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

Networking Working Group                                  T. Winter, Ed.
Internet-Draft
Intended status: Standards Track                         P. Thubert, Ed.
Expires: September 9, 2010                                 Cisco Systems
                                                        ROLL Design Team
                                                            IETF ROLL WG
                                                           March 8, 2010


      RPL: IPv6 Routing Protocol for Low power and Lossy Networks
                         draft-ietf-roll-rpl-07

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.

Status of this Memo

   This Internet-Draft is submitted to IETF in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt.



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   The list of Internet-Draft Shadow Directories can be accessed at
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   This Internet-Draft will expire on September 9, 2010.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  6
     1.1.  Design Principles  . . . . . . . . . . . . . . . . . . . .  6
     1.2.  Expectations of Link Layer Type  . . . . . . . . . . . . .  7
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Topology . . . . . . . . . . . . . . . . . . . . . . . . .  9
       3.1.1.  Topology Identifiers . . . . . . . . . . . . . . . . .  9
       3.1.2.  DODAG Information  . . . . . . . . . . . . . . . . . . 10
     3.2.  Instances, DODAGs, and DODAG Iterations  . . . . . . . . . 11
     3.3.  Traffic Flows  . . . . . . . . . . . . . . . . . . . . . . 13
       3.3.1.  Multipoint-to-Point Traffic  . . . . . . . . . . . . . 13
       3.3.2.  Point-to-Multipoint Traffic  . . . . . . . . . . . . . 13
       3.3.3.  Point-to-Point Traffic . . . . . . . . . . . . . . . . 13
     3.4.  Upward Routes and DODAG Construction . . . . . . . . . . . 13
       3.4.1.  DODAG Information Object (DIO) . . . . . . . . . . . . 14
       3.4.2.  DAG Repair . . . . . . . . . . . . . . . . . . . . . . 14
       3.4.3.  Grounded and Floating DODAGs . . . . . . . . . . . . . 15
       3.4.4.  Administrative Preference  . . . . . . . . . . . . . . 15
       3.4.5.  Objective Function (OF)  . . . . . . . . . . . . . . . 15
       3.4.6.  Distributed Algorithm Operation  . . . . . . . . . . . 15
     3.5.  Downward Routes and Destination Advertisement  . . . . . . 16
       3.5.1.  Destination Advertisement Object (DAO) . . . . . . . . 16
     3.6.  Routing Metrics and Constraints Used By RPL  . . . . . . . 17
       3.6.1.  Loop Avoidance . . . . . . . . . . . . . . . . . . . . 18
       3.6.2.  Rank Properties  . . . . . . . . . . . . . . . . . . . 19
   4.  ICMPv6 RPL Control Message . . . . . . . . . . . . . . . . . . 21
   5.  Upward Routes  . . . . . . . . . . . . . . . . . . . . . . . . 22
     5.1.  DODAG Information Object (DIO) . . . . . . . . . . . . . . 22
       5.1.1.  DIO Base Format  . . . . . . . . . . . . . . . . . . . 22
       5.1.2.  DIO Base Rules . . . . . . . . . . . . . . . . . . . . 24
       5.1.3.  DIO Suboptions . . . . . . . . . . . . . . . . . . . . 25
     5.2.  DODAG Information Solicitation (DIS) . . . . . . . . . . . 30
     5.3.  Upward Route Discovery and Maintenance . . . . . . . . . . 30
       5.3.1.  RPL Instance . . . . . . . . . . . . . . . . . . . . . 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  . . . . . . . . . . . . . . 36
       5.3.5.  DIO Transmission . . . . . . . . . . . . . . . . . . . 36
       5.3.6.  DODAG Selection  . . . . . . . . . . . . . . . . . . . 39
     5.4.  Operation as a Leaf Node . . . . . . . . . . . . . . . . . 39
     5.5.  Administrative Rank  . . . . . . . . . . . . . . . . . . . 40
     5.6.  Collision  . . . . . . . . . . . . . . . . . . . . . . . . 40
   6.  Downward Routes  . . . . . . . . . . . . . . . . . . . . . . . 40
     6.1.  Destination Advertisement Object (DAO) . . . . . . . . . . 41
       6.1.1.  DAO Suboptions . . . . . . . . . . . . . . . . . . . . 42
     6.2.  Downward Route Discovery and Maintenance . . . . . . . . . 43



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       6.2.1.  Overview . . . . . . . . . . . . . . . . . . . . . . . 43
       6.2.2.  Mode of Operation  . . . . . . . . . . . . . . . . . . 44
       6.2.3.  Destination Advertisement Parents  . . . . . . . . . . 44
       6.2.4.  Operation of DAO Storing Nodes . . . . . . . . . . . . 45
       6.2.5.  Operation of DAO Non-storing Nodes . . . . . . . . . . 48
       6.2.6.  Scheduling to Send DAO (or no-DAO) . . . . . . . . . . 49
       6.2.7.  Triggering DAO Message from the Sub-DODAG  . . . . . . 49
       6.2.8.  Sending DAO Messages to DAO Parents  . . . . . . . . . 51
       6.2.9.  Multicast Destination Advertisement Messages . . . . . 52
   7.  Packet Forwarding and Loop Avoidance/Detection . . . . . . . . 52
     7.1.  Suggestions for Packet Forwarding  . . . . . . . . . . . . 53
     7.2.  Loop Avoidance and Detection . . . . . . . . . . . . . . . 54
       7.2.1.  Source Node Operation  . . . . . . . . . . . . . . . . 55
       7.2.2.  Router Operation . . . . . . . . . . . . . . . . . . . 55
   8.  Multicast Operation  . . . . . . . . . . . . . . . . . . . . . 57
   9.  Maintenance of Routing Adjacency . . . . . . . . . . . . . . . 58
   10. Guidelines for Objective Functions . . . . . . . . . . . . . . 59
   11. RPL Constants and Variables  . . . . . . . . . . . . . . . . . 61
   12. Manageability Considerations . . . . . . . . . . . . . . . . . 62
     12.1. Control of Function and Policy . . . . . . . . . . . . . . 62
       12.1.1. Initialization Mode  . . . . . . . . . . . . . . . . . 62
       12.1.2. DIO Base option  . . . . . . . . . . . . . . . . . . . 63
       12.1.3. Trickle Timers . . . . . . . . . . . . . . . . . . . . 63
       12.1.4. DAG Sequence Number Increment  . . . . . . . . . . . . 64
       12.1.5. Destination Advertisement Timers . . . . . . . . . . . 64
       12.1.6. Policy Control . . . . . . . . . . . . . . . . . . . . 64
       12.1.7. Data Structures  . . . . . . . . . . . . . . . . . . . 65
     12.2. Information and Data Models  . . . . . . . . . . . . . . . 65
     12.3. Liveness Detection and Monitoring  . . . . . . . . . . . . 65
       12.3.1. Candidate Neighbor Data Structure  . . . . . . . . . . 65
       12.3.2. Directed Acyclic Graph (DAG) Table . . . . . . . . . . 65
       12.3.3. Routing Table  . . . . . . . . . . . . . . . . . . . . 66
       12.3.4. Other RPL Monitoring Parameters  . . . . . . . . . . . 67
       12.3.5. RPL Trickle Timers . . . . . . . . . . . . . . . . . . 67
     12.4. Verifying Correct Operation  . . . . . . . . . . . . . . . 67
     12.5. Requirements on Other Protocols and Functional
           Components . . . . . . . . . . . . . . . . . . . . . . . . 67
     12.6. Impact on Network Operation  . . . . . . . . . . . . . . . 67
   13. Security Considerations  . . . . . . . . . . . . . . . . . . . 67
   14. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 67
     14.1. RPL Control Message  . . . . . . . . . . . . . . . . . . . 68
     14.2. New Registry for RPL Control Codes . . . . . . . . . . . . 68
     14.3. New Registry for the Control Field of the DIO Base . . . . 68
     14.4. DODAG Information Object (DIO) Suboption . . . . . . . . . 69
   15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 69
   16. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 70
   17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 71
     17.1. Normative References . . . . . . . . . . . . . . . . . . . 71



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     17.2. Informative References . . . . . . . . . . . . . . . . . . 72
   Appendix A.  Requirements  . . . . . . . . . . . . . . . . . . . . 74
     A.1.  Protocol Properties Overview . . . . . . . . . . . . . . . 74
       A.1.1.  IPv6 Architecture  . . . . . . . . . . . . . . . . . . 74
       A.1.2.  Typical LLN Traffic Patterns . . . . . . . . . . . . . 74
       A.1.3.  Constraint Based Routing . . . . . . . . . . . . . . . 74
     A.2.  Deferred Requirements  . . . . . . . . . . . . . . . . . . 75
   Appendix B.  Examples  . . . . . . . . . . . . . . . . . . . . . . 75
     B.1.  DAO Operation When Only the Root Node Stores DAO
           Information  . . . . . . . . . . . . . . . . . . . . . . . 75
     B.2.  DAO Operation When All Nodes Fully Store DAO
           Information  . . . . . . . . . . . . . . . . . . . . . . . 77
     B.3.  DAO Operation When Nodes Have Mixed Capabilities . . . . . 79
   Appendix C.  Outstanding Issues  . . . . . . . . . . . . . . . . . 81
     C.1.  Additional Support for P2P Routing . . . . . . . . . . . . 81
     C.2.  Destination Advertisement / DAO Fan-out  . . . . . . . . . 81
     C.3.  Source Routing . . . . . . . . . . . . . . . . . . . . . . 81
     C.4.  Address / Header Compression . . . . . . . . . . . . . . . 81
     C.5.  Managing Multiple Instances  . . . . . . . . . . . . . . . 82
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 82































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1.  Introduction

   Low power and Lossy Networks (LLNs) 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 (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



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   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

   RPL does not rely on any particular features of a specific link layer
   technology.  RPL is designed to be able to operate over a variety of
   different link layers, including but not limited 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].

   Additionally, this document uses 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 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.

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

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

   Rank: The rank of a node in a DAG identifies the nodes position with
         respect to a DODAG root.  The farther away a node is from a
         DODAG root, the higher is the rank of that node.  The rank of a
         node may be a simple topological distance, or may more commonly
         be calculated as a function of other properties as described



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         later.

   DODAG parent:  A parent of a node within a DODAG is one of the
         immediate successors of the node on a path towards the DODAG
         root.  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 DODAG is defined in this
         specification to be any neighboring node which is located at
         the same rank within a DODAG.  Note that siblings defined in
         this manner do not necessarily share a common DODAG parent.
         (See Section 3.6.2.1).

   Sub-DODAG  The sub-DODAG of a node is the set of other nodes in the
         DODAG that might use a path towards the DODAG root that
         contains that node.  Nodes in the 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-DODAG of that node).
         (See Section 3.6.2.1).

   DODAGID:  The identifier of a DODAG root.  The DODAGID must be unique
         within the scope of a RPL Instance in the LLN.

   DODAG Iteration:  A specific sequence number iteration ("version") of
         a 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.




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   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].

   Objective Function (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 DODAG is said to be grounded, when the root can reach
         the Goal of the objective function.

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

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


3.  Protocol 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.  Topology

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

3.1.1.  Topology Identifiers

   RPL uses four identifiers to track and control the topology:

   o  The first is a RPLInstanceID.  A RPLInstanceID identifies a set of
      one or more DODAGs.  All DODAGs in the same RPL Instance use the



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      same OF.  A network may have multiple RPLInstanceIDs, each of
      which defines an independent set of DODAGs, which may be optimized
      for different OFs and/or applications.  The set of DODAGs
      identified by a RPLInstanceID is called a RPL Instance.

   o  The second is a DODAGID.  The scope of a DODAGID is a RPL
      Instance.  The combination of RPLInstanceID and DODAGID uniquely
      identifies a single DODAG in the network.  A RPL Instance may have
      multiple DODAGs, each of which has an unique DODAGID.

   o  The third is a DODAGSequenceNumber.  The scope of a
      DODAGSequenceNumber is a DODAG.  A DODAG is sometimes
      reconstructed from the DODAG root, by incrementing the
      DODAGSequenceNumber.  The combination of RPLInstanceID, DODAGID,
      and DODAGSequenceNumber uniquely identifies a DODAG Iteration.

   o  The fourth is rank.  The scope of rank is a DODAG Iteration.  Rank
      establishes a 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 may become, a member of, the
   implementation should conceptually keep track of the following
   information.  The data structures described in this 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 RPL control messages







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3.2.  Instances, DODAGs, and DODAG Iterations

   Each RPL Instance constructs a routing topology optimized for a
   certain Objective Function (OF).  A RPL Instance may provide routes
   to certain destination prefixes, reachable via the DODAG 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 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



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   DODAG) is depicted in Figure 2.


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

                          Figure 1: RPL Instance



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


                         Figure 2: DODAG Iteration








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3.3.  Traffic Flows

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.

3.3.2.  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.

3.3.3.  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'
   neighbor.

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

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 DODAG
   Information Object (DIO) messages.  Undirected links between siblings
   are also identified during this process, which can be used to provide
   additional diversity.





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3.4.1.  DODAG Information Object (DIO)

   A DIO identifies the RPL Instance, the DODAGID, the values used to
   compute the 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 Control Message",
   which is capable of carrying a DIO.

3.4.2.  DAG Repair

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

   RPL supports mechanisms which may be used for local repair within the
   DODAG iteration.  The DIO message 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-DODAG by advertising an effective rank of INFINITY
      to its sub-DODAG, OR detach and form a floating DODAG in order to
      preserve inner connectivity within its 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, if at all possible.







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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.

3.4.4.  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.

3.4.5.  Objective Function (OF)

   The Objective Function (OF) implements the optimization objectives of
   route selection within the 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-of0], and related companion specifications.

   By using defined OFs that are understood by all nodes in a particular
   deployment, and by referencing 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 RPL
   Instance using a known Objective Function, it may be configured to
   join a RPL Instance using an unknown Objective Function - but in that
   case only acting as a leaf node.

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.




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   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  Nodes provision routing table entries, for the destinations
      specified by the 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 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.

3.5.  Downward Routes and Destination Advertisement

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



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   operation.

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

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' route to local neighbors.

3.6.  Routing Metrics and Constraints Used By RPL

   Routing metrics are used by routing protocols to compute 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 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' from the candidate
   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 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.







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   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

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.

3.6.1.1.  Greediness and Rank-based Instabilities

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

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

   Suppose a node is willing to receive and process a DIO messages from
   a node in its own sub-DODAG, and in general a node deeper than
   itself.  In this case, a possibility exists that a feedback loop is
   created, wherein two or more nodes continue to try and move in the
   DODAG iteration while attempting to optimize against each other.  In
   some cases, this will result in 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 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-DODAG.

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-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.





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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 related DAO state.  This loop happens
   when a no-DAO was missed and persists until all 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.

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, in order to prevent
   sibling loops.

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 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 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 the stability of the
           routing topology.  Some dampening or filtering might be
           applied to keep the topology stable, and thus the rank does
           not necessarily change as fast as some physical metrics



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           would.  A new DODAG iteration would be a good opportunity to
           reconcile the discrepancies that might form over time between
           metrics and ranks within a DODAG iteration.

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

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

   Abstract:  Rank does not have a physical unit, but rather a range of
           increment per hop, where the assignment of each increment is
           to be determined by the implementation.

   The rank value feeds into DODAG parent 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 DODAG parent selection and
   movement within the DODAG are restricted in favor of loop avoidance.

3.6.2.1.  Rank Comparison (DAGRank())

   Rank may be thought of as a fixed point number, where the position of
   the decimal point between the integer part and the fractional part is
   determined by MinHopRankIncrease.  MinHopRankIncrease is the minimum
   increase in rank between a node and any of its DODAG parents.  When
   an objective function computes rank, the objective function operates
   on the entire (i.e. 16-bit) rank quantity.  When rank is compared,
   e.g. for determination of parent/sibling relationships or loop
   detection, the integer portion of the rank is to be used.  The
   integer portion of the Rank is computed by the DAGRank() macro as
   follows:


              DAGRank(rank) = 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.

   By convention in this document, using the macro DAGRank(node) may be
   interpreted as DAGRank(node.rank), where node.rank is the rank value



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   as maintained by the node.

   A node A has a rank less than the rank of a node B if DAGRank(A) is
   less than DAGRank(B).

   A node A has a rank equal to the rank of a node B if DAGRank(A) is
   equal to DAGRank(B).

   A node A has a rank greater than the rank of a node B if DAGRank(A)
   is greater than DAGRank(B).

3.6.2.2.  Rank Relationships

   The computation of the 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 closer to the DODAG root than the position of N. Node M
           may safely be a DODAG parent for Node N without risk of
           creating a loop.  Further, for a node N, all parents in the
           DODAG parent set must be of rank less than DAGRank(N).  In
           other words, the rank presented by a node N MUST be greater
           than that presented by any of its parents.

   DAGRank(M) equals DAGRank(N):  In this case the positions of M and N
           within the 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, which however entails the chance of
           creating a loop (which must be detected and resolved by some
           other means).

   DAGRank(M) is greater than DAGRank(N):  In this case, the position of
           M is farther from the DODAG root than the position of N.
           Further, Node M may in fact be in the sub-DODAG of Node N. If
           node N selects node M as DODAG parent there is a risk to
           create a loop.

   As an example, the 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 function being
   used within the DODAG.


4.  ICMPv6 RPL Control Message

   This document defines the RPL Control Message, a new ICMPv6 message.



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   In accordance with [RFC4443], the 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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |     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 field identifies the type of RPL Control Message.  This
   document defines three codes for the following RPL Control Message
   types:

   o  0x01: DODAG Information Solicitation (Section 5.2)

   o  0x02: DODAG Information Object (Section 5.1)

   o  0x04: Destination Advertisement Object (Section 6.1)


5.  Upward Routes

   This section describes how RPL discovers and maintains upward routes.
   It describes DODAG Information Objects (DIOs), the messages used to
   discover and maintain these routes.  It specifies how RPL generates
   and responds to DIOs.  It also describes DODAG Information
   Solicitation (DIS) messages, which are used to trigger DIO
   transmissions.

5.1.  DODAG Information Object (DIO)

   The DODAG 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 Base Format

   DIO Base is an always-present container option in a DIO message.
   Every DIO MUST include a DIO Base.



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

                            Figure 4: DIO Base

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

         Grounded (G):  The Grounded (G) flag indicates whether the
               upward routes this node advertises provide connectivity
               to the set of addresses which are application-defined
               goals.  If the 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) flag indicates whether the
               root of this DODAG can collect and use downward route
               state.  If the flag is set, nodes in the network are
               enabled to exchange destination advertisements messages
               to build downward routes (Section 6).  If the flag is
               cleared, destination advertisement messages are disabled
               and the DODAG maintains only upward routes.

         Destination Advertisement Trigger (T):  The Destination
               Advertisement Trigger (T) flag indicates a complete
               refresh of downward routes.  If the flag is set, then a
               refresh of downward route state is to take place over the
               entire DODAG.  If the flag is cleared, the downward route
               maintenance is in its normal mode of operation.  The
               further details of this process are described in
               Section 6.







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         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.  If the flag is set, then a
               non-root ancestor is known to be storing routing table
               entries learned from DAO messages.  If the flag is
               cleared, only the root node may be storing routing table
               entries learned from DAO messaging.  This flag is further
               described in Section 6.

         DODAGPreference (Prf):  A 3-bit unsigned integer that defines
               how preferable the root of this DODAG is compared to
               other DODAG roots within the instance.  DAGPreference
               ranges from 0x00 (least preferred) to 0x07 (most
               preferred).  The default is 0 (least preferred).
               Section 5.3 describes how DAGPreference affects DIO
               processing.

         Unassigned bits of the Control Field are reserved.  They MUST
         be set to zero on transmission and MUST be ignored on
         reception.

   Sequence Number:  8-bit unsigned integer set by the DODAG root.
         Section 5.3 describes the rules for sequence numbers and how
         they affect DIO processing.

   Rank: 16-bit unsigned integer indicating the DODAG rank of the node
         sending the DIO message.  Section 5.3 describes how Rank is set
         and how it affects DIO processing.

   RPLInstanceID:  8-bit field set by the DODAG root that indicates
         which RPL Instance the DODAG is part of.

   Destination Advertisement Trigger Sequence Number (DTSN):  8-bit
         unsigned integer set by the node issuing the DIO message.  The
         Destination Advertisement Trigger Sequence Number (DTSN) flag
         is used as part of the procedure to maintain downward routes.
         The details of 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 IPv6 address of
         the DODAG root.

5.1.2.  DIO Base Rules

   1.  If the 'A' flag of a DIO Base is cleared, the 'T' flag MUST also
       be cleared.




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   2.  For the following DIO Base fields, a node that is not a DODAG
       root MUST advertise the same values as 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 that DODAG root
       eventually advertises the 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 following fields at each hop:
       1.  Destination Advertisements Stored (S)
       2.  DAGRank
       3.  DTSN

   4.  The DODAGID field each root sets MUST be unique within the 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 Container,
   DAG Destination Prefix, and DAG Configuration.

5.1.3.1.  DIO Suboption Format

   The Pad N, DAG Metric Container, DAG Destination Prefix, and DAG
   Configuration suboptions all follow this format:

        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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
       |  Subopt. Type |       Suboption Length        | Suboption Data
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -

                  Figure 5: DIO Suboption Generic Format

   Suboption Type:  8-bit identifier of the type of suboption.

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





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

   The following subsections specify the DIO message suboptions which
   are currently defined for use in the DODAG Information Object.

   When processing a DIO message containing a suboption for which the
   Suboption Type value is not recognized by the receiver, the receiver
   MUST silently ignore the unrecognized option and continue to process
   the following suboption, correctly handling any remaining options in
   the message.

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

5.1.3.2.  Pad1

   The Pad1 suboption format is as follows:


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

                              Figure 6: Pad 1

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

   The Pad1 option is used to insert one or two octets of padding in the
   DIO message 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.

5.1.3.3.  PadN

   The PadN suboption format is as follows:








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        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    |       Suboption Length        | Suboption Data
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -

                              Figure 7: Pad N

   The PadN suboption is used to insert three or more octets of padding
   in the DIO message to enable suboptions alignment.  For N (N > 2)
   octets of padding, the Suboption Length field contains the value N-3,
   and the Option Data consists of N-3 zero-valued octets.  PadN Option
   data MUST be ignored by the receiver.

5.1.3.4.  Metric Container

   The Metric Container suboption 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 = 2    |       Suboption Length        | Metric Data
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -

                        Figure 8: Metric Container

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

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

5.1.3.5.  Destination Prefix

   The Destination Prefix suboption format is as follows:











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

                     Figure 9: DAG Destination Prefix

   The Destination Prefix suboption is used to indicate that
   connectivity to the specified destination prefix is available from
   the DODAG root, or from another node located upwards along the DODAG
   on the path to the DODAG root.  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 of a particular application.

   The Suboption Length is coded as the length of the suboption in
   octets, excluding the 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
   length of time in seconds (relative to the time the packet is sent)
   that the Destination Prefix is valid for route determination.  The
   lifetime is initially set by the node that owns the prefix and
   denotes the valid lifetime for that prefix (similar to
   AdvValidLifetime [RFC4861]).  The value might be reduced by the
   originator and/or en-route nodes that will not provide connectivity
   for the whole valid lifetime.  A value of all one bits (0xFFFFFFFF)
   represents infinity.  A value of all zero bits (0x00000000) indicates
   a loss of reachability.

   The Prefix Length is an 8-bit unsigned integer that indicates the
   number of leading bits in the destination prefix.

   The Destination Prefix contains Prefix Length significant bits of the
   destination prefix.  The remaining bits of the Destination Prefix, as



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   required to complete the trailing octet, are set to 0.

   In the 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 Configuration

   The DODAG Configuration suboption 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   | MinHopRankInc |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                      Figure 10: DODAG Configuration

   The DODAG Configuration suboption is used to distribute configuration
   information for DODAG Operation through the DODAG.  The information
   communicated in this suboption is generally static and unchanging
   within the DODAG, therefore it 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 unicast
   DODAG Information Solicitation (DIS) message.

   The Length is coded as 5.

   DIOIntervalDoublings is an 8-bit unsigned integer, configured on the
   DODAG root and used to configure the trickle timer (see
   Section 5.3.5.1 for details on trickle timers) 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 the trickle timer governing when DIO
   message should be sent within the DODAG.  The minimum configured
   interval for the DIO trickle timer in 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
   minimum number of relevant incoming DIOs required to suppress a DIO



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   transmission.  If the 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 mechanism is disabled.

   MinHopRankInc, 8-bit unsigned integer, is the MinHopRankIncrease as
   described in Section 3.6.2.1.

5.2.  DODAG Information 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 may use DIS to
   probe its neighborhood for nearby DODAGs.  The DODAG Information
   Solicitation carries no additional message body.  Section 5.3.5
   describes how nodes respond to a DIS.

5.3.  Upward Route Discovery and Maintenance

   Upward route discovery allows a node to join a DODAG by discovering
   neighbors that are members of the DODAG and identifying a set of
   parents.  The exact policies for selecting neighbors and parents is
   implementation-dependent.  This section specifies the set of rules
   those policies must follow for interoperability.

5.3.1.  RPL Instance

      A RPLInstanceID MUST be unique across an LLN.

      A node MAY belong to 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 three logical sets of link-local nodes.  First, the candidate
   neighbor set is a subset of the nodes that can be reached via link-
   local multicast.  The selection of this set is implementation-
   dependent and OF-dependent.  Second, the parent set is a restricted
   subset of the candidate neighbor set.  Finally, the preferred parent,
   a set of size one, is an element of the parent set that is the
   preferred next hop in upward routes.

   More precisely:



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   1.  The DODAG parent set MUST be a subset of the candidate neighbor
       set.

   2.  A DODAG root MUST have a 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 one.

   4.  A node's preferred DODAG parent MUST be a member of its DODAG
       parent set.

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

   6.  When Neighbor Unreachability Detection (NUD), or an equivalent
       mechanism, determines that a 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 eliminated from the 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 a DODAG root is loop-
   free, as rank decreases on each hop to the root.  The OF can guide
   candidate neighbor set and parent set selection, as discussed in
   [I-D.ietf-roll-routing-metrics].

5.3.3.  Neighbors and Parents across DODAG Iterations

   The above rules govern a single DODAG iteration.  The rules in this
   section define how RPL operates when there are multiple DODAG
   iterations:

5.3.3.1.  DODAG Iteration

   1.  The tuple (RPLInstanceID, DODAGID, DODAGSequenceNumber) uniquely
       defines a DODAG Iteration.  Every element of a node's DODAG
       parent set, as conveyed by the last heard DIO from each DODAG
       parent, MUST belong to the same DODAG iteration.  Elements of a
       node's candidate neighbor set MAY belong to different DODAG
       Iterations.

   2.  A 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 root of the corresponding DODAG.





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   3.  A node MUST NOT send DIOs for DODAG iterations of which it is not
       a member.

   4.  DODAG roots MAY increment the DODAGSequenceNumber that they
       advertise and thus move to a new DODAG iteration.  When a DODAG
       root increments its DODAGSequenceNumber, it MUST follow the
       conventions of Serial Number Arithmetic as described in
       [RFC1982].

   5.  Within a given DODAG, a node that is a not a root MUST NOT
       advertise a DODAGSequenceNumber higher than the highest
       DODAGSequenceNumber it has heard.  Higher is defined as the
       greater-than operator in [RFC1982].

   6.  Once a node has advertised a DODAG iteration by sending a DIO, it
       MUST NOT be member of a previous DODAG iteration of the same
       DODAG (i.e. with the same RPLInstanceID, the same DODAGID, and a
       lower DODAGSequenceNumber).  Lower is defined as the less-than
       operator in [RFC1982].

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

   When the DODAG parent set becomes empty on a node that is not a root,
   (i.e. the last parent has been removed, causing the node to no longer
   be associated with that DODAG), 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 DODAGSequenceNumber is incremented, a new DODAG Iteration
   spreads outward from the DODAG root.  Thus a parent that advertises
   the new DODAGSequenceNumber can not possibly belong to the sub-DODAG
   of a node that still advertises an older 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



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   previous DODAG Iteration (prior DODAGSequenceNumber) once it has
   committed to advertising the new DODAG Iteration.

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

5.3.3.2.  DODAG Roots

   1.  A DODAG root that does not have connectivity to the 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 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.

   An LLN node that is a 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 that backbone and use one
   router as a "backbone root".  The backbone root is the virtual root
   of the 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 itself, expose a rank of
   ROOT_RANK to the LLN, and are part of the same DODAG, coordinating
   DODAGSequenceNumber and other DODAG root determined parameters with
   the virtual root over the backbone.

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.  All else being equal, it is left to the
   implementation to determine which DODAG is most preferred, possibly
   based on additional criteria beyond Prf and the OF.

5.3.3.4.  Rank and Movement within a DODAG Iteration






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   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.

   3.  Let L be the lowest rank within a DODAG iteration that a given
       node has advertised.  Within the same DODAG Iteration, that node
       MUST NOT advertise an effective rank 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 RPL Instance.  Such a join has no rank restrictions, unless
       that different DODAG is a DODAG Iteration of which that node has
       previously been a 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
       DODAGSequenceNumber Iteration advertised from suitable DODAG
       parents, choose to migrate 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 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 for some reasonable 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 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.



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   If a node needs to move down a DODAG that it is attached to, causing
   the rank to increase, then it MAY poison its routes and delay before
   moving as described in Section 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-DODAG, by advertising an effective rank of
       INFINITE_RANK and resetting the associated DIO trickle timer to
       cause 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, before announcing a new
       rank.

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

   An implementation may choose to employ this poisoning mechanism when
   a node loses all of its current parents, i.e. the set of DODAG
   parents becomes depleted, and it can not jump to an alternate 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
   portions of the network are able to re-establish upwards routes.
   Path redundancy in the 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).

5.3.3.6.  Detaching

   1.  A node unable to stay connected to a DODAG within a given DODAG
       iteration MAY detach from this DODAG iteration.  A node that
       detaches becomes root of its own floating DODAG and SHOULD
       immediately advertise this new situation in a DIO as an alternate
       to poisoning.



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5.3.3.7.  Following a Parent

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

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

5.3.4.  DIO Message Communication

   When an DIO message is 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 the sender of the DIO message is a member of the candidate
       neighbor set, then the DIO is eligible for further processing.

5.3.4.1.  DIO Message Processing

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

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

5.3.5.  DIO Transmission

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



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   o  When a node detects or causes an inconsistency, it MUST reset the
      trickle timer.

   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 7.2, the node MUST reset the trickle timer.

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

   o  When a node receives a unicast DIS message, it MUST unicast a DIO
      message in response, and the response MUST include the DODAG
      Configuration Object.  This provides a means that an interrogating
      node may be guaranteed to receive the DODAG Configuration Object,
      which otherwise might not be included at the option of the sender.
      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
      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 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, in the absence of a grounded node, for
      example in support of LLN installation and commissioning.

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 (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]





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   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.

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.  If I is not equal to I_min:

       1.  Setting I to I_min.

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

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

   When a node learns about a DODAG through a DIO message, and makes the
   decision to join 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.







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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 DIO message from a DODAG parent that updates
      the information learned from a prior DIO message for that DODAG
      Parent

   o  A 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 DODAG parent has changed.

5.3.6.  DODAG Selection

   The DODAG selection is implementation and algorithm dependent.  Nodes
   SHOULD prefer to join DODAGs for 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 DODAG parent.

5.4.  Operation as a Leaf Node

   In some cases a RPL node may attach to a DODAG as a leaf node only.
   One example of such a case is when a node does not understand the RPL
   Instance's OF.  A leaf node does not extend DODAG connectivity but
   still needs to advertise its presence using DIOs.  A node operating



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   as a leaf node must obey the following rules:

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

   2.  Its DIOs must advertise a DAGRank of INFINITE_RANK.

   3.  It MAY transmit unicast DAOs as described in Section 6.2.

   4.  It MAY transmit multicast DAOs to the '1 hop' neighborhood as
       described in Section 6.2.9.

5.5.  Administrative Rank

   In some cases it might be beneficial to adjust the rank advertised by
   a node beyond that computed by the OF based on some implementation
   specific policy and properties of the node.  For 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.

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 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.  Downward Routes

   This section describes how RPL discovers and maintains downward
   routes.  Messages containing the Destination Advertisement Object
   (DAO), used to construct downward routes, are described.  The
   downward routes are necessary in support of P2MP flows, from the
   DODAG roots toward the leaves.  It specifies non-storing and storing
   behavior of nodes with respect to DAO messaging and DAO routing table



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   entries.  Nodes, as according to their resources and the
   implementation, may selectively store routing table entries learned
   from DAO messages, or may instead propagate the DAO information
   upwards while adding source routing information.  A further
   optimization is described whereby DAO messages may be used to
   populate routing table entries for the '1-hop' neighbors, which may
   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 information upwards along the 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 node that
         owns the prefix for each new DAO message for that prefix.

   DAO Rank:  16-bit unsigned integer indicating the DAO Rank associated
         with the advertised Destination Prefix.  The DAO Rank is
         analogous to the Rank in the DIO message in that it may be used
         to convey a relative distance to the Destination Prefix as
         computed by the Objective Function in use over the DODAG.  It
         serves as a mechanism by which an ancestor node may order
         alternate DAO paths.





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   RPLInstanceID:  8-bit field indicating the 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 capable of storing
         a limited amount of routing state.  The further specification
         of this field and its use is under investigation.

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

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

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

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

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

6.1.1.  DAO Suboptions

   The DAO message may optionally include a number of suboptions.

   The DAO suboptions are in the same format as the DIO Suboptions
   described in Section 6.1.1.

   In particular, a DAO message may include a DAG Metric Container
   suboption as described in Section 5.1.3.4.  This suboption may be
   present in implementations where the DAO Rank is insufficient to
   optimize a path to the DAO Destination Prefix.






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6.2.  Downward Route Discovery and Maintenance

6.2.1.  Overview

   Destination Advertisement operation produces DAO messages that flow
   up the DODAG, provisioning downward routing state for destination
   prefixes available in the sub-DODAG of the DODAG root, and possibly
   other nodes.  The routing state provisioned with this mechanism is in
   the form of soft-state routing table entries.  DAO messages are able
   to record loose source routing information as by propagate up the
   DODAG.  This mechanism is flexible to support the provisioning of
   paths which consist of fully specified source routes, piecewise
   source routes, or hop-by-hop routes as according to the
   implementation and the capabilities of the 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 implementation/deployment and not generally changed during the
   operation of the RPL LLN.

   When Destination Advertisement is enabled:

   1.  Some nodes in the LLN MAY store at least one routing table entry
       for a particular destination learned from a DAO.  Such a node is
       termed a 'storing node', with respect to that particular
       destination.

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

   3.  DODAG roots nodes SHOULD be fully-storing nodes.

   4.  Other nodes in the DODAG are 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 particular destination.

   5.  Non-storing nodes MUST participate in the construction of
       piecewise source routes as they propagate the DAO message, as
       described in Section 6.2.5.

   6.  Storing nodes MUST store any source route information received
       from the DAO (RRStack) in the routing table entry entry.  If a
       node is not capable to do this then it must act as a non-storing
       node with respect to that particular destination.





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   7.  Storing nodes MUST use piecewise source routes in order to
       forward data across a non-storing region of the LLN.  The source
       routing mechanism is to be described in a companion
       specification.  (If a node is not capable to do this, then that
       node MUST NOT operate as a storing node).

6.2.2.  Mode of Operation

   o  DAO Operation may not 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 table entries, and is distinct from
      the unicast DAO operation used to establish downward routes along
      the DODAG.

   1.  The 'A' flag in the DIO as conveyed from the DODAG root serves to
       enable/disable DAO operation over the 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 Operation is disabled, a node SHOULD NOT emit DAOs.

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

6.2.3.  Destination Advertisement Parents

   o  Nodes will select a subset of their DODAG Parents to whom DAOs
      will be sent

      *  This subset is the set of 'DAO Parents'

      *  Each DAO parent MUST be a DODAG Parent.  (Not all DODAG parents
         need to be DAO parents).

      *  Operation with more than DAO Parent requires consideration of
         such issues as DAO fan-out and path diversity, to be elaborated
         in a future version of this specification.

   o  The selection of DAO parents is implementation specific and may be
      based on selecting the DODAG Parents that offer the best upwards
      cost (as opposed to downwards or mixed), as determined by the



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      metrics in use and the Objective Function.

   o  When DAO messages are unicast to the DAO Parent, the identity of
      the DAO Parent (DODAGID x DAGSequenceNumber) combined with the
      RPLInstanceID in the DAO message unambiguously associates the DAO
      message, and thus the particular destination prefix, with a DODAG
      Iteration.

   o  When DAO messages are unicast to the DAO Parent, the DAO Rank may
      be updated as according to the implementation and Objective
      Function in use to reflect the relative (aggregated) cost of
      reaching the Destination Prefix through that DAO parent.  As a
      further extension, a DAO Suboption for the 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
   following states:

   CONNECTED   This entry is 'owned' by the node - it is manually
               configured and is considered as a 'self' entry for DAO
               Operation

   REACHABLE   This entry has been reported from a neighbor of the node.
               This state includes the following substates:






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               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 DAO entry to DAO Parents,
   the DAO Entry record is logically marked to indicate that an attempt
   has not yet been made for each parent.  As the unicast attempts are
   completed for each parent, this mark may be cleared.  This mechanism
   may serve to limit DAO entry updates for each parent to a subset that
   needs to 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 be provisioned outside of the
       context of RPL, e.g. through a management API.  An implementation
       SHOULD provide a means to provision/manage CONNECTED DAO entries,



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       including whether they are to be redistributed in RPL.

6.2.4.1.1.2.  Operation in the REACHABLE state

   1.  When a REACHABLE(*) entry times out, i.e. the DAO Lifetime has
       elapsed, the entry MUST be placed into the UNREACHABLE state and
       no-DAO SHOULD be scheduled to send to the node's DAO Parents.

   2.  When a no-DAO for a REACHABLE(*) entry is received with a newer
       DAO Sequence Number, the entry MUST be placed into the
       UNREACHABLE state and no-DAO SHOULD be scheduled to send to the
       node's DAO Parents.

   3.  When a REACHABLE(*) entry is to be removed because NUD or
       equivalent has determined that the next-hop neighbor is no longer
       reachable, the entry MUST be placed into the UNREACHABLE state
       and no-DAO SHOULD be scheduled to send to the node's DAO Parents.

   4.  When a REACHABLE(*) entry is to be removed because an associated
       Forwarding Error has been returned by the next-hop neighbor, the
       entry MUST be placed into the UNREACHABLE state and no-DAO SHOULD
       be scheduled to send to the node's DAO Parents.

   5.  When a DAO (or no-DAO) for a REACHABLE(*) entry is received with
       an older or unchanged DAO Sequence Number, then the DAO (or no-
       DAO) SHOULD be ignored and the associated entry MUST NOT be
       updated with the stale information.

6.2.4.1.1.2.1.  REACHABLE(Confirmed)

   1.  When a DAO for a previously unknown (or UNREACHABLE) destination
       is received and is to be stored, it MUST be entered into the
       routing table in the REACHABLE(Confirmed) state, and a DAO SHOULD
       be scheduled to send to the node's DAO Parents.  Alternately the
       node may behave as a non-storing node with respect to this
       destination.

   2.  When a DAO for a REACHABLE(Confirmed) entry is received with a
       newer DAO Sequence Number, the entry MUST be updated with the
       logical equivalent of the DAO contents and a DAO SHOULD be
       scheduled to send to the node's DAO Parents.

   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 DAO
       entry MUST be placed in the REACHABLE(Pending) state and the
       associated Retry Counter MUST be set to 0.





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6.2.4.1.1.2.2.  REACHABLE(Pending)

   1.  When a DAO for a REACHABLE(Pending) entry is received with a
       newer DAO Sequence Number, the entry MUST be updated with the
       logical equivalent of the DAO contents and the entry MUST be
       placed in the REACHABLE(Confirmed) state.

   2.  When a DAO for a REACHABLE(Pending) entry is expected, e.g.
       because DAO has (again) been triggered with respect to that
       neighbor, then the associated Retry Counter MUST be incremented.

   3.  When the associated Retry Counter for a REACHABLE(Pending) entry
       reaches a 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 the UNREACHABLE state

   1.  An implementation SHOULD bound the time that the entry is
       allocated in the UNREACHABLE state.  Upon the equivalent expiry
       of the related timer (RemoveTimer), the entry SHOULD be
       suppressed.

   2.  While the entry is in the UNREACHABLE state a node SHOULD make a
       reasonable attempt to report a no-DAO to each of the DAO parents.

   3.  When the node has completed an attempt to report a no-DAO to each
       of the DAO parents, the entry SHOULD be suppressed.

6.2.5.  Operation of DAO Non-storing Nodes

   1.  When a DAO is received from a child by a node who will not store
       a routing table entry for the DAO, the node MUST schedule to pass
       the DAO 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 DAO payload as it moves up the DODAG:

       1.  The most recent addition to the RRStack (the 'next waypoint')
           is investigated to determine if the node already has a route
           provisioned to the waypoint.  If the node already has such a
           route, then it is not necessary to add additional information
           to the RRStack.  The node SHOULD NOT modify the RRStack
           further.

       2.  If the node does not have a route provisioned to the next
           waypoint, then the node MUST append the address of the child
           to the RRStack, and increment RRCount.



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

   1.  An implementation SHOULD arrange to rate-limit the sending of
       DAOs.

   2.  When scheduling to send a DAO, an implementation SHOULD
       equivalently start a timer (DelayDAO) to delay sending the DAO.
       If the DelayDAO timer is already running then the DAO may be
       considered as already scheduled, and implementation SHOULD leave
       the timer running at its present duration.

   o  When computing the delay before sending a DAO, in order to
      increase the effectiveness of aggregation, an implementation MAY
      allow time to receive 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/DAGRank(self_rank) <= DelayDAO < DAO_LATENCY/
         DAGRank(parent_rank), where DAGRank() is defined as in
         Section 3.6.2, such that nodes deeper in the DODAG may tend to
         report DAO messages first before their parent nodes will report
         DAO messages.  Note that this suggestion is intended as an
         optimization to allow efficient aggregation -- it is not
         required for correct operation in the general case.

6.2.7.  Triggering DAO Message from the Sub-DODAG

   Triggering DAO messages from the Sub-DODAG occurs by using the
   following control fields with the rules described below:

   The DTSN field from the DIO is a sequence number that is part of the
   mechanism to trigger DAO messages.  The motivation to use a sequence
   number is to provide some means of reliable signaling to the sub-
   DODAG-- whereas a control flag that is activated for a short time may
   be unobserved by the sub-DODAG if the triggering DIO messages are
   lost, the DTSN increment may be observed later even if some DIO
   messages have been lost since the sequence number increment.

   The 'T' flag provides a way to signal the refresh of DAO information
   over the entire DODAG iteration.  Whereas a DTSN increment may only
   trigger a DAO refresh as far as the nearest storing node (because a
   storing node will not increment its own DTSN in response, as
   described in the rules below), the assertion of the 'T' flag in
   conjunction with an incremented DTSN will 'punch through' storing
   nodes to elicit a DAO refresh from the entire DODAG Iteration.

   The 'S' flag provides a way to signal to a sub-DODAG that there is at
   least one non-root node somewhere in the set of DODAG ancestors,



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   where that non-root node is a storing node.  This allows for an
   optimization-- when it is clear to a non-storing node that the root
   node can be the only storing ancestor, then that node does not
   necessarily need to trigger updates from its sub-DODAG when it
   modifies its DAO parent set.  The motivation here is that the root
   node should be able to update its stored source routing information
   for the affected sub-DODAG based only on receiving DAO information
   concerning the link that changed.  In the other case, when the 'S'
   flag is set, the non-storing node does not have a means to determine
   which DAO information may (or may not) need to be updated in the
   intermediate storing node so it must trigger DAO messages in order to
   update the intermediate storing node.  Please note that some aspects
   of the proper use of the 'S' flag remain under investigation.

   Further examples of triggering DAO messages are contained in
   Appendix B.

   The control fields are used to trigger DAO messages as follows:

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

   2.   Non-root nodes that store routing table entries learned from
        DAOs MUST set the 'S' flag when they emit DIO messages.

   3.   A node that has any DAO Parent with the 'S' flag set MUST also
        set the 'S' flag when it emits DIO messages.

   4.   A node that has all DAO Parents with cleared 'S' flags, and does
        not store routing table entries learned from DAOs, MUST clear
        the 'S' flag when it emits DIO messages.

   5.   A DAO Trigger Sequence Number (DTSN) MUST be maintained by each
        node per RPL Instance.  The DTSN, in conjunction with the 'T'
        flag from the DIO message, provides a means by which DAO
        messages may be reliably triggered in the event of topology
        change.

   6.   The DTSN MUST be advertised by the node in the DIO message.

   7.   A node keeps track of the DTSN that it has heard from the last
        DIO from each of its DAO Parents.  Note that there is one DTSN
        maintained per DAO Parent- each DAO Parent may independently
        increment it at will.

   8.   A node that is not a fully-storing node SHOULD increment its own
        DTSN when it adds a new parent, that parent having the 'S' flag
        set, to its DAO Parent set.  It MAY defer advertising the



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        increment as long as it has a DAO parent that already provides
        adequate connectivity.

   9.   A node that is not a fully-storing node MUST increment its own
        DTSN when it receives a DIO from a DAO Parent that contains a
        newly incremented DTSN.  (The newly incremented DTSN is detected
        by comparing the value received in the DIO with the value last
        recorded for that DAO parent).

   10.  A fully-storing node MUST increment its own DTSN when it
        receives a DIO from a DAO Parent that contains a newly
        incremented DTSN and a set 'T' flag.

   11.  When a storing or non-storing node joins a new DODAG iteration,
        it SHOULD increment its DTSN as if the 'T' flag has been set.

   12.  DAO Transmission SHOULD be scheduled when a new parent is added
        to the DAO Parent set.

   13.  A node that receives a newly incremented DTSN from a DAO Parent
        MUST schedule a DAO transmission.

   o  When a node that is not fully-storing sees a DTSN increment, it
      will increment its own DTSN.  This will cause the DTSN increment
      to extend down the DODAG to the first fully-storing node, which
      will send its DAOs back up, rebuilding source routes information
      along the way to the first node that incremented the DTSN, who
      then may report the new DAO information to its new parent.

   o  When a fully-storing node sees a DTSN increment, it is caused to
      reissue its entire set of routing table entries learned from DAOs
      (or an aggregated subset thereof), but will not need to increment
      its own DTSN.  The 'DTSN increment wave' stops when it encounters
      fully-storing nodes.

   o  When a fully-storing node sees a DTSN increment AND the 'T' flag
      is set, it does increment its own DTSN as well.  The 'T' flag
      'punches through' all nodes, causing all routing tables in the
      entire sub-DODAG to be refreshed.

6.2.8.  Sending DAO Messages to DAO Parents

   1.  When storing nodes send DAO messages for stored entries the
       RRStack SHOULD be cleared in the DAO message.

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





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       *  The IPv6 Source Address is the node sending the DAO message.

       *  The IPv6 Destination Address is DAO parent.

   3.  When the appointed time arrives (DelayDAO) for the transmission
       of DAO messages (with jitter as appropriate) for the requested
       entries, the implementation MAY aggregate the the entries into a
       reduced numbers of DAOs to be reported to each parent, and
       perform compression if possible.

   4.  Note: it is NOT RECOMMENDED that a DAO Transmission (No-DAO) be
       scheduled when a DAO Parent is removed from the DAO Parent set.

6.2.9.  Multicast Destination Advertisement Messages

   A special case of DAO operation, distinct from unicast DAO operation,
   is multicast DAO operation which may be used to populate '1-hop'
   routing table entries.

   1.  A node MAY multicast a DAO message to the link-local scope all-
       nodes multicast address FF02::1.

   2.  A multicast DAO message MUST be used only to advertise
       information about self, i.e. prefixes directly connected to or
       owned by this node, such as a multicast group that the node is
       subscribed to or a global address owned by the node.

   3.  A multicast DAO message MUST NOT be used to relay connectivity
       information learned (e.g. through unicast DAO) from another node.

   4.  Information obtained from a multicast DAO MAY be installed in the
       routing table and MAY be propagated by a node in unicast DAOs.

   5.  A node MUST NOT perform any other DAO related processing on a
       received multicast DAO, in particular a node MUST NOT perform the
       actions of a DAO parent upon receipt of a multicast DAO.

   o  The multicast DAO may be used to enable direct P2P communication,
      without needing the RPL routing structure to relay the packets.

   o  The multicast DAO does not presume any DODAG relationship between
      the emitter and the receiver.


7.  Packet Forwarding and Loop Avoidance/Detection






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7.1.  Suggestions for Packet Forwarding

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

   1.  In the scope of this specification, it is preferred to select a
       successor from a DODAG iteration that matches the RPLInstanceID
       marked in the IPv6 header of the packet being forwarded.

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

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

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

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

   6.  If there is a DODAG parent offering a default route then select
       that DODAG parent as a successor.

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

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

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

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



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   neared.

7.2.  Loop Avoidance and Detection

   RPL loop avoidance mechanisms are kept simple and designed to
   minimize churn and states.  Loops may form for a number of reasons,
   from control packet loss to sibling forwarding.  RPL includes a
   reactive loop detection technique that protects from meltdown and
   triggers repair of broken paths.

   RPL loop detection uses information that is placed into the packet in
   the IPv6 flow label.  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   | RPLInstanceID |
               +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                         Figure 12: RPL Flow Label

   Down 'O' bit:  1-bit flag indicating whether the packet is expected
         to progress up or down.  A router sets the 'O' bit when the
         packet is expect to progress down (using DAO routes), and
         resets it when forwarding towards the root of the DODAG
         iteration.  A host MUST set the bit to 0.

   Sibling 'S' bit:  1-bit flag indicating whether the packet has been
         forwarded via a sibling at the present rank, and denotes a risk
         of a sibling loop.  A host sets the bit to 0.

   Rank-Error 'R' bit:  1-bit flag indicating whether a rank error was
         detected.  A rank error is detected when there is a mismatch in
         the relative ranks and the direction as indicated in the 'O'
         bit.  A host MUST set the bit to 0.

   Forwarding-Error 'F' bit:  1-bit flag indicating that this node can
         not forward the packet further towards the destination.  The
         'F' bit might be set by sibling that can not forward to a
         parent a packet with the Sibling 'S' bit set, or by a child
         node that does not have a route to destination for a packet
         with the down 'O' bit set.  A host MUST set the bit to 0.






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   SenderRank:  8-bit field set to zero by the source and to
         DAGRank(rank) by a router that forwards inside the RPL network.
         (Note that the case where DAGRank(rank) does not fit into 8
         bits is under investigation.)

   RPLInstanceID:  8-bit field indicating the DODAG instance along which
         the packet is 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 RPL network MUST be issued with the
   flow label zeroed out, but for the RPLInstanceID field.

   If the source is aware of the RPLInstanceID that is preferred for the
   flow, then it MUST set the RPLInstanceID field in the flow label
   accordingly, otherwise it MUST set it to the RPL_DEFAULT_INSTANCE.

   If a compression mechanism such as 6LoWPAN is applied to the packet,
   the flow label MUST NOT be compressed even if it is set to all
   zeroes.

7.2.2.  Router Operation

7.2.2.1.  Conformance to RFC 3697

   [RFC3697] mandates that the 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 to a destination connected to a RPL
   network or that routes a packet outside the RPL network MUST zero out
   all the fields but the RPLInstanceID field that must be delivered
   without a 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 given instance.

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

   When a router receives a packet that specifies a given RPLInstanceID



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   and the node can forward the packet along the DODAG associated to
   that instance, then the router MUST do so and leave the RPLInstanceID
   flag unchanged.

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

7.2.2.3.  DAG Inconsistency Loop Detection

   The DODAG is inconsistent if the direction of a packet does not match
   the rank relationship.  A receiver detects an inconsistency if it
   receives a packet with either:

      the 'O' bit set (to down) from a node of a higher rank.

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

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

   When the DODAG root increments the DODAGSequenceNumber a temporary
   rank discontinuity may form between the next iteration and the 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 choose to
   forward a packet to a (future) parent that is 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
   the same as in the next iteration and the packet may be judged to not
   be making forward progress.  If the sending router is aware that the
   chosen successor has already joined the next iteration, then the
   sending router MUST update the SenderRank to INFINITE_RANK as it
   forwards the packets across the discontinuity into the next DODAG
   iteration in order to avoid a false detection of rank inconsistency.

   One inconsistency along the path is not considered as a critical
   error and the packet may continue.  But a second detection along the
   path of a same packet should not occur and the packet is dropped.

   This process is controlled by the Rank-Error bit in the Flow Label.
   When an inconsistency, is detected on a packet, if the Rank-Error bit
   was not set then the Rank-Error bit is set.  If it 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
   forward progress and may loop between siblings.  Experimental



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   evidence has shown that one sibling hop can be 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 forwards a packet to a
   non-sibling, the Sibling bit in the packet must be reset.  When a
   router forwards to a sibling: if the Sibling bit was not set then the
   Sibling bit is set.  If the Sibling bit was set then then the router
   SHOULD return the packet to the sibling that that passed it with the
   Forwarding-Error 'F' bit set.

7.2.2.5.  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 the child.  With DAO inconsistency loop recovery, a packet
   can be used to recursively explore and cleanup the obsolete DAO
   states along a sub-DODAG.

   In a general manner, a packet that goes down should never go up
   again.  If DAO inconsistency loop recovery is applied, then the
   router SHOULD send the packet to the parent that passed it with the
   Forwarding-Error 'F' bit set.  Otherwise the router MUST silently
   discard the packet.

7.2.2.6.  Forward Path Recovery

   Upon receiving a packet with a Forwarding-Error bit set, the node
   MUST remove the routing states that caused forwarding to that
   neighbor, clear the Forwarding-Error bit and attempt to send the
   packet again.  The packet may its way to an alternate neighbor.  If
   that alternate neighbor still has an inconsistent DAO state via this
   node, the process will recurse, this node will set the Forwarding-
   Error 'F' bit and the routing state in the alternate neighbor will be
   cleaned up as well.


8.  Multicast Operation

   This section describes further the multicast routing operations over
   an IPv6 RPL network, and specifically how unicast DAOs can be used to
   relay group registrations up.  Wherever the following text mentions
   Multicast Listener Discovery (MLD), one can read MLDv2 ([RFC3810]) or
   v3.

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



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

   A router might select to pass a listener registration DAO message to
   its preferred parent only, in which case multicast packets coming
   back might be lost for all of its sub-DODAG if the 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 router will need to prune.

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

   For unicast traffic, it is expected that the grounded root of an
   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 is 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 entries installed from the
   DAO message.

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

   As a result, the DODAG Root acts as an automatic proxy Rendezvous
   Point for the RPL network, and as source towards the Internet for all
   multicast flows started in the RPL LLN.  So regardless of whether the
   root is actually attached to the Internet, and regardless of whether
   the 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



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

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


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 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 construct the DODAG (e.g. "minimize the path cost using
   the ETX metric and avoid 'Blue' links").  The Objective Code Points
   are specified in [I-D.ietf-roll-routing-metrics],
   [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



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      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 to the rank of the candidate
      a value representing the relative locations of self and the
      candidate in the DODAG iteration.

      *  The increase in rank must be at least MinHopRankIncrease.
         (This prevents the creation of a path of sibling links
         connecting a 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 purely additive metric such as ETX,
         the increase in rank can be made proportional to the increase
         in the metric.

      *  Candidate neighbors that would cause self's rank to increase
         are not considered for parent selection

   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.




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      *  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 for parent selection

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


11.  RPL Constants and Variables

   Following is a summary of RPL constants and variables.

   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 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 0xFFFF.

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







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   DEFAULT_DIO_INTERVAL_MIN  TBD (To be determined)

   DEFAULT_DIO_INTERVAL_DOUBLINGS  TBD (To be determined)

   DEFAULT_DIO_REDUNDANCY_CONSTANT  TBD (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 5.3.5.1

   DAG Sequence Number Increment Timer  Up to one instance per DODAG
         that the node is acting as DODAG root of.  May not be supported
         in all implementations.  Expiry triggers revision of
         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 DAO parent (the subset of
         DODAG parents chosen to receive destination advertisements) per
         DODAG.  Expiry triggers sending of DAO message to the DAO
         parent.  See Section 6.2.6

   RemoveTimer  Up to one instance per DAO entry per neighbor (i.e.
         those neighbors that have given DAO messages to this node as a
         DODAG parent) Expiry triggers a change in state for the DAO
         entry, setting up to do unreachable (No-DAO) advertisements or
         immediately deallocating the DAO entry if there are no DAO
         parents.  See Section 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.

12.1.  Control of Function and Policy

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



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   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.

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 5.1.1:

   DAGPreference
   RPLInstanceID
   DAGObjectiveCodePoint
   DODAGID
   Destination Prefixes
   DIOIntervalDoublings
   DIOIntervalMin
   DIORedundancyConstant

   DAG Root behavior:  In some cases, a node may not want to permanently
         act as a DODAG root if it cannot join a grounded DODAG.  For
         example a battery-operated node may not want to act as a 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 DODAG root for a configured period of time.

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

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 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 5.3.5.1:






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   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.

12.1.4.  DAG Sequence Number Increment

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

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

12.1.6.  Policy Control

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




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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.

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.

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 3.1, an implementation is expected to
   maintain a set of data structures in support of DODAG discovery:

   o  The candidate neighbors data structure

   o  For each DODAG:

      *  A set of DODAG parents

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.

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:





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   o  DODAGID

   o  DAGObjectiveCodePoint

   o  A set of Destination Prefixes offered upwards along the DODAG

   o  A set of DODAG Parents

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

   o  DODAGSequenceNumber

   The set of 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 DODAG Parent

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






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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
   DODAG parent, e.g. if the DODAGID has changed.

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

12.3.5.  RPL Trickle Timers

   A RPL implementation operating on a 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).

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.

12.5.  Requirements on Other Protocols and Functional Components

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

12.6.  Impact on Network Operation

   To be completed.


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].


14.  IANA Considerations





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14.1.  RPL Control Message

   The RPL Control Message is an ICMP information message type that is
   to be used carry DODAG Information Objects, DODAG 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.

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 | DODAG Information Solicitation   | This document |
        | 0x02 | DODAG Information Object         | This document |
        | 0x04 | Destination Advertisement Object | This document |
        +------+----------------------------------+---------------+

                             RPL Control Codes

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

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

   o  Capability description




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   o  Defining RFC

   Four groups are currently defined:

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

                              DIO Base Flags

14.4.  DODAG 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 |
         +-------+------------------------------+---------------+

              DODAG Information Option (DIO) Base Suboptions


15.  Acknowledgements

   The authors would like to acknowledge the review, feedback, and
   comments from Emmanuel Baccelli, Dominique Barthel, Yusuf Bashir,
   Phoebus Chen, 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,



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   and Arsalan Tavakoli, which have provided useful design
   considerations to RPL.


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


   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


   Jonathan W. Hui
   Arch Rock Corporation
   501 2nd St. Ste. 410



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   San Francisco, CA  94107
   USA

   Email: 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


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

   Email: stevedh@cs.berkeley.edu




17.  References

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.







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17.2.  Informative References

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

   [I-D.ietf-roll-of0]
              Thubert, P., "RPL Objective Function 0",
              draft-ietf-roll-of0-01 (work in progress), February 2010.

   [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



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              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.

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

   [RFC3810]  Vida, R. and L. Costa, "Multicast Listener Discovery
              Version 2 (MLDv2) for IPv6", RFC 3810, June 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.




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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,



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   [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

B.1.  DAO Operation When Only the Root Node Stores DAO Information

   Consider the example of Figure 13.  In this example only the root
   node, (LBR*), will store DAO information.  This is not known, nor is
   it required to be known, to all nodes a priori.  Rather, each node is
   able to observe from the state of the 'S' flag that no ancestor, with
   the exception of the root node, stores DAO information.


                                  (LBR*)
                                   /  \
                                  /    \
                                 /      \
                             (11)        (12)
                               |          |
                               |          |
                               |          |
                             (21)        (22)
                                 \
                                  \
                                   \
                                   (31)
                                   /  \
                                  /    \
                                 /      \
                             (41)        (42)
                               :          :


                   Figure 13: Only Root Node Stores DAOs

   In this example:



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   o  The 'S' flag is cleared in DIO messages emitted by (LBR*), because
      (LBR*) is the DODAG root.

   o  The 'S' flag is cleared in all DIO messages emitted by all other
      nodes, because no other node stores DAO information.

   o  (LBR*) has learned from DAO messages how to reach node (31) with a
      source route via {(11) (21)}.

   o  All source routes to nodes in the sub-DODAG of node (31),
      including nodes (41), (42), and others will include the prefix
      {(11) (21) (31)}

   o  Node (31) maintains a DTSN, (31).DTSN, that it will advertise in
      DIO messages.

   Suppose now that there is a topology change within the same DODAG
   iteration, causing node (31) to evict node (21) as a DAO parent and
   add node (22) as a DAO parent:

   1.  Node (31) will schedule a DAO transmission because it has added a
       new node (22) to its DAO parent set.

   2.  Node (31) need not increment (31).DTSN at this event, because in
       this example no DAO parents have the 'S' flag set.  Specifically
       this indicates to Node (31) that there are no intermediate
       storing nodes that may need to be explicitly updated with DAO
       information from it's sub-DODAG.  Hence nodes (41), (42), and by
       extension the sub-DODAG of node (31) will not subsequently
       observe an incremented (31).DTSN and the sub-DODAG will not emit
       DAOs.

   3.  A new flow of DAOs for node (31) reaches the (LBR*), updating the
       source route information for node (31) to include the new path
       {(12) (22)}.

   4.  (LBR*) may implicitly update all source routes that must transit
       node (31), i.e. the sub-DODAG of node (31), to use the updated
       source route prefix {(12) (22)} instead of {(11) (21)}.

   Thus the use of the 'S' flag in the case where only the root node
   stores DAO information has allowed an optimization whereby only a DAO
   update for the node that changed its DAO parent set, (31), needs to
   be sent to the DODAG root.







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B.2.  DAO Operation When All Nodes Fully Store DAO Information

   Consider the example of Figure 14.  In this example all nodes will
   fully store DAO information.


                                  (LBR*)
                                   /  \
                                  /    \
                                 /      \
                            (11*)        (12*)
                               |          |
                               |          |
                               |          |
                            (21*)        (22*)
                                 \
                                  \
                                   \
                                  (31*)
                                   /  \
                                  /    \
                                 /      \
                            (41*)        (42*)
                               :          :


                      Figure 14: All Nodes Store DAOs

   In this example:

   o  The 'S' flag is cleared in DIO messages emitted by (LBR*), because
      (LBR*) is the DODAG root.

   o  The 'S' flag is set in DIO messages emitted by all non-root nodes
      because each non-root node stores DAO information.

   o  Source routing state is effectively not provisioned in this
      example, because each node has been able to store hop-by-hop
      routing state for each destination, possibly aggregated, as
      learned from DAOs.  For example, node (11*) will have learned and
      stored information from a DAO to the effect that node (41*) is
      routable through a next hop of node (21*).  Node (12*) on the
      other hand does not necessarily have a route provisioned to node
      (41*).

   Suppose now that there is a topology change within the same DODAG
   iteration, causing node (31*) to evict node (21*) as a DAO parent and
   add node (22*) as a DAO parent:



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   1.  Node (31*) will schedule a DAO transmission because it has added
       a new node (22*) to its DAO parent set.

   2.  Node (31) need not increment (31).DTSN, because it is a fully
       storing node and does not need to trigger DAO information from
       its sub-DODAG.

   3.  Node (31) gives a DAO Update to node (22*).  Presuming that node
       (22*) has received the update, node (22*) will store the new
       entries for routes including the sub-DODAG of node (31*),
       including nodes (41*) and (42*).  Node (22*) will schedule a DAO
       transmission for the new entries.

   4.  Similarly, node (22*) updates node (12*) and node (12*) updates
       (LBR*).  Hop-by-hop routing state for the sub-DODAG of node (31*)
       is now provisioned at nodes (12*) and (22*).

   Thus the addition to the DAO Parent set at the fully storing node
   (31*) does not elicit additional DAO-related traffic from its sub-
   DODAG.  The intermediate nodes along the 'new' downward path are
   updated by DAO messages along the new path.

   Suppose next that the DODAG root triggers a refresh of DAO
   information over the same DODAG Iteration.  (Note that the DODAG root
   might also trigger a DAO refresh but allow other topology changes at
   the same time by incrementing the DODAG Sequence Number to cause a
   move to the next DODAG Iteration).:

   1.  (LBR*) will increment its DTSN and issue a DIO with the 'T' flag
       set.

   2.  Nodes (11*) and (12*) will increment their own DTSNs in response
       to observing in the DIO from LBR a new DTSN and the 'T' flag
       being set.  They will reset their trickle timers to cause the
       issue of new DIOs with the 'T' flag set.  These nodes will also
       schedule a DAO transmission in response to observing a new DTSN
       from their DAO Parent, (LBR*).  (This DAO transmission may be
       scheduled with a sufficient delay computed based on rank to allow
       a chance for the sub-DODAGs of the nodes to report DAO messages
       prior to the nodes reporting their own DAO information to (LBR*).
       This is implementation specific and may allow a chance for DAO
       aggregation.).

   3.  Node (21*) receives a DIO from node (11*) and observes the new
       (11*).DTSN as well as the set 'T' flag.  Node (21*) increments
       its own DTSN, resets the trickle timer, and schedules a DAO
       transmission.




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   4.  Similarly, as each node observes the incremented DTSN with the
       'T' flag set from each of its parents, each node will increment
       its own DTSN, reset the DIO trickle timer, and schedule a DAO
       transmission.

   Thus the entire DODAG iteration has been re-armed to send DAO
   messages based on the (LBR*)'s assertion of the 'T' flag.  Note that
   normally a DTSN increment would cause no further action in a sub-
   DODAG beyond the first fully storing node that is encountered, but
   that in this case the 'T' flag effectively provides a means to 'punch
   through' all fully storing nodes.

B.3.  DAO Operation When Nodes Have Mixed Capabilities

   Consider the example of Figure 15.  In this example some nodes are
   capable of storing DAO information and some are not.


                                  (LBR*)
                                   /  \
                                  /    \
                                 /      \
                             (11)        (12*)
                               |          |
                               |          |
                               |          |
                             (21)        (22)
                                 \
                                  \
                                   \
                                   (31)
                                   /  \
                                  /    \
                                 /      \
                             (41)        (42*)
                               :          :


                 Figure 15: Mixed Capability DAO Operation

   In this example:

   o  The 'S' flag is cleared in DIO messages emitted by (LBR*), because
      (LBR*) is the DODAG root.

   o  The 'S' flag is set in DIO messages emitted by (12*), because it
      is a storing node.




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   o  The 'S' flag will be set in DIO messages emitted by nodes that
      contain node (12*) (or more generally any non-root storing node)
      as a DAO parent/ancestor.  This indicates that somewhere along the
      DAO path there is a non-root storing node that may need to have
      its state updated (by a DAO refresh) in certain conditions.

   Suppose that there is a topology change within the same DODAG
   iteration, causing node (31) to add node (22) as a DAO parent:

   1.  Node (31) will schedule a DAO transmission because it has added a
       new node (22) to its DAO parent set.  Node (31) will increment
       (31).DTSN because node (22) has set the 'S' flag in its DIO
       messages.  Node (31) will reset its DIO trickle timer.

   2.  Node (31)'s trickle timer will then expire and a DIO is issued
       and received by node's (41) and (42*).

   3.  Node (41) is a non-storing node.  It will increment (41).DTSN in
       response to observing the increment in (31).DTSN, and reset its
       trickle timer.  This results finally in the reliable (thanks to
       the DTSN) triggering of a DAO update from node (41)'s sub-DODAG.

   4.  As node (41) receives DAO updates from its sub-DODAG it updates
       the DAOs with source routing information as necessary and passes
       them on to node (31), along with its own (node (41)) DAO update.

   5.  Meanwhile, node (42*) is a fully storing node.  It observes the
       increment to (31).DTSN and schedules a DAO update.  Node (42*)
       does not need to increment (42*).DTSN, since it is a fully
       storing node it does not need to solicit DAO updates from its
       sub-DODAG in this case.  At the scheduled time Node (42*)
       reissues its DAO information to node (31).

   6.  Node (31) receives the DAO messages from its sub-DODAG, adds
       source routing information as necessary, and issues DAO updates
       to node (22).

   7.  Node (22) similarly receives DAO messages from node (31), updates
       source routing information as necessary, and issues DAO updates
       to node (12*).

   8.  The intermediate storing node (12*) has now received from DAO
       messages the information necessary to provision routing state for
       node (31) and its sub-DODAG.  As downwards traffic is routed
       through node (12*) it is able to consult source routing
       information that was learned from the DAO messages as needed to
       specify routes down the DAG across the non-storing nodes (22),
       (31), ...



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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 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 DODAG by the destination advertisement
   mechanism are not the most desirable downward paths for the specific
   application scenario (in part because the DODAG links may not be
   symmetric).  It may be desired to support within RPL the discovery
   and installation of more direct routes 'across' the DAG.  Such
   mechanisms need to be investigated.

C.2.  Destination Advertisement / DAO Fan-out

   When DAO messages are relayed to more than one 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.

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.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.





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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




















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