| TOC |
|
This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as “work in progress.”
The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 9, 2010.
Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document.
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, most of the time supporting only low data rates, that are usually fairly 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 a large number of nodes, up to several dozens or hundreds or more nodes in the network. 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. This document specifies the Routing Protocol for Low Power and Lossy Networks (RPL).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [RFC2119].
1.
Introduction
1.1.
Design Principles
1.2.
Expectations of Link Layer Behavior
2.
Terminology
3.
Protocol Model
3.1.
Protocol Properties Overview
3.1.1.
IPv6 Architecture
3.1.2.
Typical LLN Traffic Patterns
3.1.3.
Constraint Based Routing
3.2.
Protocol Operation
3.2.1.
DAG Construction
3.2.2.
Destination Advertisement
3.3.
Loop Avoidance and Stability
3.3.1.
Greediness and Rank-based Instabilities
3.3.2.
Merging DAGs
3.3.3.
DAG Loops
3.3.4.
DAO Loops
3.3.5.
Sibling Loops
3.4.
Local and Temporary Routing Decision
3.5.
Maintenance of Routing Adjacency
4.
Constraint Based Routing in LLNs
4.1.
Routing Metrics
4.2.
Routing Constraints
4.3.
Constraint Based Routing
5.
RPL Protocol Specification
5.1.
DAG Information Option
5.1.1.
DAG Information Option (DIO) base option
5.2.
Conceptual Data Structures
5.2.1.
Candidate Neighbors Data Structure
5.2.2.
Directed Acyclic Graphs (DAGs) Data Structure
5.3.
DAG Discovery and Maintenance
5.3.1.
DAG Discovery Rules
5.3.2.
Reception and Processing of RA-DIO messages
5.3.3.
RA-DIO Transmission
5.3.4.
Trickle Timer for RA Transmission
5.4.
DAG Heartbeat
5.5.
DAG Selection
5.6.
Administrative rank
5.7.
Candidate DAG Parent States and Stability
5.7.1.
Held-Up
5.7.2.
Held-Down
5.7.3.
Collision
5.7.4.
Instability
5.8.
Guidelines for Objective Code Points
5.8.1.
Objective Function
5.8.2.
Objective Code Point 0 (OCP 0)
5.9.
Establishing Routing State Outward Along the DAG
5.9.1.
Destination Advertisement Message Formats
5.9.2.
Destination Advertisement Operation
5.10.
Multicast Operation
5.11.
Maintenance of Routing Adjacency
5.12.
Packet Forwarding
6.
RPL Variables
7.
Manageability Considerations
7.1.
Control of Function and Policy
7.1.1.
Initialization Mode
7.1.2.
DIO Base option
7.1.3.
Trickle Timers
7.1.4.
DAG Heartbeat
7.1.5.
The Destination Advertisement Option
7.1.6.
Policy Control
7.1.7.
Data Structures
7.2.
Information and Data Models
7.3.
Liveness Detection and Monitoring
7.3.1.
Candidate Neighbor Data Structure
7.3.2.
Directed Acyclic Graph (DAG) Table
7.3.3.
Routing Table
7.3.4.
Other RPL Monitoring Parameters
7.3.5.
RPL Trickle Timers
7.4.
Verifying Correct Operation
7.5.
Requirements on Other Protocols and Functional Components
7.6.
Impact on Network Operation
8.
Security Considerations
9.
IANA Considerations
9.1.
DAG Information Option (DIO) Base Option
9.2.
New Registry for the Flag Field of the DIO Base Option
9.3.
DAG Information Option (DIO) Suboption
9.4.
Destination Advertisement Option (DAO) Option
9.5.
Objective Code Point
10.
Acknowledgements
11.
Contributors
12.
References
12.1.
Normative References
12.2.
Informative References
Appendix A.
Deferred Requirements
Appendix B.
Examples
B.1.
Moving Down a DAG
B.2.
Link Removed
B.3.
Link Added
B.4.
Node Removed
B.5.
New LBR Added
B.6.
Destination Advertisement
B.7.
Example: DAG Parent Selection
B.8.
Example: DAG Maintenance
B.9.
Example: Greedy Parent Selection and Instability
B.10.
Example: DAG Merge
Appendix C.
Additional Examples
Appendix D.
Outstanding Issues
D.1.
Additional Support for P2P Routing
D.2.
Loop Detection
D.3.
Destination Advertisement / DAO Fan-out
D.4.
Source Routing
D.5.
Address / Header Compression
§
Authors' Addresses
| TOC |
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, most of the time supporting only low data rates, that are usually fairly 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 a large number of nodes, up to several dozens or hundreds or more nodes in the network. 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,” September 2009.), [I‑D.ietf‑roll‑home‑routing‑reqs] (Brandt, A., Buron, J., and G. Porcu, “Home Automation Routing Requirements in Low Power and Lossy Networks,” September 2009.), [I‑D.ietf‑roll‑indus‑routing‑reqs] (Networks, D., Thubert, P., Dwars, S., and T. Phinney, “Industrial Routing Requirements in Low Power and Lossy Networks,” June 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 Routing Protocol for Low Power and Lossy Networks (RPL).
| TOC |
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,” September 2009.), [I‑D.ietf‑roll‑home‑routing‑reqs] (Brandt, A., Buron, J., and G. Porcu, “Home Automation Routing Requirements in Low Power and Lossy Networks,” September 2009.), [I‑D.ietf‑roll‑indus‑routing‑reqs] (Networks, D., Thubert, P., Dwars, S., and T. Phinney, “Industrial Routing Requirements in Low Power and Lossy Networks,” June 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. (Note: it is intended that as this 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 instantiation of the protocol to be optimized in terms of required code space. It must be noted that RPL is not restricted to the aforementioned applications and is expected to be used in other environments. All "MUST" application requirements that cannot be satisfied by RPL will be specifically listed in the Appendix A, accompanied by a justification.
The core set of functionalities is to be capable of operating in the most severely constrained environments, with minimal requirements for memory, energy, processing, communication, and other consumption of limited resources from nodes. Trade-offs inherent in the provisioning of protocol features will be exposed to the implementer in the form of configurable parameters, such that the implementer can further tweak and optimize the operation of RPL as appropriate to a specific application and implementation. Finally, RPL is designed to consult implementation specific policies to determine, for example, the evaluation of routing metrics.
A set of companion documents to this specification will provide further guidance in the form of applicability statements specifying a set of operating points appropriate to the Building Automation, Home Automation, Industrial, and Urban application scenarios.
| TOC |
This specification does not rely on any particular features of a specific link layer technologies. It is anticipated that an implementer should be able to operate RPL over a variety of different low power wireless or PLC (Power Line Communication) link layer technologies.
Implementers may find RFC 3819 (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.
| TOC |
The terminology used in this document is consistent with and incorporates that described in `Terminology in Low power And Lossy Networks' [I‑D.ietf‑roll‑terminology] (Vasseur, J., “Terminology in Low power And Lossy Networks,” March 2010.). The terminology is extended in this document as follows:
- Autonomous:
- The ability of a routing protocol to independently function without relying on any external influence or guidance. Includes self-organization capabilities.
- DAG:
- Directed Acyclic Graph. A directed graph having the property that all edges are oriented in such a way that no cycles exist. In the RPL context, all edges are contained in paths oriented toward and terminating at a root node (a DAG root, or sink- typically a Low Power and Lossy Network Border Router (LBR)).
- DAGID:
- DAG Identifier. A globally unique identifier for a DAG. All nodes who are part of a given DAG have knowledge of the DAGID. This knowledge is used to identify peer nodes within the DAG in order to coordinate DAG maintenance while avoiding loops.
- DAG parent:
- A parent of a node within a DAG is one of the immediate successors of the node on a path towards the DAG root. For each DAGID that a node is a member of, the node will maintain a set containing one or more DAG parents. If a node is a member of multiple DAGs then it must conceptually maintain a set of DAG parents for each DAGID.
- DAG sibling:
- A sibling of a node within a DAG is defined in this specification to be any neighboring node which is located at the same rank (depth) within a DAG. Note that siblings defined in this manner do not necessarily share a common parent. For each DAG that a node is a member of, the node will maintain a set of DAG siblings. If a node is a member of multiple DAGs then it must conceptually maintain a set of DAG siblings for each DAG.
- DAG root:
- A DAG root is a sink within the DAG. All paths in the DAG terminate at a DAG root, and all DAG edges contained in the paths terminating at a DAG root are oriented toward the DAG root. There must be at least one DAG root per DAG, and in some cases there may be more than one. In many use cases, source-sink represents a dominant traffic flow, where the sink is a DAG root or is located behind the DAG root. Maintaining routes towards DAG roots is therefore a prominent functionality for RPL.
- Grounded:
- A DAG is grounded if it contains a DAG root offering connectivity to an external routed infrastructure such as the public Internet or a private core (non-LLN) IP network.
- Floating:
- A DAG is floating if is not grounded. A floating DAG is not expected to reach any additional external routed infrastructure such as the public Internet or a private core (non-LLN) IP network.
- Inward:
- Inward refers to the direction from leaf nodes towards DAG roots, following the orientation of the edges within the DAG.
- Outward:
- Outward refers to the direction from DAG roots towards leaf nodes, going against the orientation of the edges within the DAG.
- P2P:
- Point-to-point. This refers to traffic exchanged between two nodes.
- P2MP:
- Point-to-Multipoint. This refers to traffic between one node and a set of nodes. This is similar to the P2MP concept in Multicast or MPLS Traffic Engineering ([RFC4461] (Yasukawa, S., “Signaling Requirements for Point-to-Multipoint Traffic-Engineered MPLS Label Switched Paths (LSPs),” April 2006.) and [RFC4875] (Aggarwal, R., Papadimitriou, D., and S. Yasukawa, “Extensions to Resource Reservation Protocol - Traffic Engineering (RSVP-TE) for Point-to-Multipoint TE Label Switched Paths (LSPs),” May 2007.)). A common RPL use case involves P2MP flows from or through a DAG root outward towards other nodes contained in the DAG.
- MP2P:
- Multipoint-to-Point; used to describe a particular traffic pattern. A common RPL use case involves MP2P flows collecting information from many nodes in the DAG, flowing inwards towards DAG roots. Note that a DAG root may not be the ultimate destination of the information, but it is a common transit node.
- OCP:
- Objective Code Point. In RPL, the Objective Code Point (OCP) indicates which routing metrics, optimization objectives, and related functions are in use in a DAG. Instances of the Objective Code Point are further described in [I‑D.ietf‑roll‑routing‑metrics] (Vasseur, J., Kim, M., Networks, D., and H. Chong, “Routing Metrics used for Path Calculation in Low Power and Lossy Networks,” April 2010.).
Note that in this document, the terms `node' and `LLN router' are used interchangeably.
| TOC |
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.
| TOC |
RPL demonstrates the following properties, consistent with the requirements specified by the application-specific requirements documents.
| TOC |
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).
| TOC |
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).
| TOC |
The RPL design supports constraint based routing, based on a set of routing metrics. The routing metrics 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 and related objective functions in use in a particular implementation by means of an Objective Code Point (OCP). Both the routing metrics and the OCP help determine the construction of the Directed Acyclic Graphs (DAG) using a distributed path computation algorithm.
RPL supports the computation and installation of different paths in support of and optimized for a set of application and implementation specific constraints, as guided by an OCP. Traffic may subsequently be directed along the appropriate constrained path based on traffic marking within the IPv6 header. For more details on the approach towards constraint-based routing, see Section 4 (Constraint Based Routing in LLNs).
| TOC |
A LLN deployment will consist of a number of nodes and a number of edges (links) between them, whose characteristics will depend on implementation and link layer (L2) specifics. Due to the nature of the LLN environment the L2 links are expected to demonstrate a large degree of variance as to their availability, quality, and other related parameters. Certain links, demonstrating a viability above a confidence threshold for particular node and link metrics, as based on guidelines from [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.), will be extracted from the L2 graph, and the resulting graph will be used as the basis on which to operate the routing protocol. Note that as the characteristics of the L2 topology vary over time the set of viable links is to be updated and the routing protocol thus continues to evaluate the LLN. In RPL this process happens in a distributed manner, and from the perspective of a single node running RPL this process results in a set of candidate neighbors, with associated node and link metrics as well as confidence values.
Many of the dominant traffic flows in support of the LLN application scenarios are MP2P flows ([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,” September 2009.), [I‑D.ietf‑roll‑home‑routing‑reqs] (Brandt, A., Buron, J., and G. Porcu, “Home Automation Routing Requirements in Low Power and Lossy Networks,” September 2009.), [I‑D.ietf‑roll‑indus‑routing‑reqs] (Networks, D., Thubert, P., Dwars, S., and T. Phinney, “Industrial Routing Requirements in Low Power and Lossy Networks,” June 2009.), and [RFC5548] (Dohler, M., Watteyne, T., Winter, T., and D. Barthel, “Routing Requirements for Urban Low-Power and Lossy Networks,” May 2009.)). These flows are rooted at designated nodes that have some application significance, such as providing connectivity to an external routed infrastructure. The term "external" is used top refer to the public Internet or a core private (non-LLN) IP network. In support of this dominant flow RPL constructs Directed Acyclic Graphs (DAGs) on top of the viable LLN topology, selecting and orienting links among candidate neighbors toward DAG roots which root the MP2P flows.
LLN nodes running RPL will construct Directed Acyclic Graphs (DAGs) rooted at designated nodes that generally have some application significance, such as providing connectivity to an external routed infrastructure. The term "external" is used top refer to the public Internet or a core private (non-LLN) IP network. This structure provides the routing solution for the dominant MP2P traffic flows. The DAG structure further provides each node potentially multiple successors for MP2P flows, which may be used for, e.g., local route repair or load balancing.
Nodes running RPL are able to further restrict the scope of the routing problem by using the DAG as a reference topology. By referencing a rank property that is related to the positions in the DAG, nodes are able to determine their positions in a DAG relative to each other. This information is used by RPL in part to construct rules for movement relative to the DAG that endeavor to avoid loops. It is important to note that the rank property is derived from metrics, and not directly from the position in the DAG, as will be discussed further.
As DAGs are organized, RPL will use a destination advertisement mechanism to build up routing tables in support of outward P2MP traffic flows. This mechanism, using the DAG as a reference, distributes routing information across intermediate nodes (between the DAG leaves and the root), guided along the DAG, such that the routes toward destination prefixes in the outward direction may be set up. As the DAG undergoes modification during DAG maintenance, the destination advertisement mechanism can be triggered to update the outward routing state.
A baseline support for P2P traffic in RPL is provided by the DAG, as P2P traffic may flow inward along the DAG until a common parent is reached who has stored an entry for the destination in its routing table and is capable of directing the traffic outward along the correct outward path. RPL also provides support for the trivial case where a P2P destination may be a `one-hop' neighbor. In the present specification RPL does not specify nor preclude any additional mechanisms that may be capable to compute and install more optimal routes into LLN nodes in support of arbitrary P2P traffic according to some routing metric.
| TOC |
RPL constructs one or more DAGs, over gradients defined by optimizing cost metrics along paths rooted at designated nodes.
The DAG construction algorithm is distributed; each node running RPL invokes a set of DAG construction rules and objective functions when considering its role with respect to neighboring nodes such that the DAG structure emerges.
| TOC |
The IPv6 Router Advertisement (RA) mechanism (as specified in [RFC4861] (Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” September 2007.)) is used by RPL in order to build and maintain a DAG.
The IPv6 RA message is augmented with a DAG Information Option (DIO), forming an RA-DIO message, to convey information about the DAG including:
The RA messages are issued whenever a change is detected to the DAG such that a node is able to determine that a region of the DAG has become inconsistent. As the DAG stabilizes the period at which RA messages occur is configured to taper off, reducing the steady-state overhead of DAG maintenance. The periodic issue of RA messages, along with the triggered RA messages in response to inconsistency, is one feature that enables RPL to operate in the presence of unreliable links.
| TOC |
Each DAG is identified by a particular identifier (DAGID) as well as its supported optimization objectives and available destination prefixes. The optimization objectives are conveyed as an Objective Code Point (OCP) as described further below. Available destination prefixes, which may include destinations available beyond the DAG root, multicast destinations, or IPv6 node addresses, are advertised outwards along the DAG and recipient nodes may then provision routing tables with entries inwards towards the destinations. The RPL implementation at each node will be provisioned by the application with sufficient information to determine which objectives and destinations are required, and thus the RPL implementation may determine which DAG to join.
The decision for a node to join a DAG may be optimized according to implementation specific policy functions on the node as indicated by one or more specific OCP values. For example, a node may be configured for one goal to optimize a bandwidth metric (OCP-1), and with a parallel goal to optimize for a reliability metric (OCP-2). Thus two DAGs, with two unique DAGIDs, may be constructed and maintained in the LLN: DAG-1 would be optimized according to OCP-1, whereas DAG-2 would be optimized according to OCP-2. A node may then maintain independent sets of DAG parents and related data structures for each DAG. Note that in such a case traffic may directed along the appropriate constrained DAG based on traffic marking within the IPv6 header. This specification will focus on the case where the node only joins one DAG; further elaboration on the proper operation of RPL in the presence of multiple DAGs, including traffic marking and related rules, are to be specified further in future revisions of this or companion specifications.
| TOC |
Certain LLN nodes may offer connectivity to an external routed infrastructure in support of an application scenario. These nodes are designated `grounded', and may serve as the DAG roots of a grounded DAG. DAGs that do not have a grounded DAG root are floating DAGs. In either case routes may be provisioned toward the DAG root, although in the floating case there is no expectation to reach an external infrastructure. Some applications will include permanent floating DAGs.
| TOC |
An administrative preference may be associated with each DAG root, and thereby each DAG, in order that some DAGs in the LLN may be more preferred over other DAGs. For example, a DAG root that is sinking traffic in support of a data collection application may be configured by the application to be very preferred. A transient DAG, e.g. a DAG that is only existing in support of DAG repair until a permanent DAG is found, may be configured to be less preferred. The administrative preference offers a way to engineer the formation of the DAG in support of the application.
| TOC |
The OCP serves to convey and control the optimization objectives in use within the DAG. The OCP is further 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.). Each instance of an allocated OCP indicates:
For example, an objective code point might indicate that the DAG is using the Expected Number of Transmissions (ETX) as a metric, that the optimization goal is to minimize ETX, that DAG Rank is equivalent to ETX, and that RA-DIO propagation entails adding the advertised ETX of the most preferred parent to the ETX of the link to the most preferred parent.
By using defined OCPs that are understood by all nodes in a particular implementation, and by conveying them in the RA-DIO message, RPL nodes may work to build optimized LLN using a variety of application and implementation specific metrics and goals.
A default OCP, OCP 0, is specified with a well-defined default behavior. OCP 0 is used to define RPL behaviors in the case where a node encounters a RA-DIO message containing a code point that it does not support.
| TOC |
| TOC |
When nodes select DAG parents, they will select the most preferred parent according to their implementation specific objectives, using the cost metrics conveyed in the RA-DIO messages along the DAG in conjunction with the related objective functions as specified by the OCP.
Based on this selection, the metrics conveyed by the most preferred DAG parent, the nodes own metrics and configuration, and a related function defined by the OCP, a node will be able to compute a value for its rank as a consequence of selecting a most preferred DAG parent.
The rank value feeds back into the DAG parent selection according to a loop-avoidance strategy. Once a DAG parent has been added, and a rank value for the node within the DAG has been computed, the nodes further options with regard to DAG parent selection and movement within the DAG are restricted in favor of loop avoidance.
It is important to note that the DAG Rank is not itself a metric, although its value is derived from and influenced by the use of metrics to select DAG parents and take up a position in the DAG. In other words, routing metrics and OCP (not rank directly) are used to determine the DAG structure and consequently the path cost. The only aim of the rank is to inform loop avoidance as explained hereafter. The computation of the DAG Rank MUST be done in such a way so as to maintain the following properties for any nodes M and N who are neighbors in the LLN:
For a node N, and its most preferred parent M, DAGRank(N) > DAGRank(M) must hold. Further, all parents in the DAG parent set must be of a rank less than 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.
If DAGRank(M) < DAGRank(N), then M is probably located in a more preferred position than N in the DAG 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.
For example, a Node M of rank 3 is likely located in a more optimum position than a Node N of rank 5. A packet directed inwards and forwarded from Node N to Node M will always make forward progress with respect to the DAG organization on that link; there is no risk of Node M at rank 3 forwarding the packet back into Node N's sub-DAG at rank of 5 or greater (which would be a sufficient condition for a loop to occur).
If DAGRank(M) == DAGRank(N), then M and N are located positions of relatively the same optimality within the DAG. In some cases, Node M may be used as a successor by Node N, but with related chance of creating a loop that must be detected and broken by some other means.
If Node M is at rank 3 and node N is at rank 3, then they are siblings; by definition Node M and N cannot be in each others sub-DAG. They may then forward to each other failing serviceable parents, making `sideways' progress (but not reverse progress). If another sibling or more gets involved there may then be some chance for 3 or more way loops, which is the risk of sibling forwarding.
If DAGRank(M) > DAGRank(N), then node M is located in a less preferred position than N in the DAG with respect to the metrics and optimizations defined by the objective code point. Further, Node (M) may in fact be in Node (N)'s sub-DAG. There is no advantage to Node (N) selecting Node (M) as a DAG parent, and such a selection may create a loop.
For example, if Node M is of rank 3 and Node N is of rank 5, then by definition Node N is in a less optimum position than Node N. Further, Node N at rank 5 may in fact be in Node M's own sub-DAG, and forwarding a packet directed inwards towards the DAG root from M to N will result in backwards progress and possibly a loop.
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 DAG.
The DAG rank is subsequently used to restrict the options a node has for movement within the DAG and to coordinate movements in order to avoid the creation of loops.
| TOC |
The sub-DAG of a node is the set of other nodes of greater rank in the DAG, and thus might use a path towards the DAG root that contains this node. This is an important property that is leveraged for loop avoidance- if a node has lesser rank then it is not in the sub-DAG. (An arbitrary node with greater rank may or may not be contained in the sub-DAG). Paths through siblings are not contained in this set.
As a further illustration, consider the DAG examples in Appendix B (Examples). Consider Node (24) in the DAG Example depicted in Figure 9 (Example DAG). In this example, the sub-DAG of Node (24) is comprised of Nodes (34), (44), and (45).
A frozen sub-DAG is a subset of nodes in the sub-DAG of a node who have been informed of a change to the node, and choose to follow the node in a manner consistent with the change, for example in preparation for a coordinated move. Nodes in the sub-DAG who hear of a change and have other options than to follow the node do not have to become part of the frozen sub-DAG, for example such a node may be able to remain attached to the original DAG through a different DAG parent. A further example may be found in Appendix B.8 (Example: DAG Maintenance).
| TOC |
A node may safely move `up' in the DAG, causing its DAG rank to decrease and moving closer to the DAG root without risking the formation of a loop.
| TOC |
A node may not consider to move `down' the DAG, causing its DAG rank to increase and moving further from the DAG root. In the case where a node looses connectivity to the DAG, it must first leave the DAG before it may then rejoin at a deeper point. This allows for the node to coordinate moving down, freezing its own sub-DAG and poisoning stale routes to the DAG, and minimizing the chances of re-attaching to its own sub-DAG thinking that it has found the original DAG again. If a node where allowed to re-attach into its own sub-DAG a loop would most certainly occur, and may not be broken until a count-to-infinity process elapses. The procedure of detaching before moving down eliminates the need to count-to-infinity.
| TOC |
A jump from one DAG to another DAG is attaching to a new DAGID, in such a way that an old DAGID is replaced by the new DAGID. In particular, when an old DAGID is left, all associated parents are no longer feasible, and a new DAGID is joined.
When a node in a DAG follows a DAG parent, it means that the DAG parent has changed its DAGID (e.g. by joining a new DAG) and that the node updates its own DAGID in order to keep the DAG parent.
| TOC |
A DAG may also be floating. Floating DAGs may be encountered, for example, during coordinated reconfigurations of the network topology wherein a node and its sub-DAG breaks off the DAG, temporarily becomes a floating DAG, and reattaches to a grounded DAG. (Such coordination endeavors to avoid the construction of transient loops in the LLN).
A DAG, or a sub-DAG temporarily promoted to a DAG, may also become floating because of a network element failure. If the DAG parent set of the node becomes completely depleted, the node will have detached from the DAG, and may, if so configured, become the root of its own transient floating DAG with a less desirable administrative preference (thus beginning the process of establishing the frozen sub-DAG), and then may reattach to the original DAG at a lower point if it is able (after hearing RA-DIO messages from alternate attachment points).
| TOC |
As RPL constructs DAGs, nodes may provision routes toward destinations advertised through RA-DIO messages through their selected parents, and are thus able to send traffic inward along the DAG by forwarding to their selected parents. However, this mechanism alone is not sufficient to support P2MP traffic flowing outward along the DAG from the DAG root toward nodes. A destination advertisement mechanism is employed by RPL to build up routing state in support of these outward flows. The destination advertisement mechanism may not be supported in all implementations, as appropriate to the application requirements. A DAG root that supports using the destination advertisement mechanism to build up routing state will indicate such in the RA-DIO message. A DAG root that supports using the destination advertisement mechanism must be capable of allocating enough state to store the routing state received from the LLN.
| TOC |
An IPv6 Neighbor Advertisement Message with Destination Advertisement Options (NA-DAO) is used to convey the destination information inward along the DAG toward the DAG root.
The information conveyed in the NA-DAO message includes the following:
| TOC |
As the DAG is constructed and maintained, nodes are capable to emit NA-DAO messages to a subset, or all, of their DAG parents. The selection of this subset is according to an implementation specific policy.
As a special case, a node may periodically emit a link-local multicast IPv6 NA-DAO message advertising its locally available destination prefixes. This mechanism allows for the one-hop neighbors of a node to learn explicitly of the prefixes on the node, and in some application specific scenarios this is desirable in support of provisioning a trivial `one-hop' route. In this case, nodes who receive the multicast destination advertisement may use it to provision the one-hop route only, and not engage in any additional processing (so as not to engage the mechanisms used by a DAG parent).
When a (unicast) NA-DAO message reaches a node capable of storing routing state, the node extracts information from the NA-DAO message and updates its local database with a record of the NA-DAO message and who it was received from. When the node later propagates NA-DAO messages, it selects the best (least depth) information for each destination and conveys this information again in the form of NA-DAO messages to a subset of its own DAG parents. At this time the node may perform route aggregation if it is able, thus reducing the overall number of NA-DAO messages.
When a (unicast) NA-DAO message reaches a node incapable of storing additional state, the node must append the next-hop address (from which neighbor the NA-DAO message was received) to a Reverse Route Stack carried within the NA-DAO message. The node then passes the NA-DAO message on to one or more of its DAG parents without storing any additional state.
When a node that is capable of storing routing state encounters a (unicast) NA-DAO message with a Reverse Route Stack that has been populated, the node knows that the NA-DAO message has traversed a region of nodes that did not record any routing state. The node is able to detach and store the Reverse Route State and associate it with the destination described by the NA-DAO message. Subsequently the node may use this information to construct a source route in order to bridge the region of nodes that are unable to support Hop-By-Hop routing to reach the destination.
In this way the destination advertisement mechanism is able to provision routing state in support of P2MP traffic flows outward along the DAG, and as according to the available resources in the network.
Further aggregations of NA-DAO messages prefix reachability information by destinations are possible in order to support additional scalability.
A further example of the operation of the destination advertisement mechanism is available in Appendix B.6 (Destination Advertisement)
| TOC |
The goal of a guaranteed consistent and loop free global routing solution for an LLN may not be practically achieved given the real behavior and volatility of the underlying metrics. The trade offs to achieve a stable approximation of global convergence may be too restrictive with respect to the need of the LLN to react quickly in response to the lossy environment. Globally the LLN may be able to achieve a weak convergence, in particular as link changes are able to be handled locally and result in minimal changes to global topology.
RPL does not aim to guarantee loop free path selection, or strong global convergence. In order to reduce control overhead, in particular the expense of mechanisms such as count-to-infinity, RPL does try to avoid the creation of loops when undergoing topology changes. Further mechanisms to mitigate the impact of loops, such as loop detection when forwarding, are under investigation.
| TOC |
If a node is greedy and attempts to move deeper in the DAG, beyond its most preferred parent, in order to increase the size of the DAG parent set, then an instability can result. This is illustrated in Figure 11 (Greedy DAG Parent Selection).
Suppose a node is willing to receive and process a RA-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 DAG in order to optimize against each other. In some cases this will result in an instability. It is for this reason that RPL mandates that a node never receive and process RA-DIO messages from deeper nodes. This rule creates an `event horizon', whereby a node cannot be influenced into an instability by the action of nodes that may be in its own sub-DAG.
A further example of the consequences of greedy operation, and instability related to processing RA-DIO messages from nodes of greater rank, may be found in Appendix B.9 (Example: Greedy Parent Selection and Instability)
| TOC |
The merging of DAGs is coordinated in a way such as to try and merge two DAGs cleanly, preserving as much DAG structure as possible, and in the process effecting a clean merge with minimal likelihood of forming transient DAG loops. The coordinated merge is also intended to minimize the related control cost.
When a node, and perhaps a related frozen sub-DAG, jumps to a different DAG, the move is coordinated by a set of timers (DAG Hop timers). The DAG Hop timers allow the nodes who will attach closer to the sink of the new DAG to `jump' first, and then drag dependent nodes behind them, thus endeavoring to efficiently coordinate the attachment of the frozen sub-DAG into the new DAG.
A further example of a DAG Merge operation may be found in Appendix B.10 (Example: DAG Merge)
| TOC |
A DAG loop may occur when a node detaches from the DAG and reattaches to a device in its prior sub-DAG that has missed the whole detachment sequence and kept advertising the original DAG. This may happen in particular when RA-DIO messages are missed. Use of the DAG sequence number can eliminate this type of loop. If the DAG sequence number is not in use, the protection is limited (it depends on propagation of RA-DIO messages during DAG hop timer), and temporary loops might occur. RPL will move to eliminate such a loop as soon as a RA-DIO message is received from a parent that appears to be going down, as the child has to detach from it immediately. (The alternate choice of staying attached and following the parent in its fall would have counted to infinity and led to detach as well).
Consider node (24) in the DAG Example depicted in Figure 9 (Example DAG), and its sub-DAG nodes (34), (44), and (45). An example of a DAG loop would be if node (24) were to detach from the DAG rooted at (LBR), and nodes (34) and (45) were to miss the detachment sequence. Subsequently, if the link (24)--(45) were to become viable and node (24) heard node (45) advertising the DAG rooted at (LBR), a DAG loop (45->34->24->45) may form if node (24) attaches to node (45).
| TOC |
A DAO loop may occur when the parent has a route installed upon receiving and processing a NA-DAO message from a child, but the child has subsequently cleaned up the state. This loop happens when a no-DAO was missed till a heartbeat cleans up all states. The DAO loop is not explicitly handled by the current specification. Split horizon, not forwarding a packet back to the node it came from, may mitigate the DAO loop in some cases, but does not eliminate it.
Consider node (24) in the DAG Example depicted in Figure 9 (Example DAG). Suppose node (24) has received a DA from node (34) advertising a destination at node (45). Subsequently, if node (34) tears down the routing state for the destination and node (24) did not hear a no-DAO message to clean up the routing state, a DAO loop may exist. node (24) will forward traffic destined for node (45) to node (34), who may then naively return it into a loop (if split horizon is not in place). A more complicated DAO loop may result if node (34) instead passes the traffic to it's sibling, node (33), potentially resulting in a (24->34->33->23->13->24) loop.
| TOC |
Sibling loops occur when a group of siblings keep choosing amongst themselves as successors such that a packet does not make forward progress. The current draft limits those loops to some degree by split horizon (do not send back to the same sibling) and parent preference (always prefer parents vs. siblings).
Consider the DAG Example depicted in Figure 9 (Example DAG). Suppose that Node (32) and (34) are reliable neighbors, and thus are siblings. Then, in the case where Nodes (22), (23), and (24) are transiently unavailable, and with no other guiding strategy, a sibling loop may exist, e.g. (33->34->32->33) as the siblings keep choosing amongst each other in an uncoordinated manner.
| TOC |
Although implementation specific, it is worth noting that a node may decide to implement some local routing decision based on some metrics, as observed locally or reported in the RA-DIO message. For example, the routing may reflect a set of successors (next-hop), along with various aggregated metrics used to load balance the traffic according to some local policy. Such decisions are local and implementation specific.
Routing stability is crucial in a LLN: in the presence of unstable links, the first option consists of removing the link from the DAG and triggering a DAG recomputation across all of the nodes affected by the removed link. Such a naive approach could unavoidably lead to frequent and undesirable changes of the DAG, routing instability, and high-energy consumption. The alternative approach adopted by RPL relies on the ability to temporarily not use a link toward a successor marked as valid, with no change on the DAG structure. If the link is perceived as non-usable for some period of time (locally configurable), this triggers a DAG recomputation, through the DAG discovery mechanism further detailed in Section 5.3 (DAG Discovery and Maintenance), after reporting the link failure. Note that this concept may be extended to take into account other link characteristics: for the sake of illustration, a node may decide to send a fixed number of packets to a particular successor (because of limited buffering capability of the successor) before starting to send traffic to another successor.
According to the local policy function, it is possible for the node to order the DAG parent set from `most preferred' to `least preferred'. By constructing such an ordered set, and by appending the set with siblings, the node is able to construct an ordered list of preferred next hops to assist in local and temporary routing decisions. The use of the ordered list by a forwarding engine is loosely constrained, and may take into account the dynamics of the LLN. Further, a forwarding engine implementation may decide to perform load balancing functions using hash-based mechanisms to avoid packet re-ordering. Note however, that specific details of a forwarding engine implementation are beyond the scope of this document.
These decisions may be local and/or temporary with the objective to maintain the DAG shape while preserving routing stability.
| TOC |
In order to relieve the LLN of the overhead of periodic keepalives, RPL may employ an as-needed mechanism of NS/NA in order to verify routing adjacencies just prior to forwarding data. Pending the outcome of verifying the routing adjacency, the packet may either be forwarded or an alternate next-hop may be selected.
| TOC |
This aim of this section is to make a clear distinction between routing metrics and constraints and define the term constraint based routing as used in this document.
| TOC |
Routing metrics are used by the routing protocol to compute the shortest path according to one of more defined metrics. IGPs such as IS-IS ([RFC5120] (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.)) compute the shortest path according to a Link State Data Base (LSDB) using link metrics configured by the network administrator. Such metrics can represent the link bandwidth (in which case the metric is usually inversely proportional to the bandwidth), delay, etc. Note that in some cases the metric is a polynomial function of several metrics defining different link characteristics. The resulting shortest path cost is equal to the sum (or multiplication) of the link metrics along the path: such metrics are said to be additive or multiplicative metrics.
Some routing protocols support more than one metric: in the vast majority of the cases, one metric is used per (sub)topology. Less often, a second metric may be used as a tie breaker in the presence of ECMP (Equal Cost Multiple Paths). The optimization of multiple metrics is known as an NP complete problem and is sometimes supported by some centralized path computation engine.
In the case of RPL, it is virtually impossible to define *the* metric, or even a composite, that will fit it all:
For that reason, the RPL protocol core is agnostic to the logic that handles metrics. A node will be configured with some external logic to use and prioritize certain metrics for a specific scenario. As new heterogeneous devices are installed to support the evolution of a network, or as networks form in a totally ad-hoc fashion, it will happen that nodes that are programmed with antagonistic logics and conflicting or orthogonal priorities end up participating in the same network. It is thus recommended to use consistent parent selection policy, as per Objective Code Points (OCP), to ensure consistent optimized paths.
RPL is designed to survive and still operate, though in a somewhat degraded fashion, when confronted to such heterogeneity. The key design point is that each node is solely responsible for setting the vector of metrics that it sources in the DAG, derived in part from the metrics sourced from its preferred parent. As a result, the DAG is not broken if another node makes its decisions in as antagonistic fashion, though an end-to-end path might not fully achieve any of the optimizations that nodes along the way expect. The default operation specified in OCP 0 clarifies this point.
| TOC |
A constraint is a link or a node characteristic that must be satisfied by the computed path (using boolean values or lower/upper bounds) and is by definition neither additive nor multiplicative. Examples of links constraints are "available bandwidth", "administrative values (e.g. link coloring)", "protected versus non-protected links", "link quality" whereas a node constraint can be the level of battery power, CPU processing power, etc.
| TOC |
The notion of constraint based routing consists of finding the shortest path according to some metrics satisfying a set of constraints. A technique consists of first filtering out all links and nodes that cannot satisfy the constraints (resulting in a sub-topology) and then computing the shortest path.
Example 1:
Link Metric: Bandwidth
Link Constraint: Blue
Node Constraint: Mains-powered node
Objective function 1:
"Find the shortest path (path with lowest cost where the path cost is the sum of all link costs (Bandwidth)) along the path such that all links are colored `Blue' and that only traverses Mains-powered nodes."
Example 2:
Link Metric: Delay
Link Constraint: Bandwidth
Objective function 2:
"Find the shortest path (path with lowest cost where the path cost is the sum of all link costs (Delay)) along the path such that all links provide at least X Bit/s of reservable bandwidth."
| TOC |
| TOC |
The DAG Information Option carries a number of metrics and other information that allows a node to discover a DAG, select its DAG parents, and identify its siblings while employing loop avoidance strategies.
| TOC |
The DAG Information Option is a container option carried within an IPv6 Router Advertisement message as defined in [RFC4861] (Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” September 2007.), which might contain a number of suboptions. The base option regroups the minimum information set that is mandatory in all cases.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |G|D|A| 00000 | Sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAGPreference | BootTimeRandom |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NodePref. | DAGRank | DAGDelay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DIOIntDoubl. | DIOIntMin. | DAGObjectiveCodePoint |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PathDigest |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| DAGID |
+ +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Figure 1: DIO Base Option |
- Type:
- 8-bit unsigned identifying the DIO base option. The suggested value is 140 to be confirmed by the IANA.
- Length:
- 8-bit unsigned integer set to 4 when there is no suboption. The length of the option (including the type and length fields and the suboptions) in units of 8 octets.
- Flag Field:
- Three flags are currently defined:
- Grounded (G):
- The Grounded (G) flag is set when the DAG root is offering connectivity to an external routed infrastructure such as the Internet.
- Destination Advertisement Trigger (D):
- The Destination Advertisement Trigger (D) flag is set when the DAG root or another node in the successor chain decides to trigger the sending of destination advertisements in order to update routing state for the outward direction along the DAG, as further detailed in Section 5.9 (Establishing Routing State Outward Along the DAG). Note that the use and semantics of this flag are still under investigation.
- Destination Advertisement Supported (A) :
- The Destination Supported (A) bit is set when the DAG root is capable to support the collection of destination advertisement related routing state and enables the operation of the destination advertisement mechanism within the DAG.
- Unassigned bits of the Flag 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 DAG root, incremented according to a policy provisioned at the DAG root, and propagated with no change outwards along the DAG. Each increment SHOULD have a value of 1 and may cause a wrap back to zero.
- DAGPreference:
- 8-bit unsigned integer set by the DAG root to its preference and unchanged at propagation. DAGPreference ranges from 0x00 (least preferred) to 0xFF (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 DAG while still meeting other optimization objectives, then the node will seek to join the more preferred DAG.
- BootTimeRandom:
- A random value computed at boot time and recomputed in case of a duplication with another node. The concatenation of the NodePreference and the BootTimeRandom is a 32-bit extended preference that is used to resolve collisions. It is set by each node at propagation time.
- NodePreference:
- The administrative preference of that LLN Node. Default is 0. 255 is the highest possible preference. Set by each LLN Node at propagation time. Forms a collision tiebreaker in combination with BootTimeRandom.
- DAGRank:
- 8-bit unsigned integer indicating the DAG rank of the node sending the RA-DIO message. The DAGRank of the DAG root is typically 1. DAGRank is further described in Section 5.3 (DAG Discovery and Maintenance).
- DAGDelay:
- 16-bit unsigned integer set by the DAG root indicating the delay before changing the DAG configuration, in TBD-units. A default value is TBD. It is expected to be an order of magnitude smaller than the RA-interval. It is also expected to be an order of magnitude longer than the typical propagation delay inside the LLN.
- DIOIntervalDoublings:
- 8-bit unsigned integer. Configured on the DAG root and used to configure the trickle timer governing when RA-DIO message should be sent within the DAG. DIOIntervalDoublings is the number of times that the DIOIntervalMin is allowed to be doubled during the trickle timer operation.
- DIOIntervalMin:
- 8-bit unsigned integer. Configured on the DAG root and used to configure the trickle timer governing when RA-DIO message should be sent within the DAG. The minimum configured interval for the RA-DIO trickle timer in units of ms is 2^DIOIntervalMin. For example, a DIOIntervalMin value of 16ms is expressed as 4.
- DAGObjectiveCodePoint:
- The DAG Objective Code Point is used to indicate the cost metrics, objective functions, and methods of computation and comparison for DAGRank in use in the DAG. The DAG OCP is set by the DAG root. (Objective Code Points are to be further defined in [I‑D.ietf‑roll‑routing‑metrics] (Vasseur, J., Kim, M., Networks, D., and H. Chong, “Routing Metrics used for Path Calculation in Low Power and Lossy Networks,” April 2010.).
- PathDigest:
- 32-bit unsigned integer CRC, updated by each LLN Node. This is the result of a CRC-32c computation on a bit string obtained by appending the received value and the ordered set of DAG parents at the LLN Node. DAG roots use a 'previous value' of zeroes to initially set the PathDigest. Used to determine when something in the set of successor paths has changed.
- DAGID:
- 128-bit unsigned integer which uniquely identify a DAG. This value is set by the DAG root. The global IPv6 address of the DAG root can be used, however. the DAGID MUST be unique per DAG within the scope of the LLN. In the case where a DAG root is rooting multiple DAGs the DAGID MUST be unique for each DAG rooted at a specific DAG root.
The following values MUST NOT change during the propagation of RA-DIO messages outwards along the DAG: Type, Length, G, DAGPreference, DAGDelay and DAGID. All other fields of the RA-DIO message are updated at each hop of the propagation.
| TOC |
In addition to the minimum options presented in the base option, several suboptions are defined for the RA-DIO message:
| TOC |
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subopt. Type | Subopt Length | Suboption Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Figure 2: DIO Suboption Generic Format |
- Suboption Type:
- 8-bit identifier of the type of suboption. When processing a RA-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:
- 8-bit unsigned integer, representing the length in octets of the suboption, not including the suboption Type and Length fields.
- Suboption Data:
- A variable length field that contains data specific to the option.
The following subsections specify the RA-DIO message suboptions which are currently defined for use in the DAG Information Option.
Implementations MUST silently ignore any RA-DIO message suboptions options that they do not understand.
RA-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).
| TOC |
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 |
+-+-+-+-+-+-+-+-+
| Figure 3: Pad 1 |
NOTE! the format of the Pad1 option is a special case - it has neither Option Length nor Option Data fields.
The Pad1 option is used to insert one octet of padding in the RA-DIO message to enable suboptions alignment. If more than one octet of padding is required, the PadN option, described next, should be used rather than multiple Pad1 options.
| TOC |
The PadN option does not have any alignment requirements. Its format is as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Type = 1 | Subopt Length | Subopt Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Figure 4: Pad N |
The PadN option is used to insert two or more octets of padding in the RA-DIO message to enable suboptions alignment. For N (N > 1) octets of padding, the Option Length field contains the value N-2, and the Option Data consists of N-2 zero-valued octets. PadN Option data MUST be ignored by the receiver.
| TOC |
The DAG Metric Container suboption may be aligned as necessary to support its contents. Its format is as follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Type = 2 | Container Len | DAG Metric Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Figure 5: DAG Metric Container |
The DAG Metric Container is used to report aggregated path metrics along the DAG. The DAG Metric Container may contain a number of discrete node, link, and aggregate path metrics as chosen by the implementer. The Container Length field contains the length in octets of the DAG Metric Data. The order, content, and coding of the DAG Metric Container data is as specified in [I‑D.ietf‑roll‑routing‑metrics] (Vasseur, J., Kim, M., Networks, D., and H. Chong, “Routing Metrics used for Path Calculation in Low Power and Lossy Networks,” April 2010.).
The processing and propagation of the DAG Metric Container is governed by implementation specific policy functions.
| TOC |
The Destination Prefix suboption has an alignment requirement of 4n+1. Its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 | Length | Prefix Length |Resvd|Prf|Resvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Prefix (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Figure 6: DAG Destination Prefix |
The Destination Prefix suboption is used when the DAG root, or another node located inwards along the DAG on the path to the DAG root, needs to indicate that it offers connectivity to destination prefixes other than the default. This may be useful in cases where more than one LBR is operating within the LLN and offering connectivity to different administrative domains, e.g. a home network and a utility network. In such cases, upon observing the Destination Prefixes offered by a particular DAG, a node MAY decide to join multiple DAGs in support of a particular application.
The Length is coded as the length of the suboption in octets, excluding the Type and Length fields.
The Prefix Length is an 8-bit unsigned integer that indicates the number of leading bits in the destination prefix. Prf is the Route Preference as in [RFC4191] (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. A value of all one bits (0xFFFFFFFF) represents infinity. A value of all zero bits (0x00000000) indicates a loss of reachability.
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 RA-DIO message may need to specify connectivity to more than one destination, the Destination Prefix suboption may be repeated.
| TOC |
The RPL implementation MUST maintain the following conceptual data structures in support of DAG discovery:
| TOC |
The set of candidate neighbors is to be populated by neighbors who are discovered by the neighbor discovery mechanism and further qualified as statistically stable as per the mechanisms discussed in [I‑D.ietf‑roll‑routing‑metrics] (Vasseur, J., Kim, M., Networks, D., and H. Chong, “Routing Metrics used for Path Calculation in Low Power and Lossy Networks,” April 2010.). The candidate neighbors, and related metrics, should demonstrate stability/reliability beyond a certain threshold, and it is recommended that a local confidence value be maintained with respect to the neighbor in order to track this. Implementations MAY choose to bound the maximum size of the candidate neighbor set, in which case a local confidence value will assist in ordering neighbors to determine which ones should remain in the candidate neighbor set and which should be evicted.
If Neighbor Unreachability Detection (NUD) determines that a candidate neighbor is no longer reachable, then it shall be removed from the candidate neighbor set. In the case that the candidate neighbor has associated states in the DAG parent set or active DA entries, then the removal of the candidate neighbor shall be coordinated with tearing down these states. All provisioned routes associated with the candidate neighbor should be removed.
| TOC |
A DAG may be uniquely identified by within the LLN by its unique DAGID. When a single device is capable to root multiple DAGs in support of an application need for multiple optimization objectives it is expected to produce a different and unique DAGID for each of the multiple DAGs.
For each DAG that a node is, or may become, a member of, the implementation MUST keep a DAG table with the following entries:
When a DAG is discovered for which no DAG data structure is instantiated, and the node wants to join (i.e. the neighbor is to become a candidate DAG parent in the Held-Up state), then the DAG data structure is instantiated.
When the candidate DAG parent set is depleted (i.e. the last candidate DAG parent has timed out of the Held-Down state), then the DAG data structure SHOULD be suppressed after the expiration of an implementation-specific local timer. An implementation SHOULD delay before deallocating the DAG data structure in order to observe that the DAGSequenceNumber has incremented should any new candidate DAG parents appear for the DAG.
| TOC |
When the DAG is self-rooted, the set of candidate DAG parents is empty.
In all other cases, for each candidate DAG parent in the set, the implementation MUST keep a record of:
| TOC |
Note that the subset of candidate DAG parents in the `Current' state comprises the set of DAG parents, i.e. the nodes actively acting as parents in the DAG.
DAG parents may be ordered, according to the OCP. When ordering DAG parents, in consultation with the OCP, the most preferred DAG parent may be identified. All current DAG parents must have a rank less than or equal to that of the most preferred DAG parent.
When nodes are added to or removed from the DAG parent set the most preferred DAG parent may have changed and should be reevaluated. Any nodes having a rank greater than self after such a change must be placed in the Held-Down state and evicted as per the procedures described in Section 5.7 (Candidate DAG Parent States and Stability)
An implementation may choose to keep these records as an extension of the Default Router List (DRL).
| TOC |
DAG discovery locates the nearest sink, as determined according to some metrics and constraints, and forms a Directed Acyclic Graph towards that sink, by identifying a set of DAG parents. During this process DAG discovery also identifies siblings, which may be used later to provide additional path diversity towards the DAG root. DAG discovery enables nodes to implement different policies for selecting their DAG parents in the DAG by using implementation specific policy functions. DAG discovery specifies a set of rules to be followed by all implementations in order to ensure interoperation. DAG discovery also standardizes the format that is used to advertise the most common information that is used in order to select DAG parents.
One of these information, the DAG rank, is used by DAG discovery to provide loop avoidance even if nodes implement different policies. The DAG Rank is computed as specified by the Objective Code Point in use by the DAG, demonstrating the properties described in Section 3.2.1.7 (DAG Rank). The rank should be computed in such a way so as to provide a comparable basis with other nodes which may not use the same metric at all.
The DAG discovery procedures take into account a number of factors, including:
| TOC |
In order to organize and maintain loopless structure, the DAG discovery implementation in the nodes MUST obey to the following rules and definitions:
| TOC |
When an RA-DIO message is received from a source device named SRC, the receiving node must first determine whether or not the RA-DIO message should be accepted for further processing, and subsequently present the RA-DIO message for further processing if eligible.
| TOC |
If the RA-DIO message is malformed, then the RA-DIO message is not eligible for further processing and is silently discarded. A RPL implementation MAY log the reception of a malformed RA-DIO message.
If SRC is not a member of the candidate neighbor set, then the RA-DIO is not eligible for further processing. (Further evaluation/confidence of this neighbor is necessary)
If the RA-DIO message advertises a DAG that the node is already a member of, then:
If the rank of SRC as reported in the RA-DIO message is lesser than that of the node within the DAG, then the RA-DIO message MUST be considered for further processing
If the rank of SRC as reported in the RA-DIO message is equal to that of the node within the DAG, then SRC is marked as a sibling and the RA-DIO message is not eligible for further processing.
If the rank of SRC as reported in the RA-DIO message is higher than that of the node within the DAG, and SRC is not a DAG parent, then the RA-DIO message MUST NOT be considered for further processing
If SRC is a DAG parent for any other DAG that the node is attached to, then the RA-DIO message MUST be considered for further processing (the DAG parent may have jumped).
If the RA-DIO message advertises a DAG that offers a better (new or alternate) solution to an optimization objective desired by the node, then the RA-DIO message MUST be considered for further processing.
| TOC |
If the received RA-DIO message is for a new/alternate DAG:
Instantiate a data structure for the new/alternate DAG if necessary
Place the neighbor in the candidate DAG parent set
If the node has sent an RA message within the risk window as described in Section 5.7.3 (Collision) then perform the collision detection described in Section 5.7.3 (Collision). If a collision occurs, place the candidate DAG parent in the collision state and do not process the RA-DIO message any further as described in Section 5.7 (Candidate DAG Parent States and Stability).
If the SRC node is also a DAG parent for another DAG that the node is a member of, and if the new/alternate DAG satisfies an equivalent optimization objective as the other DAG, then the DAG parent is known to have jumped.
Remove SRC as a DAG parent from the other DAG (place it in the held-down state)
If the other DAG is now empty of candidate parents, then directly follow SRC into the new DAG by adding it as a DAG parent in the Current state, else ignore the RA-DIO message (do not follow the parent).
If the new/alternate DAG offers a better solution to the optimization objectives, then prepare to jump: copy the DIO information into the record for the candidate DAG parent, place the candidate DAG parent into the Held-Up state, and start the DAG Hop timer as per Section 5.7.1 (Held-Up).
If the RA-DIO message is for a known/existing DAG:
Process the RA-DIO message as per the rules in Section 5.3 (DAG Discovery and Maintenance)
As candidate parents are identified, they may subsequently be promoted to DAG parents by following the rules of DAG discovery as described in Section 5.3 (DAG Discovery and Maintenance). When a node adds another node to its set of candidate parents, the node becomes attached to the DAG through the parent node.
In the DAG discovery implementation, the most preferred parent should 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).
| TOC |
Each node maintains a timer that governs when to multicast RA messages. This timer is implemented as a trickle timer operating over a variable interval. Trickle timers are further detailed in Section 5.3.4 (Trickle Timer for RA Transmission). The governing parameters for the timer should be configured consistently across the DAG, and are provided by the DAG root in the RA-DIO message. In addition to periodic RA messages, each LLN node will respond to Router Solicitation (RS) messages according to [RFC4861] (Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” September 2007.).
Note that if multiple DAG roots are participating in the same DAG, i.e. offering RA-DIO messages with the same DAGID, then they must coordinate with each other to ensure that their RA-DIO messages are consistent when they emit RA-DIO messages. In particular the Sequence number must be identical from each DAG root, regardless of which of the multiple DAG roots issues the RA-DIO message, and changes to the Sequence number should be issued at the same time. The specific mechanism of this coordination, e.g. along a non-LLN network between DAG roots, is beyond the scope of this specification.
| TOC |
RPL treats the construction of a DAG 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 DAG that a node is part of, the node must maintain a single trickle timer. The required state contains the following conceptual items:
- I:
- The current length of the communication interval
- T:
- A timer with a duration set to a random value in the range [I/2, I]
- C:
- Redundancy Counter
- I_min:
- The smallest communication interval in milliseconds. This value is learned from the RA-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 RA-DIO message as DIOIntervalDoublings. The default value is DEFAULT_DIO_INTERVAL_DOUBLINGS.
| TOC |
The trickle timer for a DAGID is reset by:
When node learns about a DAG through a RA-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 consistent RA for this DAG from a DAG parent, it MAY increment C.
When the timer fires at time T, the node compares C to the redundancy constant, DEFAULT_DIO_REDUNDANCY_CONSTANT. If C is less than that value, the node generates a new RA and broadcasts it. When the communication interval I expires, the node doubles the interval I so long as it has previously doubled it fewer than I_doubling times, resets C, and chooses a new T value.
| TOC |
The trickle timer is reset whenever an inconsistency is detected within the DAG, for example:
| TOC |
The DAG root makes the sole determination of when to revise the DAGSequenceNumber by incrementing it upwards. When the DAGSequenceNumber is increased an inconsistency results, causing RA-DIO messages to be sent back outwards along the DAG to convey the change. The degree to which this mechanism is relied on may be determined by the implementation- on one hand it may serve as a periodic heartbeat, refreshing the DAG states, and on the other hand it may result in a constant steady-state control cost overhead which is not desirable.
Some implementations may provide an administrative interface, such as a command line, at the DAG root whereby the DAGSequenceNumber may be caused to increment in response to some policy outside of the scope of RPL.
Other implementations may make use of a periodic timer to automatically increment the DAGSequenceNumber, resulting in a periodic DAG Heartbeat at a rate appropriate to the application and implementation.
| TOC |
The DAG selection is implementation and algorithm dependent. Nodes SHOULD prefer to join DAGs advertising OCPs and destinations compatible with their implementation specific objectives. In order to limit erratic movements, and all metrics being equal, nodes SHOULD keep their previous selection. Also, nodes SHOULD provide a means to filter out a candidate parent whose availability is detected as fluctuating, at least when more stable choices are available. Nodes MAY place the failed candidate parent in a Hold Down mode that ensures that the candidate parent will not be reused for a given period of time.
When connection to a fixed network is not possible or preferable for security or other reasons, scattered DAGs MAY aggregate as much as possible into larger DAGs in order to allow connectivity within the LLN.
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.
| TOC |
When the DAG is formed under a common administration, or when a node performs a certain role within a community, it might be beneficial to associate a range of acceptable rank with that node. For instance, a node that has limited battery should be a leaf unless there is no other choice, and may then augment the rank computation specified by the OCP in order to expose an exaggerated rank.
| TOC |
Candidate DAG parents may or may not be eligible to act as DAG parents depending on runtime conditions. The following states are defined:
- Current
- This candidate parent is in the set of DAG parents and may be used for forwarding traffic inward along the DAG. When a candidate parent is placed into the Current state, or taken out of the Current state, it is necessary to re-evaluate which of the remaining DAG parents is the most preferred DAG parent and its rank. At that time any remaining DAG parents of greater rank than this node must be placed in the Held-Down state, and the hold-down timer started, in order to be evicted as DAG parents. In the same fashion, siblings must also be reevaluated.
- Held-Up
- This parent can not be used until the DAG hop timer elapses.
- Held-Down
- This candidate parent can not be used till hold down timer elapses. At the end of the hold-down period, the candidate is removed from the candidate DAG parent set, and may be reinserted if it appears again with a RA-DIO message.
- Collision
- This candidate parent can not be used till its next RA-DIO message.
| TOC |
This state is managed by the DAG Hop timer, it serves 2 purposes:
Delay the reattachment of a sub-DAG that has been forced to detach. This is not as safe as the use of the sequence, but still covers that when a sub-DAG has detached, the RA-DIO message that is initiated by the new DAG root has a chance to spread outward along the sub-DAG, ideally forming a frozen sub-DAG that is aware of the DAG change, such that two different DAGs have formed prior to an attempted reattachment.
Limit RA-DIO message storms (control cost / churn) when two DAGs collide/merge. The idea is that between the nodes from DAG A that decide to move to DAG B, those that see the highest place (closer to the DAG root) in DAG B will move first and advertise their new locations before other nodes from DAG A actually move.
A new DAG is discovered upon receiving a RA message with or without a DIO. The node joins the DAG by selecting the source of the RA message as a DAG parent (and possibly installing the DAG parent as a default gateway). The node is then a member of the DAG and may begin to multicast RA-DIO messages containing the DIO for the DAG.
When a new DAG is discovered, the candidate parent that advertises the new DAG is placed in a held up state for the duration of a DAG Hop timer. If the resulting new set of DAG parents is more preferable than the current one, or if the node is intending to maintain a membership in the new DAG in addition to its current DAG, the node expects to jump and becomes unstable.
A node that is unstable may discover other candidate parents from the same new DAG during the instability phase. It needs to start a new DAG Hop timer for all these. The first timer that elapses for a given new DAG clears them all for that DAG, allowing the node to jump to the highest position available in the new DAG.
The duration of the DAG Hop timer depends on the DAG Delay of the new DAG and on the rank of candidate parent that triggers it: (candidates rank + random) * candidate's DAG_delay (where 0 <= random < 1). It is randomized in order to limit collisions and synchronizations.
| TOC |
When a neighboring node is 'removed' from the Default Router List, it is actually held down for a hold down timer period, in order to prevent flapping. This happens when a node disappears (upon expiration timer).
When the hold down timer elapses, the node is removed from the candidate DAG parent set.
| TOC |
A race condition occurs if 2 nodes send RA-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 DAGs. In order to detect the situation, LLN Nodes time stamp the sending of RA-DIO message. Any RA-DIO message received within a short link-layer-dependent period introduces a risk. To resolve the collision, a 32bits extended preference is constructed from the RA-DIO message by concatenating the NodePreference with the BootTimeRandom.
A node that decides to add a candidate to its DAG parents will do so between (candidate rank) and (candidate rank + 1) times the candidate DAG Delay. But since a node is unstable as soon as it receives the RA-DIO message from the desired candidate, it will restrain from sending a RA-DIO message between the time it receives the RA and the time it actually jumps. So the crossing of RA may only happen during the propagation time between the candidate and the node, plus some internal queuing and processing time within each machine. It is expected that one DAG delay normally covers that interval, but ultimately it is up to the implementation and the configuration of the candidate parent to define the duration of risk window.
There is risk of a collision when a node receives an RA, for another candidate that is more preferable than the current candidate, within the risk window. In the face of a potential collision, the node with lowest extended preference processes the RA-DIO message normally, while the router with the highest extended preference places the other in collision state, does not start the DAG hop timer, and does not become instable. It is expected that next RAs between the two will not cross anyway.
For example, consider a case where two nodes are each rooting their own transient floating DAGs and multicast RA-DIO messages towards each other in a close enough interval that the RA-DIO messages `cross'. Then each node may receive the RA-DIO message from the other node, and in some scenario decide to join each others DAG. RPL avoids this deadlock scenario via the collision mechanism described above - after each node sends the RA-DIO message they will enter the risk window. When the peer RA-DIO message is received in the risk window, the nodes will calculate the extended preferences as describe above and the node with the lowest extended preference will proceed to process the RA-DIO message, while the other node will defer, avoiding the deadlock scenario.
| TOC |
A node is instable when it is prepared to shortly replace a set of DAG parents in order to jump to a different DAGID. This happens typically when the node has selected a more preferred candidate parent in a different DAG and has to wait for the DAG hop timer to elapse before adjusting the DAG parent set. Instability may also occur when the entire current DAG parent set is lost and the next best candidates are still held up. Instability is resolved when the DAG hop timer of all the candidate(s) causing instability elapse. Such candidates then change state to Current or Held- Down.
Instability is transient (in the order of DAG hop timers). When a node is unstable, it MUST NOT send RAs with the DIO message. This avoids loops when node A decides to attach to node B and node B decides to attach to node A. Unless RAs cross (see Collision section), a node receives RA-DIO messages from stable candidate parents, which do not plan to attach to the node, so the node can safely attach to them.
| TOC |
| TOC |
An Objective Function (OF) allows for the selection of a DAG to join, and a number of peers in that DAG as parents. The OF is used to compute an ordered list of parents and provides load balancing guidance. The OF is also responsible to compute the rank of the device within the DAG.
The Objective Function is specified in the RA-DIO message using an objective code point (OCP) and indicates the objective function that has been used to compute the DAG (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.). This document specifies the OCP 0, in support of default operation.
Most Objective Functions are expected to follow the same abstract behavior:
| TOC |
Here follows the specification for the default Objective Function corresponding to OCP codepoint 0. This is a very simple reference to help design more complex Objective Functions. In particular, the Objective Function described here does not use physical metrics as described in [I‑D.ietf‑roll‑routing‑metrics] (Vasseur, J., Kim, M., Networks, D., and H. Chong, “Routing Metrics used for Path Calculation in Low Power and Lossy Networks,” April 2010.), but are only based on abstract information from the RA-DIO message such as rank and administrative preference.
This document specifies a default objective metric, called OF0, and using the OCP 0. OF0 is the default objective function of RPL, and can be used if allowed by the policy of the processing node when no objective function is included in the RA-DIO message, or if the OF indicated in the RA-DIO message is unknown to the node. If not allowed, then the RA-DIO message is simply ignored and not processed by the node.
| TOC |
OF0 favors the connectivity. That is, the Objective Function is designed to find the nearest sink into a 'grounded' topology, and if there is none then join any network per order of administrative preference. The metric in use is the rank.
OF0 selects a preferred parent and a backup next_hop if one is available. The backup next_hop might be a parent or a sibling. All the traffic is routed via the preferred parent. When the link conditions do not let a packet through to the preferred parent, the packet is passed to the backup next_hop.
The step of rank is 4 for each hop.
| TOC |
As it scans all the candidate neighbors, OF0 keeps the parent that is the best for the following criteria (in order):
| TOC |
| TOC |
The destination advertisement mechanism supports the dissemination of routing state required to support traffic flows outward along the DAG, from the DAG root toward nodes.
As a result of destination advertisement operation:
Destinations disseminated with the destination advertisement mechanism may be prefixes, individual hosts, or multicast listeners. The mechanism supports nodes of varying capabilities as follows:
Nodes that are capable of storing routing state, and finally the DAG roots, are able to learn which destinations are contained in the sub-DAG below the node, and via which next-hop neighbors. The dissemination and installation of this routing state into nodes allows for Hop-By-Hop routing from the DAG root outwards along the DAG. The mechanism is further enhance by supporting the construction of source routes across stateless `gaps' in the DAG, where nodes are incapable of storing additional routing state. An adaptation of this mechanism allows for the implementation of loose-source 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 DAG, 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, as cued by occasional RAs and other mechanisms, similarly to other protocols such as RIP [RFC2453] (Malkin, G., “RIP Version 2,” November 1998.).
| TOC |
| TOC |
RPL extends Neighbor Discovery [RFC4861] (Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” September 2007.) and RFC4191 [RFC4191] (Draves, R. and D. Thaler, “Default Router Preferences and More-Specific Routes,” November 2005.) to allow a node to include a destination advertisement option, which includes prefix information, in the Neighbor Advertisement (NA) messages. A prefix option is normally present in RA messages only, but the NA is augmented with this option in order to propagate destination information inwards along the DAG. The option is named the Destination Advertisement Option (DAO), and an NA message containing this option may be referred to as a destination advertisement, or NA-DAO. The RPL use of destination advertisements allows the nodes in the DAG to build up routing state for nodes contained in the sub-DAG in support of traffic flowing outward along the DAG.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Prefix Length | RRCount |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Depth | Reserved | DAO Sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse Route Stack (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Figure 7: The Destination Advertisement Option (DAO) |
- Type:
- 8-bit unsigned identifying the Destination Advertisement option. IANA had defined the IPv6 Neighbor Discovery Option Formats registry. The suggested type value for the Destination Advertisement Option carried within a NA message is 141, to be confirmed by IANA.
- Length:
- 8-bit unsigned integer. The length of the option (including the Type and Length fields) in units of 8 octets.
- Prefix Length:
- Number of valid leading bits in the IPv6 Prefix.
- RRCount:
- 8-bit unsigned integer. This counter is used to count the number of entries in the Reverse Route Stack. A value of `0' indicates that no Reverse Route Stack is present.
- DAO Lifetime:
- 32-bit unsigned integer. The length of time in seconds (relative to the time the packet is sent) that the prefix is valid for route determination. A value of all one bits (0xFFFFFFFF) represents infinity. A value of all zero bits (0x00000000) indicates a loss of reachability.
- Route Tag:
- 32-bit unsigned integer. The Route Tag may be used to give a priority to prefixes that should be stored. This may be useful in cases where intermediate nodes are capable of storing a limited amount of routing state. The further specification of this field and its use is under investigation.
- DAO Depth:
- Set to 0 by the node that owns the prefix and first issues the NA-DAO message. Incremented by all LLN nodes that propagate the NA-DAO message.
- Reserved:
- 8-bit unused field. The reserved field MUST be set to zero on transmission and MUST be ignored on receipt.
- DAO Sequence:
- Incremented by the node that owns the prefix for each new NA-DAO message for that prefix.
- Prefix:
- Variable-length field containing an IPv6 address or a prefix of an IPv6 address. The Prefix Length field contains the number of valid leading bits in the prefix. The bits in the prefix after the prefix length (if any) are reserved and MUST be 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 who adds on to the Reverse Route Stack will append to the list and increment the RRCount.
| TOC |
| TOC |
According to implementation specific policy, a subset or all of the feasible parents in the DAG may be selected to receive prefix information from the destination advertisement mechanism. This subset of DAG parents shall be designated the set of DA parents.
As NA-DAO messages for particular destinations move inwards along the DAG, a sequence counter is used to guarantee their freshness. The sequence counter is incremented by the source of the NA-DAO message (the node that owns the prefix, or learned the prefix via some other means), each time it issues a NA-DAO message for its prefix. Nodes who receive the NA-DAO message and, if scope allows, will be forwarding a NA-DAO message for the unmodified destination inwards along the DAG, will leave the sequence number unchanged. Intermediate nodes will check the sequence counter before processing a NA-DAO message, and if the DAO is unchanged (the sequence counter has not changed), then the NA-DAO message will be discarded without additional processing. Further, if the NA-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 NA-DAO message is discarded. A depth is also added for tracking purposes; the depth is incremented at each hop as the NA-DAO message is propagated up the DAG. Nodes who are storing routing state may use the depth to determine which possible next-hops for the destination are more optimal.
If destination advertisements are activated in the RA-DIO message as indicated by the `D' bit, the node sends unicast destination advertisements to its DA parents, and only accepts unicast destination advertisements from any nodes but those contained in the DA parent subset.
Every NA to a DA parent MAY contain one or more DAOs. Receiving a RA-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 DAG, and any connected prefixes). If the Destination Advertisement Supported (A) bit is set in the RA-DIO message for the DAG, then a destination advertisement is also sent to a DAG parent once it has been added to the DA parent set after a movement, or when the list of advertised prefixes has changed. Destination advertisements may also be scheduled for sending when the PathDigest of the RA-DIO message has changed, indicating that some aspect of the inwards paths along the DAG has been modified.
Destination advertisements may advertise positive (prefix is present) or negative (removed) NA-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 RA-DIO message) or by receiving a no-DAO. A no-DAO is a conveyed as a NA-DAO message with a DAO Lifetime of 0.
A node who is capable of recording the state information conveyed in a unicast NA-DAO message will do so upon receiving and processing the NA-DAO message, thus building up routing state concerning destinations below it in the DAG. If a node capable of recording state information receives a NA-DAO message containing a Reverse Route Stack, then the node knows that the NA-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 NA-DAO message information on.
A node who is unable to record the state information conveyed in the NA-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 NA-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 build up 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 some cases (called hybrid cases), some nodes along the path a destination advertisement follows inward along the DAG may store state and some may not. The destination advertisement mechanism allows for the provisioning of routing state such that when a packet is traversing outwards along the DAG, some nodes may be able to directly forward to the next hop, and other nodes may be able to specify a piecewise source route in order to bridge spans of stateless nodes within the path on the way to the desired destination.
In the case where no node is able to store any routing state as destination advertisements pass by, and the DAG root ends up with NA-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:
Note that nodes may receive multiple information from different neighbors for a specific destination, as different paths through the DAG may be propagating information inwards along the DAG for the same destination. A node who is recording routing state will keep track of the information from each neighbor independently, and when it comes time to propagate the NA-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.
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 NA-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 NA-DAO messages with zero lifetime (also called no-DAO) to the DA parents.
| TOC |
The destination advertisement mechanism requires 2 timers; the DelayNA timer and the RemoveTimer.
| TOC |
It is also possible for a node to multicast a NA-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 NA-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 NA-DAO message is not used for routing and does not presume any DAG relationship between the emitter and the receiver; it MUST NOT be used to relay information learned (e.g. information in the Reachable list) from another node; information obtained from a multicast NA-DAO MAY be installed in the routing table and MAY be propagated by a router in unicast NA-DAOs.
A node receiving a multicast NA-DAO message addressed to FF02::1 MAY install prefixes contained in the NA-DAO message in the routing table for local use. Such a node MUST NOT perform any other processing on the NA-DAO message (i.e. such a node does not presume it is a DA parent).
| TOC |
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 RA-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 NA-DAO message. If the NA-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 NA-DAO message via the link local address of the reporting neighbor and it SHOULD further propagate the information in a NA-DAO message.
The RA-DIO message from the DAG root is used to synchronize the whole DAG, including the periodic reporting of destination advertisements back up the DAG. Its period is expected to vary, depending on the configuration of the trickle timer that governs the RAs.
When a node receives a RA-DIO message over an LLN interface from a DA parent, the DelayNA is armed to force a full update.
When the node broadcasts a RA-DIO message on an LLN interface, for all entries on that interface:
Since the DelayNA timer has a duration that decreases with the depth, it is expected to receive all NA-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.
| TOC |
Finally, the destination advertisement mechanism responds to a series of events, such as:
| TOC |
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:
Consider a node M who is performing an aggregation, and a node N who is to be a member of the aggregation group. A node Z situated above the node M in the DAG, 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.
| TOC |
DEF_NA_LATENCY = To Be Determined
MAX_DESTROY_INTERVAL = To Be Determined
| TOC |
This section describes further the multicast routing operations over an IPv6 RPL network, and specifically how unicast NA-DAOs can be used to relay group registrations inwards. 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 root of the DAG, MLD requests are mapped and transported as NA-DAO messages within the RPL protocol; each hop coalesces the multiple requests for a same group as a single NA-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 NA-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 NA-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 NA-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 RPL DAG 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 outwards along the DAG based on the multicast routing table entries installed from the NA-DAO message.
For a source inside the DAG, the packet is passed to the preferred parents, and if that fails then to the alternates in the DAG. 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 DAG root has to further propagate the packet into the external infrastructure.
As a result, the DAG Root acts as an automatic proxy Rendez-vous 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 DAG is grounded or floating, the root can serve inner multicast streams at all times.
| TOC |
The selection of successors, along the default paths inward along the DAG, or along the paths learned from destination advertisements outward along the DAG, leads to the formation of routing adjacencies that require maintenance.
In IGPs such as OSPF [RFC4915] (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, “MANET Neighborhood Discovery Protocol (NHDP),” July 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.
| TOC |
When forwarding a packet to a destination, precedence is given to selection of a next-hop successor as follows:
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 who was the predecessor of the packet (split horizon), except in the case where it is intended for the packet to change from an inward to an outward flow, such as switching from DIO routes to DAO routes as the destination is neared.
| TOC |
- DIO Timer
- One instance per DAG that a node is a member of. Expiry triggers RA-DIO message transmission. Trickle timer with variable interval in [0, DIOIntervalMin..2^DIOIntervalDoublings]. See Section 5.3.4 (Trickle Timer for RA Transmission)
- DAG Hop Timer
- Up to one instance per candidate DAG parent in the `Held-Up' state per DAG that a node is going to jump to. Expiry triggers candidate DAG parent to become a DAG parent in the `Current' state, as well as cancellation of any other DAG Hop timers associated with other DAG parents for that DAG. Duration is computed based on the rank of the candidate DAG parent and DAG delay, as (candidates rank + random) * candidate's DAG_delay (where 0 <= random < 1). See Section 5.7.1 (Held-Up).
- Hold-Down Timer
- Up to one instance per candidate DAG parent in the `Held-Down' state per DAG. Expiry triggers the eviction of the candidate DAG parent from the candidate DAG parent set. The interval should be chosen as appropriate to prevent flapping. See Section 5.7 (Candidate DAG Parent States and Stability).
- DAG Heartbeat Timer
- Up to one instance per DAG that the node is acting as DAG root of. May not be supported in all implementations. Expiry triggers revision of DAGSequenceNumber, causing a new series of updated RA-DIO message to be sent. Interval should be chosen appropriate to propagation time of DAG and as appropriate to application requirements (e.g. response time vs. overhead). See Section 5.4 (DAG Heartbeat)
- DelayNA Timer
- Up to one instance per DA parent (the subset of DAG parents chosen to receive destination advertisements) per DAG. Expiry triggers sending of NA-DAO message to the DA parent. The interval is to be proportional to DEF_NA_LATENCY/(node rank), such that nodes of greater rank (further outward along the DAG) expire first, coordinating the sending of NA-DAO messages to allow for a chance of aggregation. See Section 5.9.2.1.1 (Destination Advertisement Timers)
- DestroyTimer
- Up to one instance per DA entry per neighbor (i.e. those neighbors who have given NA-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, RA_INTERVAL). See Section 5.9.2.1.1 (Destination Advertisement Timers)
| TOC |
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.
| TOC |
| TOC |
When a node is first powered up, it may either choose to stay silent and not send any multicast RA-DIO message until it has joined a DAG, or to immediately root a transient DAG and start sending multicast RA-DIO messages. A RPL implementation SHOULD allow configuring whether the node should stay silent or should start advertising RA-DIO messages.
Furthermore, the implementation SHOULD to allow configuring whether or not the node should start sending an RS message as an initial probe for nearby DAGs, or should simply wait until it received RA messages from other nodes that are part of existing DAGs.
| TOC |
RPL specifies a number of protocol parameters.
A RPL implementation SHOULD allow configuring the following routing protocol parameters, which are further described in Section 5.1.1 (DAG Information Option (DIO) base option):
- DAGPreference
- NodePreference
- DAGDelay
- DIOIntervalDoublings
- DIOIntervalMin:
- DAGObjectiveCodePoint
- PathDigest
- DAGID
- Destination Prefixes
- DAG Root behavior:
- In some cases, a node may not want to permanently act as a DAG root if it cannot join a grounded DAG. 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 Hop Timer:
- A RPL implementation MUST provide the ability to configure the value of the DAG Hop Timer, expressed in ms.
- 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.
| TOC |
A RPL implementation makes use of trickle timer to govern the sending of RA-DIO message. Such an algorithm is determined a by a set of configurable parameters that are then advertised by the DAG root along the DAG in RA-DIO messages.
For each DAG, a RPL implementation MUST allow for the monitoring of the following parameters, further described in Section 5.3.4 (Trickle Timer for RA 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.
| TOC |
A RPL implementation may allow by configuration at the DAG root to refresh the DAG 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.
| TOC |
The following set of parameters of the NA-DAO messages SHOULD be configurable:
| TOC |
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 DAG, 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 NA-DAO messages according to a set of rules defined by policy.
| TOC |
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.
| TOC |
The information and data models necessary for the operation of RPL will be defined in a separate document specifying the RPL SNMP MIB.
| TOC |
The aim of this section is to describe the various RPL mechanisms specified to monitor the protocol.
As specified in Section 5.2 (Conceptual Data Structures), an implementation must maintain a set of data structures in support of DAG discovery:
| TOC |
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.
| TOC |
For each DAG, a RPL implementation MUST keep track of the following DAG table values:
The set of candidate DAG parents structure is itself a table with the following entries:
| TOC |
To be completed.
| TOC |
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 RA-DIO message along with the neighbor identification if avialable.
| TOC |
A RPL implementation operating on a DAG root MUST allow for the configuration of the following trickle parameters:
A RPL implementation MAY provide a counter reporting the number of times an inconsistency (and thus the trickle timer has been reset).
| TOC |
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.
| TOC |
RPL does not have any impact on the operation of existing protocols.
| TOC |
To be completed.
| TOC |
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.).
| TOC |
| TOC |
The DAG Information Option is a container option carried within an IPv6 Router Advertisement message as defined in [RFC4861] (Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” September 2007.), which might contain a number of suboptions. The base option regroups the minimum information set that is mandatory in all cases.
IANA had defined the IPv6 Neighbor Discovery Option Formats registry. The suggested type value for the DAG Information Option (DIO) Base Option is 140, to be confirmed by IANA.
| TOC |
IANA is requested to create a registry for the Flag field of the DIO Base Option.
New bit numbers may be allocated only by an IETF Consensus action. Each bit should be tracked with the following qualities:
Three flags are currently defined:
| Bit | Description | Reference |
|---|---|---|
| 0 | Grounded DAG | This document |
| 1 | Destination Advertisement Trigger | This document |
| 2 | Destination Advertisement Supported | This document |
| DIO Base Option Flags |
| TOC |
IANA is requested to create a registry for the DIO Base Option Suboptions
| Value | Meaning | Reference |
|---|---|---|
| 0 | Pad1 - DIO Padding | This document |
| 1 | PadN - DIO suboption padding | This document |
| 2 | DAG Metric Container | This Document |
| 3 | Destination Prefix | This Document |
| DAG Information Option (DIO) Base Option Suboptions |
| TOC |
The RPL protocol extends Neighbor Discovery [RFC4861] (Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” September 2007.) and [RFC4191] (Draves, R. and D. Thaler, “Default Router Preferences and More-Specific Routes,” November 2005.) to allow a node to include a Destination Advertisement Option, which includes prefix information in the Neighbor Advertisements messages. The Neighbor Advertisement messages are augmented with the Destination Advertisement Option (DAO).
IANA had defined the IPv6 Neighbor Discovery Option Formats registry. The suggested type value for the Destination Advertisement Option carried within a Neighbor Advertisement message is 141, to be confirmed by IANA.
| TOC |
This specification requests that an Objective Code Point registry, as to be 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.), reserve the Objective Code Point value 0x0000, for the purposes designated as OCP 0 in this document.
| TOC |
The ROLL Design Team would like to acknowledge the review, feedback, and comments from Dominique Barthel, Yusuf Bashir, Mathilde Durvy, Manhar Goindi, Mukul Goyal, Quentin Lampin, Philip Levis, Jerry Martocci, Alexandru Petrescu, and Don Sturek.
The ROLL Design Team would like to acknowledge the guidance and input provided by the ROLL Chairs, David Culler and JP Vasseur.
The ROLL Design Team would like to acknowledge prior contributions of 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.
| TOC |
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/ 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
| TOC |
| TOC |
| [RFC2119] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| TOC |
| [I-D.ietf-bfd-base] | Katz, D. and D. Ward, “Bidirectional Forwarding Detection,” draft-ietf-bfd-base-09 (work in progress), February 2009 (TXT). |
| [I-D.ietf-manet-nhdp] | Clausen, T., Dearlove, C., and J. Dean, “MANET Neighborhood Discovery Protocol (NHDP),” draft-ietf-manet-nhdp-10 (work in progress), July 2009 (TXT). |
| [I-D.ietf-roll-building-routing-reqs] | Martocci, J., Riou, N., Mil, P., and W. Vermeylen, “Building Automation Routing Requirements in Low Power and Lossy Networks,” draft-ietf-roll-building-routing-reqs-07 (work in progress), September 2009 (TXT). |
| [I-D.ietf-roll-home-routing-reqs] | Brandt, A., Buron, J., and G. Porcu, “Home Automation Routing Requirements in Low Power and Lossy Networks,” draft-ietf-roll-home-routing-reqs-08 (work in progress), September 2009 (TXT). |
| [I-D.ietf-roll-indus-routing-reqs] | Networks, D., Thubert, P., Dwars, S., and T. Phinney, “Industrial Routing Requirements in Low Power and Lossy Networks,” draft-ietf-roll-indus-routing-reqs-06 (work in progress), June 2009 (TXT). |
| [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,” draft-ietf-roll-routing-metrics-06 (work in progress), April 2010 (TXT). |
| [I-D.ietf-roll-terminology] | Vasseur, J., “Terminology in Low power And Lossy Networks,” draft-ietf-roll-terminology-03 (work in progress), March 2010 (TXT). |
| [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,” draft-tsao-roll-security-framework-02 (work in progress), March 2010 (TXT). |
| [Levis08] | Levis, P., Brewer, E., Culler, D., Gay, D., Madden, S., Patel, N., Polastre, J., Shenker, S., Szewczyk, R., and A. Woo, “The Emergence of a Networking Primitive in Wireless Sensor Networks,” Communications of the ACM, v.51 n.7, July 2008 (HTML). |
| [RFC2453] | Malkin, G., “RIP Version 2,” STD 56, RFC 2453, November 1998 (TXT, HTML, XML). |
| [RFC3819] | Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. Wood, “Advice for Internet Subnetwork Designers,” BCP 89, RFC 3819, July 2004 (TXT). |
| [RFC4101] | Rescorla, E. and IAB, “Writing Protocol Models,” RFC 4101, June 2005 (TXT). |
| [RFC4191] | Draves, R. and D. Thaler, “Default Router Preferences and More-Specific Routes,” RFC 4191, November 2005 (TXT). |
| [RFC4461] | Yasukawa, S., “Signaling Requirements for Point-to-Multipoint Traffic-Engineered MPLS Label Switched Paths (LSPs),” RFC 4461, April 2006 (TXT). |
| [RFC4861] | Narten, T., Nordmark, E., Simpson, W., and H. Soliman, “Neighbor Discovery for IP version 6 (IPv6),” RFC 4861, September 2007 (TXT). |
| [RFC4875] | Aggarwal, R., Papadimitriou, D., and S. Yasukawa, “Extensions to Resource Reservation Protocol - Traffic Engineering (RSVP-TE) for Point-to-Multipoint TE Label Switched Paths (LSPs),” RFC 4875, May 2007 (TXT). |
| [RFC4915] | Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P. Pillay-Esnault, “Multi-Topology (MT) Routing in OSPF,” RFC 4915, June 2007 (TXT). |
| [RFC5120] | Przygienda, T., Shen, N., and N. Sheth, “M-ISIS: Multi Topology (MT) Routing in Intermediate System to Intermediate Systems (IS-ISs),” RFC 5120, February 2008 (TXT). |
| [RFC5548] | Dohler, M., Watteyne, T., Winter, T., and D. Barthel, “Routing Requirements for Urban Low-Power and Lossy Networks,” RFC 5548, May 2009 (TXT). |
| TOC |
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.
| TOC |
Consider the example LLN physical topology in Figure 8 (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 9 (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 RAs containing DIO, as governed by a trickle timer. Nodes (11), (12), (13), have selected (LBR) as their only parent, attached to the DAG at rank 2, and periodically advertise RA-DIO multicasts. 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.
| Figure 8: Example LLN Topology |
(LBR)
/ | \
.---` | `----.
/ | \
(11) (12) (13)
| \ | \ | \
| `----. | `----. | `----.
| \| \| \
(21) (22) (23) (24)
| /| /| |
| .----` | .----` | |
| / | / | |
(31) (32) (33) (34)
| /| \ | \ | \
| .----` | `----. | `----. | `----.
| / | \| \| \
.--------(41) (42) (43) (44) (45)
/ / /| \ | \
.----` .----` .----` | `----. | `----.
/ / / | \| \
(51) (52) (53) (54) (55) (56)
Note that the links depicted represent directed links in the DAG overlaid on top of the physical topology depicted in Figure 8 (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.
| Figure 9: Example DAG |
| TOC |
Consider node (56) in the example of Figure 8 (Example LLN Topology). In the unmodified example, node (56) is at rank 6 with one DAG parent, {(43)}, and one sibling (55). Suppose, for example, that node (56) wished to expand its DAG parent set to contain node (55), as {(43), (55)}. Such a change would require node (56) to detach from the DAG, to defer reattachment until a loop avoidance algorithm has completed, and to then reattach to the DAG with {(43), (55)} as it's DAG parents. When node (56) detaches from the DAG, it is able to act as the root of its own floating DAG and establish its frozen sub-DAG (which is empty). Node (56) can then observe that Node (55) is still attached to the original DAG, that its sequence number is able to increment, and deduce that Node (55) is safely not behind Node (56). There is then little change for a loop, and Node (56) may safely reattach to the DAG, with parents {(43), (55)}. At reattachment time, node (56) would present itself with a rank deeper than that of its deepest DAG parent (node (55) at rank 6), rank 7.
| TOC |
Consider the example of Figure 8 (Example LLN Topology) when link (13)-(24) goes down.
| TOC |
Consider the example of Figure 8 (Example LLN Topology) when link (12)-(42) appears.
| TOC |
Consider the example of Figure 8 (Example LLN Topology) when node (41) disappears.
| TOC |
Consider the example of Figure 8 (Example LLN Topology) when a new LBR, (LBR2) appears, with connectivity (LBR2)-(52), (LBR2)-(53).
| TOC |
Consider the example DAG depicted in Figure 9 (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).
| TOC |
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 IPv6 Router Solicitation (RS) messages to probe for nearby DAGs.
| TOC |
: : :
: : :
(A) (A) (A)
|\ | |
| `-----. | |
| \ | |
(B) (C) (B) (C) (B)
| | \
| | `-----.
| | \
(D) (D) (C)
|
|
|
(D)
-1- -2- -3-
| Figure 10: DAG Maintenance |
Consider the example depicted in Figure 10 (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 RA-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:
| TOC |
(A) (A) (A)
|\ |\ |\
| `-----. | `-----. | `-----.
| \ | \ | \
(B) (C) (B) \ | (C)
\ | | /
`-----. | | .-----`
\| |/
(C) (B)
-1- -2- -3-
| Figure 11: Greedy DAG Parent Selection |
Consider the example depicted in Figure 11 (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 11 (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 11 (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 11 (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 11 (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.
| TOC |
:
:
(A) (D)
| |
| |
| |
(B) (E)
| |
| |
| |
(C) (F)
| Figure 12: Merging DAGs |
Consider the example depicted in Figure 12 (Merging DAGs). Nodes (A), (B), and (C) are part of some larger grounded DAG, where Node (A) is at a rank of d, Node (B) at d+1, and Node (C) at d+2. The DAG comprised of Nodes (D), (E), and (F) is a floating, less preferred, DAG, with Node (D) as the DAG root. This floating DAG may have been formed, for example, in the absence of a grounded DAG or when Node (D) had to detach from a grounded DAG and (E) and (F) followed. All nodes are using compatible objective code points.
Nodes (D), (E), and (F) would rather join the more preferred grounded DAG if they are able than to remain in the less preferred floating DAG.
Next, let links (C)--(D) and (A)--(E) become viable. The following sequence of events may then occur in a typical case:
| TOC |
Consider the expanded example LLN physical topology in Figure 13 (Expanded LLN Topology). In this example an additional LBR is added. Suppose that all nodes are configured with an implementation specific policy function that aims to minimize the number of hops, and that both LBRs are configured to root different DAGIDs. We may now walk through the formation of the two DAGs.
(LBR) (LBR2)
/ | \ / \
.---` | `----. / \
/ | \ | |
(11)------(12)------(13) (14) (15)
| \ | \ | \ | /|
| `----. | `----. | `----. | .----` |
| \| \| \| / |
(21)------(22)------(23) (24) (25)
| /| /| | / /
| .----` | .----` | .-----]|[------` /
| / | / | / | /
(31)------(32)------(33)------(34)-----`
| /| \ | \ | \
| .----` | `----. | `----. | `----.
| / | \| \| \
.--------(41) (42) (43)------(44)------(45)
/ / /| \ | \
.----` .----` .----` | `----. | `----.
/ / / | \| \
(51)------(52)------(53)------(54)------(55)------(56)
| Figure 13: Expanded LLN Topology |
(LBR) (LBR2)
/ | \ / \
.---` | `----. / \
/ | \ | |
(11) (12) (13) (14) (15)
(21) (22) (23) (24) (25)
(31) (32) (33) (34)
(41) (42) (43) (44) (45)
(51) (52) (53) (54) (55) (56)
| Figure 14: DAG Construction Step 1 |
(LBR) (LBR2)
/ | \ / \
.---` | `----. / \
/ | \ | |
(11) (12) (13) (14) (15)
| \ | \ | | /|
| `----. | `----. | | .----` |
| \| \| | / |
(21) (22) (23) (24) (25)
(31) (32) (33) (34)
(41) (42) (43) (44) (45)
(51) (52) (53) (54) (55) (56)
| Figure 15: DAG Construction Step 2 |
(LBR) (LBR2)
/ | \ / \
.---` | `----. / \
/ | \ | |
(11) (12) (13) (14) (15)
| \ | \ | | /|
| `----. | `----. | | .----` |
| \| \| | / |
(21) (22) (23) (24) (25)
| /| / | / /
| .----` | .----` .-----]|[------` /
| / | / / | /
(31) (32) (33) (34)-----`
(41) (42) (43) (44) (45)
(51) (52) (53) (54) (55) (56)
| Figure 16: DAG Construction Step 3 |
(LBR) (LBR2)
/ | \ / \
.---` | `----. / \
/ | \ | |
(11) (12) (13) (14) (15)
| \ | \ | | /|
| `----. | `----. | | .----` |
| \| \| | / |
(21) (22) (23) (24) (25)
| /| / | / /
| .----` | .----` .-----]|[------` /
| / | / / | /
(31) (32) (33) (34)-----`
| /| | \ | \
| .----` | | `----. | `----.
| / | | \| \
(41) (42) (43) (44) (45)
(51) (52) (53) (54) (55) (56)
| Figure 17: DAG Construction Step 4 |
(LBR) (LBR2)
/ | \ / \
.---` | `----. / \
/ | \ | |
(11) (12) (13) (14) (15)
| \ | \ | | /|
| `----. | `----. | | .----` |
| \| \| | / |
(21) (22) (23) (24) (25)
| /| / | / /
| .----` | .----` .-----]|[------` /
| / | / / | /
(31) (32) (33) (34)-----`
| /| | \ | \
| .----` | | `----. | `----.
| / | | \| \
.--------(41) (42) (43) (44) (45)
/ / /| | \
.----` .----` .----` | | `----.
/ / / | | \
(51) (52) (53) (54) (55) (56)
| Figure 18: DAG Construction Step 5 |
| TOC |
This section enumerates some outstanding issues that are to be addressed in future revisions of the RPL specification.
| TOC |
In some situations the baseline mechanism to support arbitrary P2P traffic, by flowing inward along the DAG until a common parent is reached and then flowing outward, may not be suitable for all application scenarios. A related scenario may occur when the outward paths setup along the DAG by the destination advertisement mechanism are not be the most desirable outward paths for the specific application scenario (in part because the DAG links may not be symmetric). It may be desired to support within RPL the discovery and installation of more direct routes `across' the DAG. Such mechanisms need to be investigated.
| TOC |
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.
| TOC |
When NA-DAO messages are relayed to more than one DAG parent, in some cases a situation may be created where a large number of NA-DAO messages conveying information about the same destination flow inward along the DAG. It is desirable to bound/limit the multiplication/fan-out of NA-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.
| TOC |
In support of nodes who maintain minimal routing state, and to make use of the collection of piecewise source routes from the destination advertisement mechanism, there needs to be some investigation of a mechanism to specify, attach, and follow source routes for packets traversing the LLN.
| TOC |
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.
| TOC |
| Tim Winter (editor) | |
| Email: | wintert@acm.org |
| Pascal Thubert (editor) | |
| Cisco Systems | |
| Village d'Entreprises Green Side | |
| 400, Avenue de Roumanille | |
| Batiment T3 | |
| Biot - Sophia Antipolis 06410 | |
| FRANCE | |
| Phone: | +33 497 23 26 34 |
| Email: | pthubert@cisco.com |
| ROLL Design Team | |
| IETF ROLL WG | |
| Email: | dtroll@external.cisco.com |