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RFC 6550

 
 
 

RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks

Part 2 of 6, p. 13 to 30
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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.  Topologies

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

3.1.1.  Constructing Topologies

   LLNs, such as Radio Networks, do not typically have predefined
   topologies, for example, those imposed by point-to-point wires, so
   RPL has to discover links and then select peers sparingly.

   In many cases, because Layer 2 ranges overlap only partially, RPL
   forms non-transitive / Non-Broadcast Multi-Access (NBMA) network
   topologies upon which it computes routes.

   RPL routes are optimized for traffic to or from one or more roots
   that act as sinks for the topology.  As a result, RPL organizes a
   topology as a Directed Acyclic Graph (DAG) that is partitioned into

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   one or more Destination Oriented DAGs (DODAGs), one DODAG per sink.
   If the DAG has multiple roots, then it is expected that the roots are
   federated by a common backbone, such as a transit link.

3.1.2.  RPL Identifiers

   RPL uses four values to identify and maintain a topology:

   o  The first is a RPLInstanceID.  A RPLInstanceID identifies a set of
      one or more Destination Oriented DAGs (DODAGs).  A network may
      have multiple RPLInstanceIDs, each of which defines an independent
      set of DODAGs, which may be optimized for different Objective
      Functions (OFs) and/or applications.  The set of DODAGs identified
      by a RPLInstanceID is called a RPL Instance.  All DODAGs in the
      same RPL Instance use the same OF.

   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 DODAGVersionNumber.  The scope of a
      DODAGVersionNumber is a DODAG.  A DODAG is sometimes reconstructed
      from the DODAG root, by incrementing the DODAGVersionNumber.  The
      combination of RPLInstanceID, DODAGID, and DODAGVersionNumber
      uniquely identifies a DODAG Version.

   o  The fourth is Rank.  The scope of Rank is a DODAG Version.  Rank
      establishes a partial order over a DODAG Version, defining
      individual node positions with respect to the DODAG root.

3.1.3.  Instances, DODAGs, and DODAG Versions

   A RPL Instance contains one or more DODAG roots.  A RPL Instance may
   provide routes to certain destination prefixes, reachable via the
   DODAG roots or alternate paths within the DODAG.  These roots may
   operate independently, or they may coordinate over a network that is
   not necessarily as constrained as an LLN.

   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.

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   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 suitable connectivity
         to coordinate with each other or that use the formation of
         multiple DODAGs as a means to dynamically and autonomously
         partition the network.

   o  a single DODAG with a virtual root that coordinates LLN sinks
      (with the same DODAGID) over a backbone network.

      *  For example, multiple border routers operating with a reliable
         transit link, e.g., in support of an IPv6 Low-Power Wireless
         Personal Area Network (6LoWPAN) application, that are capable
         of acting as logically equivalent interfaces to the sink of the
         same DODAG.

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

   Each RPL packet is associated with a particular RPLInstanceID (see
   Section 11.2) and, therefore, RPL Instance (Section 5).  The
   provisioning or automated discovery of a mapping between a
   RPLInstanceID and a type or service of application traffic is out of
   scope for this specification (to be defined in future companion
   specifications).

   Figure 1 depicts an example of a RPL Instance comprising three DODAGs
   with DODAG roots R1, R2, and R3.  Each of these DODAG roots
   advertises the same RPLInstanceID.  The lines depict connectivity
   between parents and children.

   Figure 2 depicts how a DODAGVersionNumber increment leads to a new
   DODAG Version.  This depiction illustrates a DODAGVersionNumber
   increment that results in a different DODAG topology.  Note that a
   new DODAG Version does not always imply a different DODAG topology.
   To accommodate certain topology changes requires a new DODAG Version,
   as described later in this specification.

   In the following examples, please note that tree-like structures are
   depicted for simplicity, although the DODAG structure allows for each
   node to have multiple parents when the connectivity supports it.

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     +----------------------------------------------------------------+
     |                                                                |
     | +--------------+                                               |
     | |              |                                               |
     | |     (R1)     |            (R2)                   (R3)        |
     | |     /  \     |            /| \                  / |  \       |
     | |    /    \    |           / |  \                /  |   \      |
     | |  (A)    (B)  |         (C) |  (D)     ...    (F) (G)  (H)    |
     | |  /|\     |\  |         /   | / |\             |\  |    |     |
     | | : : :    : : |        :   (E)  : :            :  `:    :     |
     | |              |            / \                                |
     | +--------------+           :   :                               |
     |      DODAG                                                     |
     |                                                                |
     +----------------------------------------------------------------+
                                RPL Instance

                          Figure 1: RPL Instance

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

                          Figure 2: DODAG Version

3.2.  Upward Routes and DODAG Construction

   RPL provisions routes Up towards DODAG roots, forming a DODAG
   optimized according to an Objective Function (OF).  RPL nodes
   construct and maintain these DODAGs through DODAG Information Object
   (DIO) messages.

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3.2.1.  Objective Function (OF)

   The Objective Function (OF) defines how RPL nodes select and optimize
   routes within a RPL Instance.  The OF is identified by an Objective
   Code Point (OCP) within the DIO Configuration option.  An OF defines
   how nodes translate one or more metrics and constraints, which are
   themselves defined in [RFC6551], into a value called Rank, which
   approximates the node's distance from a DODAG root.  An OF also
   defines how nodes select parents.  Further details may be found in
   Section 14, [RFC6551], [RFC6552], and related companion
   specifications.

3.2.2.  DODAG Repair

   A DODAG root institutes a global repair operation by incrementing the
   DODAGVersionNumber.  This initiates a new DODAG Version.  Nodes in
   the new DODAG Version can choose a new position whose Rank is not
   constrained by their Rank within the old DODAG Version.

   RPL also supports mechanisms that may be used for local repair within
   the DODAG Version.  The DIO message specifies the necessary
   parameters as configured from and controlled by policy at the DODAG
   root.

3.2.3.  Security

   RPL supports message confidentiality and integrity.  It is designed
   such that link-layer mechanisms can be used when available and
   appropriate; yet, in their absence, RPL can use its own mechanisms.
   RPL has three basic security modes.

   In the first, called "unsecured", RPL control messages are sent
   without any additional security mechanisms.  Unsecured mode does not
   imply that the RPL network is unsecure: it could be using other
   present security primitives (e.g., link-layer security) to meet
   application security requirements.

   In the second, called "preinstalled", nodes joining a RPL Instance
   have preinstalled keys that enable them to process and generate
   secured RPL messages.

   The third mode is called "authenticated".  In authenticated mode,
   nodes have preinstalled keys as in preinstalled mode, but the
   preinstalled key may only be used to join a RPL Instance as a leaf.
   Joining an authenticated RPL Instance as a router requires obtaining
   a key from an authentication authority.  The process by which this
   key is obtained is out of scope for this specification.  Note that
   this specification alone does not provide sufficient detail for a RPL

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   implementation to securely operate in authenticated mode.  For a RPL
   implementation to operate securely in authenticated mode, it is
   necessary for a future companion specification to detail the
   mechanisms by which a node obtains/requests the authentication
   material (e.g., key, certificate) and to determine from where that
   material should be obtained.  See also Section 10.3.

3.2.4.  Grounded and Floating DODAGs

   DODAGs can be grounded or floating: the DODAG root advertises which
   is the case.  A grounded DODAG offers connectivity to hosts that are
   required for satisfying the application-defined goal.  A floating
   DODAG is not expected to satisfy the goal; in most cases, it only
   provides routes to nodes within the DODAG.  Floating DODAGs may be
   used, for example, to preserve interconnectivity during repair.

3.2.5.  Local DODAGs

   RPL nodes can optimize routes to a destination within an LLN by
   forming a Local DODAG whose DODAG root is the desired destination.
   Unlike global DAGs, which can consist of multiple DODAGs, local DAGs
   have one and only one DODAG and therefore one DODAG root.  Local
   DODAGs can be constructed on demand.

3.2.6.  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.

3.2.7.  Data-Path Validation and Loop Detection

   The low-power and lossy nature of LLNs motivates RPL's use of on-
   demand loop detection using data packets.  Because data traffic can
   be infrequent, maintaining a routing topology that is constantly up
   to date with the physical topology can waste energy.  Typical LLNs
   exhibit variations in physical connectivity that are transient and
   innocuous to traffic, but that would be costly to track closely from
   the control plane.  Transient and infrequent changes in connectivity
   need not be addressed by RPL until there is data to send.  This
   aspect of RPL's design draws from existing, highly used LLN protocols
   as well as extensive experimental and deployment evidence on its
   efficacy.

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   The RPL Packet Information that is transported with data packets
   includes the Rank of the transmitter.  An inconsistency between the
   routing decision for a packet (Upward or Downward) and the Rank
   relationship between the two nodes indicates a possible loop.  On
   receiving such a packet, a node institutes a local repair operation.

   For example, if a node receives a packet flagged as moving in the
   Upward direction, and if that packet records that the transmitter is
   of a lower (lesser) Rank than the receiving node, then the receiving
   node is able to conclude that the packet has not progressed in the
   Upward direction and that the DODAG is inconsistent.

3.2.8.  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
      configurations.

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

   o  Nodes listen for DIOs and use their information to join a new
      DODAG (thus, selecting DODAG parents), or to maintain an existing
      DODAG, according to the specified Objective Function and Rank of
      their neighbors.

   o  Nodes provision routing table entries, for the destinations
      specified by the DIO message, via their DODAG parents in the DODAG
      Version.  Nodes that decide to join a DODAG can provision one or
      more DODAG parents as the next hop for the default route and a
      number of other external routes for the associated instance.

3.3.  Downward Routes and Destination Advertisement

   RPL uses Destination Advertisement Object (DAO) messages to establish
   Downward routes.  DAO messages are an optional feature for
   applications that require point-to-multipoint (P2MP) or point-to-
   point (P2P) traffic.  RPL supports two modes of Downward traffic:
   Storing (fully stateful) or Non-Storing (fully source routed); see
   Section 9.  Any given RPL Instance is either storing or non-storing.
   In both cases, P2P packets travel Up toward a DODAG root then Down to
   the final destination (unless the destination is on the Upward
   route).  In the Non-Storing case, the packet will travel all the way
   to a DODAG root before traveling Down.  In the Storing case, the

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   packet may be directed Down towards the destination by a common
   ancestor of the source and the destination prior to reaching a DODAG
   root.

   As of the writing of this specification, no implementation is
   expected to support both Storing and Non-Storing modes of operation.
   Most implementations are expected to support either no Downward
   routes, Non-Storing mode only, or Storing mode only.  Other modes of
   operation, such as a hybrid mix of Storing and Non-Storing mode, are
   out of scope for this specification and may be described in other
   companion specifications.

   This specification describes a basic mode of operation in support of
   P2P traffic.  Note that more optimized P2P solutions may be described
   in companion specifications.

3.4.  Local DODAGs Route Discovery

   Optionally, a RPL network can support on-demand discovery of DODAGs
   to specific destinations within an LLN.  Such Local DODAGs behave
   slightly differently than Global DODAGs: they are uniquely defined by
   the combination of DODAGID and RPLInstanceID.  The RPLInstanceID
   denotes whether a DODAG is a Local DODAG.

3.5.  Rank Properties

   The Rank of a node is a scalar representation of the location of that
   node within a DODAG Version.  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.  Even
   though the specific computation of the Rank is left to the Objective
   Function, the Rank must implement generic properties regardless of
   the Objective Function.

   In particular, the Rank of the nodes must monotonically decrease as
   the DODAG Version is followed towards the DODAG destination.  In that
   regard, the Rank can be considered a scalar representation of the
   location or radius of a node within a DODAG Version.

   The details of how the Objective Function computes Rank are out of
   scope for this specification, although that computation may depend,
   for example, on parents, link metrics, node metrics, and the node
   configuration and policies.  See Section 14 for more information.

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

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   Type: The Rank is an abstract numeric value.

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

   Stability: The stability of the Rank determines the stability of the
         routing topology.  Some dampening or filtering is RECOMMENDED
         to keep the topology stable; thus, the Rank does not
         necessarily change as fast as some link or node metrics would.
         A new DODAG Version would be a good opportunity to reconcile
         the discrepancies that might form over time between metrics and
         Ranks within a DODAG Version.

   Properties: The Rank is incremented in a strictly monotonic fashion,
         and it 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: The 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 Objective Function.

   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
   node's further options with regard to DODAG parent selection and
   movement within the DODAG are restricted in favor of loop avoidance.

3.5.1.  Rank Comparison (DAGRank())

   Rank may be thought of as a fixed-point number, where the position of
   the radix 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.  A
   DODAG root provisions MinHopRankIncrease.  MinHopRankIncrease creates
   a trade-off between hop cost precision and the maximum number of hops
   a network can support.  A very large MinHopRankIncrease, for example,
   allows precise characterization of a given hop's effect on Rank but
   cannot support many hops.

   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 relationships or loop
   detection, the integer portion of the Rank is to be used.  The

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   integer portion of the Rank is computed by the DAGRank() macro as
   follows, where floor(x) is the function that evaluates to the
   greatest integer less than or equal to x:

              DAGRank(rank) = floor(rank/MinHopRankIncrease)

   For example, if a 16-bit Rank quantity is decimal 27, and the
   MinHopRankIncrease is decimal 16, then DAGRank(27) = floor(1.6875) =
   1.  The integer part of the Rank is 1 and the fractional part is
   11/16.

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

3.5.2.  Rank Relationships

   Rank computations 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 a 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 or identical.  Routing
      through a node with equal Rank may cause a routing loop (i.e., if
      that node chooses to route through a node with equal Rank as
      well).

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   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 of creating a loop.

   As an example, the Rank could be computed in such a way so as to
   closely track ETX (expected transmission count, a fairly common
   routing metric used in LLN and defined in [RFC6551]) when the metric
   that an Objective Function minimizes is ETX, or latency, or in a more
   complicated way as appropriate to the Objective Function being used
   within the DODAG.

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.  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
   tiebreaker in the presence of Equal Cost Multiple Paths (ECMPs).  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 constraint-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
   neighbor set, thus leading to a constrained shortest path.

   An Objective Function specifies the objectives used to compute the
   (constrained) path.  Furthermore, nodes are configured to support a
   set of metrics and constraints and select their parents in the DODAG
   according to the metrics and constraints advertised in the DIO
   messages.  Upstream and Downstream metrics may be merged or
   advertised separately depending on the OF and the metrics.  When they
   are advertised separately, it may happen that the set of DIO parents

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   is different from the set of DAO parents (a DAO parent is a node to
   which unicast DAO messages are sent).  Yet, all are DODAG parents
   with regard to the rules for Rank computation.

   The Objective Function is decoupled from the routing metrics and
   constraints used by RPL.  Whereas the OF dictates rules such as DODAG
   parent selection, load balancing, and so on, the set of metrics
   and/or constraints used, and thus those that determine the preferred
   path, are based on the information carried within the DAG container
   option in DIO messages.

   The set of supported link/node constraints and metrics is specified
   in [RFC6551].

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

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

3.7.  Loop Avoidance

   RPL tries to avoid creating loops when undergoing topology changes
   and includes Rank-based data-path validation mechanisms for detecting
   loops when they do occur (see Section 11 for more details).  In
   practice, this means that RPL guarantees neither loop-free path
   selection nor tight delay convergence times, but it can detect and
   repair a loop as soon as it is used.  RPL uses this loop detection to
   ensure that packets make forward progress within the DODAG Version
   and trigger repairs when necessary.

3.7.1.  Greediness and Instability

   A node is greedy if it attempts to move deeper (increase Rank) in the
   DODAG Version in order to increase the size of the parent set or
   improve some other metric.  Once a node has joined a DODAG Version,
   RPL disallows certain behaviors, including greediness, in order to
   prevent resulting instabilities in the DODAG Version.

   Suppose a node is willing to receive and process a DIO message 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
   Version 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 form of local repair.  This approach creates an

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   "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.

3.7.1.1.  Example: Greedy Parent Selection and Instability

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

              -1-                    -2-                    -3-

                  Figure 3: Greedy DODAG Parent Selection

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

   If a RPL node is too greedy, in that it attempts to optimize for an
   additional number of parents beyond its most preferred parents, then
   an instability can result.  Consider the DODAG illustrated in
   Figure 3-1.  In this example, Nodes (B) and (C) may most prefer Node
   (A) as a DODAG parent, but we will consider the case when they are
   operating under the greedy condition that will try to optimize for
   two parents.

   o  Let Figure 3-1 be the initial condition.

   o  Suppose Node (C) first is able to leave the DODAG and rejoin at a
      lower Rank, taking both Nodes (A) and (B) as DODAG parents as
      depicted in Figure 3-2.  Now Node (C) is deeper than both Nodes
      (A) and (B), and Node (C) is satisfied to have two DODAG parents.

   o  Suppose Node (B), in its greediness, is willing to receive and
      process a DIO message from Node (C) (against the rules of RPL),
      and then Node (B) leaves the DODAG and rejoins at a lower Rank,

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      taking both Nodes (A) and (C) as DODAG parents.  Now Node (B) is
      deeper than both Nodes (A) and (C) and is satisfied with two DAG
      parents.

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

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

   o  Again, Node (C) leaves and rejoins deeper.

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

   o  This cycle can be averted through mechanisms in RPL:

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

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

   These mechanisms are further described in Section 8.2.2.4.

3.7.2.  DODAG Loops

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

   For example, consider the local repair mechanism that allows a node
   to detach from the DODAG, advertise a Rank of INFINITE_RANK (in order
   to poison its routes / inform its sub-DODAG), and then reattach to
   the DODAG.  In some of these cases, the node may reattach to its own
   prior-sub-DODAG, causing a DODAG loop, because the poisoning may fail
   if the INFINITE_RANK advertisements are lost in the LLN environment.
   (In this case, the Rank-based data-path validation mechanisms would
   eventually detect and trigger correction of the loop).

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3.7.3.  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-Path (a DAO message that invalidates a previously announced
   prefix, see Section 6.4.3) was missed and persists until all state
   has been cleaned up.  RPL includes an optional mechanism to
   acknowledge DAO messages, which may mitigate the impact of a single
   DAO message being missed.  RPL includes loop detection mechanisms
   that mitigate the impact of DAO loops and trigger their repair.  (See
   Section 11.2.2.3.)

4.  Traffic Flows Supported by RPL

   RPL supports three basic traffic flows: multipoint-to-point (MP2P),
   point-to-multipoint (P2MP), and point-to-point (P2P).

4.1.  Multipoint-to-Point Traffic

   Multipoint-to-point (MP2P) is a dominant traffic flow in many LLN
   applications ([RFC5867], [RFC5826], [RFC5673], and [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.

4.2.  Point-to-Multipoint Traffic

   Point-to-multipoint (P2MP) is a traffic pattern required by several
   LLN applications ([RFC5867], [RFC5826], [RFC5673], and [RFC5548]).
   RPL supports P2MP traffic by using a destination advertisement
   mechanism that provisions Down routes toward destinations (prefixes,
   addresses, or multicast groups), and away from roots.  Destination
   advertisements can update routing tables as the underlying DODAG
   topology changes.

4.3.  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.  As pointed out later in this document, in the most
   constrained case (when nodes cannot store routes), that common
   ancestor may be the DODAG root.  In other cases, it may be a node
   closer to both the source and destination.

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   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 potentially more optimal routes to support
   arbitrary P2P traffic.

5.  RPL Instance

   Within a given LLN, there may be multiple, logically independent RPL
   Instances.  A RPL node may belong to multiple RPL Instances, and it
   may act as a router in some and as a leaf in others.  This document
   describes how a single instance behaves.

   There are two types of RPL Instances: Local and Global.  RPL divides
   the RPLInstanceID space between Global and Local instances to allow
   for both coordinated and unilateral allocation of RPLInstanceIDs.
   Global RPL Instances are coordinated, have one or more DODAGs, and
   are typically long-lived.  Local RPL Instances are always a single
   DODAG whose singular root owns the corresponding DODAGID and
   allocates the local RPLInstanceID in a unilateral manner.  Local RPL
   Instances can be used, for example, for constructing DODAGs in
   support of a future on-demand routing solution.  The mode of
   operation of Local RPL Instances is out of scope for this
   specification and may be described in other companion specifications.

   The definition and provisioning of RPL Instances are out of scope for
   this specification.  Guidelines may be application and implementation
   specific, and they are expected to be elaborated in future companion
   specifications.  Those operations are expected to be such that data
   packets coming from the outside of the RPL network can unambiguously
   be associated to at least one RPL Instance and be safely routed over
   any instance that would match the packet.

   Control and data packets within RPL network are tagged to
   unambiguously identify of which RPL Instance they are a part.

   Every RPL control message has a RPLInstanceID field.  Some RPL
   control messages, when referring to a local RPLInstanceID as defined
   below, may also include a DODAGID.

   Data packets that flow within the RPL network expose the
   RPLInstanceID as part of the RPL Packet Information that RPL
   requires, as further described in Section 11.2.  For data packets
   coming from outside the RPL network, the ingress router determines
   the RPLInstanceID and places it into the resulting packet that it
   injects into the RPL network.

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5.1.  RPL Instance ID

   A global RPLInstanceID MUST be unique to the whole LLN.  Mechanisms
   for allocating and provisioning global RPLInstanceID are out of scope
   for this specification.  There can be up to 128 Global instance in
   the whole network.  Local instances are always used in conjunction
   with a DODAGID (which is either given explicitly or implicitly in
   some cases), and up 64 Local instances per DODAGID can be supported.
   Local instances are allocated and managed by the node that owns the
   DODAGID, without any explicit coordination with other nodes, as
   further detailed below.

   A global RPLInstanceID is encoded in a RPLInstanceID field as
   follows:

        0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+
       |0|     ID      |  Global RPLInstanceID in 0..127
       +-+-+-+-+-+-+-+-+

         Figure 4: RPLInstanceID Field Format for Global Instances

   A local RPLInstanceID is autoconfigured by the node that owns the
   DODAGID and it MUST be unique for that DODAGID.  The DODAGID used to
   configure the local RPLInstanceID MUST be a reachable IPv6 address of
   the node, and it MUST be used as an endpoint of all communications
   within that Local instance.

   A local RPLInstanceID is encoded in a RPLInstanceID field as follows:

        0 1 2 3 4 5 6 7
       +-+-+-+-+-+-+-+-+
       |1|D|   ID      |  Local RPLInstanceID in 0..63
       +-+-+-+-+-+-+-+-+

         Figure 5: RPLInstanceID Field Format for Local Instances

   The 'D' flag in a local RPLInstanceID is always set to 0 in RPL
   control messages.  It is used in data packets to indicate whether the
   DODAGID is the source or the destination of the packet.  If the 'D'
   flag is set to 1, then the destination address of the IPv6 packet
   MUST be the DODAGID.  If the 'D' flag is cleared, then the source
   address of the IPv6 packet MUST be the DODAGID.

   For example, consider a Node A that is the DODAG root of a Local RPL
   Instance, and has allocated a local RPLInstanceID.  By definition,
   all traffic traversing that Local RPL Instance will either originate
   or terminate at Node A.  In this case, the DODAGID will be the

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   reachable IPv6 address of Node A.  All traffic will contain the
   address of Node A, and thus the DODAGID, in either the source or
   destination address.  Thus, the local RPLInstanceID may indicate that
   the DODAGID is equivalent to either the source address or the
   destination address by setting the 'D' flag appropriately.



(page 30 continued on part 3)

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