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

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

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Copyright Notice

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Table of Contents

1.  Introduction
    1.1.  Design Principles
    1.2.  Expectations of Link Layer Type
2.  Terminology
3.  Protocol Model
    3.1.  Instances, DODAGs, and DODAG Iterations
    3.2.  Traffic Flows
        3.2.1.  Multipoint-to-Point Traffic
        3.2.2.  Point-to-Multipoint Traffic
        3.2.3.  Point-to-Point Traffic
    3.3.  DODAG Construction
        3.3.1.  DAG Information Object (DIO)
        3.3.2.  DAG Repair
        3.3.3.  Grounded and Floating DODAGs
        3.3.4.  Administrative Preference
        3.3.5.  Objective Function (OF)
        3.3.6.  Distributed Algorithm Operation
    3.4.  Destination Advertisement
        3.4.1.  Destination Advertisement Object (DAO)
4.  Routing Metrics and Constraints Used By RPL
5.  Rank
    5.1.  Loop Avoidance
        5.1.1.  Greediness and Rank-based Instabilities
        5.1.2.  DODAG Loops
        5.1.3.  DAO Loops
        5.1.4.  Sibling Loops
    5.2.  Rank Properties
6.  RPL Protocol Specification
    6.1.  RPL Messages
        6.1.1.  ICMPv6 RPL Control Message
        6.1.2.  DAG Information Solicitation (DIS)
        6.1.3.  DAG Information Object (DIO)
        6.1.4.  Destination Advertisement Object (DAO)
    6.2.  Protocol Elements
        6.2.1.  Topological Elements
        6.2.2.  Neighbors, Parents, and Siblings
        6.2.3.  DODAG Information
    6.3.  DAG Discovery and Maintenance
        6.3.1.  DAG Discovery Rules
        6.3.2.  DIO Message Communication
        6.3.3.  DIO Transmission
        6.3.4.  Trickle Timer for DIO Transmission
    6.4.  DAG Selection
    6.5.  Operation as a Leaf Node
    6.6.  Administrative rank
    6.7.  Collision
    6.8.  Establishing Routing State Down the DODAG
        6.8.1.  Destination Advertisement Operation
    6.9.  Loop Detection
        6.9.1.  Source Node Operation
        6.9.2.  Router Operation
    6.10.  Multicast Operation
    6.11.  Maintenance of Routing Adjacency
7.  Suggestions for Packet Forwarding
8.  Guidelines for Objective Functions
9.  RPL Constants and Variables
10.  Manageability Considerations
    10.1.  Control of Function and Policy
        10.1.1.  Initialization Mode
        10.1.2.  DIO Base option
        10.1.3.  Trickle Timers
        10.1.4.  DAG Sequence Number Increment
        10.1.5.  Destination Advertisement Timers
        10.1.6.  Policy Control
        10.1.7.  Data Structures
    10.2.  Information and Data Models
    10.3.  Liveness Detection and Monitoring
        10.3.1.  Candidate Neighbor Data Structure
        10.3.2.  Directed Acyclic Graph (DAG) Table
        10.3.3.  Routing Table
        10.3.4.  Other RPL Monitoring Parameters
        10.3.5.  RPL Trickle Timers
    10.4.  Verifying Correct Operation
    10.5.  Requirements on Other Protocols and Functional Components
    10.6.  Impact on Network Operation
11.  Security Considerations
12.  IANA Considerations
    12.1.  RPL Control Message
    12.2.  New Registry for RPL Control Codes
    12.3.  New Registry for the Control Field of the DIO Base
    12.4.  DAG Information Object (DIO) Suboption
13.  Acknowledgements
14.  Contributors
15.  References
    15.1.  Normative References
    15.2.  Informative References
Appendix A.  Requirements
    A.1.  Protocol Properties Overview
        A.1.1.  IPv6 Architecture
        A.1.2.  Typical LLN Traffic Patterns
        A.1.3.  Constraint Based Routing
    A.2.  Deferred Requirements
Appendix B.  Examples
    B.1.  Destination Advertisement
    B.2.  Example: DAG Parent Selection
    B.3.  Example: DAG Maintenance
    B.4.  Example: Greedy Parent Selection and Instability
Appendix C.  Outstanding Issues
    C.1.  Additional Support for P2P Routing
    C.2.  Loop Detection
    C.3.  Destination Advertisement / DAO Fan-out
    C.4.  Source Routing
    C.5.  Address / Header Compression
§  Authors' Addresses

1.  Introduction

Low power and Lossy Networks (LLNs) are made largely of constrained nodes (with limited processing power, memory, and sometimes energy when they are battery operated). These routers are interconnected by lossy links, 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] (Martocci, J., Riou, N., Mil, P., and W. Vermeylen, “Building Automation Routing Requirements in Low Power and Lossy Networks,” December 2009.), [I‑D.ietf‑roll‑home‑routing‑reqs] (Brandt, A. and J. Buron, “Home Automation Routing Requirements in Low Power and Lossy Networks,” November 2009.), [RFC5673] (Pister, K., Thubert, P., Dwars, S., and T. Phinney, “Industrial Routing Requirements in Low-Power and Lossy Networks,” October 2009.), and [RFC5548] (Dohler, M., Watteyne, T., Winter, T., and D. Barthel, “Routing Requirements for Urban Low-Power and Lossy Networks,” May 2009.). 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] (Martocci, J., Riou, N., Mil, P., and W. Vermeylen, “Building Automation Routing Requirements in Low Power and Lossy Networks,” December 2009.), [I‑D.ietf‑roll‑home‑routing‑reqs] (Brandt, A. and J. Buron, “Home Automation Routing Requirements in Low Power and Lossy Networks,” November 2009.), [RFC5673] (Pister, K., Thubert, P., Dwars, S., and T. Phinney, “Industrial Routing Requirements in Low-Power and Lossy Networks,” October 2009.), and [RFC5548] (Dohler, M., Watteyne, T., Winter, T., and D. Barthel, “Routing Requirements for Urban Low-Power and Lossy Networks,” May 2009.). Because those requirements are heterogeneous and sometimes incompatible in nature, the approach is first taken to design a protocol capable of supporting a core set of functionalities corresponding to the intersection of the requirements. As the RPL design evolves optional features may be added to address some application specific requirements. This is a key protocol design decision providing a granular approach in order to restrict the core of the protocol to a minimal set of functionalities, and to allow each implementation of the protocol to be optimized differently. All "MUST" application requirements that cannot be satisfied by RPL will be specifically listed in the Appendix A, accompanied by a justification.

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

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

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

1.2.  Expectations of Link Layer Type

As RPL is a routing protocol, it of course does not rely on any particular features of a specific link layer technology. RPL should 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 (Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. Wood, “Advice for Internet Subnetwork Designers,” July 2004.) [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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].

This document requires readers to be familiar with the terminology described in `Terminology in Low power And Lossy Networks' [I‑D.ietf‑roll‑terminology] (Vasseur, J., “Terminology in Low power And Lossy Networks,” March 2010.).

DAG:

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

DAG Instance:

A DAG Instance is a set of possibly multiple Destination Oriented DAGs. A network may have more than one DAG Instance, and a RPL router can participate to multiple DAG instances. Each DAG Instance operates independently of other DAG Instances. This document describes operation within a single DAG instance.

InstanceID:

Unique identifier of a DAG Instance.

Destination Oriented DAG (DODAG):

A DAG rooted at a single destination, which is a node with no outgoing edges. The tuple (InstanceID, DAGID) uniquely identifies a Destination Oriented DAG (DODAG). In the RPL context, a router can can belong to at most one DODAG per DAG Instance.

DAGID:

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

DODAG Iteration:

A specific sequence number iteration of a DODAG.

DAGSequenceNumber:

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 (InstanceID, DAGID, DAGSequenceNumber) tuple.

DAG parent:

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

DAG sibling:

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

DAG root:

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

Sub-DAG

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

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.

OCP:

Objective Code Point. The Objective Code Point is used to indicate which Objective Function is in use in a DODAG. The Objective Code Point is further described in [I‑D.ietf‑roll‑routing‑metrics] (Vasseur, J., Kim, M., Networks, D., and H. Chong, “Routing Metrics used for Path Calculation in Low Power and Lossy Networks,” April 2010.).

OF:

Objective Function. The 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] (Vasseur, J., Kim, M., Networks, D., and H. Chong, “Routing Metrics used for Path Calculation in Low Power and Lossy Networks,” April 2010.).

Goal:

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

Grounded:

A DAG is grounded when the root can reach the Goal of the objective function.

Floating:

A DAG is floating if is not Grounded. A floating DAG 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 Model

The aim of this section is to describe RPL in the spirit of [RFC4101] (Rescorla, E. and IAB, “Writing Protocol Models,” June 2005.). Protocol details can be found in further sections.

3.1.  Instances, DODAGs, and DODAG Iterations

Each DAG instance constructs a routing topology optimized for a certain Objective Function (OF). A DAG instance may provide routes to certain destination prefixes. A single DAG 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 DAGID, such that nodes can identify the DODAG root.

A DAG instance may comprise:

• 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.
• multiple uncoordinated DODAGs with independent roots (differing DAGIDs)
  • For example, multiple data collection points in an urban data collection application that do not have an always-on backbone suitable to coordinate to form a single DODAG, and further use the formation of multiple DODAGs as a means to dynamically and autonomously partition the network.
• a single DODAG with a single virtual root coordinating LLN sinks (with the same DAGID) 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.
• a combination of one of the above as suited to some application scenario.

Traffic is bound to a specific DODAG 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 DAG instance, for example to follow paths optimized for low latency or low energy. The provisioning or automated discovery of a mapping between an InstanceID and a type or service of application traffic is beyond the scope of this specification.

An example of a DAG Instance comprising a number of DODAGs is depicted in Figure 1 (DAG Instance). A DODAG Iteration is depicted in Figure 2 (DODAG Iteration).

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

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

3.2.  Traffic Flows

3.2.1.  Multipoint-to-Point Traffic

Multipoint-to-Point (MP2P) is a dominant traffic flow in many LLN applications ([I‑D.ietf‑roll‑building‑routing‑reqs] (Martocci, J., Riou, N., Mil, P., and W. Vermeylen, “Building Automation Routing Requirements in Low Power and Lossy Networks,” December 2009.), [I‑D.ietf‑roll‑home‑routing‑reqs] (Brandt, A. and J. Buron, “Home Automation Routing Requirements in Low Power and Lossy Networks,” November 2009.), [RFC5673] (Pister, K., Thubert, P., Dwars, S., and T. Phinney, “Industrial Routing Requirements in Low-Power and Lossy Networks,” October 2009.), [RFC5548] (Dohler, M., Watteyne, T., Winter, T., and D. Barthel, “Routing Requirements for Urban Low-Power and Lossy Networks,” May 2009.)). The destinations of MP2P flows are designated nodes that have some application significance, such as providing connectivity to the larger Internet or core private IP network. RPL supports MP2P traffic by allowing MP2P destinations to be reached via DODAG roots.

3.2.2.  Point-to-Multipoint Traffic

Point-to-multipoint (P2MP) is a traffic pattern required by several LLN applications ([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,” December 2009.), [I‑D.ietf‑roll‑home‑routing‑reqs] (Brandt, A. and J. Buron, “Home Automation Routing Requirements in Low Power and Lossy Networks,” November 2009.), [RFC5673] (Pister, K., Thubert, P., Dwars, S., and T. Phinney, “Industrial Routing Requirements in Low-Power and Lossy Networks,” October 2009.), [RFC5548] (Dohler, M., Watteyne, T., Winter, T., and D. Barthel, “Routing Requirements for Urban Low-Power and Lossy Networks,” May 2009.)). RPL supports P2MP traffic by using a destination advertisement mechanism that provisions routes toward destination prefixes and away from roots. Destination advertisements can update routing tables as the underlying DODAG topology changes.

3.2.3.  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.

3.3.  DODAG Construction

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

3.3.1.  DAG Information Object (DIO)

A DIO identifies the DAG Instance, the DAGID, the values used to compute the DAG 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.

3.3.2.  DAG Repair

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

RPL may support mechanisms for local repair within the DODAG iteration. The DIO message will specify 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:

• Poison its sub-DAG by advertising an effective rank of INFINITY, OR detach and form a floating DODAG in order to preserve inner connectivity within its sub-DAG.
• Move down the DODAG iteration in a limited manner, no further than a bound configured 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 possible.

3.3.3.  Grounded and Floating DODAGs

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

3.3.4.  Administrative Preference

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

3.3.5.  Objective Function (OF)

The Objective Function (OF) implements the optimization objectives of route selection within the DAG 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] (Vasseur, J., Kim, M., Networks, D., and H. Chong, “Routing Metrics used for Path Calculation in Low Power and Lossy Networks,” April 2010.) and related companion specifications.

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

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

3.3.6.  Distributed Algorithm Operation

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

• Some nodes are configured to be DODAG roots, with associated DODAG configuration.
• Nodes advertise their presence, affiliation with a DODAG, routing cost, and related metrics by sending link-local multicast DIO messages.
• Nodes may adjust the rate at which DIO messages are sent in response to stability or detection of routing inconsistencies.
• 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.
• The nodes provision routing table entries for the destinations specified by the DIO towards their parents in the DODAG iteration. Nodes may provision a parent as a default gateway.
• Nodes may identify siblings within the DODAG iteration to increase path diversity.
• Using both DIOs and possibly information in data packets, RPL nodes detect possible routing loops. When a RPL node detects a possible routing loop, it may adapt its DIO transmission rate to apply a local repair to the topology. This process and its limitations is discussed in greater detail in 3.4.

3.4.  Destination Advertisement

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

3.4.1.  Destination Advertisement Object (DAO)

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

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

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

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

3.4.1.1.  `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.

4.  Routing Metrics and Constraints Used By RPL

Routing metrics are used by routing protocols to compute the shortest paths. Interior Gateway Protocols (IGPs) such as IS-IS ([RFC5120] (Przygienda, T., Shen, N., and N. Sheth, “M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs),” February 2008.)) and OSPF ([RFC4915] (Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P. Pillay-Esnault, “Multi-Topology (MT) Routing in OSPF,” June 2007.)) 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, that will satisfy all use cases.

In addition, RPL supports constrained-based routing where constraints may be applied to 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] (Vasseur, J., Kim, M., Networks, D., and H. Chong, “Routing Metrics used for Path Calculation in Low Power and Lossy Networks,” April 2010.).

The role of the Objective Function is to advertise routing metrics and constraints in addition to the objectives used to compute the (constrained) shortest path.

Example 1:

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

Example 2:

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

5.  Rank

5.1.  Loop Avoidance

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

5.1.1.  Greediness and Rank-based Instabilities

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

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

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

A further example of the consequences of greedy operation, and instability related to processing DIO messages from nodes of greater rank, may be found in Appendix B.4 (Example: Greedy Parent Selection and Instability)

5.1.2.  DODAG Loops

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

5.1.3.  DAO Loops

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

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

5.1.4.  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 to prevent sibling loops.

5.2.  Rank Properties

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

The rank is not a cost metric, although its value can be derived from and influenced by metrics. The rank has properties of its own that are not necessarily that 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, not necessarily a good indication or a proper expression of a distance or a cost to the root.

Stability:

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

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 such property.

Abstract:

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

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

The computation of the DAG rank MUST be done in such a way so as to maintain the following properties for any nodes M and N that are neighbors in the LLN:

DAGRank(M) is less than DAGRank(N):

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

DAGRank(M) equals DAGRank(N):

In this case M and N are located positions of relatively the same optimality within the DODAG. In some cases, Node M may be used as a successor by Node N, but with related chance of creating a loop that must be detected and broken by some other means.

DAGRank(M) is greater than DAGRank(N):

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

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

6.  RPL Protocol Specification

6.1.  RPL Messages

6.1.1.  ICMPv6 RPL Control Message

This document defines the RPL Control Message, a new ICMPv6 message. The RPL Control Message has the following general format, in accordance with [RFC4443] (Conta, A., Deering, S., and M. Gupta, “Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification,” March 2006.):

     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                          +
    |                                                               |

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

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

• 0x01: DAG Information Solicitation (Section 6.1.2 (DAG Information Solicitation (DIS)))
• 0x02: DAG Information Object (Section 6.1.3 (DAG Information Object (DIO)))
• 0x04: Destination Advertisement Object (Section 6.1.4 (Destination Advertisement Object (DAO)))

6.1.2.  DAG Information Solicitation (DIS)

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

6.1.3.  DAG Information Object (DIO)

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

6.1.3.1.  DIO Base

The DIO Base is a container option, which is always present, and might contain a number of suboptions. The base option regroups the minimum information set that is mandatory in all cases.

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |G|D|A|0|0| Prf |   Sequence    |  InstanceID   |    DAGRank    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    +                                                               +
    |                            DAGID                              |
    +                                                               +
    |                                                               |
    +                                                               +
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   sub-option(s)...
    +-+-+-+-+-+-+-+-+-+-+-+-+-+

Control Field:

The DAG Control Field is currently allocated as follows:

Grounded (G):

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

Destination Advertisement Trigger (D):

The Destination Advertisement Trigger (D) flag is set when the DODAG root or another node in the successor chain decides to trigger the sending of destination advertisements in order to update routing state for the down direction along the DODAG, as further detailed in Section 6.8 (Establishing Routing State Down the DODAG). Note that the use and semantics of this flag are still under investigation.

Destination Advertisement Supported (A):

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

DAGPreference (Prf):

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

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

Sequence Number:

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

InstanceID:

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

DAGRank:

8-bit unsigned integer indicating the DAG rank of the node sending the DIO message. The DAGRank of the DODAG root is ROOT_RANK. DAGRank is further described in Section 6.3 (DAG Discovery and Maintenance).

DAGID:

128-bit unsigned integer which uniquely identify a DODAG. This value is set by the DODAG root. The global IPv6 address of the DODAG root can be used. the DAGID MUST be unique per DAG Instance within the scope of the LLN.

The following values MUST NOT change during the propagation of DIO messages down the DAG:

Grounded (G)

Destination Advertisement Supported (A)

DAGPreference (Prf)

Sequence

InstanceID

DAGID

All other fields of the DIO message may be updated at each hop of the propagation.

6.1.3.1.1.  DIO Suboptions

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

6.1.3.1.1.1.  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 |         Subopt Length         | Subopt Data
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -

Suboption Type:

8-bit identifier of the type of suboption. 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, continue to process the following suboption, correctly handling any remaining options in the message.

Suboption Length:

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

Suboption Data:

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

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

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

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

6.1.3.1.1.2.  Pad1

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

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

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.

6.1.3.1.1.3.  PadN

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

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

The PadN option 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 Option 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.

6.1.3.1.1.4.  DAG Metric Container

The DAG Metric Container suboption may be aligned as necessary to support its contents. Its format 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    |       Container Length        | DAG Metric Data
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -

The DAG Metric Container is used to report aggregated path metrics along the DODAG. The DAG Metric Container may contain a number of discrete node, link, and aggregate path metrics as chosen by the implementer. The Container Length field contains the length in octets of the DAG Metric Data. The order, content, and coding of the DAG Metric Container data is as specified in [I‑D.ietf‑roll‑routing‑metrics] (Vasseur, J., Kim, M., Networks, D., and H. Chong, “Routing Metrics used for Path Calculation in Low Power and Lossy Networks,” April 2010.).

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

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

6.1.3.1.1.5.  Destination Prefix

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

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

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

The 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] (Draves, R. and D. Thaler, “Default Router Preferences and More-Specific Routes,” November 2005.). 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] (Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” September 2007.)). 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 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.

6.1.3.1.1.6.  DAG Configuration

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

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

The DAG Configuration suboption is used to distribute configuration information for DAG 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 DAG 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 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 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.

6.1.4.  Destination Advertisement Object (DAO)

The Destination Advertisement Object (DAO) is used to propagate destination information upwards along the DODAG. The RPL use of the DAO allows the nodes in the DODAG to provision routing state for nodes contained in the sub-DAG in support of traffic flowing down 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          |  InstanceID   |   DAO Rank    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          DAO Lifetime                         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           Route Tag                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Prefix Length |    RRCount    |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               |
    |                   Prefix (Variable Length)                    |
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |             Reverse Route Stack (Variable Length)             |
    .                                                               .
    .                                                               .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

DAO Sequence:

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

InstanceID:

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

DAO Rank:

Set by the node that owns the prefix and first issues the DAO message to its rank.

DAO Lifetime:

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

Route Tag:

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

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.

Prefix:

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

Reverse Route Stack:

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

6.2.  Protocol Elements

6.2.1.  Topological Elements

RPL uses four identifiers to track and control the routing topology

• The first is an InstanceID. An InstanceID defines what OF a DAG uses and may also indicate what destinations are offered. A network may have multiple InstanceIDs, each of which defines an independent DAG optimized for a different OF and/or application. The DAG defined by an InstanceID is called a DAG Instance.
• The second is a DAGID. The scope of a DAGID is a DAG Instance. A combination of InstanceID and DAGID defines a DODAG. A DAG Instance may have multiple DODAGs.
• The third value is a DAG Sequence Number. The scope of a DAG Sequence Number is a DODAG. A DODAG is sometimes reconstructed from the root, by incrementing the DAGSequenceNumber. A combination of InstanceID, DAGID, and DAG Sequence Number defines a DODAG Iteration.
• The fourth value is rank. The scope of rank is a DODAG Iteration. Rank establishes a partial order over a DODAG Iteration, defining individual node positions.

6.2.2.  Neighbors, Parents, and Siblings

1. A node that is not a DODAG root MAY maintain multiple DAG parents for a single DAG Instance.
2. The set of DAG parents MUST be a conceptual subset of the set of candidate neighbors. (This does not dictate implementation, e.g., to use a certain data structure).
3. If Neighbor Unreachability Detection (NUD), or an equivalent mechanism, determines that a neighbor is no longer reachable, then a RPL node MUST NOT consider this node in the neighbor set when calculating and advertising routes until the node determines it is reachable again.
4. Routes via that unreachable neighbor MUST be eliminated from the routing table, and the node SHOULD poison using no-DAO all DAO routes that it has advertised via DAO and that it can reach only via that neighbor.

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

The OF guides in the selection and maintains a number of neighbors to interact with, which neighbors being qualified as statistically stable and presenting adequate properties as per the the OF logic, for instance following mechanisms discussed in [I‑D.ietf‑roll‑routing‑metrics] (Vasseur, J., Kim, M., Networks, D., and H. Chong, “Routing Metrics used for Path Calculation in Low Power and Lossy Networks,” April 2010.). Those neighbors are referred to as candidate neighbors.

Candidate neighbors may take the role of Parent or Siblings, in part as determined by rank.

For the purpose of inheriting metrics and computing rank, the OF might select one preferred parent. In that case, the rank of this node is computed as the rank of the preferred parent plus a rank increment as determined by the OF.

6.2.3.  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 for each DODAG. 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.

• InstanceID
• DAGID
• DAGSequenceNumber
• DAG Metric Container, including DAGObjectiveCodePoint
• A set of Destination Prefixes offered upwards along the DODAG
• A set of DAG parents
• A set of DAG siblings
• A timer to govern the sending of DIO messages

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

6.2.3.1.  DAG Parents/Siblings Structure

When the DODAG is self-rooted, the set of DAG parents is empty.

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

• a reference to the neighboring device which is the DAG parent/sibling
• a record of most recent information taken from the DAG Information Object last processed in the case where the neighboring device is a DAG parent

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

When nodes are added to or removed from the DAG parent/sibling sets the most preferred DAG parent may have changed. The role of all the nodes in the list should be reevaluated. In particular, any nodes having a rank greater than self after such a change must be evicted from the set.

6.3.  DAG Discovery and Maintenance

DAG discovery allows a node to join a DODAG rooted at a DODAG root by discovering neighbors that are members of the DODAG, and identifying a set of parents. DAG discovery also identifies siblings, which may be used later to provide additional path diversity towards the DODAG root.

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

DAG discovery enables nodes to implement different policies for selecting their DAG parents in the DODAG by using implementation specific policy functions. DAG discovery specifies a set of rules to be followed by all implementations to enable interoperation.

6.3.1.  DAG Discovery Rules

The following rules define the RPL DAG Discovery procedures:

6.3.1.1.  DODAG Iteration

1. An InstanceID SHOULD be administratively provisioned on a DODAG root that is significant RPL objective. The InstanceID MUST be unique to that purpose across the scope of the LLN.
2. A DAGID MUST be unique within the scope of the InstanceID. It MAY be derived from the IPv6 address of the DODAG root.
3. A node MAY belong to multiple DAG instances. The related details of operation are outside the scope of this specification.
4. DODAG roots MAY increment the DAGSequenceNumber that they advertise.
5. When a DODAG root increments its DAGSequenceNumber, it MUST follow the conventions of Serial Number Arithmetic as described in [RFC1982] (Elz, R. and R. Bush, “Serial Number Arithmetic,” August 1996.).
6. The tuple (InstanceID, DAGID, DAGSequenceNumber) uniquely defines a DODAG Iteration. All of a node's parents within a DODAG MUST belong to the same DODAG iteration, as conveyed by the last heard DIO from each parent.
7. A node MUST NOT propagate DIOs for a DODAG Iteration unless it is the DODAG root of the DODAG iteration or has selected DODAG parents in that DODAG iteration.
8. A node acting as a leaf SHOULD NOT propagate DIOs for a DODAG Iteration.
9. A node MUST belong at most to one DODAG Iteration per InstanceID.
10. Within a given DODAG, a node that is a not a root MUST NOT advertise a DAGSequenceNumber higher than the highest DAGSequenceNumber it has heard.

Within a particular implementation, a DODAG root may increment the DAGSequenceNumber periodically, at a rate that depends on the deployment. In other implementations loop detection may be considered sufficient to solve the routing issues, and the DODAG root may increment the DAGSequenceNumber only upon administrative intervention. Another possibility is that nodes within the LLN have some means to signal the DODAG root in order to request an on-demand increment when routing issues are detected.

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

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

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

6.3.1.2.  DODAG Roots

1. A DODAG root that does not have connectivity to a network outside of the LLN MUST NOT set the Grounded bit.
2. A DODAG root MUST advertise a rank of ROOT_RANK.
3. A node that does not have any DODAG parent MAY become the DODAG root of a floating DODAG. It MAY also set its DAGPreference such that it is less preferred. This behavior may be a desired alternate to poisoning.

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

In a deployment that uses a backbone link to federate a number of LLN roots, it is possible to run RPL over the 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, expose a rank of ROOT_RANK over the LLN and are part of the same DODAG, coordinated with the virtual root over the backbone.

6.3.1.3.  Rank and Movement within a DODAG Iteration

1. A node MUST NOT advertise a rank less than or equal to any member of its parent set within the DODAG Iteration.
2. A node MAY advertise a rank lower than its prior advertisement within the DODAG Iteration. (This corresponds to a node moving up within the DODAG Iteration).
3. Let L be the lowest rank within a DODAG iteration that a given node has advertised. Within a DODAG Iteration, that node MUST NOT advertise an effective rank deeper than L + DAGMaxRankIncrease. INFINITE_RANK is an exception to this rule: a node MAY advertise an INFINITE_RANK at any time. (This corresponds to a limited rank increase for the purpose of local repair within the DODAG Iteration.)
4. A node MAY, at any time, choose to join a different DODAG within a DAG Instance. Such a join has no rank restrictions, unless that different DODAG is a DODAG Iteration that the node has been a prior member of, in which case the rule of the previous bullet (3) must be observed. Until a node transmits a DIO indicating its new DODAG membership, it MUST forward packets along the previous DODAG.
5. A node MAY, at any time after hearing the next DAGSequenceNumber Iteration advertised from suitable parents, choose to migrate up to the next DODAG Iteration within the DODAG.

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

When a node migrates into the next DODAG Iteration, the parent and sibling sets need to be rebuilt for the new iteration. An implementation could defer to migrate until for some reasonable 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 parent/sibling sets for the new DODAG.

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

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

6.3.1.4.  Poisoning a Broken Path

1. A node MAY poison, in order to avoid being used as an ancestor by the nodes in its sub-DAG, by advertising an effective rank of INFINITE_RANK and resetting the associated DIO trickle timer to cause the 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 6.3.1.3 (Rank and Movement within a DODAG Iteration), the node MUST advertise INFINITE_RANK within the DODAG iteration if its revised rank would exceed the maximum DAG rank increase.

An implementation may choose to employ this poisoning mechanism when a node that loses all of its current parents, i.e. the set of DAG parents becomes depleted, and it can not jump onto 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 lower portions of the network are able to re-establish up routes. Path redundancy in the DAG reduces the significance of either effect, since children with alternate parents should be able to utilize those alternates and retain rank while the detached parent re-establishes its rank.

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

6.3.1.5.  Detaching

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

6.3.1.6.  Following a Parent

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

A DAG parent may have moved, migrated forward into 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.

6.3.2.  DIO Message Communication

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

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

6.3.2.1.  DIO Message Processing

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

Process the DIO message as per the rules in Section 6.3 (DAG Discovery and Maintenance)

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

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

6.3.3.  DIO Transmission

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

• When a node detects or causes an inconsistency, it MUST reset the interval of the trickle timer to a minimum value.
• When a node migrates to a new DODAG Iteration it MUST reset the trickle timer to its minimum value
• When a node detects an inconsistency when forwarding a packet, as detailed in Section 6.9 (Loop Detection), the node MUST reset the trickle timer to its minimum value.
• When a node receives a multicast DIS message, it MUST reset the trickle timer to the minimum value.
• When a node receives a unicast DIS message, it MUST unicast a DIO message in response, and include the DAG Configuration Object. In this case the node SHOULD NOT reset the trickle timer.
• If a node is not a member of a DODAG, it MUST suppress transmitting DIO messages.
• 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.

6.3.4.  Trickle Timer for DIO Transmission

RPL treats the construction of a DODAG as a consistency problem, and uses a trickle timer [Levis08] (Levis, P., Brewer, E., Culler, D., Gay, D., Madden, S., Patel, N., Polastre, J., Shenker, S., Szewczyk, R., and A. Woo, “The Emergence of a Networking Primitive in Wireless Sensor Networks,” July 2008.) to control the rate of control broadcasts.

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

I:

The current length of the communication interval

T:

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

C:

Redundancy Counter

I_min:

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

I_doublings:

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

6.3.4.1.  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 DIO message.
2. Setting C to zero.
3. Setting I to I_min.
4. Setting T to a random value as described above.
5. Restarting the trickle timer to expire after a duration T

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

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

6.3.4.2.  Determination of Inconsistency

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

• The node joins a new DODAG
• The node moves within a DODAG
• The node receives a modified DIO message from a DAG parent
• A DAG parent forwards a packet intended to move up, indicating an inconsistency and possible loop.
• A metric communicated in the DIO message is determined to be inconsistent, as according to a implementation specific path metric selection engine.
• The rank of a DAG parent has changed.

6.4.  DAG Selection

The DAG selection is implementation and algorithm dependent. Nodes SHOULD prefer to join DODAGs for InstanceIDs advertising OCPs and destinations compatible with their implementation specific objectives. In order to limit erratic movements, and all metrics being equal, nodes SHOULD keep their previous selection. Also, nodes SHOULD provide a means to filter out a parent whose availability is detected as fluctuating, at least when more stable choices are available.

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

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

6.5.  Operation as a Leaf Node

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

1. MAY receive and process DIOs for that DODAG
2. SHOULD NOT transmit DIOs for that DODAG
3. MUST NOT transmit DIOs containing the DAG Metric Container for that DODAG
4. MAY transmit unicast DAOs to the chosen parents for that DODAG
5. MAY transmit multicast DAOs to the `1 hop' neighborhood.

6.6.  Administrative rank

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

6.7.  Collision

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

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

6.8.  Establishing Routing State Down the DODAG

The destination advertisement mechanism supports the dissemination of routing state required to support traffic flows down along the DODAG, from the DODAG root toward nodes.

As a result of destination advertisement operation:

• Destination advertisement establishes down routes along the DODAG. Such paths consist of:
• Hop-By-Hop routing state within islands of `stateful' nodes.
• Source Routing `bridges' across nodes that do not retain state.

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

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

Nodes that are capable of storing routing state, and finally the DODAG roots, are able to learn which destinations are contained in the sub-DAG below the node, and via which next-hop neighbors. The dissemination and installation of this routing state into nodes allows for Hop-By-Hop routing from the DODAG root down the DODAG. The mechanism is further enhance by supporting the construction of source routes across stateless `gaps' in the DODAG, where nodes are incapable of storing additional routing state. An adaptation of this mechanism allows for the implementation of loose-source routing.

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

A design choice behind advertising routes via destination advertisements is not to synchronize the parent and children databases along the DODAG, but instead to update them regularly to recover from the loss of packets. The rationale for that choice is time variations in connectivity across unreliable links. If the topology can be expected to change frequently, synchronization might be an excessive goal in terms of exchanges and protocol complexity. The approach used here results in a simple protocol with no real peering. The destination advertisement mechanism hence provides for periodic updates of the routing state, similarly to other protocols such as RIP [RFC2453] (Malkin, G., “RIP Version 2,” November 1998.).

6.8.1.  Destination Advertisement Operation

6.8.1.1.  Overview

According to implementation specific policy, a subset or all of the feasible parents in the DODAG may be selected to receive prefix information from the destination advertisement mechanism. This subset of DAG parents shall be designated the set of DA parents.

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

If destination advertisements are activated in the DIO message as indicated by the `D' bit, the node sends unicast destination advertisements to one of its DA parents, that is selected as most favored for incoming down traffic. The node only accepts unicast destination advertisements from any nodes but those contained in the DA parent subset.

Receiving a DIO message with the `D' destination advertisement bit set from a DAG parent stimulates the sending of a delayed destination advertisement back, with the collection of all known prefixes (that is the prefixes learned via destination advertisements for nodes lower in the DODAG, and any connected prefixes). If the Destination Advertisement Supported (A) bit is set in the DIO message for the DODAG, then a destination advertisement is also sent to a DAG parent once it has been added to the DA parent set after a movement, or when the list of advertised prefixes has changed.

A node that modifies its DAG Parent set may set the `D' bit in subsequent DIO propagation in order to trigger destination advertisements to be updated to its DAG Parents and other ancestors on the DODAG. Additional recommendations and guidelines regarding the use of this mechanism are still under consideration and will be elaborated in a future revision of this specification.

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

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

A node that is unable to record the state information conveyed in the DAO message will append the next-hop address to the Reverse Route Stack, increment the RRCount, and then pass the destination advertisement on without recording any additional state. In this way the Reverse Route Stack will contain a vector of next hops that must be traversed along the reverse path that the DAO message has traveled. The vector will be ordered such that the node closest to the destination will appear first in the list. In such cases, if it is useful to the implementation to try and provision redundant paths, the node may choose to convey the destination advertisement to one or more DAG parents in order of preference as guided by an implementation specific policy.

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

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

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

• A reference to the ND entry that was created for the advertising neighbor.
• The IPv6 address and interface for the advertising neighbor.
• The logical equivalent of the full destination advertisement information (including the prefixes, depth, and Reverse Route Stack, if any).
• A 'reported' Boolean to keep track whether this prefix was reported already, and to which of the DA parents.
• A counter of retries to count how many DIO messages were sent on the interface to the advertising neighbor without reachability confirmation for the prefix.

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

When a node loses connectivity to a child that is used as next hop for a route learned from a DAO, the node should cleanup all routes and DAO states that are related to that child. If the lost child was the only adjacency leading to the DAO prefix, the node should poison the route by sending no-DAOs to the parents to which it has advertised the DAO prefixes.

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

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

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

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

6.8.1.1.1.  Destination Advertisement Timers

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

• The DelayDAO timer is armed upon a stimulation to send a destination advertisement (such as a DIO message from a DA parent). When the timer is armed, all entries in the Reachable list as well as all entries for Connected list are set to not be reported yet for that particular DA parent.
• For a root, the DIO timer has a duration of DEF_DAO_LATENCY. For a node in a DODAG iteration, the DelayDAO timer has a duration that is randomized between (DEF_DAO_LATENCY divided by the Rank of the node) and (DEF_DAO_LATENCY divided by the Rank of the parent). The intention is that nodes located deeper in the DODAG iteration should have a shorter DelayDAO timer, allowing DAO messages a chance to be reported from deeper in the DODAG and potentially aggregated along sub-DAGs before propagating further up.
• The RemoveTimer is used to clean up entries for which DAO messages are no longer being received from the sub-DAG.
• When a DIO message is sent that is requesting destination advertisements, a flag is set for all DAO entries in the routing table.
• If the flag has already been set for a DAO entry, the retry count is incremented.
• If a DAO message is received to confirm the entry, the entry is refreshed and the flag and count may be cleared.
• If at least one entry has reached a threshold value and the RemoveTimer is not running, the entry is considered to be probably gone and the RemoveTimer is started.
• When the RemoveTimer elapse, DAO messages with lifetime 0, i.e. no-DAOs, are sent to explicitly inform DA parents that the entries which have reached the threshold are no longer available, and the related routing states may be propagated and cleaned up.
• The RemoveTimer has a duration of min (MAX_DESTROY_INTERVAL, TBD(DIO Trickle Timer Interval)).

6.8.1.2.  Multicast Destination Advertisement Messages

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

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

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

6.8.1.3.  Unicast Destination Advertisement Messages from Child to Parent

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

The DIO message from the DODAG root is used to synchronize the whole DODAG iteration, including the periodic reporting of destination advertisements back up the DODAG. Its period is expected to vary, depending on the configuration of the DIO trickle timer.

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

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

• If the entry is CONFIRMED, it goes PENDING with the retry count set to 0.
• If the entry is PENDING, the retry count is incremented. If it reaches a maximum threshold, the entry goes ELAPSED If at least one entry is ELAPSED at the end of the process: if the RemoveTimer is not running then it is armed with a jitter.

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

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

6.8.1.4.  Other Events

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

• Destination advertisement operation stopped: All entries in the abstract lists are freed. All the routes learned from DAO messages are removed.
• Interface going down: for all entries in the Reachable list on that interface, the associated route is removed, and the entry is scheduled to be removed.
• Loss of routing adjacency: When the routing adjacency for a neighbor is lost, as per the procedures described in Section 6.11 (Maintenance of Routing Adjacency), and if the associated entries are in the Reachable list, the associated routes are removed, and the entries are scheduled to be destroyed.
• Changes to DA parent set: all entries in the Reachable list are set to not 'reported' and DelayDAO is armed.

6.8.1.5.  Aggregation of Prefixes by a Node

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

Other cases might occur for which additional support is required:

1. The aggregating node is attached within the sub-DAG of the nodes it is aggregating for.
2. A node that is to be aggregated for is located somewhere else within the DODAG iteration, not in the sub-DAG of the aggregating node.
3. A node that is to be aggregated for is located somewhere else in the LLN.

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

Additional protocols may be applied beyond the scope of this specification to dynamically elect/provision an aggregating node and groups of nodes eligible to be aggregated in order to provide route summarization for a sub-DAG.

6.9.  Loop 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] (Deering, S. and R. Hinden, “Internet Protocol, Version 6 (IPv6) Specification,” December 1998.) and its operation is further specified in [RFC3697] (Rajahalme, J., Conta, A., Carpenter, B., and S. Deering, “IPv6 Flow Label Specification,” March 2004.). For the purpose of RPL operations, the flow label is constructed as follows:

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

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.

SenderRank:

8-bit field set to zero by the source and to its rank by a router that forwards inside the RPL network.

InstanceID:

8-bit field indicating the DODAG instance along which the packet is sent.

6.9.1.  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 InstanceID field.

If the source is aware of the InstanceID that is preferred for the flow, then it MUST set the InstanceID 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.

6.9.2.  Router Operation

6.9.2.1.  Conformance to RFC 3697

[RFC3697] (Rajahalme, J., Conta, A., Carpenter, B., and S. Deering, “IPv6 Flow Label Specification,” March 2004.) 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 InstanceID field that must be delivered without a change.

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

When a router receives a packet that is flagged with a given InstanceID and the node can forward the packet along the DODAG associated to that instance, then the router MUST do so and leave the InstanceID flag unchanged.

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

6.9.2.3.  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 DAG Sequence Number 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.

6.9.2.4.  Sibling Loop Avoidance

When a packet is forwarded along siblings, it cannot be checked for forward progress and may loop between siblings. Experimental 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.

6.9.2.5.  DAO Inconsistency Loop Detection and Recovery

A DAO inconsistency happens when router that has an down DAO route via a child that is a remnant from an obsolete state that is not matched in the child. With DAO inconsistency loop recovery, a packet can be used to recursively explore and cleanup the obsolete DAO states along a sub-DAG.

In a general manner, a packet that goes down should never go up again. So rather than routing up a packet with the down bit set, the router MUST discard the packet. 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.

6.9.2.6.  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.

6.10.  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 MLD, one can read MLDv2 or v3.

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

Along the path between the router and the 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-DAG 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.

6.11.  Maintenance of Routing Adjacency

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

In IGPs such as OSPF [RFC4915] (Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P. Pillay-Esnault, “Multi-Topology (MT) Routing in OSPF,” June 2007.) or IS-IS [RFC5120] (Przygienda, T., Shen, N., and N. Sheth, “M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs),” February 2008.), the maintenance of a routing adjacency involves the use of Keepalive mechanisms (Hellos) or other protocols such as BFD ([I‑D.ietf‑bfd‑base] (Katz, D. and D. Ward, “Bidirectional Forwarding Detection,” February 2009.)) and MANET Neighborhood Discovery Protocol (NHDP [I‑D.ietf‑manet‑nhdp] (Clausen, T., Dearlove, C., and J. Dean, “Mobile Ad Hoc Network (MANET) Neighborhood Discovery Protocol (NHDP),” October 2009.)). 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] (Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” September 2007.)). 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.

7.  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 InstanceID 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-DAG), 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 DAG parent offering a default route then select that DAG parent as a successor.
7. If there is a DODAG iteration offering a route to a prefix matching the destination, but all DAG parents have been tried and are temporarily unavailable (as determined by the forwarding procedure), then select a DAG sibling as a successor.
8. Finally, if no DAG siblings are available, the packet is dropped. ICMP Destination Unreachable may be invoked. An inconsistency is detected.

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

Note that the chosen successor MUST NOT be the neighbor 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 neared.

8.  Guidelines for Objective Functions

An Objective Function (OF) allows for the selection of a DODAG to join, and a number of peers in that DAG 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] (Vasseur, J., Kim, M., Networks, D., and H. Chong, “Routing Metrics used for Path Calculation in Low Power and Lossy Networks,” April 2010.), and indicates the method that must be used to compute the DODAG (e.g. "minimize the path cost using the ETX metric and avoid `Blue' links"). The Objective Code Points are specified in [I‑D.ietf‑roll‑routing‑metrics] (Vasseur, J., Kim, M., Networks, D., and H. Chong, “Routing Metrics used for Path Calculation in Low Power and Lossy Networks,” April 2010.) and related companion specifications.

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

• The parent selection is triggered each time an event indicates that a potential next hop information is updated. This might happen upon the reception of a DIO message, a timer elapse, or a trigger indicating that the state of a candidate neighbor has changed.
• 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.
• 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.
• An OF computes self's rank by adding the step of rank to that candidate to the rank of that candidate. The step of rank is computed by estimating the link as follows:
• The step of rank might vary from 1 to 16.
• 1 indicates a unusually good link, for instance a link between powered devices in a mostly battery operated environment.
• 4 indicates a `normal'/typical link, as qualified by the implementation.
• 16 indicates a link that can hardly be used to forward any packet, for instance a radio link with quality indicator or expected transmission count that is close to the acceptable threshold.
• Candidate neighbors that would cause self's rank to increase are ignored
• Candidate neighbors that advertise an OF incompatible with the set of OF specified by the policy functions are ignored.
• As it scans all the candidate neighbors, the OF keeps the current best parent and compares its capabilities with the current candidate neighbor. The OF defines a number of tests that are critical to reach the objective. A test between the routers determines an order relation.
• If the routers are roughly equal for that relation then the next test is attempted between the routers,
• Else the best of the 2 becomes the current best parent and the scan continues with the next candidate neighbor
• Some OFs may include a test to compare the ranks that would result if the node joined either router
• 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.
• Other rounds of scans might be necessary to elect alternate parents and siblings. In the next rounds:
• Candidate neighbors that are not in the same DODAG are ignored
• Candidate neighbors that are of greater rank than self are ignored
• Candidate neighbors of an equal rank to self (siblings) are ignored
• Candidate neighbors of a lesser rank than self (non-siblings) are preferred

9.  RPL Constants and Variables

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

ZERO_LIFETIME

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

BASE_RANK

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

ROOT_RANK

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

INFINITE_RANK

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

RPL_DEFAULT_INSTANCE

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

DEFAULT_DIO_INTERVAL_MIN

To be determined

DEFAULT_DIO_INTERVAL_DOUBLINGS

To be determined

DEFAULT_DIO_REDUNDANCY_CONSTANT

To be determined

DEF_DAO_LATENCY

To be determined

MAX_DESTROY_INTERVAL

To be determined

DIO Timer

One instance per DODAG that a node is a member of. Expiry triggers DIO message transmission. Trickle timer with variable interval in [0, DIOIntervalMin..2^DIOIntervalDoublings]. See Section 6.3.4 (Trickle Timer for DIO Transmission)

DAG Sequence Number Increment Timer

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

DelayDAO Timer

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

RemoveTimer

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

10.  Manageability Considerations

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

10.1.  Control of Function and Policy

10.1.1.  Initialization Mode

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

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

10.1.2.  DIO Base option

RPL specifies a number of protocol parameters.

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

DAGPreference

InstanceID

DAGObjectiveCodePoint

DAGID

Destination Prefixes

DIOIntervalDoublings

DIOIntervalMin

DIORedundancyConstant

DAG Root behavior:

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

DAG Table Entry Suppression

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

10.1.3.  Trickle Timers

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

For each DODAG, a RPL implementation MUST allow for the monitoring of the following parameters, further described in Section 6.3.4 (Trickle Timer for DIO Transmission):

I

T

C

I_min

I_doublings

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

10.1.4.  DAG Sequence Number Increment

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

10.1.5.  Destination Advertisement Timers

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

• The DelayDAO timer
• The Remove timer

10.1.6.  Policy Control

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

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

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

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

10.1.7.  Data Structures

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

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

10.2.  Information and Data Models

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

10.3.  Liveness Detection and Monitoring

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

As specified in Section 6.2 (Protocol Elements), an implementation is expected to maintain a set of data structures in support of DAG discovery:

• The candidate neighbors data structure
• For each DODAG:
• A set of DAG parents

10.3.1.  Candidate Neighbor Data Structure

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

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

10.3.2.  Directed Acyclic Graph (DAG) Table

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

• DAGID
• DAGObjectiveCodePoint
• A set of Destination Prefixes offered upwards along the DODAG
• A set of DAG Parents
• timer to govern the sending of DIO messages for the DODAG
• DAGSequenceNumber

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

• A reference to the neighboring device which is the DAG parent
• A record of most recent information taken from the DAG Information Object last processed from the DAG Parent
• A flag reporting if the Parent is a DA Parent as described in Section 6.8 (Establishing Routing State Down the DODAG)

10.3.3.  Routing Table

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

• Destination Prefix
• Destination Prefix Length
• Lifetime Timer
• Next Hop
• Next Hop Interface
• Flag indicating that the route was provisioned from one of:
  • Unicast DAO message
  • DIO message
  • Multicast DAO message

10.3.4.  Other RPL Monitoring Parameters

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

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

10.3.5.  RPL Trickle Timers

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

• The DIOIntervalMin expressed in ms
• The DIOIntervalDoublings
• The DIORedundancyConstant

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

10.4.  Verifying Correct Operation

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

10.5.  Requirements on Other Protocols and Functional Components

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

10.6.  Impact on Network Operation

To be completed.

11.  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] (Tsao, T., Alexander, R., Daza, V., and A. Lozano, “A Security Framework for Routing over Low Power and Lossy Networks,” March 2010.).

12.  IANA Considerations

12.1.  RPL Control Message

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

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

12.2.  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:

• Code
• Description
• Defining RFC

Three codes are currently defined:

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

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

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

• Bit number (counting from bit 0 as the most significant bit)
• Capability description
• Defining RFC

Four groups are currently defined:

12.4.  DAG Information Object (DIO) Suboption

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

13.  Acknowledgements

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

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

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

14.  Contributors

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

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

Email: jpv@cisco.com


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

Email: jhui@archrock.com


Thomas Heide Clausen
LIX, Ecole Polytechnique, France

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


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

Email: pal@cs.stanford.edu


Richard Kelsey
Ember Corporation
Boston, MA
USA

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


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

Email: stevedh@cs.berkeley.edu


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

Email: kpister@dustnetworks.com


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

Email: abr@zen-sys.com

15.  References

15.1. Normative References

15.2. Informative References

Appendix A.  Requirements

A.1.  Protocol Properties Overview

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

A.1.1.  IPv6 Architecture

RPL is strictly compliant with layered IPv6 architecture.

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

A.1.2.  Typical LLN Traffic Patterns

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

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

A.1.3.  Constraint Based Routing

The RPL design supports constraint based routing, based on a set of routing metrics and constraints. The routing metrics and constraints for links and nodes with capabilities supported by RPL are specified in a companion document to this specification, [I‑D.ietf‑roll‑routing‑metrics] (Vasseur, J., Kim, M., Networks, D., and H. Chong, “Routing Metrics used for Path Calculation in Low Power and Lossy Networks,” April 2010.). RPL signals the metrics, constraints, and related Objective Functions (OFs) in use in a particular implementation by means of an Objective Code Point (OCP). Both the routing metrics, constraints, and the OF help determine the construction of the Directed Acyclic Graphs (DAG) using a distributed path computation algorithm.

A.2.  Deferred Requirements

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

Appendix B.  Examples

Consider the example LLN physical topology in Figure 13 (Example LLN Topology). In this example the links depicted are all usable L2 links. Suppose that all links are equally usable, and that the implementation specific policy function is simply to minimize hops. This LLN physical topology then yields the DAG depicted in Figure 14 (Example DAG), where the links depicted are the edges toward DAG parents. This topology includes one DAG, rooted by an LBR node (LBR) at rank 1. The LBR node will issue DIO messages, as governed by a trickle timer. Nodes (11), (12), (13), have selected (LBR) as their only parent, attached to the DAG at rank 2, and periodically multicast DIOs. Node (22) has selected (11) and (12) in its DAG parent set, and advertises itself at rank 3. Node (22) thus has a set of DAG parents {(11), (12)} and siblings {((21), (23)}.

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

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

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

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

B.1.  Destination Advertisement

Consider the example DAG depicted in Figure 14 (Example DAG). Suppose that Nodes (22) and (32) are unable to record routing state. Suppose that Node (42) is able to perform prefix aggregation on behalf of Nodes (53), (54), and (55).

• Node (53) would send a DAO message to Node (42), indicating the availability of destination (53).
• Node (54) and Node (55) would similarly send DAO messages to Node (42) indicating their own destinations.
• Node (42) would collect and store the routing state for destinations (53), (54), and (55).
• In this example, Node (42) may then be capable of representing destinations (42), (53), (54), and (55) in the aggregation (42').
• Node (42) sends a DAO message advertising destination (42') to Node 32.
• Node (32) does not want to maintain any routing state, so it adds onto to the Reverse Route Stack in the DAO message and passes it on to Node (22) as (42'):[(42)]. It may send a separate DAO message to indicate destination (32).
• Node (22) does not want to maintain any routing state, so it adds on to the Reverse Route Stack in the DAO message and passes it on to Node (12) as (42'):[(42), (32)]. It also relays the DAO message containing destination (32) to Node 12 as (32):[(32)], and finally may send a DAO message for itself indicating destination (22).
• Node (12) is capable to maintain routing state again, and receives the DAO messages from Node (22). Node (12) then learns:
• Destination (22) is available via Node (22)
• Destination (32) is available via Node (22) and the piecewise source route to (32)
• Destination (42') is available via Node (22) and the piecewise source route to (32), (42').
• Node (12) sends DAO messages to (LBR), allowing (LBR) to learn routes to the destinations (12), (22), (32), and (42'). (42), (53), (54), and (55) are available via the aggregation (42'). It is not necessary for Node (12) to propagate the piecewise source routes to (LBR).

B.2.  Example: DAG Parent Selection

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

• Node (N) transmits a RPL DIS message.
• Node (B) responds. Node (N) investigates the DIO message, and learns that Node (B) is a member of DAGID 1 at rank 4, and not `Blue'. Node (N) takes note of this, but is not yet confident.
• Similarly, Node (N) hears from Node (A) at rank 9, Node (C) at rank 5, and Node (E) at rank 4.
• Node (D) responds. Node (D) has a DIO message that indicates that it is a member of DAGID 1 at rank 2, but it carries the attribute `Blue'. Node (N)'s policy function rejects Node (D), and no further consideration is given.
• This process continues until Node (N), based on implementation specific policy, builds enough confidence to trigger a decision to join DAGID 1. Let Node (N) determine its most preferred parent to be Node (E).
• Node (N) adds Node (E) (rank 4) to its set of DAG parents for DAGID 1. Following the mechanisms specified by the OCP, and given that the ETX is 1 for the link between (N) and (E), Node (N) is now at rank 5 in DAGID 1.
• Node (N) adds Node (B) (rank 4) to its set of DAG parents for DAGID 1.
• Node (N) is a sibling of Node (C), both are at rank 5.
• Node (N) may now forward traffic intended for the default destination upwards along DAGID 1 via nodes (B) and (E). In some cases, e.g. if nodes (B) and (E) are tried and fail, node (N) may also choose to forward traffic to its sibling node (C), without making upwards progress but with the intention that node (C) or a following successor can make upwards progress. Should Node (C) not have a viable parent, it should never send the packet back to Node (N) (to avoid a 2-node loop).

B.3.  Example: DAG Maintenance

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

           -1-                    -2-                    -3-

Consider the example depicted in Figure 15 (DAG Maintenance)-1. In this example, Node (A) is attached to a DAG at some rank d. Node (A) is a DAG parent of Nodes (B) and (C). Node (C) is a DAG parent of Node (D). There is also an undirected sibling link between Nodes (B) and (C).

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

Consider the case where Node (C) hears a DIO message from a Node (Z) at a lesser rank and superior position in the DAG than node (A). Node (C) may safely undergo the process to evict node (A) from its DAG parent set and attach directly to Node (Z) without creating a loop, because its rank will decrease.

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

• Node (C) must first detach from the DAG by removing Node (A) from its DAG parent set, leaving an empty DAG parent set. Node (C) may become the root of its own floating, less preferred, DAG.
• Node (D), hearing a modified DIO message from Node (C), follows Node (C) into the floating DAG. This is depicted in Figure 15 (DAG Maintenance)-2. In general, any node with no other options in the sub-DAG of Node (C) will follow Node (C) into the floating DAG, maintaining the structure of the sub-DAG.
• Node (C) hears a DIO message with an incremented DAGSequenceNumber from Node (B) and determines it is able to rejoin the grounded DAG by reattaching at a deeper rank to Node (B). Node (C) adds Node (B) to its DAG parent set. Node (C) has now safely moved deeper within the grounded DAG without creating any loops.
• Node (D), and any other sub-DAG of Node (C), will hear the modified DIO message sourced from Node (C) and follow Node (C) in a coordinated manner to reattach to the grounded DAG. The final DAG is depicted in Figure 15 (DAG Maintenance)-3

B.4.  Example: Greedy Parent Selection and Instability

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

           -1-                    -2-                    -3-

Consider the example depicted in Figure 16 (Greedy DAG Parent Selection). A DAG is depicted in 3 different configurations. A usable link between (B) and (C) exists in all 3 configurations. In Figure 16 (Greedy DAG Parent Selection)-1, Node (A) is a DAG parent for Nodes (B) and (C), and (B)--(C) is a sibling link. In Figure 16 (Greedy DAG Parent Selection)-2, Node (A) is a DAG parent for Nodes (B) and (C), and Node (B) is also a DAG parent for Node (C). In Figure 16 (Greedy DAG Parent Selection)-3, Node (A) is a DAG parent for Nodes (B) and (C), and Node (C) is also a DAG parent for Node (B).

If a RPL node is too greedy, in that it attempts to optimize for an additional number of parents beyond its preferred parent, then an instability can result. Consider the DAG illustrated in Figure 16 (Greedy DAG Parent Selection)-1. In this example, Nodes (B) and (C) may most prefer Node (A) as a DAG parent, but are operating under the greedy condition that will try to optimize for 2 parents.

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

• Let Figure 16 (Greedy DAG Parent Selection)-1 be the initial condition.
• Suppose Node (C) first is able to leave the DAG and rejoin at a lower rank, taking both Nodes (A) and (B) as DAG parents as depicted in Figure 16 (Greedy DAG Parent Selection)-2. Now Node (C) is deeper than both Nodes (A) and (B), and Node (C) is satisfied to have 2 DAG parents.
• Suppose Node (B), in its greediness, is willing to receive and process a DIO message from Node (C) (against the rules of RPL), and then Node (B) leaves the DAG and rejoins at a lower rank, taking both Nodes (A) and (C) as DAG parents. Now Node (B) is deeper than both Nodes (A) and (C) and is satisfied with 2 DAG parents.
• Then Node (C), because it is also greedy, will leave and rejoin deeper, to again get 2 parents and have a lower rank then both of them.
• Next Node (B) will again leave and rejoin deeper, to again get 2 parents
• And again Node (C) leaves and rejoins deeper...
• The process will repeat, and the DAG will oscillate between Figure 16 (Greedy DAG Parent Selection)-2 and Figure 16 (Greedy DAG Parent Selection)-3 until the nodes count to infinity and restart the cycle again.
• This cycle can be averted through mechanisms in RPL:
  • Nodes (B) and (C) stay at a rank sufficient to attach to their most preferred parent (A) and don't go for any deeper (worse) alternate parents (Nodes are not greedy)
  • Nodes (B) and (C) do not process DIO messages from nodes deeper than themselves (because such nodes are possibly in their own sub-DAGs)

Appendix C.  Outstanding Issues

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

C.1.  Additional Support for P2P Routing

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

C.2.  Loop Detection

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

C.3.  Destination Advertisement / DAO Fan-out

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

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

The use of suitable triggers, such as the `D' bit, to trigger DA operation within an affected sub-DAG, is under investigation. Further, the ability to limit scope of the affected depth within the sub-DAG is under investigation (e.g. if a stateful node can proxy for all nodes `behind' it, then there may be no need to propagate the triggered `D' bit further).

C.4.  Source Routing

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

C.5.  Address / Header Compression

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

Authors' Addresses