Networking Working Group                                  T. Winter, Ed.
Internet-Draft
Intended status: Standards Track                         P. Thubert, Ed.
Expires: June 10, August 7, 2010                                    Cisco Systems
                                                        ROLL Design Team
                                                            IETF ROLL WG
                                                        December 7, 2009
                                                       February 03, 2010

      RPL: IPv6 Routing Protocol for Low power and Lossy Networks
                         draft-ietf-roll-rpl-05
                         draft-ietf-roll-rpl-06

Abstract

   Low power and Lossy Networks (LLNs) are a class of network in which
   both the routers and their interconnect are constrained: LLN routers
   typically operate with constraints on (any subset of) processing
   power, memory and energy (battery), and their interconnects are
   characterized by (any subset of) high loss rates, low data rates and
   instability.  LLNs are comprised of anything from a few dozen and up
   to thousands of LLN routers, and support point-to-point traffic
   (between devices inside the LLN), point-to-multipoint traffic (from a
   central control point to a subset of devices inside the LLN) and
   multipoint-to-point traffic (from devices inside the LLN towards a
   central control point).  This document specifies the IPv6 Routing
   Protocol for LLNs (RPL), which provides a mechanism whereby
   multipoint-to-point traffic from devices inside the LLN towards a
   central control point, as well as point-to-multipoint traffic from
   the central control point to the devices inside the LLN, is
   supported.  Support for point-to-point traffic is also available.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  5  6
     1.1.  Design Principles  . . . . . . . . . . . . . . . . . . . .  5  6
     1.2.  Expectations of Link Layer Type  . . . . . . . . . . . . .  6  7
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6  7
   3.  Protocol Model Overview  . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Topology . .  8
     3.1. . . . . . . . . . . . . . . . . . . . . . . .  9
       3.1.1.  Topology Identifiers . . . . . . . . . . . . . . . . .  9
       3.1.2.  DODAG Information  . . . . . . . . . . . . . . . . . . 10
     3.2.  Instances, DODAGs, and DODAG Iterations  . . . . . . . . .  8
     3.2. 11
     3.3.  Traffic Flows  . . . . . . . . . . . . . . . . . . . . . . 10
       3.2.1. 13
       3.3.1.  Multipoint-to-Point Traffic  . . . . . . . . . . . . . 10
       3.2.2. 13
       3.3.2.  Point-to-Multipoint Traffic  . . . . . . . . . . . . . 10
       3.2.3. 13
       3.3.3.  Point-to-Point Traffic . . . . . . . . . . . . . . . . 10
     3.3. 13
     3.4.  Upward Routes and DODAG Construction . . . . . . . . . . . . . . . . . . . . 11
       3.3.1. 13
       3.4.1.  DAG Information Object (DIO) . . . . . . . . . . . . . 11
       3.3.2. 14
       3.4.2.  DAG Repair . . . . . . . . . . . . . . . . . . . . . . 11
       3.3.3. 14
       3.4.3.  Grounded and Floating DODAGs . . . . . . . . . . . . . 12
       3.3.4. 15
       3.4.4.  Administrative Preference  . . . . . . . . . . . . . . 12
       3.3.5. 15
       3.4.5.  Objective Function (OF)  . . . . . . . . . . . . . . . 12
       3.3.6. 15
       3.4.6.  Distributed Algorithm Operation  . . . . . . . . . . . 13
     3.4. 15
     3.5.  Downward Routes and Destination Advertisement  . . . . . . . . . . . . . . . . 13
       3.4.1. 16
       3.5.1.  Destination Advertisement Object (DAO) . . . . . . . . 13
   4. 16
     3.6.  Routing Metrics and Constraints Used By RPL  . . . . . . . . . 14
   5.  Rank . . 17
       3.6.1.  Loop Avoidance . . . . . . . . . . . . . . . . . . . . 18
       3.6.2.  Rank Properties  . . . . . . . 15
     5.1.  Loop Avoidance . . . . . . . . . . . . 19
   4.  ICMPv6 RPL Control Message . . . . . . . . . . 15
       5.1.1.  Greediness and Rank-based Instabilities . . . . . . . 15
       5.1.2.  DODAG Loops . 21
   5.  Upward Routes  . . . . . . . . . . . . . . . . . . . . 16
       5.1.3.  DAO Loops . . . . 22
     5.1.  DODAG Information Object (DIO) . . . . . . . . . . . . . . 22
       5.1.1.  DIO Base Format  . . . . 16
       5.1.4.  Sibling Loops . . . . . . . . . . . . . . . 22
       5.1.2.  DIO Base Rules . . . . . 16
     5.2.  Rank Properties . . . . . . . . . . . . . . . 24
       5.1.3.  DIO Suboptions . . . . . . 16
   6.  RPL Protocol Specification . . . . . . . . . . . . . . 25
     5.2.  DODAG Information Solicitation (DIS) . . . . 18
     6.1.  RPL Messages . . . . . . . 30
     5.3.  Upward Route Discovery and Maintenance . . . . . . . . . . 30
       5.3.1.  RPL Instance . . . . . . 18
       6.1.1.  ICMPv6 RPL Control Message . . . . . . . . . . . . . . 18
       6.1.2.  DAG Information Solicitation (DIS) . 30
       5.3.2.  Neighbors and Parents within a DODAG Iteration . . . . 30
       5.3.3.  Neighbors and Parents across DODAG Iterations  . . . . 31
       5.3.4.  DIO Message Communication  . 19
       6.1.3.  DAG Information Object (DIO) . . . . . . . . . . . . . 19
       6.1.4.  Destination Advertisement Object (DAO) 36
       5.3.5.  DIO Transmission . . . . . . . . 26
     6.2.  Protocol Elements . . . . . . . . . . . 36
       5.3.6.  DODAG Selection  . . . . . . . . . 28
       6.2.1.  Topological Elements . . . . . . . . . . 39
     5.4.  Operation as a Leaf Node . . . . . . . 28
       6.2.2.  Neighbors, Parents, and Siblings . . . . . . . . . . 39
     5.5.  Administrative Rank  . 28
       6.2.3.  DODAG Information . . . . . . . . . . . . . . . . . . 29
     6.3.  DAG Discovery and Maintenance 39
     5.6.  Collision  . . . . . . . . . . . . . . 30
       6.3.1.  DAG Discovery Rules . . . . . . . . . . 40
   6.  Downward Routes  . . . . . . . 31
       6.3.2.  DIO Message Communication . . . . . . . . . . . . . . 35
       6.3.3.  DIO Transmission . . 40
     6.1.  Destination Advertisement Object (DAO) . . . . . . . . . . 40
       6.1.1.  DAO Suboptions . . . . . . . 36
       6.3.4.  Trickle Timer for DIO Transmission . . . . . . . . . . 37
     6.4.  DAG Selection . . . 42
     6.2.  Downward Route Discovery and Maintenance . . . . . . . . . 42
       6.2.1.  Overview . . . . . . . . . . . . . . . . . . . . . . 38
     6.5. . 42
       6.2.2.  Mode of Operation as a Leaf Node  . . . . . . . . . . . . . . . . . 39
     6.6.  Administrative rank . 43
       6.2.3.  Destination Advertisement Parents  . . . . . . . . . . 44
       6.2.4.  Operation of DAO Storing Nodes . . . . . . . . 39
     6.7.  Collision . . . . 45
       6.2.5.  Operation of DAO Non-storing Nodes . . . . . . . . . . 48
       6.2.6.  Scheduling to Send DAO (or no-DAO) . . . . . . . . . . 39
     6.8.  Establishing Routing State Down 48
       6.2.7.  Triggering DAO Message from the DODAG Sub-DODAG  . . . . . . 49
       6.2.8.  Sending DAO Messages to DAO Parents  . . 40
       6.8.1.  Destination Advertisement Operation . . . . . . . 50
       6.2.9.  Multicast Destination Advertisement Messages . . 41
     6.9.  Loop Detection . . . 51
   7.  Packet Forwarding and Loop Avoidance/Detection . . . . . . . . 51
     7.1.  Suggestions for Packet Forwarding  . . . . . . . . . . . 48
       6.9.1.  Source Node Operation . 51
     7.2.  Loop Avoidance and Detection . . . . . . . . . . . . . . . 49
       6.9.2.  Router 52
       7.2.1.  Source Node Operation  . . . . . . . . . . . . . . . . 53
       7.2.2.  Router Operation . . . . . . . 49
     6.10. Multicast Operation . . . . . . . . . . . . 54
   8.  Multicast Operation  . . . . . . . 51
     6.11. Maintenance of Routing Adjacency . . . . . . . . . . . . . 52
   7.  Suggestions for Packet Forwarding . 56
   9.  Maintenance of Routing Adjacency . . . . . . . . . . . . . . 53
   8. . 57
   10. Guidelines for Objective Functions . . . . . . . . . . . . . . 54
   9. 58
   11. RPL Constants and Variables  . . . . . . . . . . . . . . . . . 56
   10. 60
   12. Manageability Considerations . . . . . . . . . . . . . . . . . 58
     10.1. 61
     12.1. Control of Function and Policy . . . . . . . . . . . . . . 58
       10.1.1. 61
       12.1.1. Initialization Mode  . . . . . . . . . . . . . . . . . 58
       10.1.2. 61
       12.1.2. DIO Base option  . . . . . . . . . . . . . . . . . . . 58
       10.1.3. 62
       12.1.3. Trickle Timers . . . . . . . . . . . . . . . . . . . . 59
       10.1.4. 62
       12.1.4. DAG Sequence Number Increment  . . . . . . . . . . . . 59
       10.1.5. 63
       12.1.5. Destination Advertisement Timers . . . . . . . . . . . 59
       10.1.6. 63
       12.1.6. Policy Control . . . . . . . . . . . . . . . . . . . . 59
       10.1.7. 63
       12.1.7. Data Structures  . . . . . . . . . . . . . . . . . . . 60
     10.2. 63
     12.2. Information and Data Models  . . . . . . . . . . . . . . . 60
     10.3. 64
     12.3. Liveness Detection and Monitoring  . . . . . . . . . . . . 60
       10.3.1. 64
       12.3.1. Candidate Neighbor Data Structure  . . . . . . . . . . 61
       10.3.2. 64
       12.3.2. Directed Acyclic Graph (DAG) Table . . . . . . . . . . 61
       10.3.3. 64
       12.3.3. Routing Table  . . . . . . . . . . . . . . . . . . . . 61
       10.3.4. 65
       12.3.4. Other RPL Monitoring Parameters  . . . . . . . . . . . 62
       10.3.5. 65
       12.3.5. RPL Trickle Timers . . . . . . . . . . . . . . . . . . 62
     10.4. 66
     12.4. Verifying Correct Operation  . . . . . . . . . . . . . . . 62
     10.5. 66
     12.5. Requirements on Other Protocols and Functional
           Components . . . . . . . . . . . . . . . . . . . . . . . . 63
     10.6. 66
     12.6. Impact on Network Operation  . . . . . . . . . . . . . . . 63
   11. 66
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 63
   12. 66
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 63
     12.1. 66
     14.1. RPL Control Message  . . . . . . . . . . . . . . . . . . . 63
     12.2. 66
     14.2. New Registry for RPL Control Codes . . . . . . . . . . . . 63
     12.3. 67
     14.3. New Registry for the Control Field of the DIO Base . . . . 64
     12.4. 67
     14.4. DAG Information Object (DIO) Suboption . . . . . . . . . . 64
   13. 68
   15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 65
   14. 68
   16. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 65
   15. 69
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 67
     15.1. 70
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 67
     15.2. 70
     17.2. Informative References . . . . . . . . . . . . . . . . . . 67 70
   Appendix A.  Requirements  . . . . . . . . . . . . . . . . . . . . 69 72
     A.1.  Protocol Properties Overview . . . . . . . . . . . . . . . 69 72
       A.1.1.  IPv6 Architecture  . . . . . . . . . . . . . . . . . . 69 72
       A.1.2.  Typical LLN Traffic Patterns . . . . . . . . . . . . . 69 73
       A.1.3.  Constraint Based Routing . . . . . . . . . . . . . . . 70 73
     A.2.  Deferred Requirements  . . . . . . . . . . . . . . . . . . 70 73
   Appendix B.  Examples  . . . . . . . . . . . . . . . . . . . . . . 70 74
     B.1.  Destination Advertisement  . . . . . . . . . . . . . . . . 72 75
     B.2.  Example: DAG DODAG Parent Selection  . . . . . . . . . . . . . . 73 76
     B.3.  Example: DAG DODAG Maintenance . . . . . . . . . . . . . . . . . 75 78
     B.4.  Example: Greedy Parent Selection and Instability . . . . . 76 79
   Appendix C.  Outstanding Issues  . . . . . . . . . . . . . . . . . 78 81
     C.1.  Additional Support for P2P Routing . . . . . . . . . . . . 78 81
     C.2.  Loop Detection  Destination Advertisement / DAO Fan-out  . . . . . . . . . 81
     C.3.  Source Routing . . . . . . . . . . . . . 78
     C.3.  Destination Advertisement / DAO Fan-out . . . . . . . . . 78 81
     C.4.  Source Routing . . . . . . .  Address / Header Compression . . . . . . . . . . . . . . . 79 82
     C.5.  Address / Header Compression  Managing Multiple Instances  . . . . . . . . . . . . . . . 79 82
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 79 82

1.  Introduction

   Low power and Lossy Networks (LLNs) are made consist of largely of constrained
   nodes (with limited processing power, memory, and sometimes energy
   when they are battery operated).  These routers are interconnected by
   lossy links, typically supporting only low data rates, that are
   usually unstable with relatively low packet delivery rates.  Another
   characteristic of such networks is that the traffic patterns are not
   simply unicast, but in many cases point-to-multipoint or multipoint-
   to-point.  Furthermore such networks may potentially comprise up to
   thousands of nodes.  These characteristics offer unique challenges to
   a routing solution: the IETF ROLL Working Group has defined
   application-specific routing requirements for a Low power and Lossy
   Network (LLN) routing protocol, specified in
   [I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548].  This
   document specifies the IPv6 Routing Protocol for Low power and Lossy
   Networks lossy
   networks (RPL).

1.1.  Design Principles

   RPL was designed with the objective to meet the requirements spelled
   out in [I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548].  Because
   those requirements are heterogeneous and sometimes incompatible in
   nature, the approach is first taken to design a protocol capable of
   supporting a core set of functionalities corresponding to the
   intersection of the requirements.  As the RPL design evolves optional
   features may be added to address some application specific
   requirements.  This is a key protocol design decision providing a
   granular approach in order to restrict the core of the protocol to a
   minimal set of functionalities, and to allow each implementation of
   the protocol to be optimized differently.  All "MUST" application
   requirements that cannot be satisfied by RPL will be specifically
   listed in the Appendix A, accompanied by a justification.

   A network may run multiple instances of RPL concurrently.  Each such
   instance may serve different and potentially antagonistic constraints
   or performance criteria.  This document defines how a single instance
   operates.

   RPL is a generic protocol that is to be deployed by instantiating the
   generic operation described in this document with a specific
   objective function (OF) (which ties together metrics, constraints,
   and an optimization objective) to realize a desired objective in a
   given environment.

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

1.2.  Expectations of Link Layer Type

   As

   RPL is a routing protocol, it of course does not rely on any particular features of a specific link layer
   technology.  RPL should is designed to be able to operate over a variety of
   different link layers, including but not limited to to, low power
   wireless or PLC (Power Line Communication) technologies.

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

2.  Terminology

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

   This

   Additionally, this document requires readers to be familiar with the uses terminology
   described in `Terminology in Low power And Lossy Networks'
   [I-D.ietf-roll-terminology]. from
   [I-D.ietf-roll-terminology], and introduces the following
   terminology:

   DAG:  Directed Acyclic Graph.  A directed graph having the property
         that all edges are oriented in such a way that no cycles exist.
         All edges are contained in paths oriented toward and
         terminating at one or more root nodes.

   DAG Instance: root:  A DAG Instance root is a set of possibly multiple
         Destination Oriented DAGs.  A network may node within the DAG that has no outgoing
         edges.  Because the graph is acyclic, by definition all DAGs
         must have more than at least one DAG Instance, root and a RPL router can participate to multiple DAG
         instances.  Each DAG Instance operates independently of other
         DAG Instances.  This document describes operation within a
         single DAG instance.

   InstanceID:  Unique identifier of all paths terminate at a
         DAG Instance. root.

   Destination Oriented DAG (DODAG):  A DAG rooted at a single
         destination, which is i.e. at a node single DAG root (the DODAG root) with no
         outgoing edges.

   DODAG root:  A DODAG root is the DAG root of a DODAG.

   Rank: The tuple
         (InstanceID, DAGID) uniquely identifies rank of a node in a Destination Oriented DAG (DODAG).  In identifies the RPL context, a router can can belong nodes position with
         respect to at
         most one DODAG per DAG Instance.

   DAGID:  The identifier of a DODAG root.  The DAGID must be unique
         within the scope of farther away a DAG Instance in the LLN.

   DODAG Iteration:  A specific sequence number iteration of node is from a DODAG.

   DAGSequenceNumber:  A sequential counter that
         DODAG root, the higher is incremented by the
         root to form a new Iteration rank of that node.  The rank of a DODAG.  A
         node may be a simple topological distance, or may more commonly
         be calculated as a function of other properties as described
         later.

   DODAG Iteration is
         identified uniquely by the (InstanceID, DAGID,
         DAGSequenceNumber) tuple.

   DAG parent:  A parent of a node within a DAG DODAG is one of the
         immediate successors of the node on a path towards the DAG DODAG
         root.

   DAG  The DODAG parent of a node will have a lower rank than
         the node itself.  (See Section 3.6.2.1).

   DODAG sibling:  A sibling of a node within a DAG DODAG is defined in this
         specification to be any neighboring node which is located at
         the same rank within a DAG. DODAG.  Note that siblings defined in
         this manner do not necessarily share a common DODAG parent.

   DAG root:  A DAG root is a node within the DAG that has no outgoing
         edges.  Because the graph is acyclic, by definition all DAGs
         must have at least one DAG root and all paths terminate at a
         DAG root.

   Sub-DAG
         (See Section 3.6.2.1).

   Sub-DODAG  The sub-DAG sub-DODAG of a node is the set of other nodes in the DAG
         DODAG that might use a path towards the DAG DODAG root that
         contains the that node.  Nodes in the sub-DAG sub-DODAG of a node have a
         greater rank than that node itself (although not all nodes of
         greater rank are necessarily in the sub-DAG).

   Up:   Up refers to the direction from leaf nodes towards DODAG roots,
         following the orientation sub-DODAG of the edges that node).
         (See Section 3.6.2.1).

   DODAGID:  The identifier of a DODAG root.  The DODAGID must be unique
         within the DODAG.

   Down: Down refers to scope of a RPL Instance in the direction from LLN.

   DODAG roots towards leaf
         nodes, going against the orientation Iteration:  A specific sequence number iteration ("version") of the edges within the
         DODAG.

   OCP:  Objective Code Point.  The Objective Code Point is used to
         indicate which Objective Function is in use in
         a DODAG.  The
         Objective Code Point is DODAG with a given DODAGID.

   RPL Instance:  A set of possibly multiple DODAGs.  A network may have
         more than one RPL Instance, and a RPL node can participate in
         multiple RPL Instances.  Each RPL Instance operates
         independently of other RPL Instances.  This document describes
         operation within a single RPL Instance.  In RPL, a node can
         belong to at most one DODAG per RPL Instance.  The tuple
         (RPLInstanceID, DODAGID) uniquely identifies a DODAG.

   RPLInstanceID:  Unique identifier of a RPL Instance.

   DODAGSequenceNumber:  A sequential counter that is incremented by the
         root to form a new Iteration of a DODAG.  A DODAG Iteration is
         identified uniquely by the (RPLInstanceID, DODAGID,
         DODAGSequenceNumber) tuple.

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

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

   Objective Code Point (OCP):  An identifier, used to indicate which
         Objective Function is in use for forming a DODAG.  The
         Objective Code Point is further described in
         [I-D.ietf-roll-routing-metrics].

   OF:   Objective Function.  The

   Objective Function (OF) defines (OF):  Defines which routing metrics, optimization
         objectives, and related functions are in use in a DODAG.  The
         Objective Function is further described in
         [I-D.ietf-roll-routing-metrics].

   Goal: The Goal is a host or set of hosts that satisfy a particular
         application objective / OF.  Whether or not a DODAG can provide
         connectivity to a goal is a property of the DODAG.  For
         example, a goal might be a host serving as a data collection
         point, or a gateway providing connectivity to an external
         infrastructure.

   Grounded:  A DAG DODAG is grounded said to be grounded, when the root can reach
         the Goal of the objective function.

   Floating:  A DAG DODAG is floating if is not Grounded.  A floating DAG DODAG
         is not expected to reach the Goal defined for the OF.

   As they form networks, LLN devices often mix the roles of `host' 'host' and
   `router'
   'router' when compared to traditional IP networks.  In this document,
   `host'
   'host' refers to an LLN device that can generate but does not forward
   RPL traffic, `router' 'router' refers to an LLN device that can forward as
   well as generate RPL traffic, and `node' 'node' refers to any RPL device,
   either a host or a router.

3.  Protocol Model Overview

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

3.1.  Instances, DODAGs,  Topology

   This section describes how the basic RPL topologies, and the rules by
   which these are constructed, i.e. the rules governing DODAG Iterations

   Each DAG instance constructs a routing topology optimized for a
   certain Objective Function (OF).  A DAG instance may provide routes
   formation.

3.1.1.  Topology Identifiers

   RPL uses four identifiers to certain destination prefixes. track and control the topology:

   o  The first is a RPLInstanceID.  A single DAG instance contains RPLInstanceID identifies a set of
      one or more Destination Oriented DAG (DODAG) roots.  These roots may
   operate independently, or may coordinate over a non-LLN backchannel.

   Each root has a unique identifier, DODAGs.  All DODAGs in the DAGID, such that nodes can
   identify same RPL Instance use the DODAG root.
      same OF.  A DAG instance network may comprise:

   o  a single DODAG with a single root

      *  For example, a DODAG have multiple RPLInstanceIDs, each of
      which defines an independent set of DODAGs, which may be optimized to minimize latency rooted at
      for different OFs and/or applications.  The set of DODAGs
      identified by a
         single centralized lighting controller in RPLInstanceID is called a home automation
         application. RPL Instance.

   o  multiple uncoordinated DODAGs with independent roots (differing
      DAGIDs)
      *  For example, multiple data collection points in an urban data
         collection application that do not have an always-on backbone
         suitable to coordinate to form  The second is a single DODAG, and further use
         the formation DODAGID.  The scope of multiple DODAGs as a means to dynamically DODAGID is a RPL
      Instance.  The combination of RPLInstanceID and
         autonomously partition the network.

   o DODAGID uniquely
      identifies a single DODAG with a single virtual root coordinating LLN sinks
      (with in the same DAGID) over some non-LLN backbone

      *  For example, network.  A RPL Instance may have
      multiple border routers operating with a reliable
         backbone, e.g. in support DODAGs, each of a 6LowPAN application, that are
         capable to act as logically equivalent sinks to the same DODAG. which has an unique DODAGID.

   o  The third is a combination of one DODAGSequenceNumber.  The scope of the above as suited to some application
      scenario.

   Traffic a
      DODAGSequenceNumber is bound to a specific DODAG.  A DODAG Instance is sometimes
      reconstructed from the DODAG root, by a marking in incrementing the
   flow label of the IPv6 header.  Traffic originating in support
      DODAGSequenceNumber.  The combination of RPLInstanceID, DODAGID,
      and DODAGSequenceNumber uniquely identifies a
   particular application may be tagged to follow an appropriate DAG
   instance, for example to follow paths optimized for low latency or
   low energy. DODAG Iteration.

   o  The provisioning or automated discovery fourth is rank.  The scope of rank is a mapping
   between an InstanceID and DODAG Iteration.  Rank
      establishes a type partial order over a DODAG Iteration, defining
      individual node positions with respect to the DODAG root.

3.1.2.  DODAG Information

   For each DODAG that a node is, or service of application traffic is
   beyond may become, a member of, the scope
   implementation should conceptually keep track of the following
   information for each DODAG.  The data structures described in this specification.

   An example of
   section are intended to illustrate a possible implementation to aid
   in the description of the protocol, but are not intended to be
   normative.

   o  RPLInstanceID

   o  DODAGID

   o  DODAGSequenceNumber

   o  DAG Metric Container, including DAGObjectiveCodePoint

   o  A set of Destination Prefixes offered by the DODAG root and
      available via paths upwards along the DODAG

   o  A set of DODAG parents

   o  A set of DODAG siblings

   o  A timer to govern the sending of DIO messages

3.2.  Instances, DODAGs, and DODAG Iterations

   Each RPL Instance comprising constructs a number of DODAGs is
   depicted in Figure 1. routing topology optimized for a
   certain Objective Function (OF).  A RPL Instance may provide routes
   to certain destination prefixes, reachable via the DODAG Iteration is depicted in Figure 2.

     +----------------------------------------------------------------+
     |                                                                |
     | +--------------+                                               |
     | |              |                                               |
     | |     (R1)     |            (R2)                   (Rn)        |
     | |     /  \     |            /| \                  / |  \       |
     | |    /    \    | roots.  A
   single RPL Instance contains one or more Destination Oriented DAG
   (DODAG) roots.  These roots may operate independently, or may
   coordinate over a non-LLN backchannel.

   Each root has a unique identifier, the DODAGID.

   A RPL Instance may comprise:

   o  a single DODAG with a single root

      *  For example, a DODAG optimized to minimize latency rooted at a
         single centralized lighting controller in a home automation
         application.

   o  multiple uncoordinated DODAGs with independent roots (differing
      DODAGIDs)

      *  For example, multiple data collection points in an urban data
         collection application that do not have an always-on backbone
         suitable to coordinate to form a single DODAG, and further use
         the formation of multiple DODAGs as a means to dynamically and
         autonomously partition the network.

   o  a single DODAG with a single virtual root coordinating LLN sinks
      (with the same DODAGID) over some non-LLN backbone

      *  For example, multiple border routers operating with a reliable
         backbone, e.g. in support of a 6LowPAN application, that are
         capable to act as logically equivalent sinks to the same DODAG.

   o  a combination of one of the above as suited to some application
      scenario.

   Traffic is bound to a specific RPL Instance by a marking in the flow
   label of the IPv6 header.  Traffic originating in support of a
   particular application may be tagged to follow an appropriate RPL
   instance which enables certain (path) properties, for example to
   follow paths optimized for low latency or low energy.  The
   provisioning or automated discovery of a mapping between a
   RPLInstanceID and a type or service of application traffic is beyond
   the scope of this specification.

   An example of a RPL Instance comprising a number of DODAGs is
   depicted in Figure 1.  A DODAG Iteration (two "versions" of the same
   DODAG) is depicted in Figure 2.

     +----------------------------------------------------------------+
     |                                                                |
     | +--------------+                                               |
     | |              |                                               |
     | |     (R1)     |            (R2)                   (Rn)        |
     | |     /  \     |            /| \                  / |  \       |
     | |    /    \    |           / |  \                /  |   \      |
     | |  (A)    (B)  |         (C) |  (D)     ...    (F) (G)  (H)    |
     | |  /|\     |\  |         /   |   |\             |   |    |     |
     | | : : :    : : |        :   (E)  : :            :   :    :     |
     | |              |            / \                                |
     | +--------------+           :   :                               |
     |      DODAG                                                     |
     |                                                                |
     +----------------------------------------------------------------+
                                DAG
                                RPL Instance

                          Figure 1: DAG RPL Instance

            +----------------+                +----------------+
            |                |                |                |
            |      (R1)      |                |      (R1)      |
            |      /  \      |                |      /         |
            |     /    \     |                |     /          |
            |   (A)    (B)   |         \      |   (A)          |
            |   /|\     |\   |    ------\     |   /|\          |
            |  : : (C)  : :  |           \    |  : : (C)       |
            |                |           /    |        \       |
            |                |    ------/     |         \      |
            |                |         /      |         (B)    |
            |                |                |          |\    |
            |                |                |          : :   |
            |                |                |                |
            +----------------+                +----------------+
                Sequence N                       Sequence N+1

                         Figure 2: DODAG Iteration

3.2.

3.3.  Traffic Flows

3.2.1.

3.3.1.  Multipoint-to-Point Traffic

   Multipoint-to-Point (MP2P) is a dominant traffic flow in many LLN
   applications ([I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs], [RFC5673], [RFC5548]).  The
   destinations of MP2P flows are designated nodes that have some
   application significance, such as providing connectivity to the
   larger Internet or core private IP network.  RPL supports MP2P
   traffic by allowing MP2P destinations to be reached via DODAG roots.

3.2.2.

3.3.2.  Point-to-Multipoint Traffic

   Point-to-multipoint (P2MP) is a traffic pattern required by several
   LLN applications ([I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs], [RFC5673], [RFC5548]).  RPL
   supports P2MP traffic by using a destination advertisement mechanism
   that provisions routes toward destination prefixes and away from
   roots.  Destination advertisements can update routing tables as the
   underlying DODAG topology changes.

3.2.3.

3.3.3.  Point-to-Point Traffic

   RPL DODAGs provide a basic structure for point-to-point (P2P)
   traffic.  For a RPL network to support P2P traffic, a root must be
   able to route packets to a destination.  Nodes within the network may
   also have routing tables to destinations.  A packet flows towards a
   root until it reaches an ancestor that has a known route to the
   destination.

   RPL also supports the case where a P2P destination is a `one-hop' 'one-hop'
   neighbor.

   RPL neither specifies nor precludes additional mechanisms for
   computing and installing more optimal routes to support arbitrary P2P
   traffic.

3.3.

3.4.  Upward Routes and DODAG Construction

   RPL provisions routes up towards DODAG roots, forming a DODAG
   optimized according to the Objective Function (OF) in use.  RPL nodes
   construct and maintain these DODAGs through exchange of DAG DODAG
   Information Object (DIO) messages.  Undirected links between siblings
   are also identified during this process, which are can be used to provide
   additional diversity.

3.3.1.

3.4.1.  DAG Information Object (DIO)

   A DIO identifies the DAG RPL Instance, the DAGID, DODAGID, the values used to
   compute the DAG RPL Instance's objective function, and the present DODAG
   Sequence Number.  It can also include additional routing and
   configuration information.  The DIO includes a measure derived from
   the position of the node within the DODAG, the rank, which is used
   for nodes to determine their positions relative to each other and to
   inform loop avoidance/detection procedures.  RPL exchanges DIO
   messages to establish and maintain routes.

   RPL adapts the rate at which nodes send DIO messages.  When a DODAG
   is detected to be inconsistent or needs repair, RPL sends DIO
   messages more frequently.  As the DODAG stabilizes, the DIO message
   rate tapers off, reducing the maintenance cost of a steady and well-
   working DODAG.

   This document defines an ICMPv6 Message Type RPL "RPL Control Message, Message",
   which is capable of carrying a DIO.

3.3.2.

3.4.2.  DAG Repair

   RPL supports global repair over the DODAG.  A DODAG Root may
   increment the DODAG Sequence Number to institute Number, thereby initiating a new DODAG
   iteration.  This institutes a global repair, repair operation, revising the
   DODAG and allowing nodes to choose an arbitrary new position within
   the new DODAG iteration.

   RPL may support supports mechanisms which may be used for local repair within the
   DODAG iteration.  The DIO message will specify specifies the necessary parameters
   as configured from the DODAG root.  Local repair options include the
   allowing a node, upon detecting a loss of connectivity to a DODAG it
   is a member of, to:

   o  Poison its sub-DAG sub-DODAG by advertising an effective rank of INFINITY, INFINITY
      to its sub-DODAG, OR detach and form a floating DODAG in order to
      preserve inner connectivity within its sub-DAG. sub-DODAG.

   o  Move down within the DODAG iteration (i.e. increase its rank) in a
      limited manner, no further than a bound configured by the DODAG
      root via the DIO so as not to count all the way to infinity.  Such
      a move may be undertaken after waiting an appropriate poisoning
      interval, and should allow the node to restore connectivity to the
      DODAG Iteration Iteration, if at all possible.

3.3.3.

3.4.3.  Grounded and Floating DODAGs

   DODAGs can be grounded or floating.  A grounded DODAG offers
   connectivity to to a goal.  A floating DODAG offers no such
   connectivity, and provides routes only to nodes within the DODAG.
   Floating DODAGs may be used, for example, to preserve inner
   connectivity during repair.

3.3.4.

3.4.4.  Administrative Preference

   An implementation/deployment may specify that some DODAG roots should
   be used over others through an administrative preference.
   Administrative preference offers a way to control traffic and
   engineer DODAG formation in order to better support application
   requirements or needs.

3.3.5.

3.4.5.  Objective Function (OF)

   The Objective Function (OF) implements the optimization objectives of
   route selection within the DAG RPL Instance.  The OF is identified by an
   Objective Code Point (OCP) within the DIO, and its specification also
   indicates the metrics and constraints in use.  The OF also specifies
   the procedure used to compute rank within a DODAG iteration.  Further
   details may be found in [I-D.ietf-roll-routing-metrics] [I-D.ietf-roll-routing-metrics],
   [I-D.ietf-roll-of0], and related companion specifications.

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

   In the case where a node is unable to encounter a suitable DAG RPL
   Instance using a known Objective Function, it may be configured to
   join DAG a RPL Instance using and an unknown Objective Function - but in that
   case only acting as a leaf node.

3.3.6.

3.4.6.  Distributed Algorithm Operation

   A high level overview of the distributed algorithm which constructs
   the DODAG is as follows:

   o  Some nodes are configured to be DODAG roots, with associated DODAG
      configuration.

   o  Nodes advertise their presence, affiliation with a DODAG, routing
      cost, and related metrics by sending link-local multicast DIO
      messages.

   o  Nodes may adjust the rate at which DIO messages are sent in
      response to stability or detection of routing inconsistencies.

   o  Nodes listen for DIOs and use their information to join a new
      DODAG, or to maintain an existing DODAG, as according to the
      specified Objective Function and rank-based loop avoidance rules.

   o  The nodes  Nodes provision routing table entries entries, for the destinations
      specified by the DIO towards DIO, via their DODAG parents in the DODAG
      iteration.  Nodes may provision a DODAG parent as a default
      gateway.

   o  Nodes may identify DODAG siblings within the DODAG iteration to
      increase path diversity.

   o  Using both DIOs DIOs, and possibly information in data packets, RPL nodes
      detect possible routing loops.  When a RPL node detects a possible
      routing loop, it may adapt its DIO transmission rate to apply a
      local repair to the topology.  This process

3.5.  Downward Routes and its
      limitations is discussed in greater detail in 3.4.

3.4. Destination Advertisement

   As

   RPL constructs and maintains DODAGs with DIO messages to establish
   upward routes, routes: it may use uses Destination Advertisement Object (DAO)
   messages to establish downward routes along the DODAG. DODAG as well as
   other routes.  DAO messages
   and support for downward routes are an optional feature for applications
   that require P2MP or P2P traffic.  DIO messages advertise whether the
   destination advertisement mechanism is enabled.

3.4.1. advertisements are enabled within a given DODAG.

3.5.1.  Destination Advertisement Object (DAO)

   A Destination Advertisement Object (DAO) conveys destination
   information upwards along the DODAG so that a DODAG root (an other
   intermediate nodes) can provision downward routes.  A DAO message
   includes prefix information to identify destinations, a capability to
   record routes in support of source routing, and information to
   determine the freshness of a particular advertisement.

   Nodes that are capable of maintaining routing state may aggregate
   routes from DAO messages that they receive before transmitting a DAO
   message.  Nodes that are not capable to maintain of maintaining routing state may
   attach a next-hop address to the Reverse Route Stack contained within
   the DAO message.  The Reverse Route Stack is subsequently used to
   generate piecewise source routes over regions of the LLN that are
   incapable of storing downward routing state.

   A special case of the DAO message, termed a no-DAO, is used to clear
   downward routing state that has been provisioned through DAO
   operation.

   This document defines an ICMPv6 Message Type RPL "RPL Control Message, Message",
   which is capable to carry the of carrying a DAO.

3.4.1.1.  `One-Hop'

3.5.1.1.  'One-Hop' Neighbors

   In addition to sending DAOs toward DODAG roots, RPL nodes may
   occasionally emit a link-local multicast DAO message advertising
   available destination prefixes.  This mechanism allow provisioning a
   trivial `one-hop' 'one-hop' route to local neighbors.

4.

3.6.  Routing Metrics and Constraints Used By RPL

   Routing metrics are used by routing protocols to compute the shortest
   paths.  Interior Gateway Protocols (IGPs) such as IS-IS ([RFC5120])
   and OSPF ([RFC4915]) use static link metrics.  Such link metrics can
   simply reflect the bandwidth or can also be computed according to a
   polynomial function of several metrics defining different link
   characteristics; in all cases they are static 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 Equal Cost
   Multiple Paths (ECMP).  The optimization of multiple metrics is known
   as an NP complete problem and is sometimes supported by some
   centralized path computation engine.

   In contrast, LLNs do require the support of both static and dynamic
   metrics.  Furthermore, both link and node metrics are required.  In
   the case of RPL, it is virtually impossible to define one metric, or
   even a composite, composite metric, that will satisfy all use cases.

   In addition, RPL supports constrained-based routing where constraints
   may be applied to both link and nodes.  If a link or a node does not
   satisfy a required constraint, it is `pruned' 'pruned' from the candidate list
   list, thus leading to a constrained shortest path.

   The set of supported link/node constraints and metrics is specified
   in [I-D.ietf-roll-routing-metrics].

   The role of the Objective Function is to advertise specify which routing
   metrics and constraints are in use, and how these are used, in
   addition to the objectives used to compute the (constrained) shortest
   path.

   Example 1: Shortest path: path offering the shortest end-to-end delay

   Example 2: Constrained shortest path: the path that does not traverse
              any battery-operated node and that optimizes the path
              reliability

5.  Rank

5.1.

3.6.1.  Loop Avoidance

   RPL guarantees neither loop free path selection nor strong global
   convergence.  In order to reduce control overhead, however, such as
   the cost of the count-to-infinity problem, RPL avoids creating loops
   when undergoing topology changes.  Furthermore, RPL includes rank-
   based mechanisms for detecting loops when they do occur.  RPL uses
   this loop detection to ensure that packets make forward progress
   within the DODAG iteration and trigger repairs when necessary.

5.1.1.

3.6.1.1.  Greediness and Rank-based Instabilities

   Once a node has joined a DODAG, DODAG iteration, RPL disallows certain
   behaviors, including greediness, in order to prevent resulting
   instabilities in the DODAG. DODAG iteration.

   If a node is allowed to be greedy and attempts to move deeper in the
   DODAG,
   DODAG iteration, beyond its most preferred parent, in order to
   increase the size of the parent set, then an instability can result.
   This is illustrated in Figure 16.

   Suppose a node is willing to receive and process a DIO messages from
   a node in its own sub-DAG, sub-DODAG, and in general a node deeper than it.
   itself.  In
   such cases this case, a chance possibility exists to create that a feedback loop, loop is
   created, wherein two or more nodes continue to try and move in the
   DODAG in order iteration while attempting to optimize against each other.  In
   some cases cases, this will result in an instability.  It is for this reason
   that RPL limits the cases where a node may process DIO messages from
   deeper nodes to some forms of local repair.  This approach creates an `event
   'event horizon', whereby a node cannot be influenced beyond some
   limit into an instability by the action of nodes that may be in its
   own sub-DAG. sub-DODAG.

   A further example of the consequences of greedy operation, and
   instability related to processing DIO messages from nodes of greater
   rank, may be found in Appendix B.4

5.1.2.

3.6.1.2.  DODAG Loops

   A DODAG loop may occur when a node detaches from the DODAG and
   reattaches to a device in its prior sub-DAG. sub-DODAG.  This may happen in
   particular when DIO messages are missed.  Strict use of the DAG
   sequence number can eliminate this type of loop, but this type of
   loop may possibly be encountered when using some local repair
   mechanisms.

5.1.3.

3.6.1.3.  DAO Loops

   A DAO loop may occur when the parent has a route installed upon
   receiving and processing a DAO message from a child, but the child
   has subsequently cleaned up the state.  This loop happens when a no-
   DAO was missed and persists until a heartbeat cleans up all states. state has been cleaned up.  RPL
   includes loop detection mechanisms that may mitigate the impact of
   DAO loops and trigger their repair.

   In the case where stateless DAO operation is used, i.e. source
   routing specifies the down routes, then DAO Loops should not occur on
   the stateless portions of the path.

5.1.4.

3.6.1.4.  Sibling Loops

   Sibling loops could occur if a group of siblings kept choosing
   amongst themselves as successors such that a packet does not make
   forward progress.  This specification limits the number of times that
   sibling forwarding may be used at a given rank rank, in order to prevent
   sibling loops.

5.2.

3.6.2.  Rank Properties

   The rank of a node is a scalar representation of the location of that
   node within a DODAG iteration.  The rank is used to avoid and detect
   loops, and as such must demonstrate certain properties.  The exact
   calculation of the rank is left to the Objective Function, and may
   depend on parents, link metrics, and the node configuration and
   policies.

   The rank is not a cost metric, although its value can be derived from
   and influenced by metrics.  The rank has properties of its own that
   are not necessarily that those of all metrics:

   Type:   Rank is an abstract scalar.  Some metrics are boolean (e.g.
           grounded), others are statistical and better expressed as a
           tuple like an expected value and a variance.  Some OCPs use
           not one but a set of metrics bound by a piece of logic.

   Function:  Rank is the expression of a relative position within a
           DODAG iteration with regard to neighbors and, and is not
           necessarily a good indication or a proper expression of a
           distance or a cost to the root.

   Stability:  The stability of the rank determines that the stability of the
           routing topology.  Some dampening or filtering might be
           applied to keep the topology stable stable, and thus the rank does
           not necessarily change as fast as some physical metrics
           would.  A new DODAG iteration is would be a good opportunity to
           reconcile the discrepancies that might form over time between the
           metrics and the ranks. ranks within a DODAG iteration.

   Granularity:  Rank is coarse grained.  A fine granularity would
           prevent the selection of siblings.

   Properties:  Rank is strictly monotonic monotonic, and can be used to validate
           a progression from or towards the root.  A metric metric, like
           bandwidth or jitter jitter, does not necessarily exhibit such this
           property.

   Abstract:  Rank does not have a physical unit, but rather a range of
           increment per hop that varies from 1 (best) to 16 (worst),
           where the assignment of each value is to be determined by the
           implementation.

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

3.6.2.1.  Rank Comparison

   Rank may be thought of as a fixed point number, where the position of
   the decimal point is determined by MinHopRankIncrease.  The integer
   portion of the Rank is determined by floor(Rank/MinHopRankIncrease).

   MinHopRankIncrease is provisioned at the DODAG Root and propagated in
   the DIO message.  For efficient implementation the MinHopRankIncrease
   SHOULD be a power of 2.  An implementation may configure a value
   MinHopRankIncrease as appropriate to balance between the loop
   avoidance logic of RPL (i.e. selection of eligible parents and
   siblings) and the metrics in use.

   A node A has a rank less than the rank of a node B if floor(Rank(A) /
   MinHopRankIncrease) is less than floor (Rank(B) /
   MinHopRankIncrease).

   A node A has a rank equal to the rank of a node B if floor(Rank(A) /
   MinHopRankIncrease) is equal to floor (Rank(B) / MinHopRankIncrease).

   A node A has a rank greater than the rank of a node B if
   floor(Rank(A) / MinHopRankIncrease) is greater than floor (Rank(B) /
   MinHopRankIncrease).

3.6.2.2.  Rank Relationships

   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 that are
   neighbors in the LLN:

   DAGRank(M) is less than DAGRank(N):  In this case, the position of M
           is probably
           located in a more preferred position than N in the DODAG with
           respect closer to the metrics and optimizations defined by DODAG root than the
           objective code point.  In any fashion, position of N. Node M
           may safely be a
           DAG DODAG parent for Node N without risk of
           creating a loop.  Further, for a node N, all parents in the DAG
           DODAG parent set must be of rank less than self's DAGRank(N).  In
           other words, the rank presented by a node N MUST be greater (deeper)
           than that presented by any of its parents.

   DAGRank(M) equals DAGRank(N):  In this case the positions of M and N are located
           positions of relatively the same optimality
           within the DODAG. DODAG and with respect to the DODAG root are
           similar (identical).  In some cases, Node M may be used as a
           successor by Node N,
           but with related chance which however entails the probability of
           creating a loop that (which must be detected and broken resolved by some
           other means. means).

   DAGRank(M) is greater than DAGRank(N):  In this case, then node the position of
           M is
           located in a less preferred position than N in farther from the DODAG with
           respect to the metrics and optimizations defined by root than the
           objective code point. position of N.
           Further, Node (M) M may in fact be in the sub-DODAG of Node (N)'s sub-DAG.  There N. If
           node N selects node M as DODAG parent there is a higher risk to Node (N)
           selecting Node (M) as a DAG parent, as such a selection may
           create a loop.

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

6.  RPL Protocol Specification

6.1.  RPL Messages

6.1.1.

4.  ICMPv6 RPL Control Message

   This document defines the RPL Control Message, a new ICMPv6 message.
   The RPL Control Message has the following general format, in
   In accordance with [RFC4443]:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     Type      |     Code      |          Checksum             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                         Message Body                          +
       |                                                               |

                       Figure 3: RPL Control Message

   The RPL Control message is an ICMPv6 information message with a
   requested Type of 155.

   The Code will be used to identify RPL Control Messages as follows:

   o  0x01: DAG Information Solicitation (Section 6.1.2)

   o  0x02: DAG Information Object (Section 6.1.3)

   o  0x04: Destination Advertisement Object (Section 6.1.4)

6.1.2.  DAG Information Solicitation (DIS)

   The DAG Information Solicitation (DIS) message may be used to solicit
   a DAG Information Object from a RPL node.  Its use is analogous to
   that of a Router Solicitation; a node may use DIS to probe its
   neighborhood for nearby DAGs.  The DAG Information Solicitation
   carries no additional message body.

6.1.3.  DAG Information Object (DIO)

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

6.1.3.1.  DIO Base

   The DIO Base is a container option, which is always present, and
   might contain a number of suboptions.  The base option regroups with [RFC4443], the
   minimum information set that is mandatory in all cases. RPL Control Message has the
   following 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |G|D|A|0|0| Prf
       |   Sequence     Type      |  InstanceID     Code      |    DAGRank          Checksum             |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                         Message Body                          +
       |                            DAGID                              |
       +                                                               +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                                                               |   sub-option(s)...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+

                       Figure 4: DIO Base 3: RPL Control Field: Message

   The DAG RPL Control Field is currently allocated as
         follows:

         Grounded (G):  The Grounded (G) flag is set when the DODAG root
               is a Goal for the OF.

         Destination Advertisement Trigger (D):  The Destination
               Advertisement Trigger (D) flag is set when the DODAG root
               or another node in the successor chain decides to trigger
               the sending of destination advertisements in order to
               update routing state for the down direction along the
               DODAG, as further detailed in Section 6.8.  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 DODAG root is capable
               to support the collection of destination advertisement
               related routing state and enables the operation of the
               destination advertisement mechanism within the DODAG.

         DAGPreference (Prf):  3-bit unsigned integer set by the DODAG
               root to its preference and unchanged at propagation.
               DAGPreference ranges from 0x00 (least preferred) to 0x07
               (most preferred).  The default message is 0 (least preferred).
               The DAG preference provides an administrative mechanism
               to engineer the self-organization of the LLN, for example
               indicating the most preferred LBR.  If a node has the
               option to join a more preferred DODAG while still meeting
               other optimization objectives, then the node will
               generally seek to join the more preferred DODAG as
               determined by the OF.

         Unassigned bits ICMPv6 information message with a
   requested Type of 155.

   The Code field identifies the type of RPL Control Field are considered as
         reserved.  They MUST be set to zero on transmission Message.  This
   document defines three types:

   o  0x01: DAG Information Solicitation (Section 5.2)

   o  0x02: DAG Information Object (Section 5.1)

   o  0x04: Destination Advertisement Object (Section 6.1)

5.  Upward Routes

   This section describes how RPL discovers and MUST be
         ignored on receipt.

   Sequence Number:  8-bit unsigned integer set by the maintains upward routes.
   It describes DODAG root,
         incremented according to a policy provisioned at Information Objects (DIOs), the DODAG
         root, messages used to
   discover and propagated with no change down the DODAG.  Each
         increment SHOULD have a value of 1 maintain these routes.  It specifies how RPL generates
   and may cause a wrap back responds to
         zero.

   InstanceID:  8-bit field indicating the topology instance associated
         with the DODAG, as provisioned at the DODAG root.

   DAGRank:  8-bit unsigned integer indicating the DIOs.  It also describes DAG rank of the node
         sending the DIO message.  The DAGRank of the DODAG root is
         ROOT_RANK.  DAGRank is further described in Section 6.3.

   DAGID:  128-bit unsigned integer Information Solicitation
   (DIS) messages, which uniquely identify a DODAG.
         This value is set by the are used to trigger DIO transmissions.

5.1.  DODAG root. Information Object (DIO)

   The global IPv6 address
         of the DODAG root can be used. the DAGID MUST be unique per DAG
         Instance within the scope of the LLN.

   The following values MUST NOT change during the propagation of DIO
   messages down the DAG:
      Grounded (G)
      Destination Advertisement Supported (A)
      DAGPreference (Prf)
      Sequence
      InstanceID
      DAGID
   All other fields of Information Object carries information that allows a node
   to discover a RPL Instance, learn its configuration parameters,
   select a DODAG parent set, and maintain the upward routing topology.

5.1.1.  DIO message may be updated at each hop of the
   propagation.

6.1.3.1.1. Base Format

   DIO Suboptions

   In addition to the minimum options presented Base is an always-present container option in the base option,
   several suboptions are defined for the a DIO message:

6.1.3.1.1.1.  Format message.
   Every DIO MUST include a DIO Base.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |G|A|T|S|0| Prf |  Subopt. Type   Sequence    |         Subopt Length             Rank              | Subopt Data
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | RPLInstanceID |     DTSN      |                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
       |                                                               |
       +                                                               +
       |                            DODAGID                            |
       +                                                               +
       |                                                               |
       +                               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                               |   sub-option(s)...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                            Figure 5: 4: DIO Suboption Generic Format

   Suboption Type:  8-bit identifier of Base

   Control Field:  The DAG Control Field has three flags and one field:

         Grounded (G):  The Grounded (G) flag indicates whether the type
               upward routes this node advertises provide connectivity
               to the set of suboption.  When
         processing a DIO message containing a suboption for addresses which are application-defined
               goals.  If the
         Suboption Type value flag is set, the DODAG is grounded and
               provides such connectivity.  If the flag is cleared, the
               DODAG is floating and may not provide such connectivity.

         Destination Advertisement Supported (A):  The Destination
               Advertisement Supported (A) bit indicates whether the
               root of this DODAG can collect and use downward route
               state.  The flag is set when nodes in the network are to
               exchange destination advertisements messages to build
               downward routes (Section 6).  The flag is cleared when
               the DODAG maintains only upward routes.

         Destination Advertisement Trigger (T):  The Destination
               Advertisement Trigger (T) flag is not recognized by the receiver, the
         receiver MUST silently ignore the unrecognized option, continue used to trigger a
               complete refresh of downward routes.  The details of this
               process the following suboption, correctly handling any
         remaining options in the message.

   Suboption Length:  16-bit unsigned integer, representing the length are described in octets of the suboption, not including the suboption Type Section 6.

         Destination Advertisements Stored (S):  The Destination
               Advertisements Stored (S) flag is used to indicate that a
               non-root ancestor is storing routing table entries
               learned from DAO messaging.  The meaning and Length fields.

   Suboption Data: further use
               of this flag is described in Section 6.

         DODAGPreference (Prf):  A variable length field 3-bit unsigned integer that contains data specific defines
               how preferable the root of this DODAG is compared to
               other DODAG roots within the option. DODAG.  DAGPreference ranges
               from 0x00 (least preferred) to 0x07 (most preferred).
               The following subsections specify the default is 0 (least preferred).  Section 5.3
               describes how DAGPreference affects DIO message suboptions which
   are currently defined for use in processing.

         Unassigned bits of the DAG Information Object.

   Implementations Control Field are reserved.  They MUST silently ignore any DIO message suboptions
   options that they do not understand.

   DIO message suboptions may have alignment requirements.  Following
         be set to zero on transmission and MUST be ignored on
         reception.

   Sequence Number:  8-bit unsigned integer set by the convention in IPv6, these options are aligned in a packet such
   that multi-octet values within DODAG root.
         Section 5.3 describes the Option Data field of each option
   fall on natural boundaries (i.e., fields of width n octets are placed
   at an rules for sequence numbers and how
         they affect DIO processing.

   Rank: 8-bit unsigned integer multiple of n octets from indicating the start DODAG rank of the header, for
   n = 1, 2, 4, or 8).

6.1.3.1.1.2.  Pad1

   The Pad1 suboption does not have any alignment requirements.  Its
   format node
         sending the DIO message.  Section 5.3 describes how Rank is as follows:

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

                              Figure 6: Pad 1

   NOTE! set
         and how it affects DIO processing.

   RPLInstanceID:  8-bit field set by the format of DODAG root that indicates
         which RPL Instance the Pad1 option DODAG is a special case - it has
   neither Option Length nor Option Data fields. part of.

   Destination Advertisement Trigger Sequence Number (DTSN):  8-bit
         unsigned integer set by the node issuing the DIO message.  The Pad1 option
         Destination Advertisement Trigger Sequence Number (DTSN) flag
         is used to insert one or two octets as part of padding in the
   DIO message procedure to enable suboptions alignment.  If more than two octets
   of padding is required, the PadN option, described next, should be
   used rather than multiple Pad1 options.

6.1.3.1.1.3.  PadN

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

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

                              Figure 7: Pad N maintain downward routes.
         The PadN option is used to insert three or more octets details of padding this process are described in Section 6.

   DODAGID:  128-bit unsigned integer set by a DODAG root which uniquely
         identifies a DODAG.  Possibly derived from the DIO message to enable suboptions alignment.  For N (N > 2) octets IPv6 address of padding, the Option Length field contains
         the value N-3, and DODAG root.

5.1.2.  DIO Base Rules

   1.  If the
   Option Data consists 'A' flag of N-3 zero-valued octets.  PadN Option data a DIO Base is cleared, the 'T' flag MUST also
       be ignored by cleared.

   2.  For the receiver.

6.1.3.1.1.4.  DAG Metric Container

   The DAG Metric Container suboption may be aligned as necessary to
   support its contents.  Its format following DIO Base fields, a node that is not a DODAG
       root MUST advertise the same values as follows:

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

                      Figure 8: DAG Metric Container

   The DAG Metric Container is used its preferred DODAG parent
       (defined in Section 5.3.2).  Therefore, if a DODAG root does not
       change these values, every node in a route to report aggregated path metrics
   along that DODAG root
       eventually advertises the DODAG.  The DAG Metric Container may contain a number of
   discrete node, link, and aggregate path metrics as chosen by same values for these fields.  These
       fields are:
       1.  Grounded (G)
       2.  Destination Advertisement Supported (A)
       3.  Destination Advertisement Trigger (T)
       4.  DAGPreference (Prf)
       5.  Sequence
       6.  RPLInstanceID
       7.  DODAGID

   3.  A node MAY update the
   implementer. following fields at each hop:
       1.  DAGRank
       2.  DTSN

   4.  The Container Length DODAGID field contains each root sets MUST be unique within the length in
   octets RPL
       Instance.

5.1.3.  DIO Suboptions

   This section describes the format of DIO suboptions and the five
   suboptions this document defines: Pad 1, Pad N, DAG Metric Data.  The order, content, Container,
   DAG Destination Prefix, and coding of the DAG Metric Container data is as specified in

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

5.1.3.1.  DIO Suboption Format

   The Pad N, DAG Metric Container MUST include the value for the Container, DAG Objective
   Code Point.

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

6.1.3.1.1.5.  Destination Prefix

   The Destination Prefix suboption does not have any alignment
   requirements.  Its format is as follows:
   Configuration suboptions all follow this 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 = 3    |            Length             |Resvd|Prf|Resvd|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                        Prefix Lifetime                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Prefix         Subopt Length         |                                               |
       +-+-+-+-+-+-+-+-+                                               |
       |             Destination Prefix (Variable Length)              |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Subopt Data
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -

                  Figure 9: DAG Destination Prefix

   The Destination Prefix suboption is used when the DODAG root, or
   another node located upwards along the DODAG on the path to the DODAG
   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 DODAG, a node MAY decide to join
   multiple DODAGs in support 5: DIO Suboption Generic Format

   Suboption Type:  8-bit identifier of a particular application.

   The Length is coded as the length type of suboption.

   Suboption Length:  16-bit unsigned integer, representing the suboption length
         in octets,
   excluding octets of the suboption, not including the suboption Type
         and Length fields.

   Prf is the Route Preference as in [RFC4191].  The reserved fields
   MUST be set to zero on transmission and MUST be ignored on receipt.

   The Prefix Lifetime is a 32-bit unsigned integer representing the

   Suboption Data:  A variable length of time in seconds (relative to the time the packet is sent) field that contains data specific
         to the Destination Prefix is valid for route determination. option.

   The
   lifetime is initially set by the node that owns the prefix and
   denotes following subsections specify the valid lifetime DIO message suboptions which
   are currently defined for that prefix (similar to
   AdvValidLifetime [RFC4861]).  The value might be reduced by use in the
   originator and/or en-route nodes that will not provide connectivity DAG Information Object.

   When processing a DIO message containing a suboption for which the whole valid lifetime.  A value of all one bits (0xFFFFFFFF)
   represents infinity.  A
   Suboption Type value of all zero bits (0x00000000) indicates
   a loss of reachability.

   The Prefix Length is an 8-bit unsigned integer that indicates not recognized by the
   number of leading bits in receiver, the destination prefix.

   The Destination Prefix contains Prefix Length significant bits of receiver
   MUST silently ignore the
   destination prefix.  The unrecognized option and continue to process
   the following suboption, correctly handling any remaining bits of options in
   the Destination Prefix, as
   required to complete message.

   DIO message suboptions may have alignment requirements.  Following
   the convention in IPv6, these options are aligned in a packet such
   that multi-octet values within the trailing octet, Option Data field of each option
   fall on natural boundaries (i.e., fields of width n octets are set to 0.

   In placed
   at an integer multiple of n octets from the event that a DIO message may need to specify connectivity to
   more than one destination, start of the Destination Prefix suboption may be
   repeated.

6.1.3.1.1.6.  DAG Configuration header, for
   n = 1, 2, 4, or 8).

5.1.3.2.  Pad1

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

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9
        0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+
       |   Type = 4    |            Length             | DIOIntDoubl.  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  DIOIntMin.   |   DIORedun.   |  MaxRankInc 0    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+

                              Figure 10: DAG Configuration

   The DAG Configuration suboption is used to distribute configuration
   information for DAG Operation through 6: Pad 1

   NOTE! the DODAG.  The information
   communicated in this suboption is generally static and unchanging
   within format of the DODAG, therefore it Pad1 option is not necessary to include in every
   DIO.  This suboption MAY be included occasionally by the DODAG Root,
   and MUST be included in response to a unicast request, e.g. a DAG
   Information Solicitation (DIS) message.

   The special case - it has
   neither Option Length nor Option Data fields.

   The Pad1 option is coded as 5.

   DIOIntervalDoublings is an 8-bit unsigned integer, configured on the
   DODAG root and used to configure the trickle timer governing when DIO
   message should be sent within the DODAG.  DIOIntervalDoublings is the
   number of times that the DIOIntervalMin is allowed to be doubled
   during the trickle timer operation.

   DIOIntervalMin is an 8-bit unsigned integer, configured on the DODAG
   root and used to configure insert one or two octets of padding in the trickle timer governing when
   DIO message to enable suboptions alignment.  If more than two octets
   of padding is required, the PadN option, described next, should be sent within the DODAG.
   used rather than multiple Pad1 options.

5.1.3.3.  PadN

   The minimum configured
   interval for the DIO trickle timer in units of ms is
   2^DIOIntervalMin.  For example, a DIOIntervalMin value of 16ms PadN option does not have any alignment requirements.  Its format
   is
   expressed as 4.

   DIORedundancyConstant follows:

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

                              Figure 7: Pad N

   The PadN option is an 8-bit unsigned integer used to configure
   suppression insert three or more octets of DIO transmissions.  DIORedundancyConstant is padding in
   the
   minimum number of relevant incoming DIOs required to suppress a DIO
   transmission.  If message to enable suboptions alignment.  For N (N > 2) octets
   of padding, the value is 0xFF then Option Length field contains the suppression mechanism is
   disabled.

   MaxRankInc, 8-bit unsigned integer, is value N-3, and the DAGMaxRankIncrease.  This
   is
   Option Data consists of N-3 zero-valued octets.  PadN Option data
   MUST be ignored by the allowable increase in rank in receiver.

5.1.3.4.  Metric Container

   The Metric Container suboption may be aligned as necessary to support of local repair.  If
   DAGMaxRankIncrease
   its contents.  Its format is as follows:

        0 then this mechanism is disabled.

6.1.4.  Destination Advertisement Object (DAO)                   1                   2
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
       |   Type = 2    |         Subopt Length         | Metric Data
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -

                        Figure 8: Metric Container

   The Destination Advertisement Object (DAO) Metric Container is used to propagate
   destination information upwards report aggregated path metrics along
   the DODAG.  The RPL use Metric Container may contain a number of discrete
   node, link, and aggregate path metrics as chosen by the
   DAO allows implementer.
   The Suboption Length field contains the nodes length in octets of the DODAG to provision routing state for
   nodes contained in
   Metric Data.  The order, content, and coding of the sub-DAG Metric Container
   data is as specified in support [I-D.ietf-roll-routing-metrics].

   The processing and propagation of traffic flowing down
   along the DODAG. Metric Container is governed by
   implementation specific policy functions.

5.1.3.5.  Destination Prefix

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

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         DAO Sequence          |  InstanceID   |   DAO Rank   Type = 3    |        Subopt Length          |Resvd|Prf|Resvd|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          DAO                        Prefix Lifetime                        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                           Route Tag                           |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | Prefix Length |    RRCount    |                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       +-+-+-+-+-+-+-+-+                                               |
       |             Destination Prefix (Variable Length)              |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Reverse Route Stack (Variable Length)             |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                     Figure 11: 9: DAG Destination Prefix

   The Destination Advertisement Object (DAO)

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

   InstanceID:  8-bit field indicating the topology instance associated
         with the DODAG, as learned from the DIO.

   DAO Rank:  Set by Prefix suboption is used when the DODAG root, or
   another node that owns located upwards along the prefix and first issues DODAG on the
         DAO message to its rank.

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

   Route Tag:  32-bit unsigned integer.  The Route Tag may be used DODAG
   root, needs to
         give a priority indicate that it offers connectivity to destination
   prefixes that should be stored. other than the default.  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
   more than one LBR is under investigation.

   Prefix Length:  8-bit unsigned integer.  Number of valid leading bits
         in operating within the IPv6 Prefix.

   RRCount:  8-bit unsigned integer.  This counter is used LLN and offering
   connectivity to count different administrative domains, e.g. a home network
   and a utility network.  In such cases, upon observing the
         number of entries Destination
   Prefixes offered by a particular DODAG, a node MAY decide to join
   multiple DODAGs in the Reverse Route Stack.  A value support of `0'
         indicates that no Reverse Route Stack is present.

   Prefix:  Variable-length field containing an IPv6 address or a prefix
         of an IPv6 address. particular application.

   The Prefix Suboption Length field contains is coded as the
         number length of valid leading bits in the prefix.  The bits suboption in
   octets, excluding the
         prefix after Type and Length fields.

   Prf is the prefix length (if any) are Route Preference as in [RFC4191].  The reserved and fields
   MUST be set to zero on transmission and MUST be ignored on receipt.

   Reverse Route Stack:  Variable-length field containing

   The Prefix Lifetime is a sequence of
         RRCount (possibly compressed) IPv6 addresses.  A node that adds
         on to 32-bit unsigned integer representing the Reverse Route Stack will append
   length of time in seconds (relative to the list and
         increment time the RRCount.

6.2.  Protocol Elements

6.2.1.  Topological Elements

   RPL uses four identifiers to track and control packet is sent)
   that the routing topology

   o Destination Prefix is valid for route determination.  The first
   lifetime is an InstanceID.  An InstanceID defines what OF a DAG
      uses initially set by the node that owns the prefix and may also indicate what destinations are offered.  A
      network may have multiple InstanceIDs, each of which defines an
      independent DAG optimized
   denotes the valid lifetime for a different OF and/or application. that prefix (similar to
   AdvValidLifetime [RFC4861]).  The DAG defined value might be reduced by an InstanceID is called a DAG Instance.

   o  The second is a DAGID.  The scope the
   originator and/or en-route nodes that will not provide connectivity
   for the whole valid lifetime.  A value of a DAGID is a DAG Instance. all one bits (0xFFFFFFFF)
   represents infinity.  A
      combination value of InstanceID and DAGID defines all zero bits (0x00000000) indicates
   a DODAG.  A DAG
      Instance may have multiple DODAGs.

   o loss of reachability.

   The third value Prefix Length is a DAG Sequence Number. an 8-bit unsigned integer that indicates the
   number of leading bits in the destination prefix.

   The scope Destination Prefix contains Prefix Length significant bits of a DAG
      Sequence Number is a DODAG.  A DODAG is sometimes reconstructed
      from the root, by incrementing
   destination prefix.  The remaining bits of the Destination Prefix, as
   required to complete the trailing octet, are set to 0.

   In the DAGSequenceNumber.  A
      combination of InstanceID, DAGID, and DAG Sequence Number defines event that a DIO message may need to specify connectivity to
   more than one destination, the Destination Prefix suboption may be
   repeated.

5.1.3.6.  DODAG Iteration.

   o Configuration

   The fourth value DODAG Configuration suboption does not have any alignment
   requirements.  Its format is rank. as follows:

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Type = 4    |            Length             | DIOIntDoubl.  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |  DIOIntMin.   |   DIORedun.   |  MaxRankInc   | MinHopRankInc |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 10: DODAG Configuration

   The scope of rank is a DODAG Iteration.
      Rank establishes a partial order over a Configuration suboption is used to distribute configuration
   information for DODAG Iteration, defining
      individual node positions.

6.2.2.  Neighbors, Parents, Operation through the DODAG.  The information
   communicated in this suboption is generally static and Siblings

   1.  A node that unchanging
   within the DODAG, therefore it is not a DODAG root necessary to include in every
   DIO.  This suboption MAY maintain multiple DAG parents
       for a single DAG Instance.

   2.  The set of DAG parents MUST be a conceptual subset of included occasionally by the set of
       candidate neighbors.  (This does not dictate implementation,
       e.g., DODAG Root,
   and MUST be included in response to use a certain data structure).

   3.  If Neighbor Unreachability Detection (NUD), or an equivalent
       mechanism, determines that unicast request, e.g. a neighbor DODAG
   Information Solicitation (DIS) message.

   The Length is no longer reachable,
       then a RPL node MUST NOT consider this node in coded as 5.

   DIOIntervalDoublings is an 8-bit unsigned integer, configured on the neighbor set
       when calculating
   DODAG root and advertising routes until used to configure the node determines
       it is reachable again.

   4.  Routes via that unreachable neighbor MUST trickle timer (see
   Section 5.3.5.1 for details on trickle timers) governing when DIO
   message should be eliminated from sent within the
       routing table, and DODAG.  DIOIntervalDoublings is the node SHOULD poison using no-DAO all DAO
       routes that it has advertised via DAO and that it can reach only
       via
   number of times that neighbor.

   A node's neighbor set the DIOIntervalMin is an unconstrained subset of allowed to be doubled
   during the nodes that it
   can reach with a link-local multicast.

   The OF guides in trickle timer operation.

   DIOIntervalMin is an 8-bit unsigned integer, configured on the selection DODAG
   root and maintains a number of neighbors used to
   interact with, which neighbors being qualified as statistically
   stable and presenting adequate properties as per configure the trickle timer governing when DIO
   message should be sent within the OF logic, DODAG.  The minimum configured
   interval for instance following mechanisms discussed in
   [I-D.ietf-roll-routing-metrics].  Those neighbors are referred to as
   candidate neighbors.

   Candidate neighbors may take the role of Parent or Siblings, DIO trickle timer in part
   as determined by rank. units of ms is
   2^DIOIntervalMin.  For example, a DIOIntervalMin value of 16ms is
   expressed as 4.

   DIORedundancyConstant is an 8-bit unsigned integer used to configure
   suppression of DIO transmissions.  DIORedundancyConstant is the purpose
   minimum number of inheriting metrics and computing rank, relevant incoming DIOs required to suppress a DIO
   transmission.  If the OF
   might select one preferred parent.  In that case, value is 0xFF then the suppression mechanism is
   disabled.

   MaxRankInc, 8-bit unsigned integer, is the DAGMaxRankIncrease.  This
   is the allowable increase in rank in support of local repair.  If
   DAGMaxRankIncrease is 0 then this
   node mechanism is disabled.

   MinHopRankInc, 8-bit unsigned integer, is computed as the rank of the preferred parent plus a rank
   increment MinHopRankIncrease as determined by the OF.

6.2.3.
   described in Section 3.6.2.1.

5.2.  DODAG Information

   For each Solicitation (DIS)

   The DODAG Information Solicitation (DIS) message may be used to
   solicit a DODAG Information Object from a RPL node.  Its use is
   analogous to that of a Router Solicitation; a node is, or may become, a member of, the
   implementation should conceptually keep track of the following
   information use DIS to
   probe its neighborhood for each DODAG. nearby DODAGs.  The data structures described in this
   section are intended DODAG Information
   Solicitation carries no additional message body.  Section 5.3.5
   describes how nodes respond to illustrate a possible implementation DIS.

5.3.  Upward Route Discovery and Maintenance

   Upward route discovery allows a node to aid
   in the description of the protocol, but join a DODAG by discovering
   neighbors that are not intended to be
   normative.

   o  InstanceID

   o  DAGID

   o  DAGSequenceNumber

   o  DAG Metric Container, including DAGObjectiveCodePoint
   o  A set members of Destination Prefixes offered upwards along the DODAG

   o  A and identifying a set of DAG
   parents.  The exact policies for selecting neighbors and parents

   o  A is
   implementation-dependent.  This section specifies the set of DAG siblings

   o rules
   those policies must follow for interoperability.

5.3.1.  RPL Instance

      A timer RPLInstanceID MUST be unique across an LLN.

      A node MAY belong to govern the sending multiple RPL Instances.

   Within a given LLN, there may be multiple, logically independent RPL
   instances.  This document describes how a single instance behaves.

5.3.2.  Neighbors and Parents within a DODAG Iteration

   RPL's upward route discovery algorithms and processing are in terms
   of DIO messages

   When three logical sets of link-local nodes.  First, the DAG parent candidate
   neighbor set is depleted on a node that is not a root,
   (i.e. the last parent is removed), then subset of the DAG information should
   not nodes that can be suppressed until after the expiration of an implementation-
   specific reached via link-
   local timer in order to observe that the DAGSequenceNumber
   has incremented should any new parents appear for the DODAG.

6.2.3.1.  DAG Parents/Siblings Structure

   When the DODAG multicast.  The selection of this set is self-rooted, implementation-
   dependent and OF-dependent.  Second, the parent set of DAG parents is empty.

   For each node in a DAG parent/sibling set, the implementation should
   conceptually keep track of:

   o  a reference to restricted
   subset of the neighboring device which is candidate neighbor set.  Finally, the DAG parent/
      sibling

   o preferred parent,
   a record set of most recent information taken from the DAG Information
      Object last processed in the case where the neighboring device size one, is
      a DAG parent

   DAG parents may be ordered, according to the OF.  When ordering DAG
   parents, in consultation with an element of the OF, parent set that is the most
   preferred DAG next hop in upward routes.

   More precisely:

   1.  The DODAG parent
   may set MUST be identified.  All current DAG parents must have a rank less
   than self.  All current DAG siblings must subset of the candidate neighbor
       set.

   2.  A DODAG root MUST have a rank DODAG parent set of size zero.

   3.  A node that is not a DODAG root MAY maintain a DODAG parent set
       of size greater than or equal to self.

   When nodes are added to or removed from the DAG parent/sibling sets
   the most one.

   4.  A node's preferred DAG parent may have changed.  The role of all the
   nodes in the list should be reevaluated.  In particular, any nodes
   having DODAG parent MUST be a member of its DODAG
       parent set.

   5.  A node's rank MUST be greater than self after such all elements of its DODAG
       parent set.

   6.  When Neighbor Unreachability Detection (NUD), or an equivalent
       mechanism, determines that a change must neighbor is no longer reachable, a
       RPL node MUST NOT consider this node in the candidate neighbor
       set when calculating and advertising routes until it determines
       that it is again reachable.  Routes through an unreachable
       neighbor MUST be evicted eliminated from the set.

6.3.  DAG Discovery and Maintenance

   DAG discovery allows routing table.

   These rules ensure that there is a consistent partial order on nodes
   within the DODAG.  As long as node ranks do not change, following the
   above rules ensures that every node's route to join a DODAG rooted at a DODAG root by
   discovering neighbors that are members of the DODAG, and identifying
   a set of parents.  DAG discovery also identifies siblings, which may
   be used later is loop-
   free, as rank decreases on each hop to provide additional path diversity towards the DODAG root.

   DODAG discovery may avoid loops by constraining how and when nodes  The OF can increase their rank, guide
   candidate neighbor set and by statistically poisoning the nodes
   that present the highest risk.

   DAG discovery enables nodes to implement different policies for
   selecting their DAG parents parent set selection, as discussed in the
   [I-D.ietf-roll-routing-metrics].

5.3.3.  Neighbors and Parents across DODAG by using implementation
   specific policy functions.  DAG discovery specifies a set of Iterations

   The above rules to
   be followed by all implementations to enable interoperation.

6.3.1.  DAG Discovery Rules govern a single DODAG iteration.  The following rules in this
   section define the how RPL DAG Discovery procedures:

6.3.1.1. operates when there are multiple DODAG
   iterations:

5.3.3.1.  DODAG Iteration

   1.   An InstanceID SHOULD be administratively provisioned on  The tuple (RPLInstanceID, DODAGID, DODAGSequenceNumber) uniquely
       defines a DODAG
        root that is significant RPL objective.  The InstanceID Iteration.  Every element of a node's DODAG
       parent set, as conveyed by the last heard DIO from each DODAG
       parent, MUST be
        unique belong to that purpose across the scope same DODAG iteration.  Elements of the LLN. a
       node's candidate neighbor set MAY belong to different DODAG
       Iterations.

   2.  A DAGID MUST be unique within the scope node is a member of a DODAG iteration if every element of its
       DODAG parent set belongs to that DODAG iteration, or if that node
       is the InstanceID.  It
        MAY be derived from the IPv6 address root of the DODAG root. corresponding DODAG.

   3.  A node MAY belong to multiple DAG instances.  The related
        details of operation are outside the scope MUST NOT send DIOs for DODAG iterations of this
        specification. which it is not
       a member.

   4.  DODAG roots MAY increment the DAGSequenceNumber DODAGSequenceNumber that they
        advertise.

   5.
       advertise and thus move to a new DODAG iteration.  When a DODAG
       root increments its DAGSequenceNumber, DODAGSequenceNumber, it MUST follow the
       conventions of Serial Number Arithmetic as described in
       [RFC1982].

   6.   The tuple (InstanceID, DAGID, DAGSequenceNumber) uniquely
        defines a DODAG Iteration.  All of

   5.  Within a node's parents within given DODAG, a
        DODAG MUST belong to the same DODAG iteration, as conveyed by
        the last heard DIO from each parent.

   7.   A node that is a not a root MUST NOT propagate DIOs for
       advertise a DODAG Iteration unless DODAGSequenceNumber higher than the highest
       DODAGSequenceNumber it has heard.  Higher is defined as the DODAG root of the DODAG iteration or has selected DODAG
        parents
       greater-than operator in that DODAG iteration.

   8.   A [RFC1982].

   6.  Once a node acting as has advertised a leaf SHOULD DODAG iteration by sending a DIO, it
       MUST NOT propagate DIOs for be member of a previous DODAG
        Iteration.

   9.   A node MUST belong at most to one iteration of the same
       DODAG Iteration per
        InstanceID.

   10.  Within a given DODAG, (i.e. with the same DODAGID and a node that lower
       DODAGSequenceNumber).  Lower is a not a root MUST NOT
        advertise a DAGSequenceNumber higher than defined as the highest
        DAGSequenceNumber it has heard. less-than operator
       in [RFC1982].

   Within a particular implementation, a DODAG root may increment the
   DAGSequenceNumber
   DODAGSequenceNumber periodically, at a rate that depends on the
   deployment.  In other implementations implementations, loop detection may be
   considered sufficient to solve the routing issues, and the DODAG root may
   increment the DAGSequenceNumber DODAGSequenceNumber only upon administrative
   intervention.  Another possibility is that nodes within the LLN have
   some means to by which they can signal detected routing inconsistencies
   or suboptimalities to the DODAG root root, in order to request an on-demand on-
   demand DODAGSequenceNumber increment when routing issues are detected. (i.e. request a global repair of
   the DODAG).

   When the DODAG parent set is depleted on a node that is not a root,
   (i.e. the last parent is removed), then the DODAG information should
   not be suppressed until after the expiration of an implementation-
   specific local timer in order to observe if the DODAGSequenceNumber
   has been incremented, should any new parents appear for the DODAG.

   As the DAGSequenceNumber DODAGSequenceNumber is incremented, a new DODAG Iteration
   spreads outward from the DODAG root.  Thus a parent that advertises
   the new DAGSequenceNumber DODAGSequenceNumber can not possibly belong to the sub-DAG sub-DODAG
   of a node that still advertises an older DAGSequenceNumber. DODAGSequenceNumber.  A node
   may safely add such a parent, without risk of forming a loop, without
   regard to its relative rank in the prior DODAG Iteration.  This is
   equivalent to jumping to a different DODAG.

   As a node transitions to new DODAG Iterations as a consequence of
   following these rules, the node will be unable to advertise the
   previous DODAG Iteration (prior DAGSequenceNumber) DODAGSequenceNumber) once it has
   committed to advertising the new DODAG Iteration.

   During a transition to a new DODAG Iteration, a node may decide to
   forward packets via 'future parents' that belong to the same DODAG
   (same InstanceID RPLInstanceID and DAGID), DODAGID), but are observed to advertise a
   more recent (incremented) DAGSequenceNumber.

6.3.1.2. DODAGSequenceNumber.

5.3.3.2.  DODAG Roots

   1.  A DODAG root that does not have connectivity to a network outside
       of the LLN set of
       addresses described as application-level goals, MUST NOT set the
       Grounded bit.

   2.  A DODAG root MUST advertise a rank of ROOT_RANK.

   3.  A node that does not have any whose DODAG parent set is empty MAY become the DODAG root
       of a floating DODAG.  It MAY also set its DAGPreference such that
       it is less preferred.  This behavior may be a desired
       alternate to poisoning.

   An LLN node that is a Goal goal for the Objective Function is the root of
   its own grounded DODAG, at rank ROOT_RANK.

   In a deployment that uses a backbone link to federate a number of LLN
   roots, it is possible to run RPL over the that backbone and use one
   router as a backbone root. "backbone root".  The backbone root is the virtual root
   of the
   DODAG DODAG, and exposes a rank of BASE_RANK over the backbone.  All
   the LLN roots that are parented to that backbone root, including the
   backbone root if it also serves as LLN root, root itself, expose a rank of
   ROOT_RANK over to the LLN LLN, and are part of the same DODAG, coordinated coordinating
   DODAGSequenceNumber and other DODAG root determined parameters with
   the virtual root over the backbone.

6.3.1.3.

5.3.3.3.  DODAG Selection

   The DODAGPreference (Prf) provides an administrative mechanism to
   engineer the self-organization of the LLN, for example indicating the
   most preferred LBR.  If a node has the option to join a more
   preferred DODAG while still meeting other optimization objectives,
   then the node will generally seek to join the more preferred DODAG as
   determined by the OF.

5.3.3.4.  Rank and Movement within a DODAG Iteration

   1.  A node MUST NOT advertise a rank less than or equal to any member
       of its parent set within the DODAG Iteration.

   2.  A node MAY advertise a rank lower than its prior advertisement
       within the DODAG Iteration.  (This corresponds to a node moving
       up within the DODAG Iteration).

   3.  Let L be the lowest rank within a DODAG iteration that a given
       node has advertised.  Within a the same DODAG Iteration, that node
       MUST NOT advertise an effective rank deeper higher than L +
       DAGMaxRankIncrease.  INFINITE_RANK is an exception to this rule:
       a node MAY advertise an INFINITE_RANK at any time.  (This
       corresponds to a limited rank increase for the purpose of local
       repair within the DODAG Iteration.)

   4.  A node MAY, at any time, choose to join a different DODAG within
       a DAG RPL Instance.  Such a join has no rank restrictions, unless
       that different DODAG is a DODAG Iteration of which that the node has
       previously been a prior member of, member, in which case the rule of the previous
       bullet (3) must be observed.  Until a node transmits a DIO
       indicating its new DODAG membership, it MUST forward packets
       along the previous DODAG.

   5.  A node MAY, at any time after hearing the next DAGSequenceNumber
       DODAGSequenceNumber Iteration advertised from suitable DODAG
       parents, choose to migrate up to the next DODAG Iteration within the
       DODAG.

   Conceptually, an implementation is maintaining a DODAG parent set
   within the DODAG Iteration.  Movement entails changes to the DODAG
   parent set.  Moving up does not present the risk to create a loop but
   moving down might, so that operation is subject to additional
   constraints.

   When a node migrates into to the next DODAG Iteration, the DODAG parent
   and sibling sets need to be rebuilt for the new iteration.  An
   implementation could defer to migrate until for some reasonable time amount of
   time, to see if some other neighbors with potentially better metrics
   but higher rank announce themselves.  Similarly, when a node jumps
   into a new DODAG it needs to construct new DODAG parent/sibling sets
   for the this new DODAG.

   When a node moves to improve its position, it must conceptually
   abandon all DODAG parents and siblings with a rank larger than
   itself.  As a consequence of the movement it may also add new
   siblings.  Such a movement may occur at any time to decrease the
   rank, as per the calculation indicated by the OF.  Maintenance of the
   parent and sibling sets occurs as the rank of candidate neighbors is
   observed as reported in their DIOs.

   If a node needs to move down a DODAG that it is attached to, causing
   the DAG rank to increase, then it MAY poison its routes and delay before
   moving as described in Section 6.3.1.4.

6.3.1.4. 5.3.3.5.

5.3.3.5.  Poisoning a Broken Path

   1.  A node MAY poison, in order to avoid being used as an ancestor by
       the nodes in its sub-DAG, sub-DODAG, by advertising an effective rank of
       INFINITE_RANK and resetting the associated DIO trickle timer to
       cause the this INFINITE_RANK to be announced promptly.

   2.  The node MAY advertise an effective rank of INFINITE_RANK for an
       arbitrary number of DIO timer events events, before announcing a new
       rank.

   3.  As per Section 6.3.1.3, 5.3.3.4, the node MUST advertise INFINITE_RANK
       within the DODAG iteration in which it participates, if its
       revised rank would exceed the maximum DAG rank increase.

   An implementation may choose to employ this poisoning mechanism when
   a node that loses all of its current parents, i.e. the set of DAG DODAG
   parents becomes depleted, and it can not jump onto to an alternate DODAG DODAG.
   An alternate mechanism is to form a floating DODAG.

   The motivation for delaying announcement of the revised route through
   multiple DIO events is to (i) increase tolerance to DIO loss, (ii)
   allow time for the poisoning action to propagate, and (iii) to
   develop an accurate assessment of its new rank.  Such gains are
   obtained at the expense of potentially increasing the delay before
   lower
   portions of the network are able to re-establish up upwards routes.
   Path redundancy in the DAG DODAG reduces the significance of either
   effect, since children with alternate parents should be able to
   utilize those alternates and retain their rank while the detached
   parent re-establishes its rank.

   Although an implementation may advertise INFINITE_RANK for the
   purposes of poisoning, it is not expected to be equivalent to setting
   the rank to INFINITE_RANK, and an implementation would likely retain
   its rank value prior to the poisoning in some form, for purpose of
   maintaining its effective position within (L + DAGMaxRankIncrease).

6.3.1.5.

5.3.3.6.  Detaching

   1.  A node that does not have a solution unable to stay connected to a DODAG within a given DODAG
       iteration MAY detach from its current this DODAG iteration.  A node that
       detaches becomes root of its own floating DODAG and SHOULD
       immediately advertise its this new situation in a DIO as an alternate
       to poisoning.

6.3.1.6.

5.3.3.7.  Following a Parent

   1.  If a node receives a DIO from one of its parents DODAG parents,
       indicating that the parent has left the DODAG, it that node SHOULD
       stay in its current DODAG through an alternate DAG parent alternative DODAG parent, if that is
       possible.  It MAY follow that the leaving parent.

   A DAG DODAG parent may have moved, migrated forward into to the next DODAG Iteration,
   or jumped to a different DODAG.  A node should give some preference
   to remaining in the current DODAG DODAG, if possible, but ought to follow
   the parent if there are no other options.

6.3.2.

5.3.4.  DIO Message Communication

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

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

   2.  If SRC the sender of the DIO message is a member of the candidate
       neighbor set, then the DIO is eligible for further processing.

6.3.2.1.

5.3.4.1.  DIO Message Processing

      If the node has sent an DIO message within the risk window as
      described in Section 6.7 then a collision has occurred; do not
      process the DIO message any further.

      Process the DIO message as per the rules in Section 6.3

   As DIO messages are received from candidate neighbors, the neighbors
   may be promoted to DAG DODAG parents by following the rules of DAG DODAG
   discovery as described in Section 6.3. 5.3.  When a node places a neighbor
   into the DAG Parent DODAG parent set, the node becomes attached to the DODAG
   through the new DODAG parent node.

   In the DAG discovery implementation, the

   The most preferred parent should be used to restrict which other
   nodes may become DAG DODAG parents.  Some nodes in the DAG DODAG parent set
   may be of a rank less than or equal to the most preferred DAG DODAG
   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).

6.3.3.

5.3.5.  DIO Transmission

   Each node maintains a timer timer, that governs when to multicast DIO
   messages.  This timer is a trickle timer, as detailed in
   Section 6.3.4. 5.3.5.1.  The DIO Configuration Option includes the
   configuration of a DAG RPL Instance's trickle timer.

   o  When a node detects or causes an inconsistency, it MUST reset the
      interval of the trickle timer to a its minimum value.

   o  When a node migrates to a new DODAG Iteration it MUST reset the
      trickle timer to its minimum value

   o  When a node detects an inconsistency when forwarding a packet, as
      detailed in Section 6.9, 7.2, the node MUST reset the trickle timer to
      its minimum value.

   o  When a node receives a multicast DIS message, it MUST reset the
      trickle timer to the its minimum value.

   o  When a node receives a unicast DIS message, it MUST unicast a DIO
      message in response, and MUST include the DAG DODAG Configuration
      Object.  In this case the node SHOULD NOT reset the trickle timer.

   o  If a node is not a member of a DODAG, it MUST suppress
      transmitting
      transmission of DIO messages.

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

6.3.4.

5.3.5.1.  Trickle Timer for DIO Transmission

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

   For each DODAG that a node is part of, of (i.e. one DODAG per RPL
   Instance), 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 message as (2^DIOIntervalMin)ms.
         The default value is DEFAULT_DIO_INTERVAL_MIN.

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

6.3.4.1.

5.3.5.1.1.  Resetting the Trickle Timer

   The trickle timer for a DODAG is reset by:

   1.  Setting I_min and I_doublings to the values learned from the
       DODAG root via a received DIO message.

   2.  Setting C to zero.

   3.  Setting I to I_min.

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

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

   When a node learns about a DODAG through a DIO message message, and makes the
   decision to join it, this DODAG, it initializes the state of the trickle
   timer by resetting the trickle timer and listening.  Each time it
   hears a redundant DIO message for this DODAG, it MAY increment C. The
   exact determination of what constitutes a redundant DIO message is
   left to an implementation; it could for example include DIOs that
   advertise the same rank.

   When the timer fires at time T, the node compares C to the redundancy
   constant, DIORedundancyConstant.  If C is less than that value, or if
   the DIORedundancyConstant value is 0xFF, the node generates a new DIO
   message and multicasts it.  When the communication interval I
   expires, the node doubles the interval I so long as it has previously
   doubled it fewer than I_doubling times, resets C, and chooses a new T
   value.

6.3.4.2.

5.3.5.1.2.  Determination of Inconsistency

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

   o  The node joins a new DODAG

   o  The node moves within a DODAG

   o  The node receives a modified DIO message from a DAG DODAG parent

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

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

   o  The rank of a DAG DODAG parent has changed.

6.4.  DAG

5.3.6.  DODAG Selection

   The DAG DODAG selection is implementation and algorithm dependent.  Nodes
   SHOULD prefer to join DODAGs for InstanceIDs RPLInstanceIDs 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 parent whose availability is
   detected as fluctuating, at least when more stable choices are
   available.

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

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

6.5.

5.4.  Operation as a Leaf Node

   In some cases it a RPL node may attach to a DODAG for DAG Instance as a leaf node only; the node in this case is not to extend connectivity
   to the DODAG to other nodes under any circumstances.  Such only.
   One example of such a case may
   occur, for example, is when a node is attaching to a DODAG that is using
   an unknown Objective Function.  When operating as a does not understand the RPL
   Instance's OF.  A leaf node, a
   node:

   1.  MAY receive and process DIOs for that DODAG

   2.  SHOULD NOT transmit DIOs for that node does not extend DODAG

   3. connectivity but
   still needs to advertise its presence using DIOs.  A node operating
   as a leaf node must obey the following rules:

   1.  It MUST NOT transmit DIOs containing the DAG Metric Container for
       that DODAG

   4. Container.

   2.  Its DIOs must advertise a DAGRank of INFINITE_RANK.

   3.  It MAY transmit unicast DAOs to the chosen parents for that DODAG

   5. as described in Section 6.2.

   4.  It MAY transmit multicast DAOs to the `1 '1 hop' neighborhood.

6.6. neighborhood as
       described in Section 6.2.9.

5.5.  Administrative rank

   When the DODAG is formed under a common administration, or when a
   node performs a certain role within a community, Rank

   In some cases it might be beneficial to associate a range of acceptable adjust the rank with advertised by
   a node beyond that computed by the OF based on some implementation
   specific policy and properties of the node.  For instance, example, 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 OF in
   order to expose an exaggerated rank.

6.7.

5.6.  Collision

   A race condition occurs if 2 nodes send DIO messages at the same time
   and then attempt to join each other.  This might happen, for example,
   between nodes which act as DAG DODAG root of their own DODAGs.  In order
   to detect the situation, LLN Nodes time stamp the sending of DIO
   message.  Any DIO message received within a short link-layer-
   dependent period introduces a risk.  It left to the implementation to
   define the duration of the risk window.

   There is risk of a collision when a node receives and processes a DIO
   within the risk window.  For example, it may occur that two nodes are
   associated with different DODAGs and near-simultaneously send DIO
   messages, which are received and processed by both, and possibly
   result in both nodes simultaneously deciding to attach to each other.
   As a remedy, in the face of a potential collision, as determined by
   receiving a DIO within the risk window, the DIO message is not
   processed.  It is expected that subsequent DIOs would not cross.

6.8.  Establishing Routing State Down the DODAG

   The destination advertisement mechanism supports

6.  Downward Routes

   This section describes how RPL discovers and maintains downward
   routes.  Messages containing the dissemination of
   routing state required Destination Advertisement Object
   (DAO), used to construct downward routes, are described.  The
   downward routes are necessary in support traffic flows down along the DODAG, of P2MP flows, from the
   DODAG root roots toward nodes.

   As a result of destination advertisement operation:

   o  Destination advertisement establishes down routes along the DODAG.
      Such paths consist of:
      *  Hop-By-Hop routing state within islands leaves.  It specifies non-storing and storing
   behavior of `stateful' nodes.
      *  Source Routing `bridges' across nodes that 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 respect to DAO messaging and learn hop-by-hop DAO routing state
      toward destinations by populating table
   entries.  Nodes, as according to their routing tables with resources and the
      routes
   implementation, may selectively store routing table entries learned from nodes in their sub-DAG.  In this process they
   from DAO messages, or may also learn necessary piecewise source routes to traverse
      regions of instead propagate the LLN that do not maintain DAO information
   upwards while adding source routing state.  They information.  A further
   optimization is described whereby DAO messages may
      perform route aggregation on known destinations before emitting
      Destination Advertisements.

   o  When nodes are incapable of storing be used to
   populate routing state, they table entries for the '1-hop' neighbors, which may
      forward
   be useful in some cases as a shortcut for P2P flows.

6.1.  Destination Advertisement Object (DAO)

   The Destination Advertisement Object (DAO) is used to propagate
   destination advertisements, recording information upwards along the reverse route as DODAG.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |         DAO Sequence          |           DAO Rank            |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | RPLInstanceID |   Route Tag   | Prefix Length |    RRCount    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                          DAO Lifetime                         |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Destination Prefix (Variable Length)              |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |             Reverse Route Stack (Variable Length)             |
       .                                                               .
       .                                                               .
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   sub-option(s)...
       +-+-+-+-+-+-+-+-+

           Figure 11: The Destination Advertisement Object (DAO)

   DAO Sequence:  16-bit unsigned integer.  Incremented by the go in order to support node that
         owns the construction of piecewise source
      routes.

   Nodes prefix for each new DAO message for that are capable of storing routing state, and finally prefix.

   DAO Rank:  16-bit unsigned integer indicating the
   DODAG roots, are able DAO Rank associated
         with the advertised Destination Prefix.  The DAO Rank is
         analogous to learn which destinations are contained the Rank in the sub-DAG below DIO message in that it may be used
         to convey a relative distance to 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 Destination Prefix as
         computed by the DODAG root down Objective Function in use over the DODAG.
   The  It
         serves as a mechanism is further enhance by supporting which an ancestor node may order
         alternate DAO paths.

   RPLInstanceID:  8-bit field indicating the construction of
   source routes across stateless `gaps' in topology instance
         associated with the DODAG, as learned from the DIO.

   Route Tag:  8-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
   incapable capable of storing additional
         a limited amount of routing state.  An adaptation  The further specification
         of this
   mechanism allows for the implementation of loose-source routing.

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

   A design choice behind advertising routes via destination
   advertisements use is not to synchronize the parent and children
   databases along under investigation.

   Prefix Length:  8-bit unsigned integer.  Number of valid leading bits
         in the DODAG, but instead to update them regularly IPv6 Prefix.

   RRCount:  8-bit unsigned integer.  This counter is used to
   recover from count the loss
         number of packets.  The rationale for that choice is
   time variations entries in connectivity across unreliable links.  If the
   topology can be expected to change frequently, synchronization might
   be an excessive goal in terms Reverse Route Stack.  A value of exchanges and protocol complexity.
   The approach used here results in a simple protocol with '0'
         indicates that no real
   peering. Reverse Route Stack is present.

   DAO Lifetime:  32-bit unsigned integer.  The destination advertisement mechanism hence provides for
   periodic updates length of the routing state, similarly to other protocols
   such as RIP [RFC2453].

6.8.1.  Destination Advertisement Operation

6.8.1.1.  Overview

   According time in
         seconds (relative to implementation specific policy, 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 subset loss of reachability.

   Destination Prefix:  Variable-length field identifying an IPv6
         destination address, prefix, or all multicast group.  The Prefix
         Length field contains the number of valid leading bits in the
   feasible parents
         prefix.  The bits in the DODAG may be selected to receive prefix
   information from after the destination advertisement mechanism.  This
   subset of DAG parents shall prefix length (if
         any) are reserved and MUST be designated the set of DA parents.

   As DAO messages for particular destinations move up the DODAG, a
   sequence counter is used to guarantee their freshness.  The zero on transmission and
         MUST be ignored on receipt.

   Reverse Route Stack:  Variable-length field containing a sequence
   counter is incremented by the source of the DAO message (the
         RRCount (possibly compressed) IPv6 addresses.  A node that owns adds
         on to the prefix, or learned Reverse Route Stack will append to the prefix via some other means),
   each time it issues a DAO message for its prefix.  Nodes that receive list and
         increment the RRCount.

6.1.1.  DAO message and, if scope allows, will be forwarding a Suboptions

   The DAO message for the unmodified destination up the DODAG, will leave the
   sequence number unchanged.  Intermediate nodes will check the
   sequence counter before processing may optionally include a number of suboptions.

   The DAO message, and if the DAO is
   unchanged (the sequence counter has not changed), then suboptions are in the DAO
   message will be discarded without additional processing.  Further, if same format as the DIO Suboptions
   described in Section 6.1.1.

   In particular, a DAO message appears to may include a DAG Metric Container
   suboption as described in Section 5.1.3.4.  This suboption may be out of synch (the sequence counter is 2
   or more behind the
   present value) then in implementations where the DAO state Rank is considered insufficient to
   optimize a path to
   be stale and may be purged, and the DAO message is discarded.  The
   rank is also added for tracking purposes; nodes Destination Prefix.

6.2.  Downward Route Discovery and Maintenance

6.2.1.  Overview

   Destination Advertisement operation produces DAO messages that are storing flow
   up the DODAG, provisioning downward routing state may use it to determine which possible next-hops for
   the destination are more optimal.

   If destination advertisements are activated
   prefixes available in the DIO message as
   indicated by the `D' bit, the node sends unicast destination
   advertisements to one sub-DODAG of its DA parents, that is selected as most
   favored for incoming down traffic. the DODAG root, and possibly
   other nodes.  The node only accepts unicast
   destination advertisements from any nodes but those contained routing state provisioned with this mechanism is in
   the
   DA parent subset.

   Receiving a DIO message with form of soft-state routing table entries.  DAO messages are able
   to record loose source routing information as by propagate up the `D' destination advertisement bit
   set from a DAG parent stimulates
   DODAG.  This mechanism is flexible to support the sending provisioning of a delayed destination
   advertisement back, with
   paths which consist of fully specified source routes, piecewise
   source routes, or hop-by-hop routes as according to the collection
   implementation and the capabilities of all known prefixes (that
   is the prefixes learned via destination advertisements for nodes
   lower in nodes.

   Destination Advertisement may or may not be enabled over a DODAG
   rooted at a DODAG root.  This is an a priori configuration determined
   by the DODAG, implementation/deployment and any connected prefixes).  If not generally changed during the
   operation of the RPL LLN.

   When Destination Advertisement Supported (A) bit is set enabled:

   1.  Some nodes in the DIO message LLN MAY store at least one routing table entry
       for the
   DODAG, then a particular destination advertisement is also sent to learned from a DAG parent
   once it has been added to the DA parent set after DAO.  Such a movement, or when
   the list of advertised prefixes has changed.

   A node is
       termed a 'storing node', with respect to that modifies its DAG Parent set may set the `D' bit in
   subsequent DIO propagation in order particular
       destination.

   2.  Some nodes are capable to trigger store at least one routing table entry
       for every unique destination
   advertisements to observed from all DAOs that pass
       through.  Such a node is termed a 'fully storing node'.

   3.  DODAG roots nodes SHOULD be updated to its DAG Parents and other ancestors
   on the DODAG.  Additional recommendations and guidelines regarding fully-storing nodes.

   4.  Other nodes in the use of this mechanism DODAG are still under consideration and will be
   elaborated not required to store routing table
       entries for any particular destinations observed in DAOs.  Nodes
       that do not store routing table entries from DAOs are termed
       'non-storing nodes', with respect to a future revision particular destination.

   5.  Non-storing nodes MUST participate in the construction of this specification.

   Destination advertisements may advertise positive (prefix is present)
   or negative (removed)
       piecewise source routes as they propagate the DAO messages, termed message, as no-DAOs.  A no-DAO is
   stimulated by
       described in Section 6.2.5.

   6.  Storing nodes MUST store any source route information received
       from the disappearance of a prefix below.  This is
   discovered by timing out after a request (a DIO message) or by
   receiving DAO (RRStack) in the routing table entry entry.  If a no-DAO.  A no-DAO
       node is a conveyed not capable to do this then it must act as a DAO message with a
   DAO Lifetime of ZERO_LIFETIME.

   A non-storing
       node with respect to that is capable of recording the state information conveyed particular destination.

   7.  Storing nodes MUST use piecewise source routes in order to
       forward data across a unicast DAO message will do so upon receiving and processing non-storing region of the
   DAO message, thus provisioning LLN.  The source
       routing state concerning destinations
   located downwards along the DODAG.  If mechanism is to be described in a companion
       specification.  (If a node is not capable of recording
   state information receives a DAO message containing a Reverse Route
   Stack, to do this, then the node knows that the
       node MUST NOT operate as a storing node).

6.2.2.  Mode of Operation

   o  DAO message has traversed one or
   more nodes that did Operation may not retain any be required for all use cases.

   o  Some applications may only need support for collection/upward/MP2P
      flow with no acknowledgement/reciprocal traffic.

   o  Some DODAGs may not support DAO Operation, which could mean that
      DAO Operation is wasteful overhead.

   o  As a special case, multicast DAO operation may be used to populate
      'one-hop' neighborhood routing state as it traversed the
   path table entries, and is distinct from
      the unicast DAO source operation used to establish downward routes along
      the node. DODAG.

   1.  The node may then extract the
   Reverse Route Stack and retain the included state 'A' flag in order to specify
   Source Routing instructions along the return path towards the
   destination.  The node MUST set DIO as conveyed from the RRCount back DODAG root serves to zero and clear
       enable/disable DAO operation over the Reverse Route Stack prior to passing entire DODAG.  This flag
       should be administratively provisioned a priori at the DODAG root
       as a function of the implementation/deployment and not tend to
       change.

   2.  When DAO message information
   on.

   A Operation is disabled, a node that SHOULD NOT emit DAOs.

   3.  When DAO Operation is unable disabled, a node MAY ignore received DAOs.

6.2.3.  Destination Advertisement Parents

   o  Nodes will select a subset of their DODAG Parents to record the state information conveyed in the
   DAO message whom DAOs
      will append be sent

      *  This subset is the next-hop address set of 'DAO Parents'

      *  Each DAO parent MUST be a DODAG Parent.  (Not all DODAG parents
         need to the Reverse Route
   Stack, increment the RRCount, be DAO parents).

      *  Operation with more than DAO Parent requires consideration of
         such issues as DAO fan-out and then pass the destination
   advertisement on without recording any additional state.  In this way
   the Reverse Route Stack will contain path diversity, to be elaborated
         in a vector future version of next hops that must
   be traversed along the reverse path that the DAO message has
   traveled. this specification.

   o  The vector will selection of DAO parents is implementation specific and may be ordered such
      based on selecting the DODAG Parents that offer the node closest best upwards
      cost (as opposed to downwards or mixed), as determined by the destination will appear first
      metrics in use and the list.  In such cases, if it
   is useful Objective Function.

   o  When DAO messages are unicast to the implementation to try and provision redundant paths, DAO Parent, the node may choose to convey identity of
      the destination advertisement to one or
   more DAG parents DAO Parent (DODAGID x DAGSequenceNumber) combined with the
      RPLInstanceID in order of preference as guided by an
   implementation specific policy.

   In certain cases (called hybrid cases), some nodes along the path a
   destination advertisement follows up DAO message unambiguously associates the DODAG may store state DAO
      message, and
   some may not.  The destination advertisement mechanism allows for thus the
   provisioning of routing state such that when particular destination prefix, with a packet is traversing
   down DODAG
      Iteration.

   o  When DAO messages are unicast to the DODAG, some nodes DAO Parent, the DAO Rank may
      be able to directly forward updated as according to the
   next hop, implementation and other nodes may be able to specify a piecewise source
   route Objective
      Function in order use to bridge spans reflect the relative (aggregated) cost of stateless nodes within
      reaching the path on Destination Prefix through that DAO parent.  As a
      further extension, a DAO Suboption for the way Metric Container may be
      included.

6.2.4.  Operation of DAO Storing Nodes

6.2.4.1.  DAO Routing Table Entry

   A DAO Routing Table Entry conceptually contains the following
   elements:

   o  Advertising Neighbor Information
      *  IPv6 Addr
      *  Interface ID
   o  To which DAO Parents has this entry been reported
   o  Retry Counter
   o  Logical equivalent of DAO Content:
      *  DAO Sequence
      *  DAO Rank
      *  DAO Lifetime
      *  Route tag (used to prioritize which destination entries should
         be stored)
      *  Destination Prefix (or Address or Mcast Group)
      *  RR Stack*

   The DAO Routing Table Entry is logically associated with the desired destination.

   In
   following states:

   CONNECTED   This entry is 'owned' by the case where no node - it is able to store any routing state as
   destination advertisements pass by, manually
               configured and the DAG root ends up with is considered as a 'self' entry for DAO
   messages that contain
               Operation

   REACHABLE   This entry has been reported from a completely specified route back to neighbor of the
   originating node in node.
               This state includes the following substates:

               CONFIRMED This entry is active, newly validated, and
                         usable

               PENDING   This entry is active, awaiting validation, and
                         usable.  A Retry Counter is associated with
                         this substate

   UNREACHABLE This entry is being cleaned up.  This entry may be
               suppressed when the cleanup process is complete.

   When an attempt is to be made to report the form of DAO entry to DAO Parents,
   the inverted Reverse Route Stack.  A
   DAG root should not request (Destination Advertisement Trigger) nor
   indicate support (Destination Advertisement Supported) for
   destination advertisements if it DAO Entry record is not able logically marked to store indicate that an attempt
   has not yet been made for parent.  As the Reverse
   Route Stack information in unicast attempts are
   completed for each parent, this case.

   The destination advertisement mark may be cleared.  This mechanism requires stateful nodes
   may serve to
   maintain lists of known prefixes.  A prefix limit DAO entry contains the
   following abstract information:

   o  A reference updates for each parent to the ND entry a subset 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
   needs to keep track whether this prefix was
      reported already, and be reported.

6.2.4.1.1.  DAO Routing Table Entry Management

           +---------------------------------+
           |                                 |
           |            REACHABLE            |    +-------------+
           |                                 |    |             |
           |        +-----------+            |    |  CONNECTED  |
     (*)----------->|           |-------+    |    |             |
           |        | Confirmed |       |    |    +-------------+
           |    +-->|           |---+   |    |
           |    |   +-----------+   |   |    |
           |    |                   |   |    |
           |    |                   |   |    |
           |    |                   |   |    |
           |    |   +-----------+   |   |    |    +-------------+
           |    |   |           |<--+   +-------->|             |
           |    +---|  Pending  |            |    | UNREACHABLE |
           |        |           |---------------->|             |--->(*)
           |        +-----------+            |    +-------------+
           |                                 |
           +---------------------------------+

                        DAO Routing Table Entry FSM

6.2.4.1.1.1.  Operation in the CONNECTED state

   1.  CONNECTED DAO entries are to which be provisioned outside of the DA parents.

   o  A counter
       context of retries to count how many DIO messages were sent on
      the interface RPL, e.g. through a management API.  An implementation
       SHOULD provide a means to provision/manage CONNECTED DAO entries,
       including whether they are to be redistributed in RPL.

6.2.4.1.1.2.  Operation in the advertising neighbor without reachability
      confirmation for the prefix.

   Note that nodes may receive multiple information from different
   neighbors for REACHABLE state

   1.  When a specific destination, as different paths through REACHABLE(*) entry times out, the
   DODAG may entry MUST be propagating information up the DODAG for placed
       into the same
   destination.  A node that is recording routing UNREACHABLE state will keep track
   of the information from each neighbor independently, and when it
   comes time no-DAO SHOULD be scheduled to send
       to propagate the node's DAO message Parents.  (TBD MUST?)

   2.  When a no-DAO for a particular prefix to
   the DA parents, then the REACHABLE(*) entry is received with a newer
       DAO information will be selected from among Sequence Number, the advertising neighbors who offer entry MUST be placed into the least depth
       UNREACHABLE state and no-DAO SHOULD be scheduled to send to the
   destination.
       node's DAO Parents.

   3.  When a node loses connectivity REACHABLE(*) entry is to a child be removed because NUD or
       equivalent has determined that the next-hop neighbor is used as next hop
   for a route learned from a DAO, no longer
       reachable, the node should cleanup all routes entry MUST be placed into the UNREACHABLE state
       and no-DAO SHOULD be scheduled to send to the node's DAO states that are related Parents.

   4.  When a REACHABLE(*) entry is to that child.  If be removed because an associated
       Forwarding Error has been returned by the lost child was next-hop neighbor, the only adjacency leading
       entry MUST be placed into the UNREACHABLE state and no-DAO SHOULD
       be scheduled to send to the node's DAO prefix, the node should poison Parents.

   5.  When a DAO (or no-DAO) for a REACHABLE(*) entry is received with
       an older or unchanged DAO Sequence Number, then the route by sending no-DAOs to DAO (or no-
       DAO) SHOULD be ignored and the parents to which it has
   advertised associated entry MUST NOT be
       updated with the stale information.

6.2.4.1.1.2.1.  REACHABLE(Confirmed)

   1.  When a DAO prefixes.

   The for a previously unknown (or UNREACHABLE) destination advertisement mechanism stores the prefix entries in
   one of 3 abstract lists; the Connected, the Reachable
       is received and the
   Unreachable lists.

   The Connected list corresponds is to be stored, it MUST be entered into the prefixes owned and managed by
       routing table in the local node.

   The Reachable list contains prefixes for which REACHABLE(Confirmed) state.  Alternately the
       node keeps
   receiving may behave as a non-storing node with respect to this
       destination.

   2.  When a DAO messages, and for those prefixes which have not yet
   timed out.

   The Unreachable list keeps track of prefixes which are no longer
   valid and in a REACHABLE(Confirmed) entry is received with a
       newer DAO Sequence Number the process entry MUST be updated with the
       logical equivalent of being deleted, in order to send the DAO
   messages with zero lifetime (also called no-DAO) contents.

   3.  When a DAO for a REACHABLE(Confirmed) entry is expected, e.g.
       because a DIO to request a DAO refresh is sent, then the DA parents.

6.8.1.1.1.  Destination Advertisement Timers

   The destination advertisement mechanism requires 2 timers; DAO
       entry MUST be placed in the
   DelayDAO timer REACHABLE(Pending) state and the RemoveTimer.

   o  The DelayDAO timer is armed upon a stimulation
       associated Retry Counter MUST be set to send 0.

6.2.4.1.1.2.2.  REACHABLE(Pending)

   1.  When a
      destination advertisement (such as DAO for a DIO message from REACHABLE(Pending) entry is received with a DA
      parent).  When
       newer DAO Sequence Number, the timer is armed, all entries entry MUST be updated with the
       logical equivalent of the DAO contents and the entry MUST be
       placed in the Reachable
      list as well as all entries REACHABLE(Confirmed) state.

   2.  When a DAO for Connected list are set a REACHABLE(Pending) entry is expected, e.g.
       because DAO has (again) been triggered with respect to not be
      reported yet for that particular DA parent.

   o  For
       neighbor, then the associated Retry Counter MUST be incremented.

   3.  When a root, the DIO timer has associated Retry Counter for a duration of DEF_DAO_LATENCY.  For REACHABLE(Pending)
       entry reaches a node maximum threshold, the entry MUST be placed into
       the UNREACHABLE state and no-DAO SHOULD be scheduled to send to
       the node's DAO Parents.

6.2.4.1.1.3.  Operation in a DODAG iteration, the DelayDAO timer has a duration UNREACHABLE state

   1.  An implementation SHOULD bound the time that the entry is randomized between (DEF_DAO_LATENCY divided by
       allocated in the Rank UNREACHABLE state.  Upon the equivalent expiry
       of the node) and (DEF_DAO_LATENCY divided by related timer (RemoveTimer), the Rank of entry SHOULD be
       suppressed.

   2.  While the parent).
      The intention entry is that nodes located deeper in the DODAG iteration
      should have UNREACHABLE state a shorter DelayDAO timer, allowing DAO messages node SHOULD make a
      chance
       reasonable attempt to be reported from deeper in report a no-DAO to each of the DODAG and potentially
      aggregated along sub-DAGs before propagating further up.

   o  The RemoveTimer is used DAO parents.

   3.  When the node has completed an attempt to clean up entries for which report a no-DAO to each
       of the DAO messages
      are no longer being received from parents, the sub-DAG.

      * entry SHOULD be suppressed.

6.2.5.  Operation of DAO Non-storing Nodes

   1.  When a DIO message is sent that DAO is requesting destination
         advertisements, received from a flag is set child by a node who will not store
       a routing table entry for all the DAO, the node MUST schedule to pass
       the DAO entries contents along to its DAO parents.  Prior to passing the
       DAO along, the node MUST process the DAO as follows, in order
       that information necessary to construct a loose source route may
       be accumulated within the
         routing table.

      *  If DAO payload as it moves up the flag has DODAG:

       1.  The most recent addition to the RRStack (the 'next waypoint')
           is investigated to determine if the node already been set for has a DAO entry, route
           provisioned to the retry
         count is incremented.

      * waypoint.  If the node already has such a DAO message
           route, then it is received not necessary to add additional information
           to confirm the entry, the entry is
         refreshed and RRStack.  The node SHOULD NOT modify the flag and count may be cleared.

      * RRStack
           further.

       2.  If at least one entry has reached a threshold value and the
         RemoveTimer is node does not running, have a route provisioned to the entry is considered next
           waypoint, then the node MUST append the address of the child
           to be
         probably gone the RRStack, and increment RRCount.

6.2.6.  Scheduling to Send DAO (or no-DAO)

   1.  An implementation SHOULD arrange to rate-limit the RemoveTimer is started.

      * sending of
       DAOs.

   2.  When the RemoveTimer elapse, DAO messages with lifetime 0, i.e.
         no-DAOs, are sent scheduling to explicitly inform DA parents that send a DAO, an implementation SHOULD
       equivalently start a timer (DelayDAO) to delay sending the
         entries which have reached DAO.
       If the threshold are no longer
         available, and timer has already been armed then the related routing states DAO may be propagated
       considered as already scheduled, and
         cleaned up. implementation SHOULD leave
       the timer running at its present duration.

   o  The RemoveTimer has a duration  In order to increase the effectiveness of min (MAX_DESTROY_INTERVAL,
      TBD(DIO Trickle Timer Interval)).

6.8.1.2.  Multicast Destination Advertisement Messages

   It is also possible for a node aggregation, an
      implementation MAY allow time to multicast a DAO message receive no-DAOs from its sub-
      DODAG prior to emitting DAOs to its DAO Parents.

      *  The scheduled delay in such cases may be, for example, such
         that DAO_LATENCY/f(self_rank) <= delay < DAO_LATENCY/
         f(parent_rank), where f(rank) is floor(rank/
         MinHopRankIncrease), such that nodes deeper in the
   link-local scope all-nodes multicast address FF02::1.  This message DODAG may
         tend to report DAO messages first before their parent nodes
         will be received by all node listening report DAO messages.  Note that this suggestion is
         intended as an optimization to allow efficient aggregation --
         it is not required for correct operation in range of the emitting node. general case.

6.2.7.  Triggering DAO Message from the Sub-DODAG

   Note: The objective DIO is modified to enable direct P2P communication, between
   destinations directly supported by neighboring nodes, without needing
   the RPL routing structure to relay the packets.

   A multicast DAO message MUST be add a 'S' flag, which is used only to advertise information
   about self, i.e. prefixes
   indicate if a non-root ancestor storing routing table entries learned
   from DAOs.  This allows an optimization in the Connected list or addresses owned by
   this node.  This would typically be a multicast group that this case where ONLY the
   root node is listening to or a global address owned by this node, though storing such routing table entries, then it can
   be used to advertise any prefix owned by this node as well.  A
   multicast DAO message is not used
   necessary for an intermediate node to trigger DAO messages from its
   sub-DODAG when it changes its DAO Parent.

   1.   The DODAG root MUST clear the 'S' flag when it emits DIO
        messages.

   2.   Non-root nodes that store routing and does not presume table entries learned from
        DAOs MUST set the 'S' flag when they emit DIO messages.

   3.   A node that has any DODAG relationship between DAO Parent with the emitter and 'S' flag set MUST also
        set the receiver; 'S' flag when it emits DIO messages.

   4.   A node that has all DAO Parents with cleared 'S' flags MUST
        clear the 'S' flag when it emits DIO messages.

   5.   A DAO Trigger Sequence Number (DTSN) MUST
   NOT be used to relay information learned (e.g. information maintained by each
        node per RPL Instance.  The DTSN, in conjunction with the
   Reachable list) from another node; information obtained 'T'
        flag from the DIO message, provides a
   multicast means by which DAO MAY
        messages may be installed reliably triggered in the routing table and MAY event of topology
        change.

   6.   The DTSN MUST be
   propagated advertised by a router in unicast DAOs.

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

   7.   A node MUST NOT perform any other processing on keeps track of the DAO message (i.e. such a node does not presume DTSN that it is a DA
   parent).

6.8.1.3.  Unicast Destination Advertisement Messages has heard from Child to
          Parent

   When sending a destination advertisement to a DA parent, a node
   includes the DAOs for prefix entries not already reported (since the last DA Trigger from an
        DIO message) in the Reachable and Connected
   lists, as well as no-DAOs for all the entries in the Unreachable
   list.  Depending on from each of its policy and ability DAO Parents.  Note that there is one DTSN
        maintained per DAO Parent-- each DAO Parent may independently
        increment it at will.  (TBD A change to retain routing state,
   the receiving DTSN does not indicate
        DAG inconsistency?).

   8.   A node that is not a fully-storing node SHOULD keep increment its own
        DTSN when it adds a record of the reported DAO message.
   If the DAO message offers new parent, that parent having the best route 'S' flag
        set, to its DAO Parent set.  It MAY defer advertising the prefix
        increment as determined
   by policy and other prefix records, the node SHOULD install long as it has a route
   to the prefix reported in the DAO message via the link local address
   of the reporting neighbor and parent that already provides
        adequate connectivity.

   9.   A node that is not a fully-storing node MUST increment its own
        DTSN when it SHOULD further propagate the
   information in receives a DAO message.

   The DIO message from the DODAG root is used to synchronize the whole
   DODAG iteration, including the periodic reporting of destination
   advertisements back up the DODAG.  Its period a DAO Parent that contains a
        newly incremented DTSN.  (The newly incremented DTSN is expected to vary,
   depending on detected
        by comparing the configuration of value received in the DIO trickle timer.

   When a with the value last
        recorded for that DAO parent).

   10.  A fully-storing node MUST increment its own DTSN when it
        receives a DIO message over an LLN interface from a DA
   parent, the DelayDAO is armed to force DAO Parent that contains a full update. newly
        incremented DTSN and a set 'T' flag.

   11.  When the a storing or non-storing node broadcasts joins a DIO message on an LLN interface, for all
   entries on that interface:

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

   o  If the entry is PENDING, the retry count is incremented.  If new DODAG iteration,
        it
      reaches a maximum threshold, the entry goes ELAPSED If at least
      one entry is ELAPSED at the end of the process: SHOULD increment its DTSN as if the RemoveTimer
      is not running then it is armed with a jitter.

   Since the DelayDAO timer 'T' flag has been set.

   12.  DAO Transmission SHOULD be scheduled when a duration that decreases with the
   depth, it new parent is expected added
        to receive all the DAO messages Parent set.

   13.  A node that receives a newly incremented DTSN from all children
   before the timer elapses and the full update is sent to the DA
   parents.

   Once the RemoveTimer a DAO Parent
        MUST schedule a DAO transmission.

   o  When a node that is elapsed, not fully-storing sees a DTSN increment, it
      will increment its own DTSN.  This will cause the prefix entry is scheduled DTSN increment
      to be
   removed and moved extend down the DODAG to the Unreachable list if there are any DA parents
   that need first fully-storing node, which
      will send its DAOs back up, rebuilding source routes information
      along the way to be informed of the change in status for first node that incremented the prefix,
   otherwise DTSN, who
      then may report the prefix entry is cleaned up right away.  The prefix
   entry new DAO information to its new parent.

   o  When a fully-storing node sees a DTSN increment, it is removed caused to
      reissue its entire set of routing table entries learned from the Unreachable list when no more DA parents DAOs
      (or an aggregated subset thereof), but will not need to be informed.  This condition may be satisfied increment
      its own DTSN.  The 'DTSN increment wave' stops when it encounters
      fully-storing nodes.

   o  When a no-DAO fully-storing node sees a DTSN increment AND the 'T' flag
      is sent to set, it does increment its own DTSN as well.  The 'T' flag
      'punches through' all current DA parents indicating the loss of the prefix,
   and noting that nodes, causing all routing tables in some cases parents may have been removed from the
   set of DA parents.

6.8.1.4.  Other Events

   Finally, the destination advertisement mechanism responds
      entire sub-DODAG to a series
   of events, such as:

   o  Destination advertisement operation stopped: All entries in the
      abstract lists are freed.  All the routes learned from be refreshed.

6.2.8.  Sending DAO Messages to DAO Parents

   1.  When storing nodes send DAO messages are removed.

   o  Interface going down: for all stored entries in the Reachable list on
      that interface,
       RRStack SHOULD be cleared in the associated route DAO message.

   2.  DAO Messages sent to DAO Parents MUST be unicast.

       *  The IPv6 Source Address is removed, and the entry node sending the DAO message.

       *  The IPv6 Destination Address is
      scheduled to be removed.

   o  Loss of routing adjacency: DAO parent.

   3.  When the routing adjacency appointed time arrives (DelayDAO) for a
      neighbor is lost, as per the procedures described in Section 6.11,
      and if the associated entries are in transmission
       of DAO messages (with jitter as appropriate) for the Reachable list, requested
       entries, the implementation MAY aggregate the
      associated routes are removed, and the entries are scheduled into a
       reduced numbers of DAOs to be
      destroyed.

   o  Changes to DA parent set: all entries in the Reachable list are
      set reported to not 'reported' each parent, and DelayDAO
       perform compression if possible.

   4.  Note: it is armed.

6.8.1.5.  Aggregation of Prefixes by a Node

   There may be number of cases where NOT RECOMMENDED that a aggregation may DAO Transmission (No-DAO) be shared within
       scheduled when a group DAO Parent is removed from the DAO Parent set.

6.2.9.  Multicast Destination Advertisement Messages

   A special case of nodes.  In such a case, it DAO operation, distinct from unicast DAO operation,
   is possible to use aggregation
   techniques with destination advertisements and improve scalability.

   Other cases might occur for multicast DAO operation which additional support is required: may be used to populate '1-hop'
   routing table entries.

   1.  The aggregating  A node is attached within the sub-DAG of MAY multicast a DAO message to the link-local scope all-
       nodes
       it is aggregating for. multicast address FF02::1.

   2.  A node that is to multicast DAO message MUST be aggregated for is located somewhere else
       within the DODAG iteration, not in the sub-DAG of the aggregating
       node.

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

   Consider a node M that is performing an aggregation, and a node N
   that is advertise
       information about self, i.e. prefixes directly connected to be a member of the aggregation group.  A node Z situated
   above the node M in the DODAG, but not above node N, will see the
   advertisements for the aggregation or
       owned by M but not that of the
   individual prefix for N. Such a node Z will route all the packets for
   node N towards node M, but this node, such as a multicast group that the node M will have no route is
       subscribed to or a global address owned by the node N
   and will fail node.

   3.  A multicast DAO message MUST NOT be used to forward.

   Additional protocols may relay connectivity
       information learned (e.g. through unicast DAO) from another node.

   4.  Information obtained from a multicast DAO MAY be applied beyond installed in the scope of this
   specification to dynamically elect/provision an aggregating node
       routing table and
   groups of nodes eligible to MAY be aggregated propagated by a node in order to provide route
   summarization for unicast DAOs.

   5.  A node MUST NOT perform any other DAO related processing on a sub-DAG.

6.9.  Loop Detection

   RPL loop avoidance mechanisms are kept simple and designed to
   minimize churn and states.  Loops may form for
       received multicast DAO, in particular a number node MUST NOT perform the
       actions of reasons,
   from control packet loss to sibling forwarding.  RPL includes a
   reactive loop detection technique that protects from meltdown and
   triggers repair DAO parent upon receipt of broken paths.

   RPL loop detection uses information that is placed into the packet in
   the IPv6 flow label. a multicast DAO.

   o  The IPv6 flow label is defined in [RFC2460] and
   its operation is further specified in [RFC3697].  For the purpose of
   RPL operations, the flow label is constructed as follows:

        0                   1                   2
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               |O|S|R|F|  SenderRank   |  InstanceID   |
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 12: RPL Flow Label

   Down 'O' bit:  1-bit flag indicating whether the packet is expected multicast DAO may be used to progress up or down.  A router sets the 'O' bit when enable direct P2P communication,
      without needing the
         packet is expect RPL routing structure to progress down (using DAO routes), and
         resets it when forwarding towards the root of relay the packets.

   o  The multicast DAO does not presume any DODAG
         iteration.  A host MUST set the bit to 0.

   Sibling 'S' bit:  1-bit flag indicating whether relationship between
      the packet has been
         forwarded via a sibling at emitter and the present rank, receiver.

7.  Packet Forwarding and denotes Loop Avoidance/Detection

7.1.  Suggestions for Packet Forwarding

   When forwarding a risk packet to a destination, precedence is given to
   selection of a sibling loop.  A host sets next-hop successor as follows:

   1.  In the bit scope of this specification, it is preferred to 0.

   Rank-Error 'R' bit:  1-bit flag indicating whether select a rank error was
         detected.  A rank error is detected when there is
       successor from a mismatch in
         the relative ranks and DODAG iteration that matches the direction as indicated RPLInstanceID
       marked in the 'O'
         bit.  A host MUST set IPv6 header of the bit to 0.

   Forwarding-Error 'F' bit:  1-bit flag indicating packet being forwarded.

   2.  If a local administrative preference favors a route that this node can
         not forward has been
       learned from a different routing protocol than RPL, then use that
       successor.

   3.  If there is an entry in the packet further towards routing table matching the destination.  The
         'F' bit might be set by sibling
       destination that can not forward to has been learned from a
         parent multicast destination
       advertisement (e.g. the destination is a packet with one-hop neighbor), then
       use that successor.

   4.  If there is an entry in the Sibling 'S' bit set, or by a child
         node routing table matching the
       destination that does not have has been learned from a route to unicast destination for a packet
         with
       advertisement (e.g. the destination is located down 'O' bit set.  A host MUST set the bit to 0.

   SenderRank:  8-bit field set to zero by the source and sub-
       DODAG), then use that successor.

   5.  If there is a DODAG iteration offering a route to its rank by a router that forwards inside the RPL network.

   InstanceID:  8-bit field indicating prefix
       matching the destination, then select one of those DODAG instance along which
         the packet parents
       as a successor.

   6.  If there is sent.

6.9.1.  Source Node Operation

   A packet a DODAG parent offering a default route then select
       that is sourced at DODAG parent as a node connected to successor.

   7.  If there is a RPL network or
   destined to DODAG iteration offering a node connected route to a RPL network MUST be issued with prefix
       matching the
   flow label zeroed out, destination, but for all DODAG parents have been tried
       and are temporarily unavailable (as determined by the InstanceID field.

   If forwarding
       procedure), then select a DODAG sibling as a successor.

   8.  Finally, if no DODAG siblings are available, the source packet is aware of the InstanceID that
       dropped.  ICMP Destination Unreachable may be invoked.  An
       inconsistency is preferred for the
   flow, then it MUST set the InstanceID field in the flow label
   accordingly, otherwise it detected.

   TTL MUST set it to the RPL_DEFAULT_INSTANCE. be decremented when forwarding.  If a compression mechanism such as 6LoWPAN the packet is applied being
   forwarded via a sibling, then the TTL MAY be decremented more
   aggressively (by more than one) to limit the packet, impact of possible
   loops.

   Note that the flow label chosen successor MUST NOT be compressed even if it is set to all
   zeroes.

6.9.2.  Router Operation

6.9.2.1.  Conformance to RFC 3697

   [RFC3697] mandates the neighbor that was the Flow Label value set by
   predecessor of the source MUST
   be delivered unchanged to packet (split horizon), except in the destination node(s).

   In order to restore case where
   it is intended for the flow label packet to its original value, change from an RPL
   router that delivers a packet up to a destination connected an down flow,
   such as switching from DIO routes to a RPL
   network or that DAO routes a packet outside as the destination is
   neared.

7.2.  Loop Avoidance and Detection

   RPL network MUST zero out
   all the fields but the InstanceID field that must be delivered
   without a change.

6.9.2.2.  Instance Forwarding

   Instance IDs loop avoidance mechanisms are used kept simple and designed to avoid loops between DODAGs from different
   origins.  DODAGs that constructed
   minimize churn and states.  Loops may form for antagonistic constraints might
   contain paths that, if mixed together, would yield loops.  Those
   loops are avoided by forwarding a number of reasons,
   from control packet along the DODAG that is
   associated loss to sibling forwarding.  RPL includes a given instance.

   The InstanceID is placed by the source in the flow label.  This
   InstanceID MUST match the DODAG instance onto which the packet is
   placed by any node, be it a host or router.

   When a router receives a packet that is flagged with a given
   InstanceID and the node can forward the packet along the DODAG
   associated to
   reactive loop detection technique that instance, then the router MUST do so protects from meltdown and leave
   triggers repair of broken paths.

   RPL loop detection uses information that is placed into the
   InstanceID flag unchanged.

   If any node can not forward a packet along the DODAG associated to
   the InstanceID in
   the IPv6 flow label, then the node SHOULD discard the
   packet.

6.9.2.3.  DAG Inconsistency Loop Detection label.  The DODAG IPv6 flow label is inconsistent if defined in [RFC2460] and
   its operation is further specified in [RFC3697].  For the direction purpose of a packet does not match
   RPL operations, the flow label is constructed as follows:

        0                   1                   2
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
               |O|S|R|F|  SenderRank   | RPLInstanceID |
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 12: RPL Flow Label

   Down 'O' bit:  1-bit flag indicating whether the rank relationship.  A receiver detects an inconsistency if it
   receives a packet with either: is expected
         to progress up or down.  A router sets the 'O' bit set (to down) from a node of a higher rank. when the 'O' bit reset (for up) from a node of a lesser rank.
         packet is expect to progress down (using DAO routes), and
         resets it when forwarding towards the 'S' bit set (to sibling) from a node root of a different rank.

   When the DODAG root increments the DAG Sequence Number a temporary
   rank discontinuity may form between the next iteration and
         iteration.  A host MUST set the prior
   iteration, in particular if nodes are adjusting their rank in bit to 0.

   Sibling 'S' bit:  1-bit flag indicating whether the
   next iteration and deferring their migration into packet has been
         forwarded via a sibling at the next iteration.
   A router that is still present rank, and denotes a member risk
         of the prior iteration may choose to
   forward a packet sibling loop.  A host sets the bit to 0.

   Rank-Error 'R' bit:  1-bit flag indicating whether a (future) parent that rank error was
         detected.  A rank error is detected when there is a mismatch in
         the next iteration.
   In some cases this could cause the parent to detect an inconsistency
   because the rank-ordering in the prior iteration is not necessarily relative ranks and the same direction as indicated in the next iteration and 'O'
         bit.  A host MUST set the packet may be judged bit to 0.

   Forwarding-Error 'F' bit:  1-bit flag indicating that this node can
         not
   be making forward progress.  If the sending router is aware that packet further towards the
   chosen successor has already joined destination.  The
         'F' bit might be set by sibling that can not forward to a
         parent a packet with the next iteration, then Sibling 'S' bit set, or by a child
         node that does not have a route to destination for a packet
         with the
   sending router down 'O' bit set.  A host MUST update set the SenderRank bit to INFINITE_RANK as it
   forwards 0.

   SenderRank:  8-bit field set to zero by the packets across source and to its rank by
         a router that forwards inside the discontinuity into RPL network.

   RPLInstanceID:  8-bit field indicating the next DODAG
   iteration in order to avoid a false detection of rank inconsistency.

   One inconsistency instance along which
         the path packet is not considered as sent.

7.2.1.  Source Node Operation

   A packet that is sourced at a node connected to a RPL network or
   destined to a node connected to a critical
   error and RPL network MUST be issued with the packet may continue.  But a second detection along
   flow label zeroed out, but for the
   path of a same packet should not occur and RPLInstanceID field.

   If the packet is dropped.

   This process source is controlled by the Rank-Error bit in aware of the Flow Label.
   When an inconsistency, RPLInstanceID that is detected on a packet, if preferred for the Rank-Error bit
   was not set
   flow, then the Rank-Error bit is set.  If it was MUST set the packet
   is discarded and RPLInstanceID field in the trickle timer is reset.

6.9.2.4.  Sibling Loop Avoidance

   When flow label
   accordingly, otherwise it MUST set it to the RPL_DEFAULT_INSTANCE.

   If a packet compression mechanism such as 6LoWPAN is forwarded along siblings, it cannot be checked for
   forward progress and may loop between siblings.  Experimental
   evidence has shown that one sibling hop can applied to the packet,
   the flow label MUST NOT be very useful but compressed even if it is
   generally sufficient set to avoid loops.  Based on all
   zeroes.

7.2.2.  Router Operation

7.2.2.1.  Conformance to RFC 3697

   [RFC3697] mandates that evidence, this
   specification enforces the simple rule Flow Label value set by the source MUST
   be delivered unchanged to the destination node(s).

   In order to restore the flow label to its original value, an RPL
   router that delivers a packet may not make 2
   sibling hops in to a row.

   When destination connected to a host issues RPL
   network or that routes a packet or when outside the RPL network MUST zero out
   all the fields but the RPLInstanceID field that must be delivered
   without a router forwards change.

7.2.2.2.  Instance Forwarding

   Instance IDs are used to avoid loops between DODAGs from different
   origins.  DODAGs that constructed for antagonistic constraints might
   contain paths that, if mixed together, would yield loops.  Those
   loops are avoided by forwarding a packet along the DODAG that is
   associated to a
   non-sibling, given instance.

   The RPLInstanceID is placed by the Sibling bit source in the flow label.  This
   RPLInstanceID MUST match the RPL Instance onto which the packet must is
   placed by any node, be reset. it a host or router.

   When a router forwards to receives a sibling: if the Sibling bit was not set then the
   Sibling bit packet that is set.  If the Sibling bit was set then then flagged with a given
   RPLInstanceID and the router
   SHOULD return node can forward the packet to along the sibling that DODAG
   associated to that passed it with instance, then the
   Forwarding-Error 'F' bit set.

6.9.2.5.  DAO Inconsistency Loop Detection and Recovery

   A DAO inconsistency happens when router that has an down DAO route
   via a child that is a remnant from an obsolete state that is not
   matched in MUST do so and leave the child.  With DAO inconsistency loop recovery,
   RPLInstanceID flag unchanged.

   If any node can not forward a packet
   can be used along the DODAG associated to recursively explore and cleanup
   the obsolete DAO
   states along a sub-DAG.

   In a general manner, a packet that goes down should never go up
   again.  So rather than routing up a packet with RPLInstanceID in the down bit set, flow label, then the
   router MUST node SHOULD discard the
   packet.  If DAO inconsistency loop recovery

7.2.2.3.  DAG Inconsistency Loop Detection

   The DODAG is applied, then the router SHOULD send inconsistent if the direction of a packet to does not match
   the parent that
   passed rank relationship.  A receiver detects an inconsistency if it
   receives a packet with either:

      the Forwarding-Error 'F' 'O' bit set.

6.9.2.6.  Forward Path Recovery

   Upon receiving set (to down) from a packet with node of a Forwarding-Error higher rank.

      the 'O' bit set, reset (for up) from a node of a lesser rank.

      the 'S' bit set (to sibling) from a node
   MUST remove of a different rank.

   When the routing states that caused forwarding to that
   neighbor, clear DODAG root increments the Forwarding-Error bit DODAGSequenceNumber a temporary
   rank discontinuity may form between the next iteration and attempt to send the
   packet again.  The packet prior
   iteration, in particular if nodes are adjusting their rank in the
   next iteration and deferring their migration into the next iteration.
   A router that is still a member of the prior iteration may its way choose to an alternate neighbor.  If
   forward a packet to a (future) parent that alternate neighbor still has an inconsistent DAO state via this
   node, is in the process will recurse, next iteration.
   In some cases this node will set could cause the Forwarding-
   Error 'F' bit and parent to detect an inconsistency
   because the routing state rank-ordering in the alternate neighbor will be
   cleaned up prior iteration is not necessarily
   the same as well.

6.10.  Multicast Operation

   This section describes further in the multicast routing operations over
   an IPv6 RPL network, next iteration and specifically how unicast DAOs can the packet may be used judged to
   relay group registrations up.  Wherever not
   be making forward progress.  If the following text mentions
   MLD, one can read MLDv2 or v3.

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

   Along is aware that the
   chosen successor has already joined the path between next iteration, then the
   sending router and MUST update the DODAG root, MLD requests
   are mapped and transported SenderRank to INFINITE_RANK as DAO messages within it
   forwards the RPL protocol;
   each hop coalesces packets across the multiple requests for discontinuity into the next DODAG
   iteration in order to avoid a same group false detection of rank inconsistency.

   One inconsistency along the path is not considered as a single
   DAO message to critical
   error and the parent(s), in packet may continue.  But a fashion similar to proxy IGMP, but
   recursively between child router and parent up to second detection along the root.

   A router might select to pass
   path of a listener registration DAO message to
   its preferred parent only, same packet should not occur and the packet is dropped.

   This process is controlled by the Rank-Error bit in which case multicast packets coming
   back might be lost for all of its sub-DAG the Flow Label.
   When an inconsistency, is detected on a packet, if the transmission fails
   over that link.  Alternatively Rank-Error bit
   was not set then the router might select to copy
   additional parents as Rank-Error bit is set.  If it would do was set the packet
   is discarded and the trickle timer is reset.

7.2.2.4.  Sibling Loop Avoidance

   When a packet is forwarded along siblings, it cannot be checked for DAO messages advertising
   unicast destinations, in which case there might
   forward progress and may loop between siblings.  Experimental
   evidence has shown that one sibling hop can be duplicates very useful but is
   generally sufficient to avoid loops.  Based on that evidence, this
   specification enforces the simple rule that a packet may not make 2
   sibling hops in a row.

   When a host issues a packet or when a router will need forwards a packet to prune.

   As a result, multicast routing states are installed
   non-sibling, the Sibling bit in each the packet must be reset.  When a
   router on forwards to a sibling: if the way from Sibling bit was not set then the listeners to
   Sibling bit is set.  If the root, enabling Sibling bit was set then then the router
   SHOULD return the root to copy a
   multicast packet to all its children routers the sibling that had issued a that passed it with the
   Forwarding-Error 'F' bit set.

7.2.2.5.  DAO
   message including a Inconsistency Loop Detection and Recovery

   A DAO for inconsistency happens when router that multicast group, as well as all the
   attached nodes has an down DAO route
   via a child that registered over MLD.

   For unicast traffic, it is expected that the grounded root of a remnant from an
   DODAG terminates RPL and MAY redistribute the RPL routes over the
   external infrastructure using whatever routing protocol is used
   there.  For multicast traffic, the root MAY proxy MLD for all the
   nodes attached to the RPL routers (this would be needed if the
   multicast source obsolete state that is located not
   matched in the external infrastructure).  For
   such child.  With DAO inconsistency loop recovery, a source, the packet will
   can be replicated as it flows down the
   DODAG based on the multicast routing table entries installed from used to recursively explore and cleanup the obsolete DAO message.

   For
   states along a sub-DODAG.

   In a general manner, a packet that goes down should never go up
   again.  So rather than routing up a source inside packet with the DODAG, down bit set, the packet is passed to
   router MUST discard the preferred
   parents, and if that fails packet.  If DAO inconsistency loop recovery
   is applied, then to the alternates in router SHOULD send the DODAG.  The packet is also copied to all the registered children, except for the
   one parent that
   passed it with the packet.  Finally, if there is Forwarding-Error 'F' bit set.

7.2.2.6.  Forward Path Recovery

   Upon receiving a listener in packet with a Forwarding-Error bit set, the
   external infrastructure then node
   MUST remove the DODAG root has routing states that caused forwarding to further propagate
   the packet into that
   neighbor, clear the external infrastructure.

   As a result, Forwarding-Error bit and attempt to send the DODAG Root acts as
   packet again.  The packet may its way to an automatic proxy Rendezvous
   Point for alternate neighbor.  If
   that alternate neighbor still has an inconsistent DAO state via this
   node, the RPL network, process will recurse, this node will set the Forwarding-
   Error 'F' bit and as source towards the Internet for all
   multicast flows started routing state in the RPL LLN.  So regardless of whether the
   root is actually attached to alternate neighbor will be
   cleaned up as well.

8.  Multicast Operation

   This section describes further the Internet, multicast routing operations over
   an IPv6 RPL network, and regardless of whether
   the DODAG is grounded or floating, the root specifically how unicast DAOs can serve inner multicast
   streams at all times.

6.11.  Maintenance of Routing Adjacency

   The selection of successors, along the default paths up along be used to
   relay group registrations up.  Wherever the
   DODAG, following text mentions
   MLD, one can read MLDv2 or along v3.

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

   Along the paths learned from destination advertisements
   down along path between the DODAG, leads to router and the formation of routing adjacencies
   that require maintenance.

   In IGPs such DODAG root, MLD requests
   are mapped and transported as OSPF [RFC4915] or IS-IS [RFC5120], DAO messages within the maintenance of
   a routing adjacency involves RPL protocol;
   each hop coalesces 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 multiple requests for a same group as LLN and would lead a single
   DAO message to excessive control traffic
   in light of the data traffic with parent(s), in a negative impact on both link
   loads and nodes resources.  Overhead fashion similar to maintain the routing
   adjacency should be minimized.  Furthermore, it is not always
   possible proxy IGMP, but
   recursively between child router and parent up to rely on the link or transport layer root.

   A router might select to provide
   information of the associated link state.  The network layer needs pass a listener registration DAO message to
   fall back on
   its own mechanism.

   Thus RPL makes use of a different approach consisting preferred parent only, in which case multicast packets coming
   back might be lost for all of probing its sub-DODAG if the
   neighbor using a Neighbor Solicitation message (see [RFC4861]).  The
   reception of a Neighbor Advertisement (NA) message with transmission fails
   over that link.  Alternatively the router might select to copy
   additional parents as it would do for DAO messages advertising
   unicast destinations, in which case there might be duplicates that
   the
   "Solicited Flag" set is used router will need to verify prune.

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

7.  Suggestions for Packet Forwarding

   When forwarding copy a
   multicast packet to all its children routers that had issued a destination, precedence is given to
   selection of DAO
   message including a next-hop successor DAO for that multicast group, as follows:

   1.  In well as all the scope of this specification,
   attached nodes that registered over MLD.

   For unicast traffic, it is preferred to select a
       successor from a DODAG iteration expected that matches the InstanceID
       marked in the IPv6 header grounded root of an
   DODAG terminates RPL and MAY redistribute the packet being forwarded.

   2.  If a local administrative preference favors a route that has been
       learned from a different RPL routes over the
   external infrastructure using whatever routing protocol than RPL, then use that
       successor.

   3.  If there is an entry in used
   there.  For multicast traffic, the routing table matching root MAY proxy MLD for all the
       destination that has been learned from a multicast destination
       advertisement (e.g.
   nodes attached to the destination is a one-hop neighbor), then
       use that successor.

   4.  If there RPL routers (this would be needed if the
   multicast source is an entry located in the external infrastructure).  For
   such a source, the packet will be replicated as it flows down the
   DODAG based on the multicast routing table matching the
       destination that has been learned entries installed from the
   DAO message.

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

   5.  If passed the packet.  Finally, if there is a listener in the
   external infrastructure then the DODAG iteration offering a route root has to further propagate
   the packet into the external infrastructure.

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

   6.  If there is a DAG parent offering a default route then select
       that DAG parent an automatic proxy Rendezvous
   Point for the RPL network, and as a successor.

   7.  If there source towards the Internet for all
   multicast flows started in the RPL LLN.  So regardless of whether the
   root is a DODAG iteration offering a route actually attached to a prefix
       matching the destination, but all DAG parents have been tried Internet, and
       are temporarily unavailable (as determined by regardless of whether
   the forwarding
       procedure), then select a DAG sibling DODAG is grounded or floating, the root can serve inner multicast
   streams at all times.

9.  Maintenance of Routing Adjacency

   The selection of successors, along the default paths up along the
   DODAG, or along the paths learned from destination advertisements
   down along the DODAG, leads to the formation of routing adjacencies
   that require maintenance.

   In IGPs such as OSPF [RFC4915] or IS-IS [RFC5120], the maintenance of
   a successor.

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

   TTL MUST be decremented when forwarding.  If not desirable in constrained
   environments such as LLN and would lead to excessive control traffic
   in light of the packet is being
   forwarded via data traffic with a sibling, then negative impact on both link
   loads and nodes resources.  Overhead to maintain the TTL MAY routing
   adjacency should be decremented more
   aggressively (by more than one) minimized.  Furthermore, it is not always
   possible to limit rely on the impact link or transport layer to provide
   information of possible
   loops.

   Note that the chosen successor MUST NOT be associated link state.  The network layer needs to
   fall back on its own mechanism.

   Thus RPL makes use of a different approach consisting of probing the
   neighbor that was using a Neighbor Solicitation message (see [RFC4861]).  The
   reception of a Neighbor Advertisement (NA) message with the
   predecessor
   "Solicited Flag" set is used to verify the validity of the packet (split horizon), except in routing
   adjacency.  Such mechanism MAY be used prior to sending a data
   packet.  This allows for detecting whether or not the case where
   it routing
   adjacency is intended for still valid, and should it not be the packet to change from an up to an down flow,
   such as switching from DIO routes case, select
   another feasible successor to DAO routes as forward the destination is
   neared.

8. packet.

10.  Guidelines for Objective Functions

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

   The Objective Function is indicated in the DIO message using an
   Objective Code Point (OCP), as specified in
   [I-D.ietf-roll-routing-metrics], and indicates the method that must
   be used to compute the DODAG (e.g. "minimize the path cost using the
   ETX metric and avoid `Blue' 'Blue' links").  The Objective Code Points are
   specified in [I-D.ietf-roll-routing-metrics] [I-D.ietf-roll-routing-metrics], [I-D.ietf-roll-of0],
   and related companion specifications.

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

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

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

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

   o  An OF computes self's rank by adding the step of rank to that
      candidate to the rank of that candidate.  The step the candidate
      a value representing the relative locations of rank is
      computed by estimating self and the link as follows:
      candidate in the DODAG iteration.

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

         +  1 indicates must be at least MinHopRankIncrease.
         (This prevents the creation of a unusually good link, for instance path of sibling links
         connecting a link
            between powered devices child with its parent.)

      *  To keep loop avoidance and metric optimization in alignment,
         the increase in rank should reflect any increase in the metric
         value.  For example, with a mostly battery operated
            environment.

         +  4 indicates a `normal'/typical link, purely additive metric such as qualified by ETX,
         the
            implementation.

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

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

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

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

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

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

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

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

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

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

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

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

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

9.

11.  RPL Constants and Variables

   Following is a summary of RPL constants and variables.  Some default
   values are to be determined in companion applicability statements.

   ZERO_LIFETIME  This is the special value of a lifetime that indicates
         immediate death and removal.  ZERO_LIFETIME has a value of 0.

   BASE_RANK  This is the rank for a virtual root that might be used to
         coordinate multiple roots.  BASE_RANK has a value of 0.

   ROOT_RANK  This is the rank for a DAG DODAG root.  ROOT_RANK has a value
         of 1.

   INFINITE_RANK  This is the constant maximum for the rank.
         INFINITE_RANK has a value of 0xFF.

   RPL_DEFAULT_INSTANCE  This is the InstanceID RPLInstanceID that is used by this
         protocol by a node without any overriding policy.
         RPL_DEFAULT_INSTANCE has a value of 0.

   DEFAULT_DIO_INTERVAL_MIN  To be determined

   DEFAULT_DIO_INTERVAL_DOUBLINGS  To be determined

   DEFAULT_DIO_REDUNDANCY_CONSTANT  To be determined

   DEF_DAO_LATENCY  To be determined

   MAX_DESTROY_INTERVAL  To be determined
   DIO Timer  One instance per DODAG that a node is a member of.  Expiry
         triggers DIO message transmission.  Trickle timer with variable
         interval in [0, DIOIntervalMin..2^DIOIntervalDoublings].  See
         Section 6.3.4 5.3.5.1

   DAG Sequence Number Increment Timer  Up to one instance per DODAG
         that the node is acting as DAG DODAG root of.  May not be supported
         in all implementations.  Expiry triggers revision of
         DAGSequenceNumber,
         DODAGSequenceNumber, causing a new series of updated DIO
         message to be sent.  Interval should be chosen appropriate to
         propagation time of DODAG and as appropriate to application
         requirements (e.g. response time vs. overhead).

   DelayDAO Timer  Up to one instance per DA DAO parent (the subset of DAG
         DODAG parents chosen to receive destination advertisements) per
         DODAG.  Expiry triggers sending of DAO message to the DA
         parent.  The interval is to be proportional to DEF_DAO_LATENCY/
         (node rank), such that nodes of greater rank (further down
         along the DODAG) expire first, coordinating the sending of DAO
         messages to allow for a chance of aggregation.
         parent.  See Section 6.8.1.1.1 6.2.6

   RemoveTimer  Up to one instance per DA DAO entry per neighbor (i.e.
         those neighbors that have given DAO messages to this node as a DAG
         DODAG parent) Expiry triggers a change in state for the DA DAO
         entry, setting up to do unreachable (No-DAO) advertisements or
         immediately deallocating the DA DAO entry if there are no DA DAO
         parents.  The interval is min(MAX_DESTROY_INTERVAL, TBD(DIO
         Trickle Timer Interval)).  See Section 6.8.1.1.1

10. 6.2.4.1.1.3

12.  Manageability Considerations

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

10.1.

12.1.  Control of Function and Policy

10.1.1.

12.1.1.  Initialization Mode

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

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

10.1.2.

12.1.2.  DIO Base option

   RPL specifies a number of protocol parameters.

   A RPL implementation SHOULD allow configuring the following routing
   protocol parameters, which are further described in Section 6.1.3.1: 5.1.1:

   DAGPreference
   InstanceID
   RPLInstanceID
   DAGObjectiveCodePoint
   DAGID
   DODAGID
   Destination Prefixes
   DIOIntervalDoublings
   DIOIntervalMin
   DIORedundancyConstant

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

   DAG

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

10.1.3.

12.1.3.  Trickle Timers

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

   For each DODAG, a RPL implementation MUST allow for the monitoring of
   the following parameters, further described in Section 6.3.4: 5.3.5.1:

   I
   T
   C
   I_min
   I_doublings

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

10.1.4.

12.1.4.  DAG Sequence Number Increment

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

10.1.5.

12.1.5.  Destination Advertisement Timers

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

   o  The DelayDAO timer

   o  The Remove timer

10.1.6.

12.1.6.  Policy Control

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

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

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

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

10.1.7.

12.1.7.  Data Structures

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

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

10.2.

12.2.  Information and Data Models

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

10.3.

12.3.  Liveness Detection and Monitoring

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

   As specified in Section 6.2, 3.1, an implementation is expected to
   maintain a set of data structures in support of DAG DODAG discovery:

   o  The candidate neighbors data structure

   o  For each DODAG:

      *  A set of DAG DODAG parents

10.3.1.

12.3.1.  Candidate Neighbor Data Structure

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

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

10.3.2.

12.3.2.  Directed Acyclic Graph (DAG) Table

   For each DAG, a RPL implementation is expected to keep track of the
   following DODAG table values:

   o  DAGID  DODAGID

   o  DAGObjectiveCodePoint

   o  A set of Destination Prefixes offered upwards along the DODAG

   o  A set of DAG DODAG Parents

   o  timer to govern the sending of DIO messages for the DODAG
   o  DAGSequenceNumber  DODAGSequenceNumber

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

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

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

   o  A flag reporting if the Parent is a DA DAO Parent as described in
      Section 6.8

10.3.3. 6

12.3.3.  Routing Table

   For each route provisioned by RPL operation, a RPL implementation
   MUST keep track of the following:

   o  Destination Prefix

   o  Destination Prefix Length

   o  Lifetime Timer

   o  Next Hop

   o  Next Hop Interface

   o  Flag indicating that the route was provisioned from one of:

      *  Unicast DAO message

      *  DIO message

      *  Multicast DAO message

10.3.4.

12.3.4.  Other RPL Monitoring Parameters

   A RPL implementation SHOULD provide a counter reporting the number of
   a times the node has detected an inconsistency with respect to a DAG
   DODAG parent, e.g. if the DAGID DODAGID has changed.

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

10.3.5.

12.3.5.  RPL Trickle Timers

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

   o  The DIOIntervalMin expressed in ms

   o  The DIOIntervalDoublings

   o  The DIORedundancyConstant

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

10.4.

12.4.  Verifying Correct Operation

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

10.5.

12.5.  Requirements on Other Protocols and Functional Components

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

10.6.

12.6.  Impact on Network Operation

   To be completed.

11.

13.  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].

12.

14.  IANA Considerations

12.1.

14.1.  RPL Control Message

   The RPL Control Message is an ICMP information message type that is
   to be used carry DAG Information Objects, DAG Information
   Solicitations, and Destination Advertisement Objects in support of
   RPL operation.

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

12.2.

14.2.  New Registry for RPL Control Codes

   IANA is requested to create a registry, RPL Control Codes, for the
   Code field of the ICMPv6 RPL Control Message.

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

   o  Code

   o  Description

   o  Defining RFC

   Three codes are currently defined:

        +------+----------------------------------+---------------+
        | Code | Description                      | Reference     |
        +------+----------------------------------+---------------+
        | 0x01 | DAG Information Solicitation     | This document |
        | 0x02 | DAG Information Object           | This document |
        | 0x04 | Destination Advertisement Object | This document |
        +------+----------------------------------+---------------+

                             RPL Control Codes

12.3.

14.3.  New Registry for the Control Field of the DIO Base

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

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

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

   o  Capability description

   o  Defining RFC

   Four groups are currently defined:

      +-------+-------------------------------------+---------------+

    +-------+-----------------------------------------+---------------+
    |  Bit  | Description                             | Reference     |
      +-------+-------------------------------------+---------------+
    +-------+-----------------------------------------+---------------+
    |   0   | Grounded DODAG (G)                      | This document |
    |   1   | Destination Advertisement Trigger Supported (A) | This document |
    |   2   | Destination Advertisement Supported Trigger (T)   | This document |
    |   3   | Destination Advertisements Stored (S)   | This document |
    | 5,6,7 | DAG DODAG Preference (Prf)                  | This document |
      +-------+-------------------------------------+---------------+
    +-------+-----------------------------------------+---------------+

                              DIO Base Flags

12.4.

14.4.  DAG Information Object (DIO) Suboption

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

         +-------+------------------------------+---------------+
         | Value | Meaning                      | Reference     |
         +-------+------------------------------+---------------+
         |   0   | Pad1 - DIO Padding           | This document |
         |   1   | PadN - DIO suboption padding | This document |
         |   2   | DAG Metric Container         | This Document |
         |   3   | Destination Prefix           | This Document |
         |   4   | DAG Timer Configuration      | This Document |
         +-------+------------------------------+---------------+

               DAG Information Option (DIO) Base Suboptions

13.

15.  Acknowledgements

   The authors would like to acknowledge the review, feedback, and
   comments from Emmanuel Baccelli, Dominique Barthel, Yusuf Bashir,
   Mathilde Durvy, Manhar Goindi, Mukul Goyal, Anders Jagd, Quentin
   Lampin, Jerry Martocci, Alexandru Petrescu, and Don Sturek.

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

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

14.

16.  Contributors

   RPL is the result of the contribution of the following members of the
   ROLL Design Team, including the editors, and additional contributors
   as listed below:

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

   Email: jpv@cisco.com

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

   Email: jhui@archrock.com

   Thomas Heide Clausen
   LIX, Ecole Polytechnique, France

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

   Philip Levis
   Stanford University
   358 Gates Hall, Stanford University
   Stanford, CA  94305-9030
   USA

   Email: pal@cs.stanford.edu

   Richard Kelsey
   Ember Corporation
   Boston, MA
   USA

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

   Stephen Dawson-Haggerty
   UC Berkeley
   Soda Hall, UC Berkeley
   Berkeley,

   Jonathan W. Hui
   Arch Rock Corporation
   501 2nd St. Ste. 410
   San Francisco, CA  94720  94107
   USA

   Email: stevedh@cs.berkeley.edu jhui@archrock.com
   Kris Pister
   Dust Networks
   30695 Huntwood Ave.
   Hayward,   94544
   USA

   Email: kpister@dustnetworks.com

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

   Email: abr@zen-sys.com

15.

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

   Email: stevedh@cs.berkeley.edu

17.  References

15.1.

17.1.  Normative References

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

15.2.

17.2.  Informative References

   [I-D.ietf-bfd-base]
              Katz, D. and D. Ward, "Bidirectional Forwarding
              Detection", draft-ietf-bfd-base-09 draft-ietf-bfd-base-11 (work in progress),
              February 2009.
              January 2010.

   [I-D.ietf-manet-nhdp]
              Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc
              Network (MANET) Neighborhood Discovery Protocol (NHDP)",
              draft-ietf-manet-nhdp-11 (work in progress), October 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-08 draft-ietf-roll-building-routing-reqs-09
              (work in progress), December 2009. January 2010.

   [I-D.ietf-roll-home-routing-reqs]
              Brandt, A. and J. Buron, "Home Automation Routing
              Requirements in Low Power and Lossy Networks",
              draft-ietf-roll-home-routing-reqs-09
              draft-ietf-roll-home-routing-reqs-11 (work in progress),
              November
              January 2010.

   [I-D.ietf-roll-of0]
              Thubert, P., "RPL Objective Function 0",
              draft-ietf-roll-of0-00 (work in progress), December 2009.

   [I-D.ietf-roll-routing-metrics]
              Vasseur, J. and D. Networks, "Routing Metrics used for
              Path Calculation in Low Power and Lossy Networks",
              draft-ietf-roll-routing-metrics-04 (work in progress),
              December 2009.

   [I-D.ietf-roll-terminology]
              Vasseur, J., "Terminology in Low power And Lossy
              Networks", draft-ietf-roll-terminology-02 (work in
              progress), October 2009.

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

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

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              August 1996.

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

   [RFC3697]  Rajahalme, J., Conta, A., Carpenter, B., and S. Deering,
              "IPv6 Flow Label Specification", RFC 3697, March 2004.

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

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

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

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

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

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

   [RFC5120]  Przygienda, T., Shen, N., and 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.

   [RFC5673]  Pister, K., Thubert, P., Dwars, S., and T. Phinney,
              "Industrial Routing Requirements in Low-Power and Lossy
              Networks", RFC 5673, October 2009.

Appendix A.  Requirements

A.1.  Protocol Properties Overview

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

A.1.1.  IPv6 Architecture

   RPL is strictly compliant with layered IPv6 architecture.

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

A.1.2.  Typical LLN Traffic Patterns

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

   As required by the aforementioned routing requirements documents, RPL
   supports the installation of multiple paths.  The use of multiple
   paths include sending duplicated traffic along diverse paths, as well
   as to support advanced features such as Class of Service (CoS) based
   routing, or simple load balancing among a set of paths (which could
   be useful for the LLN to spread traffic load and avoid fast energy
   depletion on some, e.g. battery powered, nodes).  Conceptually,
   multiple instances of RPL can be used to send traffic along different
   topology instances, the construction of which is governed by
   different Objective Functions (OF).  Details of RPL operation in
   support of multiple instances are beyond the scope of the present
   specification.

A.1.3.  Constraint Based Routing

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

A.2.  Deferred Requirements

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

Appendix B.  Examples

   Consider the example LLN physical topology in Figure 13.  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 DODAG depicted in Figure 14, where the links depicted are
   the edges toward DAG DODAG parents.  This topology includes one DAG,
   rooted by an LBR node (LBR) at rank 1.  The LBR node will issue DIO
   messages, as governed by a trickle timer.  Nodes (11), (12), (13),
   have selected (LBR) as their only parent, attached to the DAG DODAG at
   rank 2, and periodically multicast DIOs.  Node (22) has selected (11)
   and (12) in its DAG DODAG parent set, and advertises itself at rank 3.
   Node (22) thus has a set of DAG DODAG 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' 'bad' links are not depicted.

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

   Note that the links depicted represent directed links in the DAG DODAG
   overlaid on top of the physical topology depicted in Figure 13.  As
   such, the depicted edges represent the relationship between nodes and
   their DAG DODAG parents, wherein all depicted edges are directed and
   oriented `up' 'up' on the page toward the DAG DODAG root (LBR).  The DAG DODAG may
   provide default routes within the LLN, and serves as the foundation
   on which RPL builds further routing structure, e.g. through the
   destination advertisement mechanism.

                          Figure 14: Example DAG

B.1.  Destination Advertisement

   Consider the example DAG DODAG depicted in Figure 14.  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 message to Node (42), indicating the
      availability of destination (53).

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

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

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

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

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

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

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

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

B.2.  Example: DAG DODAG Parent Selection

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

   o  Node (N) transmits a RPL DIS message.

   o  Node (B) responds.  Node (N) investigates the DIO message, and
      learns that Node (B) is a member of DAGID DODAGID 1 at rank 4, and not
      `Blue'.
      '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 message that indicates that
      it is a member of DAGID DODAGID 1 at rank 2, but it carries the
      attribute
      `Blue'. '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 enough confidence to trigger a decision to
      join DAGID DODAGID 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 DODAG parents for
      DAGID
      DODAGID 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 DODAGID 1.

   o  Node (N) adds Node (B) (rank 4) to its set of DAG DODAG parents for
      DAGID
      DODAGID 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 upwards along DAGID DODAGID 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 upwards progress but with the intention that node
      (C) or a following successor can make upwards 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).

B.3.  Example: DAG DODAG Maintenance

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

              -1-                    -2-                    -3-

                        Figure 15: DAG Maintenance

   Consider the example depicted in Figure 15-1.  In this example, Node
   (A) is attached to a DAG DODAG at some rank d.  Node (A) is a DAG DODAG
   parent of Nodes (B) and (C).  Node (C) is a DAG DODAG 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, sub-DODAG, 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'
   '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 message from a Node (Z)
   at a lesser rank and superior position in the DAG DODAG than node (A).
   Node (C) may safely undergo the process to evict node (A) from its
   DAG parent set and attach directly to Node (Z) without creating a
   loop, because its rank will decrease.

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

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

   o  Node (D), hearing a modified DIO message from Node (C), follows
      Node (C) into the floating DAG.  This is depicted in Figure 15-2.
      In general, any node with no other options in the sub-DAG sub-DODAG of
      Node (C) will follow Node (C) into the floating DAG, maintaining
      the structure of the sub-DAG. sub-DODAG.

   o  Node (C) hears a DIO message with an incremented DAGSequenceNumber
      DODAGSequenceNumber from Node (B) and determines it is able to
      rejoin the grounded DAG DODAG by reattaching at a deeper rank to Node
      (B).  Node (C) adds Node (B) to its DAG DODAG parent set.  Node (C)
      has now safely moved deeper within the grounded DAG DODAG without
      creating any loops.

   o  Node (D), and any other sub-DAG sub-DODAG of Node (C), will hear the
      modified DIO message sourced from Node (C) and follow Node (C) in
      a coordinated manner to reattach to the grounded DAG.  The final
      DAG
      DODAG is depicted in Figure 15-3

B.4.  Example: Greedy Parent Selection and Instability

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

              -1-                    -2-                    -3-

                 Figure 16: Greedy DAG DODAG Parent Selection

   Consider the example depicted in Figure 16.  A DAG DODAG is depicted in 3
   different configurations.  A usable link between (B) and (C) exists
   in all 3 configurations.  In Figure 16-1, Node (A) is a DAG DODAG parent
   for Nodes (B) and (C), and (B)--(C) is a sibling link.  In
   Figure 16-2, Node (A) is a DAG DODAG parent for Nodes (B) and (C), and
   Node (B) is also a DAG DODAG parent for Node (C).  In Figure 16-3, Node
   (A) is a
   DAG DODAG parent for Nodes (B) and (C), and Node (C) is also a DAG
   DODAG 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 DODAG illustrated in
   Figure 16-1.  In this example, Nodes (B) and (C) may most prefer Node
   (A) as a DAG DODAG parent, but are operating under the greedy condition
   that will try to optimize for 2 parents.

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

   o  Let Figure 16-1 be the initial condition.

   o  Suppose Node (C) first is able to leave the DAG DODAG and rejoin at a
      lower rank, taking both Nodes (A) and (B) as DAG DODAG parents as
      depicted in Figure 16-2.  Now Node (C) is deeper than both Nodes
      (A) and (B), and Node (C) is satisfied to have 2 DAG DODAG parents.

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

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

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

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

   o  The process will repeat, and the DAG DODAG will oscillate between
      Figure 16-2 and Figure 16-3 until the nodes count to infinity and
      restart the cycle again.

   o  This cycle can be averted through mechanisms in RPL:

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

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

Appendix C.  Outstanding Issues

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

C.1.  Additional Support for P2P Routing

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

C.2.  Loop Detection

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

C.3.  Destination Advertisement / DAO Fan-out

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

   In general, the utility of providing redundancy along downwards
   routes by sending DAO messages to more than one parent is under
   investigation.

   The use of suitable triggers, such as the `D' bit, 'T' flag, to trigger DA
   operation within an affected sub-DAG, sub-DODAG, is under investigation.
   Further, the ability to limit scope of the affected depth within the
   sub-DAG
   sub-DODAG is under investigation (e.g. if a stateful node can proxy
   for all nodes `behind' 'behind' it, then there may be no need to propagate the
   triggered `D' bit 'T' flag further).

C.4.

C.3.  Source Routing

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

C.5.

C.4.  Address / Header Compression

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

C.5.  Managing Multiple Instances

   A network may run multiple instances of RPL concurrently.  Such a
   network will require methods for assigning and otherwise managing
   RPLInstanceIDs.  This will likely be addressed in a separate
   document.

Authors' Addresses

   Tim Winter (editor)

   Email: wintert@acm.org

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

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

   ROLL Design Team
   IETF ROLL WG

   Email: rpl-authors@external.cisco.com