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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                        ROLL Design Team
Expires: March 18, 2010                                     IETF ROLL WG
                                                      September 14, 2009


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

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   Copyright (c) 2009 IETF Trust and the persons identified as the
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Abstract

   This document specifies the Routing Protocol for Low Power and Lossy
   Networks (RPL), in accordance with the requirements described in



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

Requirements Language

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


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Design Principles  . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Protocol Model . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Problem  . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Protocol Properties Overview . . . . . . . . . . . . . . .  7
       3.2.1.  IPv6 Architecture  . . . . . . . . . . . . . . . . . .  7
       3.2.2.  Path Properties for LLN Traffic Flows  . . . . . . . .  7
       3.2.3.  Constraint Based Routing . . . . . . . . . . . . . . .  8
       3.2.4.  Autonomous Operation . . . . . . . . . . . . . . . . .  8
     3.3.  Protocol Operation . . . . . . . . . . . . . . . . . . . .  8
       3.3.1.  DAG Construction . . . . . . . . . . . . . . . . . . .  9
       3.3.2.  Source Routing . . . . . . . . . . . . . . . . . . . . 19
       3.3.3.  Destination Advertisement  . . . . . . . . . . . . . . 19
     3.4.  Other Considerations . . . . . . . . . . . . . . . . . . . 21
       3.4.1.  DAG Rank and Loop Avoidance  . . . . . . . . . . . . . 21
       3.4.2.  DAG Parent Selection, Stability, and Greediness  . . . 25
       3.4.3.  Merging DAGs . . . . . . . . . . . . . . . . . . . . . 27
       3.4.4.  Local and Temporary Routing Decision . . . . . . . . . 29
       3.4.5.  Scalability  . . . . . . . . . . . . . . . . . . . . . 30
       3.4.6.  Maintenance of Routing Adjacency . . . . . . . . . . . 30
   4.  Constraint Based Routing in LLNs . . . . . . . . . . . . . . . 30
     4.1.  Routing Metrics  . . . . . . . . . . . . . . . . . . . . . 30
     4.2.  Routing Constraints  . . . . . . . . . . . . . . . . . . . 32
     4.3.  Constraint Based Routing . . . . . . . . . . . . . . . . . 32
   5.  Specification of Core Protocol . . . . . . . . . . . . . . . . 32
     5.1.  DAG Information Option . . . . . . . . . . . . . . . . . . 33
       5.1.1.  DIO base option  . . . . . . . . . . . . . . . . . . . 33
     5.2.  Conceptual Data Structures . . . . . . . . . . . . . . . . 39
       5.2.1.  Candidate Neighbors  . . . . . . . . . . . . . . . . . 39
       5.2.2.  DAGs . . . . . . . . . . . . . . . . . . . . . . . . . 40
     5.3.  Initialization and Configuration . . . . . . . . . . . . . 41
     5.4.  DAG Discovery  . . . . . . . . . . . . . . . . . . . . . . 42
       5.4.1.  RA-DIO Reception . . . . . . . . . . . . . . . . . . . 45
       5.4.2.  RA-DIO Transmission  . . . . . . . . . . . . . . . . . 47



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       5.4.3.  Trickle Timer for RA Transmission  . . . . . . . . . . 48
     5.5.  DAG Heartbeat  . . . . . . . . . . . . . . . . . . . . . . 49
     5.6.  DAG Selection  . . . . . . . . . . . . . . . . . . . . . . 50
     5.7.  Administrative rank  . . . . . . . . . . . . . . . . . . . 50
     5.8.  Candidate DAG Parent States and Stability  . . . . . . . . 51
       5.8.1.  Held-Up  . . . . . . . . . . . . . . . . . . . . . . . 51
       5.8.2.  Held-Down  . . . . . . . . . . . . . . . . . . . . . . 52
       5.8.3.  Collision  . . . . . . . . . . . . . . . . . . . . . . 52
       5.8.4.  Instability  . . . . . . . . . . . . . . . . . . . . . 53
     5.9.  Guidelines for Objective Code Points . . . . . . . . . . . 53
       5.9.1.  Objective Function . . . . . . . . . . . . . . . . . . 53
       5.9.2.  Objective Code Point 0 (OCP 0) . . . . . . . . . . . . 55
     5.10. Establishing Routing State Outward Along the DAG . . . . . 57
       5.10.1. Destination Advertisement Message Formats  . . . . . . 58
       5.10.2. Destination Advertisement Operation  . . . . . . . . . 60
     5.11. Maintenance of Routing Adjacency . . . . . . . . . . . . . 67
     5.12. Packet Forwarding  . . . . . . . . . . . . . . . . . . . . 67
       5.12.1. Loop Taxonomy  . . . . . . . . . . . . . . . . . . . . 68
     5.13. Expectations of Link Layer Behavior  . . . . . . . . . . . 70
   6.  Summary of RPL Timers  . . . . . . . . . . . . . . . . . . . . 70
   7.  Protocol Extensions  . . . . . . . . . . . . . . . . . . . . . 71
   8.  Manageability Considerations . . . . . . . . . . . . . . . . . 71
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 71
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 72
     10.1. DAG Information Option . . . . . . . . . . . . . . . . . . 72
     10.2. Objective Code Point . . . . . . . . . . . . . . . . . . . 72
     10.3. Destination Advertisement Option . . . . . . . . . . . . . 72
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 72
   12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 72
   13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 74
     13.1. Normative References . . . . . . . . . . . . . . . . . . . 74
     13.2. Informative References . . . . . . . . . . . . . . . . . . 74
   Appendix A.  Deferred Requirements . . . . . . . . . . . . . . . . 76
   Appendix B.  Examples  . . . . . . . . . . . . . . . . . . . . . . 76
     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
   Appendix C.  Additional Examples . . . . . . . . . . . . . . . . . 82
   Appendix D.  Outstanding Issues  . . . . . . . . . . . . . . . . . 86
     D.1.  Additional Support for P2P Routing . . . . . . . . . . . . 86
     D.2.  Loop Detection . . . . . . . . . . . . . . . . . . . . . . 86
     D.3.  DAO Fan-out  . . . . . . . . . . . . . . . . . . . . . . . 86
     D.4.  Source Routing . . . . . . . . . . . . . . . . . . . . . . 86
     D.5.  Address / Header Compression . . . . . . . . . . . . . . . 86
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 87



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1.  Introduction

   The defining characteristics of Low Power and Lossy Networks (LLNs)
   offer unique challenges to a routing solution.  The IETF ROLL Working
   Group has defined application-specific routing requirements for a Low
   Power and Lossy Network (LLN) routing protocol
   [I-D.ietf-roll-building-routing-reqs]
   [I-D.ietf-roll-home-routing-reqs] [I-D.ietf-roll-indus-routing-reqs]
   [RFC5548].  RPL is a new routing protocol designed to meet these
   requirements.

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

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

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


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



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   in this document as follows:

   Autonomous:  Refers to the ability of a routing protocol to
         independently function without requiring 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 LBR).

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

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

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

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

   Grounded:  A DAG is grounded if it contains a DAG Root offering a
         default route to an external routed infrastructure such as the
         Internet.






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   Floating:  A DAG is floating if is not Grounded.  A floating DAG may
         install a default route, although it is not expected to reach
         any additional external routed infrastructure such as the
         Internet.

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

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

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

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

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

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

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


3.  Protocol Model

   The aim of this section is to describe RPL in the spirit of
   [RFC4101].  An architectural protocol overview (the big picture of
   the protocol) is provided in this section.  Protocol details can be
   found in further sections.





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3.1.  Problem

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

3.2.  Protocol Properties Overview

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

3.2.1.  IPv6 Architecture

   RPL is strictly compliant with layered IPv6 architecture.

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

3.2.2.  Path Properties for LLN Traffic Flows

   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 nodes).




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3.2.3.  Constraint Based Routing

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

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

3.2.4.  Autonomous Operation

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

3.3.  Protocol Operation

   LLN nodes running RPL will construct Directed Acyclic Graphs (DAGs)
   rooted at designated nodes that generally have some application
   significance, such as providing a default route to an external routed
   infrastructure.  The DAG is sufficient to support inward MP2P traffic
   flows, flowing inward along the LLN towards a sink (DAG Root), which
   is one of the dominant traffic flows described in the requirements
   documents ([I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs],
   [I-D.ietf-roll-indus-routing-reqs], and [RFC5548]).

   By utilizing a DAG for dominant MP2P flows, RPL allows each node to
   select and maintain potentially multiple successors capable of
   forwarding traffic inwards towards the root.  The DAG does not
   present as many single points of failure as a tree, and in addition
   can offer a node a set of pre-computed successors in support of, e.g.
   local route repair in case of a temporary failure, load balancing, or
   short term fluctuations in link characteristics.

   A DAG also serves to restrict the routing problem on the nodes when
   it is used as a reference topology.  This allows nodes to determine
   their positions in a DAG relative to each other and provides a means
   to coordinate route repair in a way that endeavors to avoid loops.



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   These mechanisms will be described in more detail later in this
   specification.

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

   Arbitrary P2P traffic MAY flow inward along the DAG until a common
   parent is reached who has stored routing state and is capable of
   directing the traffic outward along the correct outward path.  In the
   present specification RPL does not specify nor preclude any
   additional mechanisms that may be capable to compute and install more
   optimal routes into LLN nodes in support of arbitrary P2P traffic.
   (Note that in some application scenarios it may be important to
   support arbitrary P2P traffic along more optimal paths `across' the
   DAG).  This functionality is to be investigated further in a future
   revision.

   This section further describes the high level operation of RPL.

3.3.1.  DAG Construction

3.3.1.1.  Overview of a Typical Case

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

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

   In the context of a particular LLN application one or more nodes will
   be capable of, e.g. serving as an LBR or acting as a data collection
   point, and thus be provisioned to act as the most preferred DAG
   roots.  These nodes will begin the process of constructing a DAG by
   occasionally emitting Router Advertisements containing the necessary
   information for neighboring nodes to evaluate the DAG Root as a
   potential DAG parent.  This information will include a DAGID, a



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   DAGPreference, and an Objective Code Point (OCP).  The DAGID is an
   identifier unique to the DAG.  The DAGPreference offers a way to
   engineer the formation of the DAG in support of the application, by
   providing a mechanism by which the DAG may look attractive for other
   nodes to join.  The OCP provides information as to which metrics and
   optimization goals are being employed across the DAG.  Note that a
   single DAG Root may conceptually root different DAGs with different
   OCPs as required to support different sets of routing constraints.
   In this case the DAG Root must provision each different DAG with a
   different DAGID.  Note that if multiple nodes acting as DAG roots are
   rooting the same DAG, i.e. presenting the same DAGID, then they must
   have some means of coordinating with each other when emitting Router
   Advertisements (This may be the case, for example, when the DAG is
   provisioned with a `virtual root' through some backbone mechanism).
   This is described further below.

   Nodes who hear Router Advertisements, advertising a specific DAGID,
   will take into consideration several criteria when processing the
   extracted DAG information.  A node may seek a DAG advertising a
   specific OCP, reflecting the implementation specific routing
   constraints understood by the node.  In particular, a node will be
   seeking to find a least cost path satisfying some objective function
   as indicated by the OCP according to some routing metrics defined in
   [I-D.ietf-roll-routing-metrics].  For example, the least cost path
   may be determined in part by minimizing energy along a path, or
   latency, or avoiding the use of battery powered nodes.  A node may be
   seeking to explicitly join a grounded DAG.  Further, a node may seek
   the minimum DAGPreference when selecting a DAG, all else being equal.
   Based on the evaluation of such criteria, a node may determine if the
   node who emitted the Router Advertisement should be considered as a
   potential DAG parent.  If so, then the node may add the advertising
   node to its set of candidate DAG parents for the advertised DAGID,
   and after waiting for a designated delay, the node may follow the
   procedures to activate the advertising node as a DAG parent and may
   then be considered to have joined the DAG designated by DAGID.

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

   As nodes join the DAG they are able advertise the fact by beginning
   to multicast the DAG information in Router Advertisements (to



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

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

   In addition to identifying DAG parents, a node also may hear the
   Router Advertisements of other neighboring nodes at the same rank
   within the DAG.  In this way a node can discover DAG Siblings.

   A node may order its set of DAG parents according to some
   implementation specific preference.  To this list the node may also
   append a similarly ordered set of DAG siblings.  By forwarding
   traffic intended for the default destination towards the DAG parents,
   the node is able to support the main Multipoint-to-point (MP2P)
   traffic flows required by a typical LLN application.  By using the
   ordered set of DAG parents and DAG siblings the node is able to take
   advantage of path diversity.  For example, preferring to forward
   traffic towards parents guarantees to get the traffic inwards, closer
   to the DAG root, by definition, regardless of which parent is
   selected.  In this example, if forwarding towards parents is not
   possible, perhaps due to a transient phenomena, then a node may then
   choose to forward traffic towards siblings, moving across the DAG at
   the same level (neither inwards or outwards).  When receiving traffic
   forwarded from a sibling, the traffic should not be forwarded back to
   the same sibling in order to avoid a 2-node loop.  In a further
   example, a forwarding implementation may choose to decrease the hop
   limit more quickly when forwarding along sibling paths than along
   parent paths.  A forwarding engine may behave in a manner similar to
   these examples, however the specific implementation of a forwarding
   engine and related path diversity strategies is beyond the scope of
   this specification.  Various related techniques are currently under
   investigation to be added in a later revision of this specification.

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

   By employing this procedure, the LLN is able to set up a path-
   constrained DAG, rooted at designated nodes, with other nodes



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   organized along paths leading inward toward the DAG root.  MP2P
   traffic intended for the destinations available to or through the DAG
   root, e.g. the default destination or other advertised prefixes,
   flows inward along the DAG towards the root, and nodes forwarding
   traffic are able to leverage the path diversity of the DAG as
   necessary.

   The DAG is then used by RPL as a reference topology, constraining the
   LLN routing problem, on which to build additional routing mechanisms.

3.3.1.2.  Further Operation

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

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

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

   A node will generally join at least one DAG, typically (but not
   necessarily) to or through a grounded DAG rooted at an LBR.  In some
   cases, as suitable to the application scenario, a DAG may still
   provision the default route toward DAG Parents and not be connected



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   to a backbone network or the Internet.

   This specification does not preclude a node from joining multiple
   DAGs.  In one such case, a particular application may require the
   node to maintain membership in multiple DAGs in order to satisfy
   competing constraints, for example to support different types of
   traffic, similar to the concept of MTR (Multi-topology routing) as
   supported by other routing protocols such as IS-IS [RFC5120] or OSPF
   [RFC4915], although the RPL mechanisms will significantly differ from
   the ones specified for these protocols.  (Note that not all
   constrained traffic cases may require multiple DAGs).  In support of
   such cases the RPL implementation must independently maintain
   requisite information and state for each DAG in parallel.  In cases
   where a competing constraints must be satisfied toward the same DAG
   root, the OCP should differ by definition and may serve to coordinate
   the maintenance of the multiple DAGs.  Further, additional
   recommendations for the operation of loop avoidance/loop detection
   mechanisms in the presence of multiple DAGs are under investigation.

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

3.3.1.3.  Router Advertisement - DAG Information Option (RA-DIO)

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

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

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



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   o  Objective Code Point (OCP) as described below.

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

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

   o  Indications for the DAG, e.g. grounded or floating.

   o  DAG configuration parameters.

   o  A vector of path metrics.  As discussed in
      [I-D.ietf-roll-routing-metrics] such metrics may be cumulative,
      may report a maximum, minimum, or average scalar value, or a link
      property.

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

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

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

3.3.1.4.  Objective Code Point (OCP)

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

   o  The set of metrics used within the DAG

   o  The objective functions used to determine the least cost
      constrained paths in order to optimize the DAG



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   o  The function used to compute DAG Rank

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

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

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

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

3.3.1.5.  Selection of Feasible DAG Parents

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

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

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

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



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

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

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

   Based on such considerations, the node may incorporate the
   neighboring node into the set of DAG parents according to
   implementation specific algorithms that are outside the scope of this
   document.

   When the node inserts the first DAG parent into the empty DAG parent
   set, it is able to join the DAG.  After the DAG parent set is
   updated, the node will consult a rank computation function indicated
   by the OCP for the DAG in order to determine its rank value, which it
   will subsequently advertise when it emits its own DIOs.  A general
   property of the rank value presented by the node is that it should be
   greater than that presented by any of its DAG parents.  A node must
   maintain its DAG Parent set such that its most preferred parent from
   the OCP goals also has the greatest rank value in the DAG parent set.
   All reliable neighboring nodes of a lesser rank than the node may
   then be considered as potential DAG parents (Note that as a
   consequence of satisfying a particular OCP goal, the most preferred
   parent may not necessarily be the potential parent of least rank, for
   example a potential parent of lesser rank may also be an energy
   constrained device that is to generally be avoided and thus not the
   most preferred).  No nodes of greater rank than the most preferred
   parent may be in the DAG Parent set; to allow such nodes will
   introduce a possibility to create loops (by potentially allowing a
   packet to make backwards progress as it is forwarded in the DAG).
   All neighboring nodes of equal rank may be considered as siblings
   within the DAG (even though they may not have parents in common, they
   may still provide path diversity towards the DAG root).

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

3.3.1.5.1.  Example

   For example, suppose that a node (N) is not attached to any DAG, and
   that it is in range of nodes (A), (B), (C), (D), and (E).  Let all
   nodes be configured to use an OCP which defines a policy such that
   ETX is to be minimized and paths with the attribute `Blue' should be
   avoided.  Let the rank computation indicated by the OCP simply
   reflect the ETX aggregated along the path.  Let the links between



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   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 Router Solicitations to probe for
   nearby DAGs.

   o  Node (N) transmits a Router Solicitation.

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

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

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

3.3.1.6.  DAG Maintenance

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



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

   When a node in a DAG follows a DAG parent, it means that the DAG
   parent has changed its DAGID (e.g. by joining a new DAG) and that the
   node updates its own DAGID in order to keep the DAG parent.

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

   When the node encounters new candidate neighbors that offer higher
   positions in the DAG, it may incorporate them directly into its set
   of DAG parents.  In this case the node may update its choice of most
   preferred parent, possibly causing its own advertised rank to
   decrease, and discarding any former parents now of a deeper rank.
   This case is `moving inwards along the DAG' and does not require any
   additional coordination for loop avoidance.

   If the DAG parent set of the node becomes completely depleted, the
   node will have detached from the DAG, and may, if so configured,
   become the root of its own transient floating DAG with a high
   DAGPreference (0xFF) (thus beginning the process of establishing the
   frozen sub-DAG), and then may reattach to the original DAG at a lower
   point if it is able (after hearing RA-DIOs from alternate attachment
   points).

   When the node encounters candidate parents that are in a different
   DAG, and decides to leave the current DAG in order to join the
   different DAG, it may do so safely without regard to loop avoidance.
   However, it may not return immediately to the current DAG as such
   movement may result in the creation of loops.

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




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

   Appendix B provides additional examples of DAG discovery and
   maintenance.

3.3.2.  Source Routing

   A Source Routing mechanism for RPL is currently under investigation.

3.3.3.  Destination Advertisement

   As RPL constructs DAGs, nodes are able to learn a set of default
   routes in order to send traffic to the sink.  However, this mechanism
   alone is not sufficient to support P2MP traffic flowing outward along
   the DAG from the DAG root toward nodes.  A Destination Advertisement
   mechanism is employed by RPL to build up routing state in support of
   these outward flows.  The Destination Advertisement mechanism may not
   be supported in all implementations, as appropriate to the
   application requirements.  A DAG Root that supports using the
   Destination Advertisement mechanism to build up routing state will
   indicate such in the DIO.  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.

3.3.3.1.  Destination Advertisement Option (DAO)

   A Destination Advertisement Option (DAO) is used to convey the
   Destination information inward along the DAG toward the DAG root.

   The information conveyed in the DAO 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.







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3.3.3.2.  Destination Advertisement Operation

   As the DAG is constructed and maintained, nodes are capable to emit
   messages containing Destination Advertisement Options to a subset 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 message containing a Destination Advertisement Options
   advertising its locally available destination prefixes.  This
   mechanism allows for the one-hop neighbors of a node to learn
   explicitly of the prefixes on the node, and in some application
   specific scenarios this is desirable in support of provisioning a
   trivial `one-hop' route.  In this case, nodes who receive the
   multicast Destination Advertisement 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) DAO reaches a node capable of storing routing state,
   the node extracts information from the DAO and updates its local
   database with a record of the DAO and who it was received from.  When
   the node later propagates DAOs, it selects the best (least depth)
   information for each destination and conveys this information again
   in the form of DAOs to a subset of its own DAG parents.  At this time
   the node may perform route aggregation if it is able, thus reducing
   the overall number of DAOs.

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

   When a node that is capable of storing routing state encounters a
   (unicast) DAO with a Reverse Route Stack that has been populated, the
   node knows that the DAO has traversed a region of nodes that did not
   record any routing state.  The node is able to detach and store the
   Reverse Route State and associate it with the destination described
   by the DAO.  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 DAOs by destinations are possible in order to



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   support additional scalability.

   A further example of the operation of the Destination Advertisement
   mechanism is available in Appendix B.6

3.4.  Other Considerations

3.4.1.  DAG Rank and Loop Avoidance

   When nodes select DAG Parents, they should select the most preferred
   parent according to their implementation specific objectives, using
   the cost metrics conveyed in the DIOs along the DAG in conjunction
   with the related objective functions as specified by the OCP.

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

   It is important to note that the DAG Rank is not itself a metric,
   although its value is derived from and influenced by the use of
   metrics to select DAG parents and take up a position in the DAG.  In
   other words, routing metrics and OCP (not rank directly) are used to
   determine the DAG structure and consequently the path cost.  The only
   aim of the rank is to inform loop avoidance as explained hereafter.
   The computation of the DAG Rank MUST be done in such a way so as to
   maintain the following properties for any nodes M and N who are
   neighbors in the LLN:

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

      If DAGRank(M) < DAGRank(N), then M is located in a more optimum
      position than N in the DAG with respect to the metrics and
      optimizations defined by the objective code point.  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 located in a more optimum
      position than a Node N of rank 5.  A packet directed inwards and



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      forwarded from Node N to Node M will always make forward progress
      with respect to the DAG organization on that link; there is no
      risk of Node M at rank 3 forwarding the packet back into Node N's
      sub-DAG at rank of 5 or greater (which would be a sufficient
      condition for a loop to occur).

      If DAGRank(M) == DAGRank(N), then M and N are located positions of
      relatively the same optimality within the DAG.  In some cases,
      Node M may be used as a successor by Node N, but with related
      chance of creating a loop that must be detected and broken by some
      other means.  If Node M is at rank 3 and node N is at rank 3, then
      they are siblings; by definition Node M and N cannot be in each
      others sub-DAG.  They may then forward to each other failing
      serviceable parents, making `sideways' progress (but not reverse
      progress).  If another sibling or more gets involved there may
      then be some chance for 3 or more way loops, which is the risk of
      sibling forwarding.

      If DAGRank(M) > DAGRank(N), then node M is located in a less
      optimum position than N in the DAG with respect to the metrics and
      optimizations defined by the objective code point.  Further, Node
      (M) may in fact be in Node (N)'s sub-DAG.  There is no advantage
      to Node (N) selecting Node (M) as a DAG Parent, and such a
      selection may create a loop.  For example, if Node M is of rank 3
      and Node N is of rank 5, then by definition Node N is in a less
      optimum position than Node N. Further, Node N at rank 5 may in
      fact be in Node M's own sub-DAG, and forwarding a packet directed
      inwards towards the DAG root from M to N will result in backwards
      progress and possibly a loop.

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

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

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

   A node may not consider to move `down' the DAG, causing its DAG rank
   to increase and moving further from the DAG root.  Such a move will
   entail moving to a less optimum position in the DAG in all cases, as
   defined by the objective code point.  In the case where a node looses



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

   Any neighboring nodes of lesser or equal rank to the current most
   preferred DAG parent are eligible to be considered as alternate DAG
   parents.

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

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






















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3.4.1.1.  Example


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

              -1-                    -2-                    -3-


                         Figure 1: DAG Maintenance

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

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

   Consider the case where Node (C) hears a DIO 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.

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





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   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 from Node (C), follows Node
      (C) into the floating DAG.  This is depicted in Figure 1-2.  In
      general, any node with no other options in the sub-DAG of Node (C)
      will follow Node (C) into the floating DAG, maintaining the
      structure of the sub-DAG.

   o  Node (C) hears a RA-DIO 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 sourced from Node (C) and
      follow Node (C) in a coordinated manner to reattach to the
      grounded DAG.  The final DAG is depicted in Figure 1-3

3.4.2.  DAG Parent Selection, Stability, and Greediness

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

   Suppose a node is willing to receive and process a RA-DIOs from a
   node in its own sub-DAG, and in general a node deeper than it.  In
   such cases a chance exists to create a feedback loop, wherein two or
   more nodes continue to try and move in the DAG in order to optimize
   against each other.  In some cases this will result in an
   instability.  It is for this reason that RPL mandates that a node
   MUST NOT receive and process RA-DIOs 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.














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3.4.2.1.  Example


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

              -1-                    -2-                    -3-


                   Figure 2: Greedy DAG Parent Selection

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

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

   o  Let Figure 2-1 be the initial condition.

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

   o  Suppose Node (B), in its greediness, is willing to receive and
      process a DIO 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.





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

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

   o  ...

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

   o  This cycle can be averted through mechanisms in RPL:

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

      *  Nodes (B) and (C) don't process DIOs from nodes deeper than
         themselves (possibly in their own sub-DAGs)

3.4.3.  Merging DAGs

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

3.4.3.1.  Example


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


                          Figure 3: Merging DAGs

   Consider the example depicted in Figure 3.  Nodes (A), (B), and (C)
   are part of some larger grounded DAG, where Node (A) is at a rank of
   d, Node (B) at d+1, and Node (C) at d+2.  The DAG comprised of Nodes
   (D), (E), and (F) is a floating, less preferred, DAG, with Node (D)



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   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 from Node (C) on link
      (C)--(D).  Node (D) will consider Node (C) a candidate neighbor,
      will note that Node (C) is in a grounded DAG at rank d+2, and will
      begin the process to join the grounded DAG at rank d+3.  Node (D)
      will start a DAG Hop timer, logically associated with the grounded
      DAG at Node (C), to coordinate the jump.  The DAG Hop timer will
      have a duration proportional to d+2.

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

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

   o  Node (E)'s DAG Hop timer for the grounded DAG at Node (A) expires
      first.  Node (E), upon the DAG Hop timer expiry, is removes Node
      (D), 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 and
      will begin to emit RA-DIOs more frequently.

   o  Node (F) will receive and process a RA-DIO 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.





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   o  Node (D) will receive a RA-DIO from Node (E).  Since Node (E) is
      now in a different DAG, Node (D) may process the RA-DIO 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
      manner, avoiding the formation of loops in the process.

3.4.4.  Local and Temporary Routing Decision

   Although implementation specific, it is worth noting that a node may
   decide to implement some local routing decision based on some
   metrics, as observed locally or reported in the DIO.  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 5.4, 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



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   to order the DAG parent set from `most preferred' to `least
   preferred'.  By constructing such an ordered set, and by appending
   the set with siblings, the node is able to construct an ordered list
   of preferred next hops to assist in local and temporary routing
   decisions.  The use of the ordered list by a forwarding engine is
   loosely constrained, and may take into account the dynamics of the
   LLN.  Further, a forwarding engine implementation may decide to
   perform load balancing functions using hash-based mechanisms to avoid
   packet re-ordering.  Note however, that specific details of a
   forwarding engine implementation are beyond the scope of this
   document.

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

3.4.5.  Scalability

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

   A further example is illustrated in Appendix C.

3.4.6.  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



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   the link bandwidth (in which case the metric is usually inversely
   proportional to the bandwidth), delay, etc.  Note that in some cases
   the metric is a polynomial function of several metrics defining
   different link characteristics.  The resulting shortest path cost is
   equal to the sum (or multiplication) of the link metrics along the
   path: such metrics are said to be additive or multiplicative metrics.

   Some routing protocols support more than one metric: in the vast
   majority of the cases, one metric is used per (sub)topology.  Less
   often, a second metric may be used as a tie breaker in the presence
   of ECMP (Equal Cost Multiple Paths).  The optimization of multiple
   metrics is known as an NP complete problem and is sometimes supported
   by some centralized path computation engine.

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

   o  Some information apply to path setup time, other apply to packet
      forwarding time.

   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



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   specified in OCP 0 clarifies this point.

4.2.  Routing Constraints

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

4.3.  Constraint Based Routing

   The notion of constraint based routing consists of finding the
   shortest path according to some metrics satisfying a set of
   constraints.  A technique consists of first filtering out all links
   and nodes that cannot satisfy the constraints (resulting in a sub-
   topology) and then computing the shortest path.

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

      Objective function 1:
         "Find the shortest path (path with lowest cost where the path
         cost is the sum of all link costs (Bandwidth)) along the path
         such that all links are colored `Blue' and that only traverses
         Mains-powered nodes."



      Example 2:
         Link Metric:     Delay
         Link Constraint: Bandwidth

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


5.  Specification of Core Protocol






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5.1.  DAG Information Option

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

5.1.1.  DIO base option

   The DAG Information Option is a container option, which might contain
   a number of suboptions.  The base option regroups the minimum
   information set that is mandatory in all cases.


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

                         Figure 4: DIO Base Option

   Type: 8-bit unsigned identifying the DIO base option.  The value is
         to be assigned 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.





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

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

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

   Reserved:  5-bit unsigned integer set to 0 by the DAG root and left
         unchanged by nodes propagating the DIO.

   Sequence Number:  8-bit unsigned integer set by the DAG root,
         incremented with each new DIO it sends on a link, and
         propagated with no change outwards along the DAG.

   DAGPreference:  8-bit unsigned integer set by the DAG root to its
         preference and unchanged at propagation.  Default is 0 (lowest
         preference).  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 DAG of lower preference while still meeting
         other optimization objectives, then the node will seek the
         minimum available preference.

   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.







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

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

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

   DIOIntervalMin:  8-bit unsigned integer.  Used to configure the
         trickle timer governing when RA-DIO 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.

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



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   the propagation.

5.1.1.1.  DIO Suboptions

   In addition to the minimum options presented in the base option, a
   number of suboptions are defined for the DIO:

5.1.1.1.1.  Format


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

                  Figure 5: DIO Suboption Generic Format

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

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

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

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

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

   DIO 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:



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

                              Figure 6: Pad 1

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

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

5.1.1.1.3.  PadN

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


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

                              Figure 7: Pad N

   The PadN option is used to insert two or more octets of padding in
   the DIO 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -




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                      Figure 8: DAG Metric Container

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

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

5.1.1.1.5.  Destination Prefix

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



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


                     Figure 9: DAG Destination Prefix

   The Destination Prefix suboption is used when the DAG root needs to
   indicate that it offers connectivity to destination prefixes other
   than the default.  This may be useful in cases where more than one
   LBR is operating within the LLN and offering connectivity to
   different administrative domains, e.g. a home network and a utility
   network.  In such cases, upon observing the Destination Prefixes
   offered by a particular DAG root, 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].



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   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.

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

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

5.2.  Conceptual Data Structures

   The RPL implementation must 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

   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



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   associated with the Candidate Neighbor should be removed.

5.2.2.  DAGs

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

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

   o  DAGID

   o  DAGObjectiveCodePoint

   o  A set of Destination Prefixes offered by the DAG root

   o  A set of candidate DAG Parents

   o  A timer to govern the sending of DIOs 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 may be deallocated.  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

   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





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   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 5.8

   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 the most preferred parent after such
   a change must be placed in the Held-Down state and evicted as per the
   procedures described in Section 5.8

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

5.3.  Initialization and Configuration

   An implementation must provide a means, e.g. a set of APIs, to allow
   the node to initialize/configure the RPL implementation.  The RPL
   implementation on the node must be provisioned to know:

      Is the node serving a role in an application scenario whereby it
      should permanently act as a DAG root?  (For example, the node may
      act as an LBR, provide Internet access, serve as an application
      specific data-collection point, or provide application control to
      the LLN.)  If so,

         What is the DAGPreference value for the self-rooted DAG (likely
         0)?

         What OCP are supported?




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         Is connectivity to external infrastructure provided (is the DAG
         grounded?)

         What destination prefixes are offered?

         What is the DAGDelay?

         Is the Destination Advertisement mechanism in effect?

         What are the values for DIOIntervalDoublings, DIOIntervalMin?

         Is the node to periodically emit DIOs (e.g. revise the DAG
         Sequence Number upwards) in order to provide a heartbeat for
         the DAG?  If so, with what period?

      If the node does not permanently act as a DAG root, should it
      actively root a (floating, DAGPreference 0xFF) DAG when no other
      DAG is available?  (For example, a battery powered node may not
      wish expend energy to do this, but will instead passively listen
      for other options).

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

      What are the supported OCP (optimization goals)?

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

   When a node is provisioned with a set of optimization goals,
   effectively indicating targeted OCPs for given destinations (possibly
   including the default destination), it may conceptually organize
   these into a table where each row indicates an optimization goal.  As
   DAGs are joined in order to satisfy optimization objectives,
   references to the DAG supporting the objective may be entered into
   each row.  In this way a node may track which objectives are
   satisfied by which DAGs, as well as which objectives are unsatisfied
   by any DAG.  This will help to inform a nodes decision to join a new
   DAG, or perhaps leave an existing DAG in order to join a better
   alternate DAG, in order to meet specific optimization objectives.

5.4.  DAG Discovery

   DAG Discovery locates the nearest sink 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



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   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 3.4.1.  The rank should be computed in such a way so as to
   provide a comparable basis with other nodes which may not use the
   same metric at all.

   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 in the DIO of the lost parents for a period of time
        which covers multiple DIO.

   2.   A LLN Node that is attached to an infrastructure that does not
        support DIO, is the DAG root of its own grounded DAG.  It's rank
        is 1.

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

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

   5.   A node MAY move at any time, with no delay, within its DAG as
        long as such a move does not increase its own DAG rank, as per
        the rank calculation indicated by the OCP.  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 a node MUST NOT move outwards along the DAG



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        that it is attached, except in the special case when choosing to
        follow the last DAG parent in the set of DAG parents.  RAs
        received from other routers located higher 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 deeper within the
        same DAG.

   6.   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 DIO 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 DIOs 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.

   7.   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 and refrains from sending RA-DIOs for that
        DAG.

   8.   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
        its DAG parent set and remain in the original DAG.  If the node
        was the last DAG parent then the node SHOULD follow that parent.

   9.   When a node detects or causes a DAG inconsistency, as described
        in Section 5.4.3.2, then the node sends an unsolicited RA-DIO
        message to its one-hop neighbors.  The RA contains an updated
        DIO to propagate the new DAG information.  Such an event will
        also cause the trickle timer governing the periodic RAs to be
        reset.





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   10.  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 may chose to follow it.

5.4.1.  RA-DIO Reception

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

5.4.1.1.  Determination of Eligibility for DIO Processing

      If the DIO is malformed, then the DIO is not eligible for further
      processing.

      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 DIO advertises a DAG that the node is already a member of,
      then:

         If the rank of SRC as reported in the DIO is less then that of
         the node within the DAG, then the DIO MUST be considered for
         further processing

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

         If the rank of SRC as reported in the DIO is lesser than that
         of the node within the DAG, and SRC is not a DAG Parent, then
         the DIO is not eligible for further processing

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

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







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5.4.1.2.  Overview of DIO Processing



      If the DIO 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

         Has the node sent an RA within the risk window as described in
         Section 5.8.3?  If so, perform the collision detection
         described in Section 5.8.3.  If a collision occurs, place the
         Candidate DAG Parent in the collision state and do not process
         the DIO any further as described in Section 5.8.

         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 DIO (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
         DAG Hop timer as per Section 5.8.1.

      If the DIO is for a known/existing DAG:

         Process the DIO as per the rules in Section 5.4

   As candidate parents are identified, they may subsequently be
   promoted to DAG parents by following the rules of DAG Discovery as
   described in Section 5.4.  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



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

5.4.2.  RA-DIO Transmission

   Each node maintains a timer that governs when to multicast RAs.  This
   timer is implemented as a trickle timer operating over a variable
   interval.  Trickle timers are further detailed in Section 5.4.3.  The
   governing parameters for the timer should be configured consistently
   across the DAG, and are provided by the DAG root in the DIO.  In
   addition to periodic RAs, each LLN node will respond to Router
   Solicitation 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 may reset the interval of
      the trickle timer to a minimum value, causing RAs to be emitted
      more frequently as part of a strategy to quickly correct the
      inconsistency.  Such inconsistencies may be, for example, an
      update to a key parameter (e.g. sequence number) in the DIO or a
      point-to-point loop detected when a node located inwards along the
      DAG forwards traffic intended for the default destination.
      Inconsistencies are further detailed in Section 5.4.3.2.

   o  When a node enters a mode of consistent operation within a DAG,
      i.e.  DIOs from its DAG Parents are consistent and no other
      inconsistencies are detected, it may begin to open up the interval
      of the trickle timer towards a maximum value, causing 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 RAs until it has encountered and joined a
      DAG (perhaps initially probing for a nearby DAG with an RS).
      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.



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

5.4.3.  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.  The operation of this timer is in support of the
   procedures further discussed in Section 5.4

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

   I:    The current length of the communication interval

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

   C:    Redundancy Counter

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

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

5.4.3.1.  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.

   2.  Setting C to zero.





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   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 an LLN learns about a DAG through a RA 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 increments 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 then I_doubling times,
   resets C, and chooses a new T value.

5.4.3.2.  Determination of Inconsistency

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

   o  The node joins a new DAGID

   o  The node moves within a DAGID

   o  The node receives a modified DIO from a DAG parent

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

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

   o  The rank of a DAG parent has changed.

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

5.5.  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-
   DIOs to be sent back outwards along the DAG to convey the change.
   The degree to which this mechanism is relied on may be determined by



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

   Some implementations may provide an administrative API 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.

5.6.  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.  How to balance these DAGs is implementation dependent, and MAY
   use a specific visitor-counter suboption in the DIO.

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

5.7.  Administrative rank

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







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5.8.  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 the most
               preferred DAG parent must be placed in the Held-Down
               state, and the hold-down timer started, in order to be
               evicted as DAG Parents.

   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.

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

5.8.1.  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 Router Advertisement
      - DAG Information Option that is initiated by the new DAG root has
      a chance to spread outward along the sub-DAG so that two different
      DAGs have formed.

      Limit RA-DIO storms 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 a router advertisement message with or
   without a RA-DIO.  The node joins the DAG by selecting the source of
   the RA message as a DAG parent (and possible default gateway) and
   propagating the DIO accordingly.



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   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.

5.8.2.  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.

5.8.3.  Collision

   A race condition occurs if 2 nodes send RA-DIO at the same time and
   then attempt to join each other.  This might happen, for example,
   between nodes which act as DAG root of their own DAGs.  In order to
   detect the situation, LLN Nodes time stamp the sending of RA-DIO.
   Any RA-DIO received within a short link-layer-dependent period
   introduces a risk.  To resolve the collision, a 32bits extended
   preference is constructed from the DIO 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 from the desired candidate, it will restrain from sending a
   RA-DIO 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



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   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 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.

5.8.4.  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
   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 DIO.  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
   DIO from stable candidate parents, which do not plan to attach to the
   node, so the node can safely attach to them.

5.9.  Guidelines for Objective Code Points

5.9.1.  Objective Function

   An objective function (OF) selects a DAG to join, and a number of
   peers in that DAG as parents.  The OF computes 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.

   An Objective Function is indicated in the DIO using an objective code
   point (OCP).  The objective code point are administered by IANA that
   might delegate some ranges to other organizations.  This
   specification reserves OCP 0, in support of default operation.

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




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   o  The parent selection is triggered each time an event indicates
      that a potential next_hop information is updated.  This might
      happen upon a RA-DIO, 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.  An interface might not be ready
      for IPv6 operation with a usable link-local address.  Finally an
      interface might or not match a required criterion for an Objective
      Function, for instance a degree of security.  As a result some
      interfaces might be completely excluded from the computation,
      while others might be more or less preferred.

   o  The OF scans all the Candidate Neighbors 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 and raise the router value 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:

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

      *  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.

         +  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 flirts with the acceptable
            threshold.

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





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   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

      *  One of these tests might include comparing the resulting ranks
         but it isn't necessarily so

   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.  Self's rank is now determined by the new
      preferred parent if it has changed.  In the next rounds:

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

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

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

5.9.2.  Objective Code Point 0 (OCP 0)

   Here follows the specification for the Objective Function for OCP 0.
   This is a very simple references 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 DIO such as rank and administrative preference.

   OCP 0 is as a default fall back behavior when a node joins a DAG but
   does not support the OF that's preferred for this DAG.

5.9.2.1.  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's none then join any network per order of administrative



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   preference.

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

5.9.2.2.  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 is validated as usable is better.

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

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

   6.   A lesser resulting rank is better.

   7.   A DAG for which there is an alternate parent is better.  This
        check is optional.  It is performed by computing 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 is fresher (most recent RA) is better.

5.9.2.3.  Selection of the Backup next_hop

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





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

   o  The preferred parent is ignored

   o  Candidate Neighbors that are not in the same DAG are ignored

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

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

   o  A router that is validated as usable is better

   o  The router with a better router preference wins

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

5.10.  Establishing Routing State Outward Along the DAG

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

   Note that some aspects of the Destination Advertisement mechanism are
   still under investigation.

   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, along paths containing DA
      parents.  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.  In this process they may also learn
      necessary piecewise source routes to traverse regions of the LLN



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      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
   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 or landmark
   (waypoint) routing.

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

   The design choice behind this is not to synchronize the parent and
   children databases along the DAG, but instead to update them
   regularly to cover 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 derivative routing state, as
   cued by occasional RAs and other mechanisms, similarly to other
   protocols such as RIP [RFC2453].

5.10.1.  Destination Advertisement Message Formats

5.10.1.1.  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 Advertisements (NAs).  A
   prefix option is normally present in Router Advertisements (RAs)
   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 containing this



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   option may be referred to as a Destination Advertisement.  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.


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

             Figure 10: Destination Advertisement Option (DAO)

   Type: 8-bit unsigned identifying the Destination Advertisement
         option.  The value is to be assigned by the 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.






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

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

   Reserved:  8-bit unused field.  It MUST be initialized to zero by the
         sender and MUST be ignored by the receiver.

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

   Prefix:  Variable-length field containing an IPv6 address or a prefix
         of an IPv6 address.  The Prefix Length field contains 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 initialized to zero by the sender and ignored by the
         receiver.

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

5.10.2.  Destination Advertisement Operation

5.10.2.1.  Overview

   Note that some aspects of the Destination Advertisement mechanism are
   still under investigation

   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.

   RPL takes advantage of the DAG structure and allows a node capable of
   storing sufficient routing state to autonomously discover the
   destinations below itself through the operation of the Destination
   Advertisement mechanism.  This allows participating nodes to build up
   routing state to support traffic flowing outwards along the DAG.
   Destination Advertisement messages convey the necessary information
   to learn the destinations.




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

   If Destination Advertisements are activated in the DIO 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
   DAG Discovery RA-DIO with the `D' Destination Advertisement bit set
   from a DAG parent stimulates the sending of a delayed Destination
   Advertisement back, with the collection of all known prefixes (that
   is the prefixes learned via Destination Advertisements for nodes
   lower in the DAG, and any connected prefixes).  If the Destination
   Advertisement Supported (A) bit is set in the DIO 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 DIO 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) 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) or by receiving a no-DAO.  A no-DAO is a
   conveyed as a DAO with a DAO Lifetime of 0.

   A node who is capable of recording the state information conveyed in
   a unicast DAO will do so upon receiving and processing the DAO, thus
   building up routing state concerning destinations below it in the
   DAG.  If a node capable of recording state information receives a DAO
   containing a Reverse Route Stack, then the node knows that the DAO



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   has traversed one or more nodes that did not retain any routing state
   as it traversed the path from the DAO source to the node.  The node
   may then extract the Reverse Route Stack and retain the included
   state in order to specify Source Routing instructions along the
   return path towards the destination.  The node MUST set the RRCount
   back to zero and clear the Reverse Route Stack prior to passing the
   DAO information on.

   A node who is unable to record the state information conveyed in the
   DAO 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 come to contain a vector of next hops that must be
   traversed along the reverse path that the DAO has traveled.  The
   vector will be ordered such that the node closest to the destination
   will appear first in the list.  In such cases 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 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 degenerate case, no node is able to store any routing state as
   Destination Advertisements pass by, and the DAG Root ends up with
   DAOs 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 nor indicate support for Destination
   Advertisements if it is not able to store the Reverse Route Stack
   information in the degenerate case.

   Information learned through Destination Advertisements can be
   redistributed in a routing protocol, MANET or IGP.  But the MANET or
   the IGP SHOULD NOT be redistributed into Destination Advertisements.
   This creates a hierarchy of routing protocols where DA routes stand
   somewhere between connected and IGP routes.

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






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   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-DIOs were sent on the
      interface to the advertising neighbor without reachability
      confirmation for the prefix.

   Note that nodes may receive multiple information from different
   neighbors for a specific destination, as different paths through the
   DAG may be propagating information inwards along the DAG for the same
   destination.  A node who is recording routing state will keep track
   of the information from each neighbor independently, and when it
   comes time to propagate the DAO 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 DAOs, 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 destroyed, in order to send no-DAOs
   to the DA parents.

5.10.2.1.1.  Destination Advertisement Timers

   The Destination Advertisement mechanism requires 2 timers; the
   DelayNA timer and the DestroyTimer.

   o  The DelayNA timer is armed upon a stimulation to send a
      Destination Advertisement (such as a DIO 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 reported yet for



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      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 DAOs a chance to be reported from deeper in the
      DAG and potentially aggregated along sub-DAGs before propagating
      further inwards.

   o  The DestroyTimer is armed when at least one entry has exhausted
      its retries, which means that a number of RA-DIO were sent toward
      the reporting neighbor but that the entry was not confirmed with a
      DAO.  When the destroy timer elapses, for all exhausted entries,
      the associated route is removed, and the entry is scheduled to be
      destroyed.

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

5.10.2.2.  Multicast Destination Advertisement messages

   It is also possible for a node to multicast a DAO 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 destination
   prefixes directly supported by neighboring nodes, without needing the
   RPL routing structure to relay the packets.

   A multicast DAO MUST be used only to advertise information about
   self, i.e. prefixes in the Connected list.  This would typically be a
   multicast group that this node is listening to or a global address
   owned by this node, though it can be used to advertise any prefix
   owned by this node as well.  A multicast DAO 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.

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

5.10.2.3.  Unicast Destination Advertisement messages from child to
           parent

   When sending a Destination Advertisement to a DA parent, a LLN Node
   includes the DAOs about not already reported prefix entries in the
   Reachable and Connected lists, as well as no-DAOs for all the entries



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   in the Unreachable list.  Depending on its policy and ability to
   retain routing state, the receiving node SHOULD keep a record of the
   reported DAO.  If the DAO offers the best route to the prefix as
   determined by policy and other prefix records, the node SHOULD
   install a route to the prefix in the DAO via the link local address
   of the reporting neighbor and it SHOULD further propagate the
   information, either as a DAO or by means of redistribution into a
   routing protocol.

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

   When the node broadcasts a RA-DIO 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 has a duration that decreases with the depth, it is
   expected to receive all DAOs from all children before the timer
   elapses and the full update is sent to the DA parents.

   Once the Destroy timer is elapsed, the prefix entry is scheduled to
   be destroyed 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.

5.10.2.4.  Other events

   Finally, the Destination Advertisement mechanism responds to a series
   of events, such as:





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   o  Destination Advertisement operation stopped: All entries in the
      abstract lists are freed.  All the routes learned from DAOs are
      destroyed.

   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 destroyed.

   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.

5.10.2.5.  Aggregation of prefixes by a node

   There may be number of cases where a aggregation may be shared within
   a platoon of nodes.  In such a case, it is possible to use
   aggregation techniques with Destination Advertisements and improve
   scalability.  For example, consider a platoon formed by firefighters
   and their commander.  Specifically, the commander may be configured
   as the Destination Advertisement aggregator for a group prefix.  At
   run time, the commander absorbs the individual DAO information
   received from the platoon members down its sub-DAG and only reports
   the aggregation up the DAG.  This works fine when the whole platoon
   is attached within the commander's sub-DAG.

   Other cases might occur for which additional support is required:

   1.  The commander is attached within the sub-DAG of one of its
       platoon members.

   2.  A platoon member is somewhere else within the DAG.

   3.  A platoon member is somewhere else in the LLN.

   In all those cases, a node situated above the commander in the DAG
   but not above the platoon member will see the advertisements for the
   aggregation owned by the commander but not that of the individual
   platoon member prefix.  So it will route all the packets for the
   platoon member towards the commander, but the commander will have no
   route to the individual platoon member and will fail to forward.

   Additional protocols may be applied beyond the scope of this
   specification to dynamically elect/provision a commander and platoon



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   in order to provide route summarization for a sub-DAG.

5.10.2.6.  Default Values

   DEF_NA_LATENCY = To Be Determined

   MAX_DESTROY_INTERVAL = To Be Determined

5.11.  Maintenance of Routing Adjacency

   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.

5.12.  Packet Forwarding

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

   1.  If the packet header contains any source routing directives (TBD)
       then the highest precedence should be given to follow them.





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   2.  If there is an entry in the routing table matching the
       destination that has been provisioned outside of the context of
       RPL, e.g. through an application intervention or a co-hosted
       (P2P) routing protocol, 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 unless overridden by a source routing directive or a route
   that has been provisioned outside of RPL, the chosen successor MUST
   NOT be the neighbor who was the predecessor of the packet (split
   horizon).

5.12.1.  Loop Taxonomy

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






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5.12.1.1.  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-DIOs 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 DIOs
   during DAG hop timer), and temporary loops might occur.  RPL will
   move to eliminate such a loop as soon as a DIO 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 12, and its
   sub-DAG Nodes (34), (44), and (45).  An example of a DAG loop would
   be if Node (24) were to detach from the DAG rooted at (LBR), and Node
   (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).

5.12.1.2.  DAO Loops

   A DAO loop may occur when the parent has a route installed by a DAO
   via a child, but the child has 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 12.  Suppose
   Node (24) has received a DA from Node (34) advertising a destination
   at Node (45).  Subsequently, if Node (34) tears down the DA state for
   the destination and Node (24) did not hear a no-DAO to clean up the
   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.

5.12.1.3.  Sibling Loops

   Sibling loops occur when a group of siblings keep choosing amongst
   themselves as successors such that a packet does not make forward
   progress.  The current draft limits those loops to some degree by



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   split horizon (do not send back to the same sibling) and parent
   preference (always prefer parents vs. siblings).  Further approaches
   to mitigate sibling loops may include:

   o  aggressively dropping the TTL to limit the impact of the loops

   o  randomizing the next hop to try and exit the loop if there is one
      one

   o  maintaining per packet states

   o  tunneling or source routing (path vector)

   Consider the DAG Example depicted in Figure 12.  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.

5.13.  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.


6.  Summary of RPL Timers

   DIO Timer  One instance per DAG that a node is a member of.  Expiry
         triggers RA-DIO transmission.  Trickle timer with variable
         interval in [0, DIOIntervalMin..2^DIOIntervalDoublings].  See
         Section 5.4.3

   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



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         Section 5.8.1.

   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 5.8

   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 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 5.5

   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 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 DAOs to allow for a
         chance of aggregation.  See Section 5.10.2.1.1

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


7.  Protocol Extensions


8.  Manageability Considerations


9.  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].







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10.  IANA Considerations

10.1.  DAG Information Option

   IANA is requested to allocate a new Neighbor Discovery Option Type
   from the IPv6 Neighbor Discovery Option Formats Registry in order to
   represent the DAG Information Option as described in Section 5.1

10.2.  Objective Code Point

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

10.3.  Destination Advertisement Option

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


11.  Acknowledgements

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

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

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


12.  Contributors

   ROLL Design Team in alphabetical order:

   Anders Brandt



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

   Email: abr@zen-sys.com


   Thomas Heide Clausen
   LIX, Ecole Polytechnique, France

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


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

   Email: stevedh@cs.berkeley.edu


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

   Email: jhui@archrock.com


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

   Email: kpister@dustnetworks.com


   Pascal Thubert
   Cisco Systems
   Village d'Entreprises Green Side
   400, Avenue de Roumanille
   Batiment T3



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   Biot - Sophia Antipolis  06410
   FRANCE

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


   Tim Winter (editor)

   wintert@acm.org


13.  References

13.1.  Normative References

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

13.2.  Informative References

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

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

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

   [I-D.ietf-roll-home-routing-reqs]
              Porcu, G., "Home Automation Routing Requirements in Low
              Power and Lossy Networks",
              draft-ietf-roll-home-routing-reqs-06 (work in progress),
              November 2008.

   [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.



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

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

   [I-D.tavakoli-hydro]
              Tavakoli, A., Dawson-Haggerty, S., Hui, J., and D. Culler,
              "HYDRO: A Hybrid Routing Protocol for Lossy and Low Power
              Networks", draft-tavakoli-hydro-01 (work in progress),
              March 2009.

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

   [I-D.tsao-roll-security-framework]
              Tsao, T., Alexander, R., Dohler, M., Daza, V., and A.
              Lozano, "A Security Framework for Routing over Low Power
              and Lossy Networks", draft-tsao-roll-security-framework-00
              (work in progress), February 2009.

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

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

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

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

   [RFC4191]  Draves, R. and D. Thaler, "Default Router Preferences and



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              More-Specific Routes", RFC 4191, November 2005.

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

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

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

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

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

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


Appendix A.  Deferred Requirements

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


Appendix B.  Examples

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



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   itself at depth 3.  Node (22) thus has a set of DAG parents {(11),
   (12)} and siblings {((21), (23)}.


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


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

                      Figure 11: Example LLN Topology
















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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 11.  As
   such, the depicted edges represent the relationship between nodes and
   their DAG parents, wherein all depicted edges are directed and
   oriented `up' on the page toward the DAG root (LBR).  The DAG
   provides the default routes within the LLN, and serves as the
   foundation on which RPL builds further routing structure, e.g.
   through the Destination Advertisement mechanism.

                          Figure 12: Example DAG

B.1.  Moving Down a DAG

   Consider node (56) in the example of Figure 11.  In the unmodified
   example, node (56) is at depth 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 depth deeper than that of its deepest DAG
   parent (node (55) at depth 6), depth 7.

B.2.  Link Removed

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

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

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

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

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

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

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

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

B.3.  Link Added

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

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

   o  Node (42) may be content to leave its advertised depth at 5,
      reflecting a depth deeper than its deepest parent (32).

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



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

B.4.  Node Removed

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

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

   o  Node (52) would observe a chance to reattach to the DAG rooted at
      (LBR) by adding Node (53) to its set of DAG parents, after an
      appropriate delay to avoid creating loops.  Node (52) will then
      advertise itself in the DAG rooted at (LBR) at depth 7.

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

B.5.  New LBR Added

   Consider the example of Figure 11 when a new LBR, (LBR2) appears,
   with connectivity (LBR2)-(52), (LBR2)-(53).

   o  Nodes (52) and Node (53) will see a chance to join a new DAG
      rooted at (LBR2) with a depth 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 advertising themselves at
      depth 2 in the DAGID (LBR2).

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

   o  Node (55) may then join the new DAG at depth 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 12.  Suppose that Nodes
   (22) and (32) are unable to record routing state.  Suppose that Node
   (42) is able to perform prefix aggregation on behalf of Nodes (53),
   (54), and (55).

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

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

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

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

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

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

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

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

   o  Node (12) sends DAOs 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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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.  DAO Fan-out

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

D.4.  Source Routing

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

D.5.  Address / Header Compression

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



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   under investigation.


Authors' Addresses

   Tim Winter (editor)

   Email: wintert@acm.org


   ROLL Design Team
   IETF ROLL WG

   Email: dtroll@external.cisco.com





































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