parent has left the DAG, and if the node can remain in the
        current DAG through an alternate DAG parent, then the node
        SHOULD remove the DAG parent which has joined the new DAG from



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        its DAG parent set and remain in the original DAG.  If there is
        no alternate parent for the DAG, then the node SHOULD follow
        that parent into the new DAG.

   10.  When a node detects or causes a DAG inconsistency, as described
        in Section 5.3.4.2, then the node SHOULD send an unsolicited RA-
        DIO message to its one-hop neighbors.  The RA-DIO is updated to
        propagate the new DAG information.  Such an event MUST also
        cause the trickle timer governing the periodic sending of RA-DIO
        messages to be reset.

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

5.3.2.  Reception and Processing of RA-DIO messages

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

5.3.2.1.  Determination of Eligibility for DIO Processing

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

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

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

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

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





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

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

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

5.3.2.2.  Overview of RA-DIO Message Processing



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

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

         Place the neighbor in the candidate DAG parent set

         If the node has sent an RA message within the risk window as
         described in Section 5.7.3 then perform the collision detection
         described in Section 5.7.3.  If a collision occurs, place the
         candidate DAG parent in the collision state and do not process
         the RA-DIO message any further as described in Section 5.7.

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

            Remove SRC as a DAG parent from the other DAG (place it in
            the held-down state)

            If the other DAG is 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 RA-DIO message
            (do not follow the parent).

         If the new/alternate DAG offers a better solution to the
         optimization objectives, then prepare to jump: copy the DIO
         information into the record for the candidate DAG parent, place
         the candidate DAG parent into the Held-Up state, and start the



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         DAG Hop timer as per Section 5.7.1.

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

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

   As candidate parents are identified, they may subsequently be
   promoted to DAG parents by following the rules of DAG discovery as
   described in Section 5.3.  When a node adds another node to its set
   of candidate parents, the node becomes attached to the DAG through
   the parent node.

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

5.3.3.  RA-DIO Transmission

   Each node maintains a timer that governs when to multicast RA
   messages.  This timer is implemented as a trickle timer operating
   over a variable interval.  Trickle timers are further detailed in
   Section 5.3.4.  The governing parameters for the timer should be
   configured consistently across the DAG, and are provided by the DAG
   root in the RA-DIO message.  In addition to periodic RA messages,
   each LLN node will respond to Router Solicitation (RS) messages
   according to [RFC4861].

   o  When a node 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. the node will remain silent when the
      timer expires).

   o  When a node detects an inconsistency, it SHOULD reset the interval
      of the trickle timer to a minimum value, causing RA messages to be
      emitted more frequently as part of a strategy to quickly correct
      the inconsistency.  Such inconsistencies may be, for example, an
      update to a key parameter (e.g. sequence number) in the RA-DIO
      message or a loop detected when a node located inwards along the
      DAG forwards traffic outwards.  Inconsistencies are further
      detailed in Section 5.3.4.2.

   o  When a node enters a mode 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



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      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 initialized, it MAY be configured to remain silent
      and not multicast any RA messages until it has encountered and
      joined a DAG (perhaps initially probing for a nearby DAG with an
      RS message).  Alternately, it may choose to root its own floating
      DAG and begin multicasting RAs using a default trickle
      configuration.  The second case may be advantageous if it is
      desired for independent nodes to begin aggregating into scattered
      floating DAGs in the absence of a grounded node, for example in
      support of LLN installation and commissioning.

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

5.3.4.  Trickle Timer for RA Transmission

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

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

   I:    The current length of the communication interval

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

   C:    Redundancy Counter

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





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   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 RA-DIO message
         as DIOIntervalDoublings.  The default value is
         DEFAULT_DIO_INTERVAL_DOUBLINGS.

5.3.4.1.  Resetting the Trickle Timer

   The trickle timer for a DAGID is reset by:

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

   2.  Setting C to zero.

   3.  Setting I to I_min.

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

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

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

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

5.3.4.2.  Determination of Inconsistency

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

   o  The node joins a new DAGID

   o  The node moves within a DAGID

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

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





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

   o  The rank of a DAG parent has changed.

5.4.  DAG Heartbeat

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

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

   Other implementations may make use of a periodic timer to
   automatically increment the DAGSequenceNumber, resulting in a
   periodic DAG Heartbeat at a rate appropriate to the application and
   implementation.

5.5.  DAG Selection

   The DAG selection is implementation and algorithm dependent.  Nodes
   SHOULD prefer to join DAGs advertising OCPs and destinations
   compatible with their implementation specific objectives.  In order
   to limit erratic movements, and all metrics being equal, nodes SHOULD
   keep their previous selection.  Also, nodes SHOULD provide a means to
   filter out a candidate parent whose availability is detected as
   fluctuating, at least when more stable choices are available.  Nodes
   MAY place the failed candidate parent in a Hold Down mode that
   ensures that the candidate parent will not be reused for a given
   period of time.

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

   A node SHOULD verify that bidirectional connectivity and adequate
   link quality is available with a candidate neighbor before it



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   considers that candidate as a DAG parent.

5.6.  Administrative rank

   When the DAG is formed under a common administration, or when a node
   performs a certain role within a community, it might be beneficial to
   associate a range of acceptable rank with that node.  For instance, a
   node that has limited battery should be a leaf unless there is no
   other choice, and may then augment the rank computation specified by
   the OCP 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
   parents depending on runtime conditions.  The following states are
   defined:

   Current     This candidate parent is in the set of DAG parents and
               may be used for forwarding traffic inward along the DAG.
               When a candidate parent is placed into the Current state,
               or taken out of the Current state, it is necessary to re-
               evaluate which of the remaining DAG parents is the most
               preferred DAG parent and its rank.  At that time any
               remaining DAG parents of greater rank than this node must
               be placed in the Held-Down state, and the hold-down timer
               started, in order to be evicted as DAG parents.  In the
               same fashion, siblings must also be reevaluated.

   Held-Up     This parent can not be used until the DAG hop timer
               elapses.

   Held-Down   This candidate parent can not be used till hold down
               timer elapses.  At the end of the hold-down period, the
               candidate is 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 message.

5.7.1.  Held-Up

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

      Delay the reattachment of a sub-DAG that has been forced to
      detach.  This is not as safe as the use of the sequence, but still
      covers that when a sub-DAG has detached, the RA-DIO message that
      is initiated by the new DAG root has a chance to spread outward



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      along the sub-DAG, ideally forming a frozen sub-DAG that is aware
      of the DAG change, such that two different DAGs have formed prior
      to an attempted reattachment.

      Limit RA-DIO message storms (control cost / churn) when two DAGs
      collide/merge.  The idea is that between the nodes from DAG A that
      decide to move to DAG B, those that see the highest place (closer
      to the DAG root) in DAG B will move first and advertise their new
      locations before other nodes from DAG A actually move.

   A new DAG is discovered upon receiving a RA message with or without a
   DIO.  The node joins the DAG by selecting the source of the RA
   message as a DAG parent (and possibly installing the DAG parent as a
   default gateway).  The node is then a 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 that advertises
   the new DAG is placed in a held up state for the duration of a DAG
   Hop timer.  If the resulting new set of DAG parents is more
   preferable than the current one, or if the node is intending to
   maintain a membership in the new DAG in addition to its current DAG,
   the node expects to jump and becomes unstable.

   A node that is unstable may discover other candidate parents from the
   same new DAG during the instability phase.  It needs to start a new
   DAG Hop timer for all these.  The first timer that elapses for a
   given new DAG clears them all for that DAG, allowing the node to jump
   to the highest position available in the new DAG.

   The duration of the DAG Hop timer depends on the DAG Delay of the new
   DAG and 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 collisions and synchronizations.

5.7.2.  Held-Down

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

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

5.7.3.  Collision

   A race condition occurs if 2 nodes send RA-DIO messages at the same
   time and then attempt to join each other.  This might happen, for



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   example, between nodes which act as DAG root of their own DAGs.  In
   order to detect the situation, LLN Nodes time stamp the sending of
   RA-DIO message.  Any RA-DIO message received within a short link-
   layer-dependent period introduces a risk.  To resolve the collision,
   a 32bits extended preference is constructed from the RA-DIO message
   by concatenating the NodePreference with the BootTimeRandom.

   A node that decides to add a candidate to its DAG parents will do so
   between (candidate rank) and (candidate rank + 1) times the candidate
   DAG Delay.  But since a node is 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 only happen during
   the propagation time between the candidate and the node, plus some
   internal queuing and processing time within each machine.  It is
   expected that one DAG delay normally covers that interval, but
   ultimately it is up to the implementation and the configuration of
   the candidate parent to define the duration of risk window.

   There is risk of a collision when a node receives an RA, for another
   candidate that is more preferable than the current candidate, within
   the risk window.  In the face of a potential collision, the node with
   lowest extended preference processes the RA-DIO message normally,
   while the router with the highest extended preference places the
   other in collision state, does not start the DAG hop timer, and does
   not become instable.  It is expected that next RAs between the two
   will not cross anyway.

   For example, consider a case where two nodes are each rooting their
   own transient floating DAGs and multicast RA-DIO messages towards
   each other in a close enough interval that the RA-DIO messages
   `cross'.  Then each node may receive the RA-DIO message from the
   other node, and in some scenario decide to join each others DAG.  RPL
   avoids this deadlock scenario via the collision mechanism described
   above - after each node sends the RA-DIO message they will enter the
   risk window.  When the peer RA-DIO message is received in the risk
   window, the nodes will calculate the extended preferences as describe
   above and the node with the lowest extended preference will proceed
   to process the RA-DIO message, while the other node will defer,
   avoiding the deadlock scenario.

5.7.4.  Instability

   A node is instable when it is prepared to shortly replace a set of
   DAG parents in order to jump to a different DAGID.  This happens
   typically when the node has selected a more preferred candidate
   parent in a different DAG and has to wait for the DAG hop timer to
   elapse before adjusting the DAG parent set.  Instability may also



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   occur when the entire current DAG parent set is lost and the next
   best candidates are still held up.  Instability is resolved when the
   DAG hop timer of all the candidate(s) causing instability elapse.
   Such candidates then change state to Current or Held- Down.

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

5.8.  Guidelines for Objective Code Points

5.8.1.  Objective Function

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

   The Objective Function is specified in the 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 [I-D.ietf-roll-routing-metrics].  This document
   specifies the OCP 0, in support of default operation.

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

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

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



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      completely excluded from the computation, while others might be
      more or less preferred.

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

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

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

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

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

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

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

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

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

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

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

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



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

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

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

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

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

5.8.2.  Objective Code Point 0 (OCP 0)

   Here follows the specification for the default Objective Function
   corresponding to OCP codepoint 0.  This is a very simple reference to
   help design more complex Objective Functions.  In particular, the
   Objective Function described here does not use physical metrics as
   described in [I-D.ietf-roll-routing-metrics], but are only based on
   abstract information from the RA-DIO message 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 policy of the processing node when no
   objective function is included in the RA-DIO message, or if the OF
   indicated in the RA-DIO message is unknown to the node.  If not
   allowed, then the RA-DIO message is simply ignored and not processed
   by the node.

5.8.2.1.  OCP 0 Objective Function (OF0)

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

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

   The step of rank is 4 for each hop.




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5.8.2.2.  Selection of the Preferred Parent

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

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

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

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

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

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

   6.   A lesser resulting rank is better.

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

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

   9.   The router with a better router preference wins.

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

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

5.8.2.3.  Selection of the Backup next_hop

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

   o  The preferred parent is ignored.

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

   o  Candidate neighbors with a higher rank are ignored.

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



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

   o  The router with a better router preference wins.

   o  The backup next_hop that was in use already is better.

5.9.  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 result of destination advertisement operation:

   o  DAG discovery establishes a DAG oriented toward a 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 `stateful' nodes.
      *  Source Routing `bridges' across nodes who do not retain state.

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

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

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

   Nodes that are capable of storing routing state, and finally the DAG
   roots, are able to learn which destinations are contained in the sub-
   DAG below the node, and via which next-hop neighbors.  The
   dissemination and installation of this routing state into nodes
   allows for Hop-By-Hop routing from the DAG root outwards along the



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   DAG.  The mechanism is further enhance by supporting the construction
   of source routes across stateless `gaps' in the DAG, where nodes are
   incapable of storing additional routing state.  An adaptation of this
   mechanism allows for the implementation of loose-source routing.

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

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

5.9.1.  Destination Advertisement Message Formats

5.9.1.1.  DAO Option

   RPL extends Neighbor Discovery [RFC4861] and RFC4191 [RFC4191] to
   allow a node to include a destination advertisement option, which
   includes prefix information, in the Neighbor Advertisement (NA)
   messages.  A prefix option is normally present in RA messages only,
   but the NA is augmented with this option in order to propagate
   destination information inwards along the DAG.  The option is named
   the 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 nodes in the DAG to build up routing state for nodes
   contained in the sub-DAG in support of traffic flowing outward along
   the DAG.














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        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |    Length     | Prefix Length |    RRCount    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          DAO Lifetime                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Route Tag                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   DAO Depth   |   Reserved    |         DAO Sequence          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                   Prefix (Variable Length)                    |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Reverse Route Stack (Variable Length)             |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

           Figure 7: The Destination Advertisement Option (DAO)

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

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

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

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

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

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



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   DAO Depth:  Set to 0 by the node that owns the prefix and first
         issues the NA-DAO message.  Incremented by all LLN nodes that
         propagate the NA-DAO message.

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

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

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

   Reverse Route Stack:  Variable-length field containing a sequence of
         RRCount (possibly compressed) IPv6 addresses.  A node who adds
         on to the 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 DAG may be selected to receive prefix
   information from the destination advertisement mechanism.  This
   subset of DAG parents shall be designated the set of DA parents.

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



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   for the destination are more optimal.

   If destination advertisements are activated in the RA-DIO message as
   indicated by the `D' bit, the node sends unicast destination
   advertisements to its DA parents, and only accepts unicast
   destination advertisements from any nodes but those contained in the
   DA parent subset.

   Every NA to a DA parent MAY contain one or more DAOs.  Receiving a
   RA-DIO message with the `D' destination advertisement bit set from a
   DAG parent stimulates the sending of a delayed destination
   advertisement back, with the collection of all known prefixes (that
   is the prefixes learned via destination advertisements for nodes
   lower in the DAG, and any connected prefixes).  If the Destination
   Advertisement Supported (A) bit is set in the 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 advertised prefixes has changed.  Destination
   advertisements may also be scheduled for sending when the PathDigest
   of the RA-DIO message has changed, indicating that some aspect of the
   inwards paths along the DAG has been modified.

   Destination advertisements may advertise positive (prefix is present)
   or negative (removed) NA-DAO messages, termed as no-DAOs.  A no-DAO
   is stimulated by the disappearance of a prefix below.  This is
   discovered by timing out after a request (a RA-DIO message) or by
   receiving a no-DAO.  A no-DAO is a conveyed as a NA-DAO message with
   a DAO Lifetime of 0.

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

   A node who is unable to record the state information conveyed in the
   NA-DAO message will append the next-hop address to the Reverse Route
   Stack, increment the RRCount, and then pass the destination
   advertisement on without recording any additional state.  In this way
   the Reverse Route Stack will contain a vector of next hops that must



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   be traversed along the reverse path that the NA-DAO message has
   traveled.  The vector will be ordered such that the node closest to
   the destination will appear first in the list.  In such cases, if it
   is useful to the implementation to try and build up redundant paths,
   the node may choose to convey the destination advertisement to one or
   more DAG parents in order of preference as guided by an
   implementation specific policy.

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

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

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

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

   o  The IPv6 address and interface for the advertising neighbor.

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

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

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

   Note that nodes may receive multiple information from different



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

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

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

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

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

5.9.2.1.1.  Destination Advertisement Timers

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

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

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

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

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



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

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

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

      *  When the RemoveTimer elapse, 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 propagated and cleaned
         up.

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

5.9.2.2.  Multicast Destination Advertisement messages

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

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

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






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5.9.2.3.  Unicast Destination Advertisement messages from child to
          parent

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

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

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

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

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

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

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

   Once the RemoveTimer is elapsed, the prefix entry is scheduled to be
   removed and moved to the Unreachable list if there are any DA parents
   that need to be informed of the change in status for the prefix,
   otherwise the prefix entry is cleaned up right away.  The prefix
   entry is removed from the Unreachable list when no more DA parents
   need to be informed.  This condition may be satisfied when a no-DAO
   is sent to all current DA parents indicating the loss of the prefix,
   and noting that in some cases parents may have been removed from the
   set of DA parents.



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5.9.2.4.  Other events

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

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

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

   o  Loss of routing adjacency: When the routing adjacency for a
      neighbor is lost, as per the procedures described in Section 5.11,
      and if the associated entries are in the Reachable list, the
      associated routes are removed, and the entries are scheduled to be
      destroyed.

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

5.9.2.5.  Aggregation of prefixes by a node

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

   Other cases might occur for which additional support is required:

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

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

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

   Consider a node M who is performing an aggregation, and a node N who
   is to be a member of the aggregation group.  A node Z situated above
   the node M in the DAG, but not above node N, will see the
   advertisements for the aggregation owned by M but not that of the
   individual prefix for N. Such a node Z will route all the packets for
   node N towards node M, but node M will have no route to the node N
   and will fail to forward.

   Additional protocols may be applied beyond the scope of this



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   specification to dynamically elect/provision an aggregating node and
   groups of nodes eligible to be aggregated in order to provide route
   summarization for a sub-DAG.

5.9.2.6.  Default Values

   DEF_NA_LATENCY = To Be Determined

   MAX_DESTROY_INTERVAL = To Be Determined

5.10.  Multicast Operation

   This section describes further the 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 listener uses a protocol such as MLD with a
   router to register to a multicast group.

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

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

   As a result, multicast routing states are installed in each router on
   the way from the listeners to the root, enabling the root to copy a
   multicast packet to all its children routers that had issued a NA-DAO
   message including a DAO for that multicast group, as well as all the
   attached nodes that registered over MLD.

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



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   along the DAG based on the multicast routing table entries installed
   from the NA-DAO message.

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

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

5.11.  Maintenance of Routing Adjacency

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

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

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






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5.12.  Packet Forwarding

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

   1.  It is preferred to select a successor from a DAG who is
       supporting an OCP and related optimization that maps to an
       objective marked in the IPv6 header of the packet being
       forwarded.

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

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

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

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

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

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

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

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

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



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   destination is neared.


6.  RPL Variables

   DIO Timer  One instance per DAG that a node is a member of.  Expiry
         triggers RA-DIO message transmission.  Trickle timer with
         variable interval in [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 node is going to jump to.
         Expiry triggers candidate DAG parent to become a DAG parent in
         the `Current' state, as well as cancellation of any other DAG
         Hop timers associated with other DAG parents for that DAG.
         Duration is computed based on the rank of the candidate DAG
         parent and DAG delay, as (candidates rank + random) *
         candidate's DAG_delay (where 0 <= random < 1).  See
         Section 5.7.1.

   Hold-Down Timer  Up to one instance per candidate DAG parent in the
         `Held-Down' state per DAG.  Expiry triggers the eviction of the
         candidate DAG parent from the candidate DAG parent set.  The
         interval should be chosen as appropriate to prevent flapping.
         See Section 5.7.

   DAG Heartbeat Timer  Up to one instance per DAG that the node is
         acting as DAG root of.  May not be supported in all
         implementations.  Expiry triggers revision of
         DAGSequenceNumber, causing a new series of updated RA-DIO
         message to be sent.  Interval should be chosen appropriate to
         propagation time of DAG 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 DA parent.
         The interval is to be proportional to DEF_NA_LATENCY/(node
         rank), such that nodes of greater rank (further outward along
         the DAG) expire first, coordinating the 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 DA
         entry, setting up to do unreachable (No-DAO) advertisements or



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


7.  Manageability Considerations

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

7.1.  Control of Function and Policy

7.1.1.  Initialization Mode

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

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

7.1.2.  DIO Base option

   RPL specifies a number of protocol parameters.

   A RPL implementation SHOULD allow configuring the following routing
   protocol parameters, which are further described in Section 5.1.1:

   DAGPreference

   NodePreference

   DAGDelay

   DIOIntervalDoublings

   DIOIntervalMin:






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   DAGObjectiveCodePoint

   PathDigest

   DAGID

   Destination Prefixes

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

   DAG Hop Timer:  A RPL implementation MUST provide the ability to
         configure the value of the DAG Hop Timer, expressed in ms.

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

7.1.3.  Trickle Timers

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

   For each DAG, a RPL implementation MUST allow for the monitoring of
   the following parameters, further described in Section 5.3.4:

   I

   T

   C

   I_min

   I_doublings:

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





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7.1.4.  DAG Heartbeat

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

7.1.5.  The Destination Advertisement Option

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

   o  The DelayNA timer

   o  The Remove timer

7.1.6.  Policy Control

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

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

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

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

7.1.7.  Data Structures

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

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




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7.2.  Information and Data Models

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

7.3.  Liveness Detection and Monitoring

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

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

   o  The candidate neighbors data structure

   o  For each DAG:

      *  A set of candidate DAG parents

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

7.3.1.  Candidate Neighbor Data Structure

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

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

7.3.2.  Directed Acyclic Graph (DAG) Table

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

   o  DAGID

   o  DAGObjectiveCodePoint

   o  A set of Destination Prefixes offered inwards along the DAG

   o  A set of candidate DAG Parents




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

   o  A DAG Hop Timer, if instantiated

   o  A Held-Down Timer, if instantiated

   o  A flag reporting if 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 number of
   a times the node has detected an inconsistency with respect to a DAG
   parent, e.g. if the DAGID has changed.

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

7.3.5.  RPL Trickle Timers

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

   o  The DIOIntervalMin expressed in ms

   o  The DIOIntervalDoublings

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




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7.4.  Verifying Correct Operation

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

7.5.  Requirements on Other Protocols and Functional Components

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

7.6.  Impact on Network Operation

   To be completed.


8.  Security Considerations

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


9.  IANA Considerations

9.1.  DAG Information Option (DIO) Base Option

   The DAG Information Option is a container option carried within an
   IPv6 Router Advertisement message as defined in [RFC4861], which
   might contain a number of suboptions.  The base option regroups the
   minimum information set that is mandatory in all cases.

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

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

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

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

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

   o  Capability description





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   o  Defining RFC

   Three flags are currently defined:

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

                           DIO Base Option Flags

9.3.  DAG Information Option (DIO) Suboption

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

         +-------+------------------------------+---------------+
         | Value | Meaning                      | Reference     |
         +-------+------------------------------+---------------+
         |   0   | Pad1 - DIO Padding           | This document |
         |   1   | PadN - DIO suboption padding | This document |
         |   2   | DAG 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 a node to include a Destination Advertisement Option, which
   includes prefix information in the Neighbor Advertisements messages.
   The Neighbor Advertisement messages are augmented with the
   Destination Advertisement Option (DAO).

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



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   0 in this document.


10.  Acknowledgements

   The ROLL Design Team would like to acknowledge the review, feedback,
   and comments from Dominique Barthel, Yusuf Bashir, Mathilde Durvy,
   Manhar Goindi, Mukul Goyal, Quentin Lampin, Philip Levis, Jerry
   Martocci, Alexandru Petrescu, and Don Sturek.

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


11.  Contributors

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

   Email: jpv@cisco.com


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

   Email: jhui@archrock.com


   Thomas Heide Clausen
   LIX, Ecole Polytechnique, France

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




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   Richard Kelsey
   Ember Corporation
   Boston, MA
   USA

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


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

   Email: stevedh@cs.berkeley.edu


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

   Email: kpister@dustnetworks.com


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

   Email: abr@zen-sys.com


12.  References

12.1.  Normative References

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

12.2.  Informative References

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



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              February 2009.

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

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

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

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

   [I-D.ietf-roll-routing-metrics]
              Vasseur, J. and D. Networks, "Routing Metrics used for
              Path Calculation in Low Power and Lossy Networks",
              draft-ietf-roll-routing-metrics-00 (work in progress),
              April 2009.

   [I-D.ietf-roll-terminology]
              Vasseur, J., "Terminology in Low power And Lossy
              Networks", draft-ietf-roll-terminology-01 (work in
              progress), May 2009.

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

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



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

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

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

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

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

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

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

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

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

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


Appendix A.  Deferred Requirements

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






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Appendix B.  Examples

   Consider the example LLN physical topology in Figure 8.  In this
   example the links depicted are all usable L2 links.  Suppose that all
   links are equally usable, and that the implementation specific policy
   function is simply to minimize hops.  This LLN physical topology then
   yields the DAG depicted in Figure 9, where the links depicted are the
   edges toward DAG parents.  This topology includes one DAG, rooted by
   an LBR node (LBR) at rank 1.  The LBR node will issue RAs containing
   DIO, as governed by a trickle timer.  Nodes (11), (12), (13), have
   selected (LBR) as their only parent, attached to the DAG at rank 2,
   and periodically advertise RA-DIO multicasts.  Node (22) has selected
   (11) and (12) in its DAG parent set, and advertises itself at rank 3.
   Node (22) thus has a set of DAG parents {(11), (12)} and siblings
   {((21), (23)}.


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


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

                      Figure 8: Example LLN Topology



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


   Note that the links depicted represent directed links in the DAG
   overlaid on top of the physical topology depicted in Figure 8.  As
   such, the depicted edges represent the relationship between nodes and
   their DAG parents, wherein all depicted edges are directed and
   oriented `up' on the page toward the DAG root (LBR).  The DAG may
   provide default routes within the LLN, and serves as the foundation
   on which RPL builds further routing structure, e.g. through the
   destination advertisement mechanism.

                           Figure 9: Example DAG

B.1.  Moving Down a DAG

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



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   little change for a loop, and Node (56) may safely reattach to the
   DAG, with parents {(43), (55)}.  At reattachment time, node (56)
   would present itself with a rank deeper than that of its deepest DAG
   parent (node (55) at rank 6), rank 7.

B.2.  Link Removed

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

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

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

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

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

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

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

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

B.3.  Link Added

   Consider the example of Figure 8 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 rank at 5,
      reflecting a rank deeper than its deepest parent (32).

   o  Node (42) may now choose to remain where it is, with two parents
      {(12), (32)}.  Should there be a reason for Node (42) to evict
      Node (32) from its set of DAG parents, Node (42) would then
      advertise itself at rank 2, thus moving up the DAG.  In this case,



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      Node (53), (54), and (55) may similarly follow and advertise
      themselves at rank 3.

B.4.  Node Removed

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

   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 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 rank 8.

B.5.  New LBR Added

   Consider the example of Figure 8 when a new LBR, (LBR2) appears, with
   connectivity (LBR2)-(52), (LBR2)-(53).

   o  Nodes (52) and Node (53) will see a chance to join a new DAG
      rooted at (LBR2) with a rank of 2.  Node (52) and (53) may take
      this chance immediately, as there is no risk of forming loops when
      joining a DAG that has never before been encountered.  Note that
      the nodes may choose to join the new DAG rooted at (LBR2) if and
      only if (LBR2) offers more optimum properties in line with the
      implementation specific local policy.

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

   o  Nodes (51), (41), (42), and (54) may then choose to join the new
      DAG at 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 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).



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B.6.  Destination Advertisement

   Consider the example DAG depicted in Figure 9.  Suppose that Nodes
   (22) and (32) are unable to record routing state.  Suppose that Node
   (42) is able to perform prefix aggregation on behalf of Nodes (53),
   (54), and (55).

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

   o  Node (54) and Node (55) would similarly send 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 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 NA-DAO message and passes
      it on to Node (22) as (42'):[(42)].  It may send a separate NA-DAO
      message to indicate destination (32).

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

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




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B.7.  Example: DAG Parent Selection

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

   o  Node (N) transmits a Router Solicitation.

   o  Node (B) responds.  Node (N) investigates the 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 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 for
      DAGID 1.  Following the mechanisms specified by the OCP, and given
      that the ETX is 1 for the link between (N) and (E), Node (N) is
      now at rank 5 in DAGID 1.

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

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

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



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

B.8.  Example: DAG Maintenance


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

              -1-                    -2-                    -3-


                        Figure 10: DAG Maintenance

   Consider the example depicted in Figure 10-1.  In this example, Node
   (A) is attached to a DAG at some rank d.  Node (A) is a DAG parent of
   Nodes (B) and (C).  Node (C) is a DAG parent of Node (D).  There is
   also an undirected sibling link between Nodes (B) and (C).

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

   Consider the case where Node (C) hears a 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 set and attach directly to Node (Z) without creating a
   loop, because its rank will decrease.

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



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

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

   o  Node (D), hearing a modified RA-DIO message from Node (C), follows
      Node (C) into the floating DAG.  This is depicted in Figure 10-2.
      In general, any node with no other options in the sub-DAG of Node
      (C) will follow Node (C) into the floating DAG, maintaining the
      structure of the sub-DAG.

   o  Node (C) hears a RA-DIO message from Node (B) and determines it is
      able to 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
      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 10-3

B.9.  Example: Greedy Parent Selection and Instability


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

              -1-                    -2-                    -3-


                  Figure 11: Greedy DAG Parent Selection

   Consider the example depicted in Figure 11.  A DAG is depicted in 3
   different configurations.  A usable link between (B) and (C) exists
   in all 3 configurations.  In Figure 11-1, Node (A) is a DAG parent
   for Nodes (B) and (C), and (B)--(C) is a sibling link.  In
   Figure 11-2, Node (A) is a DAG parent for Nodes (B) and (C), and Node
   (B) is also a DAG parent for Node (C).  In Figure 11-3, Node (A) is a



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   DAG parent for Nodes (B) and (C), and Node (C) is also a DAG parent
   for Node (B).

   If a RPL node is too greedy, in that it attempts to optimize for an
   additional number of parents beyond its preferred parent, then an
   instability can result.  Consider the DAG illustrated in Figure 11-1.
   In this example, Nodes (B) and (C) may most prefer Node (A) as a DAG
   parent, but are operating under the greedy condition that will try to
   optimize for 2 parents.

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

   o  Let Figure 11-1 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 11-2.  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 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.  Now Node (B) is
      deeper than both Nodes (A) and (C) and is satisfied with 2 DAG
      parents.

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

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

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

   o  The process will repeat, and the DAG will oscillate between
      Figure 11-2 and Figure 11-3 until the nodes count to infinity and
      restart the cycle again.

   o  This cycle can be averted through mechanisms in RPL:

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



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         alternate parents (Nodes are not greedy)

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

B.10.  Example: DAG Merge


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


                          Figure 12: Merging DAGs

   Consider the example depicted in Figure 12.  Nodes (A), (B), and (C)
   are part of some larger grounded DAG, where Node (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 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



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      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, removes Node (D)
      as its parent, thus emptying the DAG parent set for the floating
      DAG, and leaving the floating DAG.  Node (E) then jumps to the
      grounded DAG by entering Node (A) into the set of DAG 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-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 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



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      manner, avoiding the formation of loops in the process.


Appendix C.  Additional Examples

   Consider the expanded example LLN physical topology in Figure 13.  In
   this example an additional LBR is added.  Suppose that all nodes are
   configured with an implementation specific policy function that aims
   to minimize the number of hops, and that both LBRs are configured to
   root different DAGIDs.  We may now walk through the formation of the
   two DAGs.


                                     (LBR)                    (LBR2)
                                     / | \                    /    \
                                .---`  |  `----.             /      \
                               /       |        \            |      |
                            (11)------(12)------(13)      (14)      (15)
                             | \       | \       | \       |        /|
                             |  `----. |  `----. |  `----. |  .----` |
                             |        \|        \|        \| /       |
                            (21)------(22)------(23)      (24)      (25)
                             |        /|        /|         |        / /
                             |  .----` |  .----` |  .-----]|[------` /
                             | /       | /       | /       |        /
                            (31)------(32)------(33)------(34)-----`
                             |        /| \       | \       | \
                             |  .----` |  `----. |  `----. |  `----.
                             | /       |        \|        \|        \
                   .--------(41)      (42)      (43)------(44)------(45)
                  /         /         /| \       | \
            .----`    .----`    .----` |  `----. |  `----.
           /         /         /       |        \|        \
        (51)------(52)------(53)------(54)------(55)------(56)


                     Figure 13: Expanded LLN Topology














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                                     (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)



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                    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























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                                     (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)



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                    Figure 18: DAG Construction Step 5


Appendix D.  Outstanding Issues

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

D.1.  Additional Support for P2P Routing

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

D.2.  Loop Detection

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

D.3.  Destination Advertisement / DAO Fan-out

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

D.4.  Source Routing

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







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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




















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