draft-ietf-roll-rpl-03.txt   draft-ietf-roll-rpl-04.txt 
Networking Working Group T. Winter, Ed. Networking Working Group T. Winter, Ed.
Internet-Draft Internet-Draft
Intended status: Standards Track P. Thubert, Ed. Intended status: Standards Track P. Thubert, Ed.
Expires: April 7, 2010 Cisco Systems Expires: April 29, 2010 Cisco Systems
ROLL Design Team ROLL Design Team
IETF ROLL WG IETF ROLL WG
October 4, 2009 October 26, 2009
RPL: Routing Protocol for Low Power and Lossy Networks RPL: IPv6 Routing Protocol for Low power and Lossy Networks
draft-ietf-roll-rpl-03 draft-ietf-roll-rpl-04
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Abstract Abstract
Low Power and Lossy Networks (LLNs) are made largely of constrained Low power and Lossy Networks (LLNs) are a class of network in which
nodes (with limited processing power, memory, and sometimes energy both the routers and their interconnect are constrained: LLN routers
when they are battery operated). These routers are interconnected by typically operate with constraints on (any subset of) processing
lossy links, most of the time supporting only low data rates, that power, memory and energy (battery), and their interconnects are
are usually fairly unstable with relatively low packet delivery characterized by (any subset of) high loss rates, low data rates and
rates. Another characteristic of such networks is that the traffic instability. LLNs are comprised of anything from a few dozen and up
patterns are not simply unicast, but in many cases point-to- to thousands of LLN routers, and support point-to- point traffic
multipoint or multipoint-to-point. Furthermore such networks may (between devices inside the LLN), point-to-multipoint traffic (from a
potentially comprise a large number of nodes, up to several dozens or central control point to a subset of devices inside the LLN) and
hundreds or more nodes in the network. These characteristics offer multipoint-to- point traffic (from devices inside the LLN towards a
unique challenges to a routing solution: the IETF ROLL Working Group central control point). This document specifies the IPv6 Routing
has defined application-specific routing requirements for a Low Power Protocol for LLNs (RPL), which provides a mechanism whereby
and Lossy Network (LLN) routing protocol. This document specifies multipoint-to-point traffic from devices inside the LLN towards a
the Routing Protocol for Low Power and Lossy Networks (RPL). central control point, as well as point-to-multipoint traffic from
the central control point to the devices inside the LLN, is
Requirements Language supported. Support for point-to-point traffic is also available.
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 [RFC2119].
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1. Design Principles . . . . . . . . . . . . . . . . . . . . 6 1.1. Design Principles . . . . . . . . . . . . . . . . . . . . 6
1.2. Expectations of Link Layer Behavior . . . . . . . . . . . 7 1.2. Expectations of Link Layer Type . . . . . . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Protocol Model . . . . . . . . . . . . . . . . . . . . . . . . 9 3. Protocol Model . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Protocol Properties Overview . . . . . . . . . . . . . . . 9 3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1. IPv6 Architecture . . . . . . . . . . . . . . . . . . 9 3.1.1. Topology Instance and Objectives . . . . . . . . . . . 9
3.1.2. Typical LLN Traffic Patterns . . . . . . . . . . . . . 10 3.1.2. Multipoint-to-Point Traffic Flows and DAGs . . . . . . 11
3.1.3. Constraint Based Routing . . . . . . . . . . . . . . . 10 3.1.3. Point-to-Multipoint Traffic Flows . . . . . . . . . . 11
3.2. Protocol Operation . . . . . . . . . . . . . . . . . . . . 10 3.1.4. Point-to-Point Traffic Flows . . . . . . . . . . . . . 12
3.2. Protocol Operation . . . . . . . . . . . . . . . . . . . . 12
3.2.1. DAG Construction . . . . . . . . . . . . . . . . . . . 12 3.2.1. DAG Construction . . . . . . . . . . . . . . . . . . . 12
3.2.2. Destination Advertisement . . . . . . . . . . . . . . 19 3.2.2. Destination Advertisement . . . . . . . . . . . . . . 15
3.3. Loop Avoidance and Stability . . . . . . . . . . . . . . . 21 3.3. Loop Avoidance and Stability . . . . . . . . . . . . . . . 17
3.3.1. Greediness and Rank-based Instabilities . . . . . . . 22 3.3.1. Greediness and Rank-based Instabilities . . . . . . . 17
3.3.2. Merging DAGs . . . . . . . . . . . . . . . . . . . . . 22 3.3.2. DAG Loops . . . . . . . . . . . . . . . . . . . . . . 18
3.3.3. DAG Loops . . . . . . . . . . . . . . . . . . . . . . 23 3.3.3. DAO Loops . . . . . . . . . . . . . . . . . . . . . . 18
3.3.4. DAO Loops . . . . . . . . . . . . . . . . . . . . . . 23 3.3.4. Sibling Loops . . . . . . . . . . . . . . . . . . . . 18
3.3.5. Sibling Loops . . . . . . . . . . . . . . . . . . . . 23 4. Routing Metrics and Constraints Used By RPL . . . . . . . . . 18
3.4. Local and Temporary Routing Decision . . . . . . . . . . . 24 5. RPL Protocol Specification . . . . . . . . . . . . . . . . . . 19
3.5. Maintenance of Routing Adjacency . . . . . . . . . . . . . 25 5.1. RPL Messages . . . . . . . . . . . . . . . . . . . . . . . 19
4. Constraint Based Routing in LLNs . . . . . . . . . . . . . . . 25 5.1.1. ICMPv6 RPL Control Message . . . . . . . . . . . . . . 19
4.1. Routing Metrics . . . . . . . . . . . . . . . . . . . . . 25 5.1.2. DAG Information Solicitation (DIS) . . . . . . . . . . 20
4.2. Routing Constraints . . . . . . . . . . . . . . . . . . . 26 5.1.3. DAG Information Object (DIO) . . . . . . . . . . . . . 20
4.3. Constraint Based Routing . . . . . . . . . . . . . . . . . 26 5.1.4. Destination Advertisement Object (DAO) . . . . . . . . 27
5. RPL Protocol Specification . . . . . . . . . . . . . . . . . . 27 5.2. Conceptual Data Structures . . . . . . . . . . . . . . . . 28
5.1. DAG Information Option . . . . . . . . . . . . . . . . . . 27 5.2.1. Candidate Neighbors Data Structure . . . . . . . . . . 28
5.1.1. DAG Information Option (DIO) base option . . . . . . . 27 5.2.2. Directed Acyclic Graphs (DAGs) Data Structure . . . . 29
5.2. Conceptual Data Structures . . . . . . . . . . . . . . . . 34 5.3. DAG Rank . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.2.1. Candidate Neighbors Data Structure . . . . . . . . . . 34 5.4. DAG Discovery and Maintenance . . . . . . . . . . . . . . 31
5.2.2. Directed Acyclic Graphs (DAGs) Data Structure . . . . 35 5.4.1. DAG Discovery Rules . . . . . . . . . . . . . . . . . 32
5.3. DAG Discovery and Maintenance . . . . . . . . . . . . . . 36 5.4.2. Reception and Processing of DIO messages . . . . . . . 36
5.3.1. DAG Discovery Rules . . . . . . . . . . . . . . . . . 37 5.4.3. DIO Transmission . . . . . . . . . . . . . . . . . . . 38
5.3.2. Reception and Processing of RA-DIO messages . . . . . 39 5.4.4. Trickle Timer for DIO Transmission . . . . . . . . . . 39
5.3.3. RA-DIO Transmission . . . . . . . . . . . . . . . . . 41 5.5. DAG Sequence Number Increment . . . . . . . . . . . . . . 40
5.3.4. Trickle Timer for RA Transmission . . . . . . . . . . 42 5.6. DAG Selection . . . . . . . . . . . . . . . . . . . . . . 41
5.4. DAG Heartbeat . . . . . . . . . . . . . . . . . . . . . . 44 5.7. Administrative rank . . . . . . . . . . . . . . . . . . . 41
5.5. DAG Selection . . . . . . . . . . . . . . . . . . . . . . 44 5.8. Collision . . . . . . . . . . . . . . . . . . . . . . . . 42
5.6. Administrative rank . . . . . . . . . . . . . . . . . . . 45 5.9. Guidelines for Objective Functions . . . . . . . . . . . . 42
5.7. Candidate DAG Parent States and Stability . . . . . . . . 45 5.9.1. Objective Function . . . . . . . . . . . . . . . . . . 42
5.7.1. Held-Up . . . . . . . . . . . . . . . . . . . . . . . 45 5.9.2. Objective Function 0 (OF0) . . . . . . . . . . . . . . 44
5.7.2. Held-Down . . . . . . . . . . . . . . . . . . . . . . 46 5.10. Establishing Routing State Outward Along the DAG . . . . . 46
5.7.3. Collision . . . . . . . . . . . . . . . . . . . . . . 46 5.10.1. Destination Advertisement Operation . . . . . . . . . 47
5.7.4. Instability . . . . . . . . . . . . . . . . . . . . . 47 5.11. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 54
5.8. Guidelines for Objective Code Points . . . . . . . . . . . 48 5.11.1. Host Basic Operation . . . . . . . . . . . . . . . . . 55
5.8.1. Objective Function . . . . . . . . . . . . . . . . . . 48 5.11.2. Instance Forwarding . . . . . . . . . . . . . . . . . 55
5.8.2. Objective Code Point 0 (OCP 0) . . . . . . . . . . . . 50 5.11.3. DAG Inconsistency Loop Detection . . . . . . . . . . . 56
5.11.4. Sibling Loop Avoidance . . . . . . . . . . . . . . . . 56
5.9. Establishing Routing State Outward Along the DAG . . . . . 52 5.11.5. DAO Inconsistency Loop Detection and Recovery . . . . 57
5.9.1. Destination Advertisement Message Formats . . . . . . 53 5.12. Multicast Operation . . . . . . . . . . . . . . . . . . . 57
5.9.2. Destination Advertisement Operation . . . . . . . . . 55 5.13. Maintenance of Routing Adjacency . . . . . . . . . . . . . 58
5.10. Multicast Operation . . . . . . . . . . . . . . . . . . . 62 5.14. Packet Forwarding . . . . . . . . . . . . . . . . . . . . 59
5.11. Maintenance of Routing Adjacency . . . . . . . . . . . . . 63 6. RPL Constants and Variables . . . . . . . . . . . . . . . . . 60
5.12. Packet Forwarding . . . . . . . . . . . . . . . . . . . . 64 7. Manageability Considerations . . . . . . . . . . . . . . . . . 61
6. RPL Variables . . . . . . . . . . . . . . . . . . . . . . . . 65 7.1. Control of Function and Policy . . . . . . . . . . . . . . 61
7. Manageability Considerations . . . . . . . . . . . . . . . . . 66 7.1.1. Initialization Mode . . . . . . . . . . . . . . . . . 61
7.1. Control of Function and Policy . . . . . . . . . . . . . . 66 7.1.2. DIO Base option . . . . . . . . . . . . . . . . . . . 61
7.1.1. Initialization Mode . . . . . . . . . . . . . . . . . 66 7.1.3. Trickle Timers . . . . . . . . . . . . . . . . . . . . 62
7.1.2. DIO Base option . . . . . . . . . . . . . . . . . . . 66 7.1.4. DAG Sequence Number Increment . . . . . . . . . . . . 63
7.1.3. Trickle Timers . . . . . . . . . . . . . . . . . . . . 67 7.1.5. Destination Advertisement Timers . . . . . . . . . . . 63
7.1.4. DAG Heartbeat . . . . . . . . . . . . . . . . . . . . 68 7.1.6. Policy Control . . . . . . . . . . . . . . . . . . . . 63
7.1.5. The Destination Advertisement Option . . . . . . . . . 68 7.1.7. Data Structures . . . . . . . . . . . . . . . . . . . 63
7.1.6. Policy Control . . . . . . . . . . . . . . . . . . . . 68 7.2. Information and Data Models . . . . . . . . . . . . . . . 64
7.1.7. Data Structures . . . . . . . . . . . . . . . . . . . 68 7.3. Liveness Detection and Monitoring . . . . . . . . . . . . 64
7.2. Information and Data Models . . . . . . . . . . . . . . . 69 7.3.1. Candidate Neighbor Data Structure . . . . . . . . . . 64
7.3. Liveness Detection and Monitoring . . . . . . . . . . . . 69 7.3.2. Directed Acyclic Graph (DAG) Table . . . . . . . . . . 64
7.3.1. Candidate Neighbor Data Structure . . . . . . . . . . 69 7.3.3. Routing Table . . . . . . . . . . . . . . . . . . . . 65
7.3.2. Directed Acyclic Graph (DAG) Table . . . . . . . . . . 69 7.3.4. Other RPL Monitoring Parameters . . . . . . . . . . . 65
7.3.3. Routing Table . . . . . . . . . . . . . . . . . . . . 70 7.3.5. RPL Trickle Timers . . . . . . . . . . . . . . . . . . 66
7.3.4. Other RPL Monitoring Parameters . . . . . . . . . . . 70 7.4. Verifying Correct Operation . . . . . . . . . . . . . . . 66
7.3.5. RPL Trickle Timers . . . . . . . . . . . . . . . . . . 70
7.4. Verifying Correct Operation . . . . . . . . . . . . . . . 71
7.5. Requirements on Other Protocols and Functional 7.5. Requirements on Other Protocols and Functional
Components . . . . . . . . . . . . . . . . . . . . . . . . 71 Components . . . . . . . . . . . . . . . . . . . . . . . . 66
7.6. Impact on Network Operation . . . . . . . . . . . . . . . 71 7.6. Impact on Network Operation . . . . . . . . . . . . . . . 66
8. Security Considerations . . . . . . . . . . . . . . . . . . . 71 8. Security Considerations . . . . . . . . . . . . . . . . . . . 66
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 71 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 66
9.1. DAG Information Option (DIO) Base Option . . . . . . . . . 71 9.1. RPL Control Message . . . . . . . . . . . . . . . . . . . 66
9.2. New Registry for the Flag Field of the DIO Base Option . . 71 9.2. New Registry for RPL Control Codes . . . . . . . . . . . . 67
9.3. DAG Information Option (DIO) Suboption . . . . . . . . . . 72 9.3. New Registry for the Control Field of the DIO Base
9.4. Destination Advertisement Option (DAO) Option . . . . . . 72 Option . . . . . . . . . . . . . . . . . . . . . . . . . . 67
9.5. Objective Code Point . . . . . . . . . . . . . . . . . . . 72 9.4. DAG Information Object (DIO) Suboption . . . . . . . . . . 68
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 73 9.5. Objective Code Point for the Default Objective
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 73 Function OF0 . . . . . . . . . . . . . . . . . . . . . . . 68
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 74 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 68
12.1. Normative References . . . . . . . . . . . . . . . . . . . 74 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 69
12.2. Informative References . . . . . . . . . . . . . . . . . . 74 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 70
Appendix A. Deferred Requirements . . . . . . . . . . . . . . . . 76 12.1. Normative References . . . . . . . . . . . . . . . . . . . 70
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 77 12.2. Informative References . . . . . . . . . . . . . . . . . . 71
B.1. Moving Down a DAG . . . . . . . . . . . . . . . . . . . . 78 Appendix A. Requirements . . . . . . . . . . . . . . . . . . . . 72
B.2. Link Removed . . . . . . . . . . . . . . . . . . . . . . . 79 A.1. Protocol Properties Overview . . . . . . . . . . . . . . . 72
B.3. Link Added . . . . . . . . . . . . . . . . . . . . . . . . 79 A.1.1. IPv6 Architecture . . . . . . . . . . . . . . . . . . 73
B.4. Node Removed . . . . . . . . . . . . . . . . . . . . . . . 80 A.1.2. Typical LLN Traffic Patterns . . . . . . . . . . . . . 73
B.5. New LBR Added . . . . . . . . . . . . . . . . . . . . . . 80 A.1.3. Constraint Based Routing . . . . . . . . . . . . . . . 73
B.6. Destination Advertisement . . . . . . . . . . . . . . . . 81 A.2. Deferred Requirements . . . . . . . . . . . . . . . . . . 74
B.7. Example: DAG Parent Selection . . . . . . . . . . . . . . 82 Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 74
B.8. Example: DAG Maintenance . . . . . . . . . . . . . . . . . 83 B.1. Destination Advertisement . . . . . . . . . . . . . . . . 76
B.9. Example: Greedy Parent Selection and Instability . . . . . 84 B.2. Example: DAG Parent Selection . . . . . . . . . . . . . . 77
B.10. Example: DAG Merge . . . . . . . . . . . . . . . . . . . . 86 B.3. Example: DAG Maintenance . . . . . . . . . . . . . . . . . 78
Appendix C. Additional Examples . . . . . . . . . . . . . . . . . 88 B.4. Example: Greedy Parent Selection and Instability . . . . . 79
Appendix D. Outstanding Issues . . . . . . . . . . . . . . . . . 92 Appendix C. Outstanding Issues . . . . . . . . . . . . . . . . . 81
D.1. Additional Support for P2P Routing . . . . . . . . . . . . 92 C.1. Additional Support for P2P Routing . . . . . . . . . . . . 81
D.2. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 92 C.2. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 81
D.3. Destination Advertisement / DAO Fan-out . . . . . . . . . 92 C.3. Destination Advertisement / DAO Fan-out . . . . . . . . . 81
D.4. Source Routing . . . . . . . . . . . . . . . . . . . . . . 92 C.4. Source Routing . . . . . . . . . . . . . . . . . . . . . . 82
D.5. Address / Header Compression . . . . . . . . . . . . . . . 93 C.5. Address / Header Compression . . . . . . . . . . . . . . . 82
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 93 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 82
1. Introduction 1. Introduction
Low Power and Lossy Networks (LLNs) are made largely of constrained Low power and Lossy Networks (LLNs) are made largely of constrained
nodes (with limited processing power, memory, and sometimes energy nodes (with limited processing power, memory, and sometimes energy
when they are battery operated). These routers are interconnected by when they are battery operated). These routers are interconnected by
lossy links, most of the time supporting only low data rates, that lossy links, typically time supporting only low data rates, that are
are usually fairly unstable with relatively low packet delivery usually unstable with relatively low packet delivery rates. Another
rates. Another characteristic of such networks is that the traffic characteristic of such networks is that the traffic patterns are not
patterns are not simply unicast, but in many cases point-to- simply unicast, but in many cases point-to-multipoint or multipoint-
multipoint or multipoint-to-point. Furthermore such networks may to-point. Furthermore such networks may potentially comprise up to
potentially comprise a large number of nodes, up to several dozens or thousands of nodes. These characteristics offer unique challenges to
hundreds or more nodes in the network. These characteristics offer a routing solution: the IETF ROLL Working Group has defined
unique challenges to a routing solution: the IETF ROLL Working Group application-specific routing requirements for a Low power and Lossy
has defined application-specific routing requirements for a Low Power Network (LLN) routing protocol, specified in
and Lossy Network (LLN) routing protocol, specified in
[I-D.ietf-roll-building-routing-reqs], [I-D.ietf-roll-building-routing-reqs],
[I-D.ietf-roll-home-routing-reqs], [I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548]. This
[I-D.ietf-roll-indus-routing-reqs], and [RFC5548]. This document document specifies the IPv6 Routing Protocol for Low power and Lossy
specifies the Routing Protocol for Low Power and Lossy Networks Networks (RPL).
(RPL).
1.1. Design Principles 1.1. Design Principles
RPL was designed with the objective to meet the requirements spelled RPL was designed with the objective to meet the requirements spelled
out in [I-D.ietf-roll-building-routing-reqs], out in [I-D.ietf-roll-building-routing-reqs],
[I-D.ietf-roll-home-routing-reqs], [I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548]. Because
[I-D.ietf-roll-indus-routing-reqs], and [RFC5548]. Because those those requirements are heterogeneous and sometimes incompatible in
requirements are heterogeneous and sometimes incompatible in nature, nature, the approach is first taken to design a protocol capable of
the approach is first taken to design a protocol capable of
supporting a core set of functionalities corresponding to the supporting a core set of functionalities corresponding to the
intersection of the requirements. (Note: it is intended that as this intersection of the requirements. (Note: it is intended that as this
design evolves optional features may be added to address some design evolves optional features may be added to address some
application specific requirements). This is a key protocol design application specific requirements). This is a key protocol design
decision providing a granular approach in order to restrict the core decision providing a granular approach in order to restrict the core
of the protocol to a minimal set of functionalities, and to allow of the protocol to a minimal set of functionalities, and to allow
each instantiation of the protocol to be optimized in terms of each implementation of the protocol to be optimized in terms of,
required code space. It must be noted that RPL is not restricted to e.g., minimizing required code space and use of limited computation
the aforementioned applications and is expected to be used in other resources.
environments. All "MUST" application requirements that cannot be
satisfied by RPL will be specifically listed in the Appendix A, Multiple instances of the protocol can be operated at the same time
accompanied by a justification. in order to serve different and potentially antagonistic constraints.
Instances run independently of one another with no required
interaction. A node might participate to multiple instances and
route independently along the associated topologies. This
specification defines only the protocol operation for the node within
one instance. Consideration is given to default behavior that
enables future extensions for the multiple instances and related
policies.
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 The core set of functionalities is to be capable of operating in the
most severely constrained environments, with minimal requirements for most severely constrained environments, with minimal requirements for
memory, energy, processing, communication, and other consumption of memory, energy, processing, communication, and other consumption of
limited resources from nodes. Trade-offs inherent in the limited resources from nodes. Trade-offs inherent in the
provisioning of protocol features will be exposed to the implementer provisioning of protocol features will be exposed to the implementer
in the form of configurable parameters, such that the implementer can in the form of configurable parameters, such that the implementer can
further tweak and optimize the operation of RPL as appropriate to a further tweak and optimize the operation of RPL as appropriate to a
specific application and implementation. Finally, RPL is designed to specific application and implementation. Finally, RPL is designed to
consult implementation specific policies to determine, for example, consult implementation specific policies to determine, for example,
the evaluation of routing metrics. the evaluation of routing metrics.
A set of companion documents to this specification will provide A set of companion documents to this specification will provide
further guidance in the form of applicability statements specifying a further guidance in the form of applicability statements specifying a
set of operating points appropriate to the Building Automation, Home set of operating points appropriate to the Building Automation, Home
Automation, Industrial, and Urban application scenarios. Automation, Industrial, and Urban application scenarios.
1.2. Expectations of Link Layer Behavior 1.2. Expectations of Link Layer Type
This specification does not rely on any particular features of a This specification does not rely on any particular features of a
specific link layer technologies. It is anticipated that an specific link layer technologies. It is anticipated that an
implementer should be able to operate RPL over a variety of different implementer should be able to operate RPL over a variety of different
low power wireless or PLC (Power Line Communication) link layer link layers, including but not limited to low power wireless or PLC
technologies. (Power Line Communication) technologies.
Implementers may find RFC 3819 [RFC3819] a useful reference when Implementers may find RFC 3819 [RFC3819] a useful reference when
designing a link layer interface between RPL and a particular link designing a link layer interface between RPL and a particular link
layer technology. layer technology.
2. Terminology 2. Terminology
The terminology used in this document is consistent with and The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
incorporates that described in `Terminology in Low power And Lossy "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
Networks' [I-D.ietf-roll-terminology]. The terminology is extended "OPTIONAL" in this document are to be interpreted as described in RFC
in this document as follows: 2119 [RFC2119].
Autonomous: The ability of a routing protocol to independently This document requires readers to be familiar with the terminology
function without relying on any external influence or guidance. described in `Terminology in Low power And Lossy Networks'
Includes self-organization capabilities. [I-D.ietf-roll-terminology].
DAG: Directed Acyclic Graph. A directed graph having the property DAG: Directed Acyclic Graph. A directed graph having the property
that all edges are oriented in such a way that no cycles exist. that all edges are oriented in such a way that no cycles exist.
In the RPL context, all edges are contained in paths oriented In the RPL context, all edges are contained in paths oriented
toward and terminating at a root node (a DAG root, or sink- toward and terminating at one or more root nodes (a DAG root,
typically a Low Power and Lossy Network Border Router (LBR)). or sink- typically a Low power and Lossy Network Border Router
(LBR)). For the purpose of this document, the term DAG is
often used to refer to a DAG Iteration as defined below.
DAGID: DAG Identifier. A globally unique identifier for a DAG. All DAG Instance: A DAG Instance is a set of possibly multiple
nodes who are part of a given DAG have knowledge of the DAGID. Destination Oriented DAGs. A network may have more than one
This knowledge is used to identify peer nodes within the DAG in DAG Instance, and a RPL router can participate to multiple DAG
order to coordinate DAG maintenance while avoiding loops. instances. Each DAG Instance operates independently of other
DAG Instances. This document describes operation within a
single DAG instance.
InstanceID: Unique identifier of a DAG Instance.
Destination Oriented DAG: A DAG rooted at a single destination,
which is a node with no outgoing edges. The tuple (InstanceID,
DAGID) uniquely identifies a Destination Oriented DAG. In the
RPL context, a router can can belong to at most one Destination
Oriented DAG per DAG Instance.
DAGID: The identifier of a DAG root. The DAGID must be unique
within the scope of a DAG Instance in the LLN.
DAG Iteration: The DAG that results from the iterative process that
reshapes the Destination Oriented DAG upon a stimulation by the
root.
DAGSequenceNumber: A sequential counter that is incremented by the
root to form a new Iteration of a DAG. A DAG Iteration is
identified uniquely by the (InstanceID, DAGID,
DAGSequenceNumber) tuple.
DAG parent: A parent of a node within a DAG is one of the immediate 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 successors of the node on a path towards the DAG root.
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 DAG sibling: A sibling of a node within a DAG is defined in this
specification to be any neighboring node which is located at specification to be any neighboring node which is located at
the same rank (depth) within a DAG. Note that siblings defined the same rank within a DAG. Note that siblings defined in this
in this manner do not necessarily share a common parent. For manner do not necessarily share a common parent.
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 DAG root: A DAG root is a node within the DAG that has no outgoing
terminate at a DAG root, and all DAG edges contained in the edges. Because the graph is acyclic, by definition all DAGs
paths terminating at a DAG root are oriented toward the DAG must have at least one DAG root and all paths terminate at a
root. There must be at least one DAG root per DAG, and in some DAG root.
cases there may be more than one. In many use cases, source-
sink represents a dominant traffic flow, where the sink is a Sub-DAG The sub-DAG of a node is the set of other nodes in the DAG
DAG root or is located behind the DAG root. Maintaining routes that might use a path towards the DAG root that contains the
towards DAG roots is therefore a prominent functionality for node. Nodes in the sub-DAG of a node have a greater rank
RPL. (although not all nodes of greater rank are in the sub-DAG).
Grounded: A DAG is grounded if it contains a DAG root offering Grounded: A DAG is grounded if it contains a DAG root offering
connectivity to an external routed infrastructure such as the connectivity to an external routed infrastructure such as the
public Internet or a private core (non-LLN) IP network. public Internet or a private core (non-LLN) IP network.
Floating: A DAG is floating if is not grounded. A floating DAG is Floating: A DAG is floating if is not grounded. A floating DAG is
not expected to reach any additional external routed not expected to reach any additional external routed
infrastructure such as the public Internet or a private core infrastructure such as the public Internet or a private core
(non-LLN) IP network. (non-LLN) IP network.
Inward: Inward refers to the direction from leaf nodes towards DAG Inward: Inward refers to the direction from leaf nodes towards DAG
roots, following the orientation of the edges within the DAG. roots, following the orientation of the edges within the DAG.
Outward: Outward refers to the direction from DAG roots towards leaf Outward: Outward refers to the direction from DAG roots towards leaf
nodes, going against the orientation of the edges within the nodes, going against the orientation of the edges within the
DAG. DAG.
P2P: Point-to-point. This refers to traffic exchanged between two OCP: Objective Code Point. The Objective Code Point is used to
nodes. indicate which Objective Function is in use in a DAG. The
Objective Code Point is further described in
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] and
[RFC4875]). 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]. [I-D.ietf-roll-routing-metrics].
OF: Objective Function. The Objective Function (OF) defines which
routing metrics, optimization objectives, and related functions
are in use in a DAG. The Objective Function is further
described in [I-D.ietf-roll-routing-metrics].
Note that in this document, the terms `node' and `LLN router' are Note that in this document, the terms `node' and `LLN router' are
used interchangeably. used interchangeably.
3. Protocol Model 3. Protocol Model
The aim of this section is to describe RPL in the spirit of The aim of this section is to describe RPL in the spirit of
[RFC4101]. Protocol details can be found in further sections. [RFC4101]. Protocol details can be found in further sections.
3.1. Protocol Properties Overview 3.1. Overview
RPL demonstrates the following properties, consistent with the 3.1.1. Topology Instance and Objectives
requirements specified by the application-specific requirements
documents.
3.1.1. IPv6 Architecture A topology instance of RPL exists over the scope of an LLN in support
of a particular application, or service, and is optimized according
to a certain objective, as determined by an Objective Function (OF),
and may be characterized by certain destination prefixes as well. A
topology instance, or DAG Instance, may be administratively
associated with an InstanceID.
RPL is strictly compliant with layered IPv6 architecture. A single topology instance may comprise:
Further, RPL is designed with consideration to the practical support o a single Destination Oriented DAG with a single DAG root
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).
3.1.2. Typical LLN Traffic Patterns * For example, a DAG optimized to minimize latency rooted at a
single centralized lighting controller in a home automation
application.
Multipoint-to-Point (MP2P) and Point-to-multipoint (P2MP) traffic o multiple uncoordinated Destination Oriented DAGs with independent
flows from nodes within the LLN from and to egress points are very DAG roots (differing DAGIDs)
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 * For example, multiple data collection points in an urban data
supports the installation of multiple paths. The use of multiple collection application that do not have an always-on backbone
paths include sending duplicated traffic along diverse paths, as well suitable to coordinate to form a single DAG, and further use
as to support advanced features such as Class of Service (CoS) based the formation of multiple DAGs as a means to dynamically and
routing, or simple load balancing among a set of paths (which could autonomously partition the network.
be useful for the LLN to spread traffic load and avoid fast energy
depletion on some, e.g. battery powered, nodes).
3.1.3. Constraint Based Routing o a single Destination Oriented DAG with multiple DAG roots
coordinating over some backbone
The RPL design supports constraint based routing, based on a set of * For example, multiple border routers operating with a reliable
routing metrics. The routing metrics for links and nodes with backbone, e.g. in support of a 6LowPAN application, that are
capabilities supported by RPL are specified in a companion document capable to act as logically equivalent sinks to the same DAG.
to this specification, [I-D.ietf-roll-routing-metrics]. 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 o a combination of one of the above as suited to some application
support of and optimized for a set of application and implementation scenario
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.
3.2. Protocol Operation The exact deployment scenario is determined as appropriate to the
application and capabilities of the LLN nodes. What is suitable for
one deployment may not be possible or necessary for another.
A LLN deployment will consist of a number of nodes and a number of Traffic is bound to a specific DAG Instance by a marking in the flow
edges (links) between them, whose characteristics will depend on label of the IPv6 header. Traffic originating in support of a
implementation and link layer (L2) specifics. Due to the nature of particular application may be tagged to follow an appropriate
the LLN environment the L2 links are expected to demonstrate a large instance, for example to follow paths optimized for low latency or
degree of variance as to their availability, quality, and other low energy. The provisioning or automated discovery of a mapping
related parameters. Certain links, demonstrating a viability above a between an InstanceID and a type or service of application traffic is
confidence threshold for particular node and link metrics, as based beyond the scope of this specification.
on guidelines from [I-D.ietf-roll-routing-metrics], will be extracted
from the L2 graph, and the resulting graph will be used as the basis Conceptually a node running RPL may capable to maintain a membership
on which to operate the routing protocol. Note that as the in multiple DAG Instances in support of different application
characteristics of the L2 topology vary over time the set of viable services and/or optimization objectives. For example, one instance
links is to be updated and the routing protocol thus continues to may optimize for minimizing latency and a separate orthogonal
evaluate the LLN. In RPL this process happens in a distributed instance may optimize for minimizing energy. This scenario does
manner, and from the perspective of a single node running RPL this introduce some additional considerations, for example loop avoidance
process results in a set of candidate neighbors, with associated node and default routing behavior. These considerations are beyond the
and link metrics as well as confidence values. scope of this specification and are intended to be elaborated on in a
future revision of this or a companion specification. As such, this
specification will deal exclusively with the scenario where a node
implements RPL in support of a single DAG Instance.
3.1.2. Multipoint-to-Point Traffic Flows and DAGs
Many of the dominant traffic flows in support of the LLN application Many of the dominant traffic flows in support of the LLN application
scenarios are MP2P flows ([I-D.ietf-roll-building-routing-reqs], scenarios are MP2P flows ([I-D.ietf-roll-building-routing-reqs],
[I-D.ietf-roll-home-routing-reqs], [I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548]). These
[I-D.ietf-roll-indus-routing-reqs], and [RFC5548]). These flows are flows are rooted at designated nodes that have some application
rooted at designated nodes that have some application significance, significance, such as providing connectivity to an external routed
such as providing connectivity to an external routed infrastructure. infrastructure. The term "external" is used to refer to the public
The term "external" is used top refer to the public Internet or a Internet or a core private (non-LLN) IP network.
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) LLN nodes running RPL will construct Directed Acyclic Graphs (DAGs)
rooted at designated nodes that generally have some application rooted at DAG roots, which may be naturally designated according to
significance, such as providing connectivity to an external routed their application significance. This structure provides the routing
infrastructure. The term "external" is used top refer to the public solution for the dominant MP2P traffic flows. The DAG structure
Internet or a core private (non-LLN) IP network. This structure further provides each node potentially multiple successors for MP2P
provides the routing solution for the dominant MP2P traffic flows. flows, which may be used for, e.g., local route repair or load
The DAG structure further provides each node potentially multiple balancing.
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 Nodes running RPL are able to further restrict the scope of the
routing problem by using the DAG as a reference topology. By routing problem by using the DAG as a reference topology. By
referencing a rank property that is related to the positions in the 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 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 each other. This information is used by RPL in part to construct
rules for movement relative to the DAG that endeavor to avoid loops. rules for movement relative to the DAG that endeavor to avoid loops.
It is important to note that the rank property is derived from 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 metrics, and not directly from the position in the DAG (Section 5.3).
discussed further.
3.1.3. Point-to-Multipoint Traffic Flows
As DAGs are organized, RPL will use a destination advertisement As DAGs are organized, RPL will use a destination advertisement
mechanism to build up routing tables in support of outward P2MP mechanism to build up routing tables in support of outward P2MP
traffic flows. This mechanism, using the DAG as a reference, traffic flows. This mechanism, using the DAG as a reference,
distributes routing information across intermediate nodes (between distributes routing information across intermediate nodes (between
the DAG leaves and the root), guided along the DAG, such that the the DAG leaves and the root), guided along the DAG, such that the
routes toward destination prefixes in the outward direction may be routes toward destination prefixes in the outward direction may be
set up. As the DAG undergoes modification during DAG maintenance, set up. As the DAG undergoes modification during DAG maintenance,
the destination advertisement mechanism can be triggered to update the destination advertisement mechanism can be triggered to update
the outward routing state. the outward routing state.
3.1.4. Point-to-Point Traffic Flows
A baseline support for P2P traffic in RPL is provided by the DAG, as 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 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 reached that has stored an entry for the destination in its routing
table and is capable of directing the traffic outward along the table and is capable of directing the traffic outward along the
correct outward path. RPL also provides support for the trivial case correct outward path. RPL also provides support for the trivial case
where a P2P destination may be a `one-hop' neighbor. In the present where a P2P destination may be a `one-hop' neighbor. In the present
specification RPL does not specify nor preclude any additional document RPL does not specify nor preclude any additional mechanisms
mechanisms that may be capable to compute and install more optimal that may be capable to compute and install more optimal routes into
routes into LLN nodes in support of arbitrary P2P traffic according LLN nodes in support of arbitrary P2P traffic according to some
to some routing metric. routing metric.
3.2.1. DAG Construction
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 3.2. Protocol Operation
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.
3.2.1.1. IP Router Advertisement - DAG Information Option (RA-DIO) 3.2.1. DAG Construction
The IPv6 Router Advertisement (RA) mechanism (as specified in 3.2.1.1. DAG Information Object (DIO)
[RFC4861]) is used by RPL in order to build and maintain a DAG.
The IPv6 RA message is augmented with a DAG Information Option (DIO), A DAG Information Object is defined and used by RPL in order to build
forming an RA-DIO message, to convey information about the DAG and maintain a DAG. This document defines an ICMPv6 Message Type RPL
including: Control Message, which is capable to carry the DIO. The DIO message
conveys information about the DAG, including:
o A DAGID used to identify the DAG as sourced from the DAG root. o A DAGID used to identify the DAG as sourced from the DAG root.
The DAGID must be unique to a single DAG in the scope of the LLN. The DAGID must be unique to a single DAG in the scope of the LLN.
o Objective Code Point (OCP) as described below. o Objective Function identified by an Objective Code Point (OCP) as
described below.
o Rank information used by nodes to determine their positions in the o Rank information used by nodes to determine their positions in the
DAG relative to each other. This is not a metric, although its DAG relative to each other.
derivation is typically closely related to one or more metrics as
specified by the OCP. The rank information is used to support
loop avoidance strategies and in support of ordering alternate
successors when engaged in path maintenance.
o Sequence number originated from the DAG root, used to aid in o Sequence number originated from the DAG root, used to aid in
identification of dependent sub-DAGs and coordinate topology identification of dependent sub-DAGs and coordinate topology
changes in a manner so as to avoid loops. changes in a manner so as to avoid loops.
o Indications and configuration for the DAG, e.g. grounded or o Indications and configuration for the DAG, e.g. grounded or
floating, administrative preference, ... floating, administrative preference, ...
o A vector of path metrics, as further described in o A set of path metrics and constraints, as further described in
[I-D.ietf-roll-routing-metrics]. [I-D.ietf-roll-routing-metrics].
o List of additional destination prefixes reachable inwards along o List of additional destination prefixes reachable inwards along
the DAG. the DAG.
The RA messages are issued whenever a change is detected to the DAG The DIO 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 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 become inconsistent. As the DAG stabilizes the period at which RA
messages occur is configured to taper off, reducing the steady-state messages occur is configured to taper off, reducing the steady-state
overhead of DAG maintenance. The periodic issue of RA messages, overhead of DAG maintenance. The periodic issue of DIO messages,
along with the triggered RA messages in response to inconsistency, is along with the triggered DIO messages in response to inconsistency,
one feature that enables RPL to operate in the presence of unreliable is one feature that enables RPL to operate in the presence of
links. unreliable links.
3.2.1.2. DAG Identification
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.
3.2.1.3. Grounded and Floating DAGs 3.2.1.2. Grounded and Floating DAGs
Certain LLN nodes may offer connectivity to an external routed Certain LLN nodes may offer connectivity to an external routed
infrastructure in support of an application scenario. These nodes infrastructure in support of an application scenario. These nodes
are designated `grounded', and may serve as the DAG roots of a 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 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, DAGs. In either case routes may be provisioned toward the DAG root,
although in the floating case there is no expectation to reach an although in the floating case there is no expectation to reach an
external infrastructure. Some applications will include permanent external infrastructure. Some applications will include permanent
floating DAGs. floating DAGs.
3.2.1.4. Administrative Preference 3.2.1.3. Administrative Preference
An administrative preference may be associated with each DAG root, 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 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 preferred over other DAGs. For example, a DAG root that is sinking
traffic in support of a data collection application may be configured 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 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 that is only existing until a permanent DAG is found, may be
is found, may be configured to be less preferred. The administrative configured to be less preferred. The administrative preference
preference offers a way to engineer the formation of the DAG in offers a way to engineer the formation of the DAG in support of the
support of the application. application.
3.2.1.5. Objective Code Point (OCP) 3.2.1.4. Objective Function (OF)
The OCP serves to convey and control the optimization objectives in The Objective Function (OF) conveys and controls the optimization
use within the DAG. The OCP is further specified in objectives in use within the DAG. The Objective Function is
[I-D.ietf-roll-routing-metrics]. Each instance of an allocated OCP indicated by an Objective Code Point (OCP), and is further specified
in [I-D.ietf-roll-routing-metrics]. Each instance of an allocated OF
indicates: indicates:
o The set of metrics used within the DAG o The set of metrics used within the DAG
o The objective functions used for least cost path determination. o The method used for least cost path determination.
o The function used to compute DAG Rank
o The functions used to accumulate metrics for propagation within a o The method used to compute DAG Rank
RA-DIO message
For example, an objective code point might indicate that the DAG is o The methods used to prepare metrics for propagation within a DIO
using the Expected Number of Transmissions (ETX) as a metric, that message
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 By using defined OCPs that are understood by all nodes in a
particular implementation, and by conveying them in the RA-DIO particular implementation, and by conveying them in the DIO message,
message, RPL nodes may work to build optimized LLN using a variety of RPL nodes may work to build optimized LLN using a variety of
application and implementation specific metrics and goals. application and implementation specific metrics and goals.
A default OCP, OCP 0, is specified with a well-defined default A default OF, OF0 (designated by OCP value of 0x0000), is specified
behavior. OCP 0 is used to define RPL behaviors in the case where a with a well-defined default behavior. OF0 may be used to define RPL
node encounters a RA-DIO message containing a code point that it does behaviors in the case where a node encounters a DIO message
not support. containing a code point that it does not support, if allowed by
policy.
3.2.1.6. Distributed Algorithm Operation 3.2.1.5. Distributed Algorithm Operation
A high level overview of the distributed algorithm which constructs
the DAG is as follows:
o Some nodes may be initially provisioned to act as DAG roots, o Some nodes may be initially provisioned to act as DAG roots,
either permanent or transient, with associated preferences. either permanent or transient, with associated preferences.
o Nodes will maintain a data structure containing their candidate o Nodes will maintain a data structure containing their candidate
(viable) neighbors, as based on guidelines in (viable) neighbors, as determined by the implementation. This
[I-D.ietf-roll-routing-metrics] This data structure will also data structure will also track DAG information as learned from and
track DAG information as learned from and associated with each associated with each neighbor.
neighbor.
o Nodes who are members of a DAG, including DAG roots, will o Nodes that are members of a DAG, including DAG roots, will
multicast RA-DIO messages as needed (when inconsistency is multicast DIO messages as needed (when inconsistency is detected),
detected), to their link-local neighbors. Nodes will also respond to their link-local neighbors. Nodes will also respond to DIS
to Router Solicitation (RS) messages. messages.
o Nodes who receive RA-DIO messages will take into consideration o Nodes that receive DIO messages may either discard the DIO based
several criteria when processing the extracted DAG information. on several criteria, including rank-based loop avoidance rules, or
The node may discount the RA-DIO according to loop avoidance rules process the DIO to maintain a position in an existing DAG or
based on rank as described further below. Nodes will consider the improve a position as according to an Objective Function (OF) and
information in the RA-DIO in order to determine whether or not current path cost.
that candidate neighbor offers a better attachment point to a DAG
(which the node may or may not be a member of) according to the
implementation specific optimization goals, OCP, and current
metrics.
o Nodes may join a new DAG or move within the current DAG, in o Nodes manage a set of DAG Parents according to the rules specified
response to the information contained in the RA-DIO message, and by RPL. This set is also augmented to include DAG siblings.
in accordance with loop avoidance rules described further in this
specification. For the successors within the DAG, a node manages
a set of DAG Parents. Joining, moving within, and leaving the DAG
is accomplished through managing this set according to the rules
specified by RPL.
o As nodes join, move within, and leave DAGs they emit updated RA- o DIO messages may be emitted more or less frequently as a function
DIOs which are received and acted on by neighboring nodes. When of DAG consistency.
inconsistencies (such as caused by movement or link loss) are
detected within the DAG structure, RA-DIO messages are emitted
more frequently. When the DAG structure becomes consistent, RA-
DIO messages taper off.
o As less preferred DAGs encounter more preferred DAGs that offer o As less preferred DAGs encounter more preferred DAGs that offer
equivalent or better optimization objectives, the nodes in the equivalent or better optimization objectives for the same
less preferred DAGs may leave to join the more preferred DAGs, InstanceID, the nodes in the less preferred DAGs may leave to join
finally leaving only the more preferred DAGs. This is an the more preferred DAGs, finally leaving only the more preferred
illustration of the mechanism by which an application may engineer DAGs. This is an illustration of the mechanism by which an
DAG construction. application may engineer DAG construction.
o As the DAG construction operation proceeds, nodes accumulate onto
the DAG in progressively outward tiers, centered around the DAG
root.
o The nodes provision routing table entries for the destinations o The nodes provision routing table entries for the destinations
specified by the RA-DIO towards their DAG Parents. Nodes may specified by the DIO towards their DAG Parents. Nodes may
provision a DAG Parent as a default gateway. provision a DAG Parent as a default gateway.
3.2.1.7. DAG Rank
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.
3.2.1.8. Sub-DAG
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.
Consider Node (24) in the DAG Example depicted in Figure 9. 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.
3.2.1.9. Moving up in a DAG
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.
3.2.1.10. Moving down in a DAG
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.
3.2.1.11. DAG Jumps
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.
3.2.1.12. Floating DAGs for DAG Repair
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).
3.2.2. Destination Advertisement 3.2.2. Destination Advertisement
As RPL constructs DAGs, nodes may provision routes toward As RPL constructs DAGs, nodes may provision routes toward
destinations advertised through RA-DIO messages through their destinations advertised through DIO messages through their selected
selected parents, and are thus able to send traffic inward along the parents, and are thus able to send traffic inward along the DAG by
DAG by forwarding to their selected parents. However, this mechanism forwarding to their selected parents. However, this mechanism alone
alone is not sufficient to support P2MP traffic flowing outward along is not sufficient to support P2MP traffic flowing outward along the
the DAG from the DAG root toward nodes. A destination advertisement DAG from the DAG root toward nodes. A destination advertisement
mechanism is employed by RPL to build up routing state in support of mechanism is employed by RPL to build up routing state in support of
these outward flows. The destination advertisement mechanism may not these outward flows. The destination advertisement mechanism may not
be supported in all implementations, as appropriate to the be supported in all implementations, as appropriate to the
application requirements. A DAG root that supports using the application requirements. A DAG root that supports using the
destination advertisement mechanism to build up routing state will destination advertisement mechanism to build up routing state will
indicate such in the RA-DIO message. A DAG root that supports using indicate such in the DIO message. A DAG root that supports using the
the destination advertisement mechanism must be capable of allocating destination advertisement mechanism must be capable of allocating
enough state to store the routing state received from the LLN. enough state to store the routing state received from the LLN.
3.2.2.1. IPv6 Neighbor Advertisement - Destination Advertisement Option 3.2.2.1. Destination Advertisement Object (DAO)
(NA-DAO)
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 A Destination Advertisement Object is defined and used by RPL in
following: order to convey the destination information inward along the DAG
toward the DAG root. This document defines an ICMPv6 Message Type
RPL Control Message, which is capable to carry the DAO. The
information conveyed in the DAO message includes the following:
o A lifetime and sequence counter to determine the freshness of the o A lifetime and sequence counter to determine the freshness of the
destination advertisement. destination advertisement.
o Depth information used by nodes to determine how far away the o Depth information used by nodes to determine how far away the
destination (the source of the destination advertisement) is destination (the source of the destination advertisement) is
o Prefix information to identify the destination, which may be a o Prefix information to identify the destination, which may be a
prefix, an individual host, or multicast listeners prefix, an individual host, or multicast listeners
o Reverse Route information to record the nodes visited (along the o Reverse Route information to record the nodes visited (along the
outward path) when the intermediate nodes along the path cannot outward path) when the intermediate nodes along the path cannot
support storing state for Hop-By-Hop routing. support storing state for Hop-By-Hop routing.
3.2.2.2. Destination Advertisement Operation 3.2.2.2. Destination Advertisement Operation
As the DAG is constructed and maintained, nodes are capable to emit 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 DAO messages to a subset of their DAG parents.
selection of this subset is according to an implementation specific
policy. 3.2.2.2.1. `One-Hop' Neighbors
As a special case, a node may periodically emit a link-local As a special case, a node may periodically emit a link-local
multicast IPv6 NA-DAO message advertising its locally available multicast IPv6 DAO message advertising its locally available
destination prefixes. This mechanism allows for the one-hop destination prefixes. This mechanism allows for the one-hop
neighbors of a node to learn explicitly of the prefixes on the node, neighbors of a node to learn explicitly of the prefixes on the node,
and in some application specific scenarios this is desirable in and in some application specific scenarios this is desirable in
support of provisioning a trivial `one-hop' route. In this case, support of provisioning a trivial `one-hop' route. In this case,
nodes who receive the multicast destination advertisement may use it nodes that receive the multicast destination advertisement may use it
to provision the one-hop route only, and not engage in any additional 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). 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 3.2.2.2.2. Operation in Support of Stateful Nodes
routing state, the node extracts information from the NA-DAO message
and updates its local database with a record of the NA-DAO message When a (unicast) DAO message reaches a node capable of storing
and who it was received from. When the node later propagates NA-DAO routing state, the node extracts information from the DAO message and
messages, it selects the best (least depth) information for each updates its local database with a record of the DAO message and the
destination and conveys this information again in the form of NA-DAO neighbor that it was received from. When the node later propagates
DAO messages, it selects the best (least depth) information for each
destination and conveys this information again in the form of DAO
messages to a subset of its own DAG parents. At this time the node 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 may perform route aggregation if it is able, thus reducing the
overall number of NA-DAO messages. overall number of DAO messages.
When a (unicast) NA-DAO message reaches a node incapable of storing 3.2.2.2.3. Operation in Support of Stateless Nodes
When a (unicast) DAO message reaches a node incapable of storing
additional state, the node must append the next-hop address (from additional state, the node must append the next-hop address (from
which neighbor the NA-DAO message was received) to a Reverse Route which neighbor the DAO message was received) to a Reverse Route Stack
Stack carried within the NA-DAO message. The node then passes the carried within the DAO message. The node then passes the DAO message
NA-DAO message on to one or more of its DAG parents without storing on to one or more of its DAG parents without storing any additional
any additional state. state.
When a node that is capable of storing routing state encounters a When a node that is capable of storing routing state encounters a
(unicast) NA-DAO message with a Reverse Route Stack that has been (unicast) DAO message with a Reverse Route Stack that has been
populated, the node knows that the NA-DAO message has traversed a populated, the node knows that the DAO message has traversed a region
region of nodes that did not record any routing state. The node is of nodes that did not record any routing state. The node is able to
able to detach and store the Reverse Route State and associate it detach and store the Reverse Route State and associate it with the
with the destination described by the NA-DAO message. Subsequently destination described by the DAO message. Subsequently the node may
the node may use this information to construct a source route in use this information to construct a source route in order to bridge
order to bridge the region of nodes that are unable to support Hop- the region of nodes that are unable to support Hop-By-Hop routing to
By-Hop routing to reach the destination. reach the destination.
In this way the destination advertisement mechanism is able to 3.2.2.2.4. Additional Considerations
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 Further aggregations of DAO messages prefix reachability information
information by destinations are possible in order to support by destinations are possible in order to support additional
additional scalability. scalability.
A special case of an DAO message, termed a `no-DAO', may be used to
tear down the routing state that has been established by the
destination advertisement mechanism in case of, e.g., unreachability
or other events that affect the outward routing state.
A further example of the operation of the destination advertisement A further example of the operation of the destination advertisement
mechanism is available in Appendix B.6 mechanism is available in Appendix B.1
3.3. Loop Avoidance and Stability 3.3. Loop Avoidance and Stability
The goal of a guaranteed consistent and loop free global routing The goal of a guaranteed consistent and loop free global routing
solution for an LLN may not be practically achieved given the real solution for an LLN may not be practically achieved given the real
behavior and volatility of the underlying metrics. The trade offs to behavior and volatility of the underlying metrics. The trade offs to
achieve a stable approximation of global convergence may be too achieve a stable approximation of global convergence may be too
restrictive with respect to the need of the LLN to react quickly in 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 response to the lossy environment. Globally the LLN may be able to
achieve a weak convergence, in particular as link changes are 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. be handled locally and result in minimal changes to global topology.
RPL does not aim to guarantee loop free path selection, or strong RPL does not aim to guarantee loop free path selection, or strong
global convergence. In order to reduce control overhead, in global convergence. In order to reduce control overhead, in
particular the expense of mechanisms such as count-to-infinity, RPL particular the expense of mechanisms such as count-to-infinity, RPL
does try to avoid the creation of loops when undergoing topology does try to avoid the creation of loops when undergoing topology
changes. Further mechanisms to mitigate the impact of loops, such as changes.
loop detection when forwarding, are under investigation.
RPL includes rank-based mechanisms for detecting loops to ensure that
packets make forward progress within the DAG and trigger DAG repair
if necessary.
3.3.1. Greediness and Rank-based Instabilities 3.3.1. Greediness and Rank-based Instabilities
If a node is greedy and attempts to move deeper in the DAG, beyond Once a node has joined a DAG, RPL disallows certain behaviors,
its most preferred parent, in order to increase the size of the DAG including greediness, in order to prevent resulting instabilities in
parent set, then an instability can result. This is illustrated in the DAG.
Figure 11.
Suppose a node is willing to receive and process a RA-DIO messages If a node is allowed to be greedy and attempts to move deeper in the
from a node in its own sub-DAG, and in general a node deeper than it. DAG, beyond its most preferred parent, in order to increase the size
In such cases a chance exists to create a feedback loop, wherein two of the DAG parent set, then an instability can result. This is
or more nodes continue to try and move in the DAG in order to illustrated in Figure 14.
optimize against each other. In some cases this will result in an
Suppose a node is willing to receive and process a DIO messages from
a node in its own sub-DAG, and in general a node deeper than it. In
such cases a chance exists to create a feedback loop, wherein two or
more nodes continue to try and move in the 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 instability. It is for this reason that RPL mandates that a node
never receive and process RA-DIO messages from deeper nodes. This never receive and process DIO messages from deeper nodes. This rule
rule creates an `event horizon', whereby a node cannot be influenced creates an `event horizon', whereby a node cannot be influenced into
into an instability by the action of nodes that may be in its own an instability by the action of nodes that may be in its own sub-DAG.
sub-DAG.
A further example of the consequences of greedy operation, and A further example of the consequences of greedy operation, and
instability related to processing RA-DIO messages from nodes of instability related to processing DIO messages from nodes of greater
greater rank, may be found in Appendix B.9 rank, may be found in Appendix B.4
3.3.2. Merging DAGs
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
3.3.3. DAG Loops 3.3.2. DAG Loops
A DAG loop may occur when a node detaches from the DAG and reattaches 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 to a device in its prior sub-DAG. This may happen in particular when
sequence and kept advertising the original DAG. This may happen in DIO messages are missed. Strict use of the DAG sequence number can
particular when RA-DIO messages are missed. Use of the DAG sequence eliminate this type of loop.
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, 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).
3.3.4. DAO Loops 3.3.3. DAO Loops
A DAO loop may occur when the parent has a route installed upon 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 receiving and processing a DAO message from a child, but the child
has subsequently cleaned up the state. This loop happens when a no- 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 DAO was missed till a heartbeat cleans up all states. RPL includes
is not explicitly handled by the current specification. Split loop detection mechanisms that may mitigate the impact of DAO loops
horizon, not forwarding a packet back to the node it came from, may and trigger their repair.
mitigate the DAO loop in some cases, but does not eliminate it.
Consider node (24) in the DAG Example depicted in Figure 9. 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.
3.3.5. Sibling Loops
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. 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.
3.4. Local and Temporary Routing Decision
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, 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.
3.5. Maintenance of Routing Adjacency In the case where stateless DAO operation is used, i.e. source
routing specifies the outwards routes, then DAO Loops should not
occur on the stateless portions of the path.
In order to relieve the LLN of the overhead of periodic keepalives, 3.3.4. Sibling Loops
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.
4. Constraint Based Routing in LLNs Sibling loops could occur if a group of siblings kept choosing
amongst themselves as successors such that a packet does not make
forward progress. This specification limits the number of times that
sibling forwarding may be used at a given rank to prevent sibling
loops.
This aim of this section is to make a clear distinction between 4. Routing Metrics and Constraints Used By RPL
routing metrics and constraints and define the term constraint based
routing as used in this document.
4.1. Routing Metrics Routing metrics are used by routing protocols to compute the shortest
paths. Interior Gateway Protocols (IGPs) such as IS-IS ([RFC5120])
and OSPF ([RFC4915]) use static link metrics. Such link metrics can
simply reflect the bandwidth or can also be computed according to a
polynomial function of several metrics defining different link
characteristics; in all cases they are static metrics. Some routing
protocols support more than one metric: in the vast majority of the
cases, one metric is used per (sub)topology. Less often, a second
metric may be used as a tie-breaker in the presence of Equal Cost
Multiple Paths (ECMP). The optimization of multiple metrics is known
as an NP complete problem and is sometimes supported by some
centralized path computation engine.
Routing metrics are used by the routing protocol to compute the In contrast, LLNs do require the support of both static and dynamic
shortest path according to one of more defined metrics. IGPs such as metrics. Furthermore, both link and node metrics are required. In
IS-IS ([RFC5120]) and OSPF ([RFC4915]) compute the shortest path the case of RPL, it is virtually impossible to define one metric, or
according to a Link State Data Base (LSDB) using link metrics even a composite, that will satisfy all use cases.
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 In addition, RPL supports constrained-based routing where constraints
majority of the cases, one metric is used per (sub)topology. Less may be applied to link and nodes. If a link or a node does not
often, a second metric may be used as a tie breaker in the presence satisfy a required constraint, it is `pruned' from the candidate list
of ECMP (Equal Cost Multiple Paths). The optimization of multiple thus leading to a constrained shortest path.
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* The set of supported link/node constraints and metrics is specified
metric, or even a composite, that will fit it all: in [I-D.ietf-roll-routing-metrics].
o Some information apply when determining routes, other information The role of the Objective Function is to advertise routing metrics
may apply only when forwarding packets along provisioned routes. and constraints in addition to the objectives used to compute the
(constrained) shortest path.
o Some values are aggregated hop-by-hop, others are triggers from Example 1: Shortest path: path offering the shortest end-to-end delay
L2.
o Some properties are very stable, others vary rapidly. Example 2: Constrained shortest path: the path that does traverse any
battery-operated node and that optimizes the path
reliability
o Some data are useful in a given scenario and useless in another. 5. RPL Protocol Specification
o Some arguments are scalar, others statistical. 5.1. RPL Messages
For that reason, the RPL protocol core is agnostic to the logic that 5.1.1. ICMPv6 RPL Control Message
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 This document defines the RPL Control Message, a new ICMPv6 message.
degraded fashion, when confronted to such heterogeneity. The key The RPL Control Message has the following general format, in
design point is that each node is solely responsible for setting the accordance with [RFC4443]:
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.
4.2. Routing Constraints 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Message Body +
| |
A constraint is a link or a node characteristic that must be Figure 1: RPL Control Message
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.
4.3. Constraint Based Routing The RPL Control message is an ICMPv6 information message with a
requested Type of 155.
The notion of constraint based routing consists of finding the The Code will be used to identify RPL Control Messages as follows:
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: o 0x01: DAG Information Solicitation (Section 5.1.2)
Link Metric: Bandwidth
Link Constraint: Blue
Node Constraint: Mains-powered node
Objective function 1: o 0x02: DAG Information Object (Section 5.1.3)
"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: o 0x04: Destination Advertisement Object (Section 5.1.4)
Link Metric: Delay
Link Constraint: Bandwidth
Objective function 2: 5.1.2. DAG Information Solicitation (DIS)
"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."
5. RPL Protocol Specification The DAG Information Solicitation (DIS) message may be used to solicit
a DAG Information Object from a RPL node. Its use is analogous to
that of a Router Solicitation; a node may use DIS to probe its
neighborhood for nearby DAGs. The DAG Information Solicitation
carries no additional message body.
5.1. DAG Information Option 5.1.3. DAG Information Object (DIO)
The DAG Information Option carries a number of metrics and other The DAG Information Object carries a number of metrics and other
information that allows a node to discover a DAG, select its DAG information that allows a node to discover a DAG, select its DAG
parents, and identify its siblings while employing loop avoidance parents, and identify its siblings while employing loop avoidance
strategies. strategies.
5.1.1. DAG Information Option (DIO) base option 5.1.3.1. DIO Base Option
The DAG Information Option is a container option carried within an The DIO Base Option is a container option, which is always present,
IPv6 Router Advertisement message as defined in [RFC4861], which and might contain a number of suboptions. The base option regroups
might contain a number of suboptions. The base option regroups the the minimum information set that is mandatory in all cases.
minimum information set that is mandatory in all cases.
0 1 2 3 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 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 | |G|D|A|0|0| Prf | Sequence | InstanceID | DAGRank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAGPreference | BootTimeRandom |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NodePref. | DAGRank | DAGDelay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DIOIntDoubl. | DIOIntMin. | DAGObjectiveCodePoint |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PathDigest |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
+ + + +
| DAGID | | DAGID |
+ + + +
| | | |
+ + + +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-option(s)... | sub-option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: DIO Base Option Figure 2: 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: Control Field: The DAG Control Field is currently allocated as
follows:
Grounded (G): The Grounded (G) flag is set when the DAG root Grounded (G): The Grounded (G) flag is set when the DAG root
is offering connectivity to an external routed is offering connectivity to an external routed
infrastructure such as the Internet. infrastructure such as the Internet.
Destination Advertisement Trigger (D): The Destination Destination Advertisement Trigger (D): The Destination
Advertisement Trigger (D) flag is set when the DAG root Advertisement Trigger (D) flag is set when the DAG root
or another node in the successor chain decides to trigger or another node in the successor chain decides to trigger
the sending of destination advertisements in order to the sending of destination advertisements in order to
update routing state for the outward direction along the update routing state for the outward direction along the
DAG, as further detailed in Section 5.9. Note that the DAG, as further detailed in Section 5.10. Note that the
use and semantics of this flag are still under use and semantics of this flag are still under
investigation. investigation.
Destination Advertisement Supported (A) : The Destination Destination Advertisement Supported (A): The Destination
Supported (A) bit is set when the DAG root is capable to Supported (A) bit is set when the DAG root is capable to
support the collection of destination advertisement support the collection of destination advertisement
related routing state and enables the operation of the related routing state and enables the operation of the
destination advertisement mechanism within the DAG. destination advertisement mechanism within the DAG.
Unassigned bits of the Flag Field are considered as reserved. DAGPreference (Prf): 3-bit unsigned integer set by the DAG
They MUST be set to zero on transmission and MUST be ignored on root to its preference and unchanged at propagation.
receipt. DAGPreference ranges from 0x00 (least preferred) to 0x07
(most preferred). The default is 0 (least preferred).
The DAG preference provides an administrative mechanism
to engineer the self-organization of the LLN, for example
indicating the most preferred LBR. If a node has the
option to join a more preferred DAG while still meeting
other optimization objectives, then the node will
generally seek to join the more preferred DAG as
determined by the OF.
Unassigned bits of the Control Field are considered as
reserved. They MUST be set to zero on transmission and MUST be
ignored on receipt.
Sequence Number: 8-bit unsigned integer set by the DAG root, Sequence Number: 8-bit unsigned integer set by the DAG root,
incremented according to a policy provisioned at the DAG root, incremented according to a policy provisioned at the DAG root,
and propagated with no change outwards along the DAG. Each and propagated with no change outwards along the DAG. Each
increment SHOULD have a value of 1 and may cause a wrap back to increment SHOULD have a value of 1 and may cause a wrap back to
zero. zero.
DAGPreference: 8-bit unsigned integer set by the DAG root to its InstanceID: 8-bit field indicating the topology instance associated
preference and unchanged at propagation. DAGPreference ranges with the DAG, as provisioned at the DAG root.
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 DAGRank: 8-bit unsigned integer indicating the DAG rank of the node
sending the RA-DIO message. The DAGRank of the DAG root is sending the DIO message. The DAGRank of the DAG root is
typically 1. DAGRank is further described in Section 5.3. ROOT_RANK. DAGRank is further described in Section 5.4.
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].
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 DAGID: 128-bit unsigned integer which uniquely identify a DAG. This
value is set by the DAG root. The global IPv6 address of the 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 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 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 rooting multiple DAGs the DAGID MUST be unique for each DAG
rooted at a specific DAG root. rooted at a specific DAG root.
The following values MUST NOT change during the propagation of RA-DIO The following values MUST NOT change during the propagation of DIO
messages outwards along the DAG: Type, Length, G, DAGPreference, messages outwards along the DAG:
DAGDelay and DAGID. All other fields of the RA-DIO message are Grounded (G)
updated at each hop of the propagation. Destination Advertisement Supported (A)
DAGPreference (Prf)
Sequence
InstanceID
DAGID
All other fields of the DIO message may be updated at each hop of the
propagation.
5.1.1.1. DAG Information Option (DIO) Suboptions 5.1.3.1.1. DIO Suboptions
In addition to the minimum options presented in the base option, In addition to the minimum options presented in the base option,
several suboptions are defined for the RA-DIO message: several suboptions are defined for the DIO message:
5.1.1.1.1. Format 5.1.3.1.1.1. Format
0 1 2 3 0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Subopt. Type | Subopt Length | Suboption Data... | Subopt. Type | Subopt Length | Subopt Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
Figure 2: DIO Suboption Generic Format Figure 3: DIO Suboption Generic Format
Suboption Type: 8-bit identifier of the type of suboption. When Suboption Type: 8-bit identifier of the type of suboption. When
processing a RA-DIO message containing a suboption for which processing a DIO message containing a suboption for which the
the Suboption Type value is not recognized by the receiver, the Suboption Type value is not recognized by the receiver, the
receiver MUST silently ignore the unrecognized option, continue receiver MUST silently ignore the unrecognized option, continue
to process the following suboption, correctly handling any to process the following suboption, correctly handling any
remaining options in the message. remaining options in the message.
Suboption Length: 8-bit unsigned integer, representing the length in Suboption Length: 16-bit unsigned integer, representing the length
octets of the suboption, not including the suboption Type and in octets of the suboption, not including the suboption Type
Length fields. and Length fields.
Suboption Data: A variable length field that contains data specific Suboption Data: A variable length field that contains data specific
to the option. to the option.
The following subsections specify the RA-DIO message suboptions which The following subsections specify the DIO message suboptions which
are currently defined for use in the DAG Information Option. are currently defined for use in the DAG Information Object.
Implementations MUST silently ignore any RA-DIO message suboptions Implementations MUST silently ignore any DIO message suboptions
options that they do not understand. options that they do not understand.
RA-DIO message suboptions may have alignment requirements. Following DIO message suboptions may have alignment requirements. Following
the convention in IPv6, these options are aligned in a packet such the convention in IPv6, these options are aligned in a packet such
that multi-octet values within the Option Data field of each option 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 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 at an integer multiple of n octets from the start of the header, for
n = 1, 2, 4, or 8). n = 1, 2, 4, or 8).
5.1.1.1.2. Pad1 5.1.3.1.1.2. Pad1
The Pad1 suboption does not have any alignment requirements. Its The Pad1 suboption does not have any alignment requirements. Its
format is as follows: format is as follows:
0 0
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
| Type = 0 | | Type = 0 |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
Figure 3: Pad 1 Figure 4: Pad 1
NOTE! the format of the Pad1 option is a special case - it has NOTE! the format of the Pad1 option is a special case - it has
neither Option Length nor Option Data fields. neither Option Length nor Option Data fields.
The Pad1 option is used to insert one octet of padding in the RA-DIO The Pad1 option is used to insert one or two octets of padding in the
message to enable suboptions alignment. If more than one octet of DIO message to enable suboptions alignment. If more than two octets
padding is required, the PadN option, described next, should be used of padding is required, the PadN option, described next, should be
rather than multiple Pad1 options. used rather than multiple Pad1 options.
5.1.1.1.3. PadN 5.1.3.1.1.3. PadN
The PadN option does not have any alignment requirements. Its format The PadN option does not have any alignment requirements. Its format
is as follows: is as follows:
0 1 0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Type = 1 | Subopt Length | Subopt Data | Type = 1 | Subopt Length | Subopt Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
Figure 4: Pad N Figure 5: Pad N
The PadN option is used to insert two or more octets of padding in The PadN option is used to insert three or more octets of padding in
the RA-DIO message to enable suboptions alignment. For N (N > 1) the DIO message to enable suboptions alignment. For N (N > 2) octets
octets of padding, the Option Length field contains the value N-2, of padding, the Option Length field contains the value N-3, and the
and the Option Data consists of N-2 zero-valued octets. PadN Option Option Data consists of N-3 zero-valued octets. PadN Option data
data MUST be ignored by the receiver. MUST be ignored by the receiver.
5.1.1.1.4. DAG Metric Container 5.1.3.1.1.4. DAG Metric Container
The DAG Metric Container suboption may be aligned as necessary to The DAG Metric Container suboption may be aligned as necessary to
support its contents. Its format is as follows: support its contents. Its format is as follows:
0 1 0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Type = 2 | Container Len | DAG Metric Data | Type = 2 | Container Length | DAG Metric Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
Figure 5: DAG Metric Container Figure 6: DAG Metric Container
The DAG Metric Container is used to report aggregated path metrics The DAG Metric Container is used to report aggregated path metrics
along the DAG. The DAG Metric Container may contain a number of along the DAG. The DAG Metric Container may contain a number of
discrete node, link, and aggregate path metrics as chosen by the discrete node, link, and aggregate path metrics as chosen by the
implementer. The Container Length field contains the length in implementer. The Container Length field contains the length in
octets of the DAG Metric Data. The order, content, and coding of the octets of the DAG Metric Data. The order, content, and coding of the
DAG Metric Container data is as specified in DAG Metric Container data is as specified in
[I-D.ietf-roll-routing-metrics]. [I-D.ietf-roll-routing-metrics].
The processing and propagation of the DAG Metric Container is The processing and propagation of the DAG Metric Container is
governed by implementation specific policy functions. governed by implementation specific policy functions.
5.1.1.1.5. Destination Prefix 5.1.3.1.1.5. Destination Prefix
The Destination Prefix suboption has an alignment requirement of The Destination Prefix suboption does not have any alignment
4n+1. Its format is as follows: requirements. Its format is as follows:
0 1 2 3 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 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| | Type = 3 | Length |Resvd|Prf|Resvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Lifetime | | Prefix Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | |
+-+-+-+-+-+-+-+-+ |
| Destination Prefix (Variable Length) | | Destination Prefix (Variable Length) |
. . . .
. . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: DAG Destination Prefix Figure 7: DAG Destination Prefix
The Destination Prefix suboption is used when the DAG root, or The Destination Prefix suboption is used when the DAG root, or
another node located inwards along the DAG on the path to the DAG another node located inwards along the DAG on the path to the DAG
root, needs to indicate that it offers connectivity to destination root, needs to indicate that it offers connectivity to destination
prefixes other than the default. This may be useful in cases where prefixes other than the default. This may be useful in cases where
more than one LBR is operating within the LLN and offering more than one LBR is operating within the LLN and offering
connectivity to different administrative domains, e.g. a home network connectivity to different administrative domains, e.g. a home network
and a utility network. In such cases, upon observing the Destination and a utility network. In such cases, upon observing the Destination
Prefixes offered by a particular DAG, a node MAY decide to join Prefixes offered by a particular DAG, a node MAY decide to join
multiple DAGs in support of a particular application. multiple DAGs in support of a particular application.
The Length is coded as the length of the suboption in octets, The Length is coded as the length of the suboption in octets,
excluding the Type and Length fields. excluding the Type and Length fields.
The Prefix Length is an 8-bit unsigned integer that indicates the Prf is the Route Preference as in [RFC4191]. The reserved fields
number of leading bits in the destination prefix. Prf is the Route MUST be set to zero on transmission and MUST be ignored on receipt.
Preference as in [RFC4191]. The reserved fields MUST be set to zero
on transmission and MUST be ignored on receipt.
The Prefix Lifetime is a 32-bit unsigned integer representing the The Prefix Lifetime is a 32-bit unsigned integer representing the
length of time in seconds (relative to the time the packet is sent) length of time in seconds (relative to the time the packet is sent)
that the Destination Prefix is valid for route determination. A that the Destination Prefix is valid for route determination. A
value of all one bits (0xFFFFFFFF) represents infinity. A value of value of all one bits (0xFFFFFFFF) represents infinity. A value of
all zero bits (0x00000000) indicates a loss of reachability. all zero bits (0x00000000) indicates a loss of reachability.
The Prefix Length is an 16-bit unsigned integer that indicates the
number of leading bits in the destination prefix.
The Destination Prefix contains Prefix Length significant bits of the The Destination Prefix contains Prefix Length significant bits of the
destination prefix. The remaining bits of the Destination Prefix, as destination prefix. The remaining bits of the Destination Prefix, as
required to complete the trailing octet, are set to 0. required to complete the trailing octet, are set to 0.
In the event that a RA-DIO message may need to specify connectivity In the event that a DIO message may need to specify connectivity to
to more than one destination, the Destination Prefix suboption may be more than one destination, the Destination Prefix suboption may be
repeated. repeated.
5.1.3.1.1.6. DAG Timer Configuration
The DAG Timer Configuration suboption does not have any alignment
requirements. Its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 4 | Length | DIOIntDoubl. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DIOIntMin. |
+-+-+-+-+-+-+-+-+
Figure 8: DAG Timer Configuration
The DAG Timer Configuration suboption is used to distribute
configuration information for DAG Timer Operation through the DAG.
The information communicated in this suboption is generally static
and unchanging within the DAG, therefore it is not necessary to
include in every DIO. This suboption MAY be included periodically by
the DAG Root, and SHOULD be included in response to a unicast
request, e.g. a DAG Information Solicitation (DIS) message.
The Length is coded as 2.
DIOIntervalDoublings is an 8-bit unsigned integer. Configured on the
DAG root and used to configure the trickle timer governing when 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 is an 8-bit unsigned integer. Configured on the DAG
root and used to configure the trickle timer governing when DIO
message should be sent within the DAG. The minimum configured
interval for the DIO trickle timer in units of ms is
2^DIOIntervalMin. For example, a DIOIntervalMin value of 16ms is
expressed as 4.
5.1.4. Destination Advertisement Object (DAO)
The Destination Advertisement Object (DAO) is used to propagate
destination information inwards along the DAG. The RPL use of the
DAO 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Sequence | InstanceID | DAO Rank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | RRCount | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Prefix (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse Route Stack (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: The Destination Advertisement Object (DAO)
DAO Sequence: Incremented by the node that owns the prefix for each
new DAO message for that prefix.
InstanceID: 8-bit field indicating the topology instance associated
with the DAG, as learned from the DIO.
DAO Rank: Set by the node that owns the prefix and first issues the
DAO message to its rank.
DAO Lifetime: 32-bit unsigned integer. The length of time in
seconds (relative to the time the packet is sent) that the
prefix is valid for route determination. A value of all one
bits (0xFFFFFFFF) represents infinity. A value of all zero
bits (0x00000000) indicates a loss of reachability.
Route Tag: 32-bit unsigned integer. The Route Tag may be used to
give a priority to prefixes that should be stored. This may be
useful in cases where intermediate nodes are capable of storing
a limited amount of routing state. The further specification
of this field and its use is under investigation.
Prefix Length: Number of valid leading bits in the IPv6 Prefix.
RRCount: 8-bit unsigned integer. This counter is used to count the
number of entries in the Reverse Route Stack. A value of `0'
indicates that no Reverse Route Stack is present.
Prefix: Variable-length field containing an IPv6 address or a prefix
of an IPv6 address. The Prefix Length field contains the
number of valid leading bits in the prefix. The bits in the
prefix after the prefix length (if any) are reserved and MUST
be set to zero on transmission and MUST be ignored on receipt.
Reverse Route Stack: Variable-length field containing a sequence of
RRCount (possibly compressed) IPv6 addresses. A node that adds
on to the Reverse Route Stack will append to the list and
increment the RRCount.
5.2. Conceptual Data Structures 5.2. Conceptual Data Structures
The RPL implementation MUST maintain the following conceptual data The RPL implementation MUST maintain the following conceptual data
structures in support of DAG discovery: structures in support of DAG discovery:
o A set of candidate neighbors o A set of candidate neighbors
o For each DAG: o For each DAG:
* A set of candidate DAG parents * A set of DAG parents and siblings
* A set of DAG parents (which are a subset of candidate DAG
parents and may be implemented as such)
5.2.1. Candidate Neighbors Data Structure 5.2.1. Candidate Neighbors Data Structure
The set of candidate neighbors is to be populated by neighbors who The set of candidate neighbors is to be populated by neighbors that
are discovered by the neighbor discovery mechanism and further are discovered by the neighbor discovery mechanism and further
qualified as statistically stable as per the mechanisms discussed in qualified as statistically stable as per the mechanisms discussed in
[I-D.ietf-roll-routing-metrics]. The candidate neighbors, and [I-D.ietf-roll-routing-metrics]. The candidate neighbors, and
related metrics, should demonstrate stability/reliability beyond a related metrics, should demonstrate stability/reliability beyond a
certain threshold, and it is recommended that a local confidence certain threshold, and it is recommended that a local confidence
value be maintained with respect to the neighbor in order to track value be maintained with respect to the neighbor in order to track
this. Implementations MAY choose to bound the maximum size of the this. Implementations MAY choose to bound the maximum size of the
candidate neighbor set, in which case a local confidence value will candidate neighbor set, in which case a local confidence value will
assist in ordering neighbors to determine which ones should remain in assist in ordering neighbors to determine which ones should remain in
the candidate neighbor set and which should be evicted. the candidate neighbor set and which should be evicted.
skipping to change at page 35, line 7 skipping to change at page 29, line 15
If Neighbor Unreachability Detection (NUD) determines that a If Neighbor Unreachability Detection (NUD) determines that a
candidate neighbor is no longer reachable, then it shall be removed candidate neighbor is no longer reachable, then it shall be removed
from the candidate neighbor set. In the case that the candidate from the candidate neighbor set. In the case that the candidate
neighbor has associated states in the DAG parent set or active DA neighbor has associated states in the DAG parent set or active DA
entries, then the removal of the candidate neighbor shall be entries, then the removal of the candidate neighbor shall be
coordinated with tearing down these states. All provisioned routes coordinated with tearing down these states. All provisioned routes
associated with the candidate neighbor should be removed. associated with the candidate neighbor should be removed.
5.2.2. Directed Acyclic Graphs (DAGs) Data Structure 5.2.2. Directed Acyclic Graphs (DAGs) Data Structure
A DAG may be uniquely identified by within the LLN by its unique At a given point of time, a DAG Iteration is uniquely identified by
DAGID. When a single device is capable to root multiple DAGs in the tuple (DagID, InstanceID, DAGSequenceNumber) where a change in
support of an application need for multiple optimization objectives the sequence denotes the iteration of a given DAG over time. When a
it is expected to produce a different and unique DAGID for each of single device is capable to root multiple DAGs in support of an
the multiple DAGs. application need for multiple optimization objectives it MUST produce
a different and unique (DagID, InstanceID) pair for each of the
multiple DAGs.
For each DAG that a node is, or may become, a member of, the For each DAG that a node is, or may become, a member of, the
implementation MUST keep a DAG table with the following entries: implementation MUST keep a DAG table with the following entries:
o InstanceID
o DAGID o DAGID
o DAGObjectiveCodePoint o DAGSequenceNumber
o A set of Destination Prefixes offered inwards along the DAG o DAG Metric Container, including DAGObjectiveCodePoint
o A set of candidate DAG parents o A set of Destination Prefixes offered inwards along the DAG
o A timer to govern the sending of RA-DIO messages for the DAG o A set of DAG parents and siblings
o DAGSequenceNumber o A timer to govern the sending of DIO messages for the DAG
When a DAG is discovered for which no DAG data structure is 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 instantiated, and the node wants to join, then the DAG data structure
become a candidate DAG parent in the Held-Up state), then the DAG is instantiated.
data structure is instantiated.
When the candidate DAG parent set is depleted (i.e. the last When the DAG parent set is depleted (i.e. the last DAG is removed),
candidate DAG parent has timed out of the Held-Down state), then the then the DAG data structure SHOULD be suppressed after the expiration
DAG data structure SHOULD be suppressed after the expiration of an of an implementation-specific local timer. An implementation SHOULD
implementation-specific local timer. An implementation SHOULD delay delay before deallocating the DAG data structure in order to observe
before deallocating the DAG data structure in order to observe that that the DAGSequenceNumber has incremented should any new DAG parents
the DAGSequenceNumber has incremented should any new candidate DAG appear for the DAG.
parents appear for the DAG.
5.2.2.1. Candidate DAG Parents Structure 5.2.2.1. DAG Parents/Siblings Structure
When the DAG is self-rooted, the set of candidate DAG parents is When the DAG is self-rooted, the set of DAG parents/siblings is
empty. empty.
In all other cases, for each candidate DAG parent in the set, the In all other cases, for each node in the set, the implementation MUST
implementation MUST keep a record of: keep a record of:
o a reference to the neighboring device which is the DAG parent o a reference to the neighboring device which is the DAG parent or
sibling
o a record of most recent information taken from the DAG Information o a record of most recent information taken from the DAG Information
Object last processed from the candidate DAG parent Object last processed from the DAG parent
o a state associated with the role of the candidate as a potential DAG parents may be ordered, according to the OF. When ordering DAG
DAG parent {Current, Held-Up, Held-Down, Collision}, further parents, in consultation with the OF, the most preferred DAG parent
described in Section 5.7 may be identified. All current DAG parents must have a rank less
than self. All current DAG siblings must have a rank equal to self.
o A DAG Hop Timer, if instantiated When nodes are added to or removed from the DAG set the most
preferred DAG parent may have changed. The role of all the nodes in
the list should be reevaluated. In particular, any nodes having a
rank greater than self after such a change must be evicted from the
set.
o A Held-Down Timer, if instantiated An implementation may choose to keep these records as an extension of
the Default Router List (DRL).
5.2.2.1.1. DAG Parents 5.3. DAG Rank
Note that the subset of candidate DAG parents in the `Current' state Based on the selection of DAG Parents, the metrics conveyed by the
comprises the set of DAG parents, i.e. the nodes actively acting as most preferred DAG parent, the nodes own metrics and configuration,
parents in the DAG. and a related function defined by the OF, a node will be able to
compute a value for its rank as a consequence of selecting a most
preferred DAG parent.
DAG parents may be ordered, according to the OCP. When ordering DAG The rank value feeds back into the DAG parent selection according to
parents, in consultation with the OCP, the most preferred DAG parent a loop-avoidance strategy. Once a DAG parent has been added, and a
may be identified. All current DAG parents must have a rank less rank value for the node within the DAG has been computed, the nodes
than or equal to that of the most preferred DAG parent. further options with regard to DAG parent selection and movement
within the DAG are restricted in favor of loop avoidance.
When nodes are added to or removed from the DAG parent set the most It is important to note that the DAG Rank is not itself a metric,
preferred DAG parent may have changed and should be reevaluated. Any although its value is derived from and influenced by the use of
nodes having a rank greater than self after such a change must be metrics to select DAG parents and take up a position in the DAG. The
placed in the Held-Down state and evicted as per the procedures only aim of the rank is to inform loop avoidance and detection.
described in Section 5.7
An implementation may choose to keep these records as an extension of The computation of the DAG Rank MUST be done in such a way so as to
the Default Router List (DRL). maintain the following properties for any nodes M and N that are
neighbors in the LLN:
5.3. DAG Discovery and Maintenance DAGRank(M) is less than DAGRank(N): In this case, 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.
Further, for a node N, all parents in the DAG parent set must
be of rank less than self's DAGRank(N). In other words, the
rank presented by a node N MUST be greater (deeper) than that
presented by any of its parents.
DAG discovery locates the nearest sink, as determined according to DAGRank(M) equals DAGRank(N): In this case M and N are located
some metrics and constraints, and forms a Directed Acyclic Graph positions of relatively the same optimality within the DAG.
towards that sink, by identifying a set of DAG parents. During this In some cases, Node M may be used as a successor by Node N,
process DAG discovery also identifies siblings, which may be used but with related chance of creating a loop that must be
later to provide additional path diversity towards the DAG root. DAG detected and broken by some other means.
discovery enables nodes to implement different policies for selecting
their DAG parents in the DAG by using implementation specific policy DAGRank(M) is greater than DAGRank(N): In this case, then node M is
functions. DAG discovery specifies a set of rules to be followed by located in a less preferred position than N in the DAG with
all implementations in order to ensure interoperation. DAG discovery respect to the metrics and optimizations defined by the
also standardizes the format that is used to advertise the most objective code point. Further, Node (M) may in fact be in
common information that is used in order to select DAG parents. Node (N)'s sub-DAG. There is a higher risk to Node (N)
selecting Node (M) as a DAG parent, as such a selection may
create a loop.
As an example, the DAG Rank could be computed in such a way so as to
closely track ETX when the objective function is to minimize ETX, or
latency when the objective function is to minimize latency, or in a
more complicated way as appropriate to the objective code point being
used within the DAG.
5.4. DAG Discovery and Maintenance
DAG discovery locates the nearest sink (aka root), 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 One of these information, the DAG rank, is used by DAG discovery to
provide loop avoidance even if nodes implement different policies. provide loop avoidance even if nodes implement different policies.
The DAG Rank is computed as specified by the Objective Code Point in The DAG Rank is computed as specified by the OF in use by the DAG,
use by the DAG, demonstrating the properties described in demonstrating the properties described in Section 5.3. The rank
Section 3.2.1.7. The rank should be computed in such a way so as to should be computed in such a way so as to provide a comparable basis
provide a comparable basis with other nodes which may not use the with other nodes which may not use the same metric at all.
same metric at all.
The DAG discovery procedures take into account a number of factors, The DAG discovery procedures take into account a number of factors,
including: including:
o RPL rules for loop avoidance based on rank o RPL rules for loop avoidance based on DAGs and ranks
o The OCP function o The Objective Function
o The advertised metrics o The advertised metrics
o Local policy functions (e.g. a bounded number of candidate o Local policy functions (e.g. a bounded number of candidate
neighbors). neighbors).
5.3.1. DAG Discovery Rules 5.4.1. DAG Discovery Rules
In order to organize and maintain loopless structure, the DAG In order to organize and maintain loopless structure, the DAG
discovery implementation in the nodes MUST obey to the following discovery implementation in the nodes MUST obey to the following
rules and definitions: rules and definitions:
1. A node that does not have any DAG parents in a DAG is the root 5.4.1.1. DAGs
of its own floating DAG. It's rank is 1. A node will end up in
that situation when it looses all of its current feasible
parents, i.e. the set of DAG parents becomes depleted. In that
case, the node SHOULD remember the DAGID and the sequence
counter of the last RA-DIO message from the lost parents for a
period of time which covers multiple RA-DIO messages. This is
done so that if the node does encounter another possible
attachment point to the lost DAGID within a period of time, the
node may observe a sequence counter change by comparing the
observed sequence counter to the last observed sequence counter
and thus verify that the new attachment point is a viable and
independent alternative to attach back to the lost DAGID.
2. A node that is attached to an infrastructure that does not 1. DAG discovery instantiates LLN topologies that are each optimized
support RA-DIO messages, is the DAG root of its own grounded for specific constraints and goals. A topology assumes the shape
DAG. It's rank is 1. (For example an LBR that is in of a DAG, and a DAG Instance is uniquely identified by its
communication with a non-LLN router not running RPL). instanceID.
3. A (non-LLN) router sending a RA messages without DIO is 2. For reasons of scalability and operations of the protocol, a DAG
considered a grounded infrastructure at rank 0. (For example, a Instance is partitioned into a set of DAGs rooted at a
router that is in communication with an LLN node but not running destination, aka Destination Oriented DAGs. A destination is
RPL such as a non-LLN public Internet router in communication uniquely identified by a DAGID so a DAG rooted at a destination
with an LBR) is uniquely identified by the pair (InstanceID, DAGID).
4. The DAG root exposes the DAG in the RA-DIO message and nodes 3. A Destination Oriented DAG is periodically reconstructed from the
propagate the RA-DIO message outwards along the DAG with the RAs root, by incrementing a DAGSequenceNumber. An Iteration of a
that they forward over their LLN links. Destination Oriented DAG is thus uniquely identified by the tuple
(InstanceID, DAGID, DAGSequenceNumber). Through this document,
the graph formed by this iterative process is referred to as the
DAG Iteration, or in short, the DAG.
5. A node MAY move at any time, with no delay, within its DAG when 4. The rank is defined within the scope of a DAG Iteration as an
the move does not cause the node to increase its own DAG rank, abstract coordinate to compare the relative position of nodes and
as per the rank calculation indicated by the OCP. ensure forward progress of the traffic.
6. A node MUST NOT move outwards along a DAG that it is attached 5. A node MUST belong at most to one DAG Iteration per InstanceID
to, causing the DAG rank to increase, except in a special case and MUST select all its parents and siblings within that same DAG
where the node MAY choose to follow the last DAG parent in the Iteration.
set of DAG parents. In the general case, if a node is required
to move such that it cannot stay within the DAG without a rank
increase, then it needs to first leave the DAG. In other words
a node that is already part of a DAG MAY move or follow a DAG
parent at any time and with no delay in order to be closer, or
stay as close, to the DAG root of its current DAG as it already
is, but may not move outwards. RAs received from other routers
located at lesser rank in the same DAG may be considered as
coming from candidate parents. RAs received from other routers
located at the same rank in the same DAG may be considered as
coming from siblings. Nodes MUST ignore RAs that are received
from other routers located at greater rank within the same DAG.
7. A node may jump from its current DAG into any different DAG if 5.4.1.2. DAG Sequence Number
it is preferred for reasons of connectivity, configured
preference, free medium time, size, security, bandwidth, DAG
rank, or whatever metrics the LLN cares to use. A node may jump
at any time and to whatever rank it reaches in the new DAG, but
it may have to wait for a DAG Hop timer to elapse in order to do
so. This allows the new higher parts (closer to the sink) of
the DAG to move first, thus allowing stepped DAG
reconfigurations and limiting relative movements. A node SHOULD
NOT join a previous DAG (identified by its DAGID) unless the
sequence number in the RA-DIO message has incremented since the
node left that DAG. A newer sequence number indicates that the
candidate parents were not attached behind this node, as they
kept getting subsequent RA-DIO messages with new sequence
numbers from the same DAG. In the event that old sequence
numbers (two or more behind the present value) are encountered
they are considered stale and the corresponding parent SHOULD be
removed from the set.
8. If a node has selected a new set of DAG parents but has not 1. The DAGSequenceNumber is incremented by the root and flooded
moved yet (because it is waiting for DAG Hop timer to elapse), through DIOs.
the node is unstable MUST NOT send RA-DIOs for that DAG.
9. If a node receives a RA-DIO from one of its DAG parents, and if 2. The root floods a new DAGSequenceNumber periodically, at a rate
the parent contains a different DAGID, indicating that the that depends on the deployment. This rate can be set to 0 if
parent has left the DAG, and if the node can remain in the other methods such as loop detection are considered sufficient to
current DAG through an alternate DAG parent, then the node solve the routing issues in that deployment.
SHOULD remove the DAG parent which has joined the new DAG from
its DAG parent set and remain in the original DAG. If there is
no alternate parent for the DAG, then the node SHOULD follow
that parent into the new DAG.
10. When a node detects or causes a DAG inconsistency, as described 3. The root MAY also flood a new DAGSequenceNumber on-demand. The
in Section 5.3.4.2, then the node SHOULD send an unsolicited RA- details of the mechanism to signal the root to do so are to be
DIO message to its one-hop neighbors. The RA-DIO is updated to specified in a future revision of this document.
propagate the new DAG information. Such an event MUST also
cause the trickle timer governing the periodic sending of RA-DIO
messages to be reset.
11. If a DAG parent increases its rank such that the node rank would 4. A parent that advertises the new DAGSequenceNumber can not
have to change, and if the node does not wish to follow (e.g. it possibly belong to the sub-DAG of a node that still advertises an
has alternate options), then the DAG parent SHOULD be evicted older DAGSequenceNumber. The node MAY thus attach to that parent
from the DAG parent set. If the DAG parent is the last in the regardless of the relative rank, and this situation is equivalent
DAG parent set, then the node SHOULD chose to follow it. to jumping onto a different Destination Oriented DAG.
5.3.2. Reception and Processing of RA-DIO messages 5. Thus, as a new DAGSequenceNumber spreads, a new DAG Iteration
forms that supersedes the previous one. During a
DAGSequenceNumber transition, a node MAY decide to forward
packets via 'future parents' that belong to the same Destination
Oriented DAG (same InstanceID and DagID), but a more recent
(incremented) DAGSequenceNumber.
When an RA-DIO message is received from a source device named SRC, 5.4.1.3. DAG Root
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.
5.3.2.1. Determination of Eligibility for DIO Processing 1. A node that does not have any DAG parent MAY become the root of
its own floating DAG. It's rank is ROOT_RANK.
If the RA-DIO message is malformed, then the RA-DIO message is not 2. A (non-LLN) router is considered connected to a grounded
eligible for further processing and is silently discarded. A RPL infrastructure at rank BASE_RANK. A LLN node that is attached to
implementation MAY log the reception of a malformed RA-DIO such an infrastructure router is the DAG root of its own grounded
message. DAG. It's rank is ROOT_RANK.
If SRC is not a member of the candidate neighbor set, then the RA- 3. In a deployment that uses a backbone link to federate a number of
DIO is not eligible for further processing. (Further evaluation/ LLN roots, it is possible to run RPL over the backbone and use
confidence of this neighbor is necessary) one router as a backbone root. The backbone root exposes a rank
of BASE_RANK over the backbone. All the LLN roots that are
parented to that backbone root, including the backbone root if it
also serves as LLN root, expose a rank of ROOT_RANK over the LLN
and act as multiple roots for a same DAG, coordinated by the
backbone root.
If the RA-DIO message advertises a DAG that the node is already a 4. The DAG root exposes the DAG in the DIO message and LLN nodes
member of, then: propagate the DIO message outwards along the DAG.
If the rank of SRC as reported in the RA-DIO message is lesser 5.4.1.4. Moving Inside a DAG
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 1. A node moves when it changes its parent selection within the same
to that of the node within the DAG, then SRC is marked as a DAG Iteration. When a node moves (within its DAG) in a fashion
sibling and the RA-DIO message is not eligible for further that cause its rank to decrease, the node MUST abandon all
processing. parents and siblings with a rank larger than self, and MAY adopt
as siblings nodes with the same rank.
If the rank of SRC as reported in the RA-DIO message is higher 2. A node MAY move at any time, with no delay, within its DAG when
than that of the node within the DAG, and SRC is not a DAG the move does not cause the node to increase its own DAG rank, as
parent, then the RA-DIO message MUST NOT be considered for per the rank calculation indicated by the OF.
further processing
If SRC is a DAG parent for any other DAG that the node is attached 3. A node MUST NOT move outwards along a DAG that it is attached to,
to, then the RA-DIO message MUST be considered for further causing the DAG rank to increase. If a node cannot stay within
processing (the DAG parent may have jumped). the DAG without a rank increase, then it MUST poison its routes
as described in Section 5.4.1.6.
If the RA-DIO message advertises a DAG that offers a better (new 4. When DIO messages are received from other routers located at
or alternate) solution to an optimization objective desired by the lesser rank in the same DAG, those routers are eligible for
node, then the RA-DIO message MUST be considered for further consideration as DAG parents. DIO messages received from other
processing. routers located at the same rank in the same DAG may be
considered as coming from siblings. DIO messages that are
received from other routers located at greater rank within the
same DAG might cause greedy behaviors and loops; such a DIO is
ignored unless:
5.3.2.2. Overview of RA-DIO Message Processing 1. The DIO comes from an existing parent or sibling; in which
case that parent must be removed.
If the received RA-DIO message is for a new/alternate DAG: 2. The DIO comes from a node that has better OF ratings than any
parent known at this point; in that case, this potential
parent MAY be remembered in order to jump at a better
position when the next sequence is flooded.
Instantiate a data structure for the new/alternate DAG if 5.4.1.5. Jumping Onto Another DAG
necessary
Place the neighbor in the candidate DAG parent set 1. A node jumps when it performs a new parent selection whereby its
DAG Iteration changes within the same DAG Instance. When a node
jumps onto a new DAG Iteration, it MUST abandon all parents and
siblings from its previous position.
If the node has sent an RA message within the risk window as 2. A node MAY jump from its current DAG onto any other DAG that
described in Section 5.7.3 then perform the collision detection provides service for the same InstanceID if it is preferred by
described in Section 5.7.3. If a collision occurs, place the the OF, for example for reasons such as connectivity, configured
candidate DAG parent in the collision state and do not process preference, free medium time, size, security, bandwidth, DAG
the RA-DIO message any further as described in Section 5.7. rank, or whatever metrics the LLN uses. This is allowed
regardless of the rank that the node reaches in the new DAG.
3. A node that jumps should attempt to transmit all the packets
received as part of the previous DAG along the previous DAG. In
other words, it should switch the parent set only after the
outstanding packet queue of packets received prior to announcing
the jump is exhausted.
4. Jumping back onto a previous DAG is equivalent to moving inside
that DAG and obeys the same rules. To satisfy this, a node
detaching from a DAG SHOULD remember its DAG as identified by the
tuple (InstanceID, DagID, DAGSequenceNumber) as well as its rank
within that DAG for long as that DAG exists.
5.4.1.6. Poisoning a Broken Path
1. A node SHOULD poison its inwards routes when it looses all of its
current feasible parents, i.e. the set of DAG parents becomes
depleted, and it can not jump onto an alternate DAG.
2. In order to poison its inwards routes, a node MAY stay at its
position within its DAG (that is maintain its InstanceID, DagID,
DAGSequenceNumber and Rank) but it SHOULD immediately advertise a
rank of INFINITE_RANK in a DIO so as to force all its children to
remove it from their parent list and try an alternate path. The
node SHOULD then wait for a new DAG Iteration (DAGSequenceNumber
increment) before resuming its operation in the same Destination
Oriented DAG.
3. Alternatively, a node MAY detach from its DAG. A node that
detaches becomes root of its own floating DAG and MUST
immediately advertise its new situation in a DIO.
4. Either way, the route poisoning will recursively be flooded
throughout the impacted sub-DAG as children lose their last
parent in the original DAG.
5. The loss of a DIO message may interrupt the flooding. This can
be compensated by cheer repetition through the trickle algorithm.
If that also fails, packet loops will be prevented by the
detection mechanism described in Section 5.11.
5.4.1.7. Following a Parent
1. If a node that receives a DIO from one of its DAG parents
indicating that the parent has left the DAG, it may either follow
that parent or stay in its current DAG through an alternate DAG
parent if that is possible.
2. If a DAG parent increases its rank such that the node rank would
have to change, and if the node does not wish to follow (e.g. it
has alternate options), then the DAG parent SHOULD be evicted
from the DAG parent set. If the DAG parent is the last in the
DAG parent set, then the node SHOULD chose to follow it.
5.4.1.8. DAG Inconsistency
1. When a node detects or causes a DAG inconsistency, as described
in Section 5.4.4.2, then the node SHOULD send an unsolicited DIO
message to its one-hop neighbors. The DIO is updated to
propagate the new DAG information. Such an event MUST also cause
the trickle timer governing the periodic sending of DIO messages
to be reset.
5.4.2. Reception and Processing of DIO messages
When an DIO message is received from a source device named SRC, the
receiving node must first determine whether or not the DIO message
should be accepted for further processing, and subsequently present
the DIO message for further processing if eligible.
1. If the DIO message is malformed, then the DIO message is not
eligible for further processing and is silently discarded. A RPL
implementation MAY log the reception of a malformed DIO message.
2. If SRC is not a member of the candidate neighbor set, then the
DIO is not eligible for further processing. (Further evaluation/
confidence of this neighbor is necessary)
3. If the DIO message advertises a DAG that the node is already a
member of, then:
* If the rank of SRC as reported in the DIO message is lesser
than that of the node within the DAG, then the DIO message
MUST be considered for further processing.
* If the rank of SRC as reported in the DIO message is equal to
that of the node within the DAG, then SRC is marked as a
sibling and the DIO message is not eligible for further
processing.
* If the rank of SRC as reported in the DIO message is higher
than that of the node within the DAG, and SRC is not a DAG
parent, then the DIO message MUST NOT be considered for
further processing
4. Even if not processed further, information from a DIO might be
remembered for instance if SRC is preferable to the current
parents per the OF selection process.
5. If SRC is a DAG parent for any other DAG that the node is
attached to, then the DIO message MUST be considered for further
processing (the DAG parent may have jumped).
6. If the DIO message advertises a DAG that offers a better (new or
alternate) solution to an optimization objective desired by the
node, then the DIO message MUST be considered for further
processing.
5.4.2.1. Overview of DIO Message Processing
If the received DIO message is for a new/alternate DAG:
If the node has sent an DIO message within the risk window as
described in Section 5.8 then a collision has occurred; do not
process the DIO message any further.
If the SRC node is also a DAG parent for another DAG that the 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 node is a member of, and if the new/alternate DAG is the same
equivalent optimization objective as the other DAG, then the InstanceID as the other DAG, then the DAG parent is known to
DAG parent is known to have jumped. have jumped.
Remove SRC as a DAG parent from the other DAG (place it in Remove SRC as a DAG parent from the other DAG
the held-down state)
If the other DAG is now empty of candidate parents, then If the other DAG is now empty of candidate parents, then
directly follow SRC into the new DAG by adding it as a DAG prepare to directly follow SRC into the new DAG by adding it
parent in the Current state, else ignore the RA-DIO message as a DAG parent for the new DAG, else ignore the DIO message
(do not follow the parent). (do not follow the parent).
Instantiate a data structure for the new/alternate DAG if
necessary
If the new/alternate DAG offers a better solution to the If the new/alternate DAG offers a better solution to the
optimization objectives, then prepare to jump: copy the DIO optimization objectives, then jump: copy the DIO information
information into the record for the candidate DAG parent, place place the neighbor into the DAG parent set.
the candidate DAG parent into the Held-Up state, and start the
DAG Hop timer as per Section 5.7.1.
If the RA-DIO message is for a known/existing DAG: If the DIO message is for a known/existing DAG:
Process the RA-DIO message as per the rules in Section 5.3 Process the DIO message as per the rules in Section 5.4
As candidate parents are identified, they may subsequently be As DIO messages are received from candidate neighbors, the neighbors
promoted to DAG parents by following the rules of DAG discovery as may be promoted to DAG parents by following the rules of DAG
described in Section 5.3. When a node adds another node to its set discovery as described in Section 5.4. When a node places a neighbor
of candidate parents, the node becomes attached to the DAG through into the DAG Parent set, the node becomes attached to the DAG through
the parent node. the new parent node.
In the DAG discovery implementation, the most preferred parent should In the DAG discovery implementation, the most preferred parent should
be used to restrict which other nodes may become DAG parents. Some 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 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 the most preferred DAG parent. (This case may occur, for example, if
an energy constrained device is at a lesser rank but should be an energy constrained device is at a lesser rank but should be
avoided as per an optimization objective, resulting in a more avoided as per an optimization objective, resulting in a more
preferred parent at a greater rank). preferred parent at a greater rank).
5.3.3. RA-DIO Transmission 5.4.3. DIO Transmission
Each node maintains a timer that governs when to multicast RA Each node maintains a timer that governs when to multicast DIO
messages. This timer is implemented as a trickle timer operating messages. This timer is implemented as a trickle timer operating
over a variable interval. Trickle timers are further detailed in over a variable interval. Trickle timers are further detailed in
Section 5.3.4. The governing parameters for the timer should be Section 5.4.4. The governing parameters for the timer should be
configured consistently across the DAG, and are provided by the DAG configured consistently across the DAG, and are provided by the DAG
root in the RA-DIO message. In addition to periodic RA messages, root in the DIO message. In addition to periodic DIO messages, each
each LLN node will respond to Router Solicitation (RS) messages node may respond to a DIS message with a DIO message.
according to [RFC4861].
o When a node is unstable, because any DAG Hop timer is running in
preparation for a jump, then the node MUST NOT transmit
unsolicited RA-DIOs (i.e. the node will remain silent when the
timer expires).
o When a node detects an inconsistency, it SHOULD reset the interval o When a node detects an inconsistency, it SHOULD reset the interval
of the trickle timer to a minimum value, causing RA messages to be of the trickle timer to a minimum value, causing DIO messages to
emitted more frequently as part of a strategy to quickly correct be emitted more frequently as part of a strategy to quickly
the inconsistency. Such inconsistencies may be, for example, an correct the inconsistency. Such inconsistencies may be, for
update to a key parameter (e.g. sequence number) in the RA-DIO example, an update to a key parameter (e.g. sequence number) in
message or a loop detected when a node located inwards along the the DIO message or a loop detected when a node located inwards
DAG forwards traffic outwards. Inconsistencies are further along the DAG forwards traffic outwards. Inconsistencies are
detailed in Section 5.3.4.2. further detailed in Section 5.4.4.2.
o When a node enters a mode of consistent operation within a DAG, o When a node enters a mode of consistent operation within a DAG,
i.e. RA-DIO messages from its DAG parents are consistent and no i.e. DIO messages from its DAG parents are consistent and no
other inconsistencies are detected, it may begin to open up the other inconsistencies are detected, it may begin to open up the
interval of the trickle timer towards a maximum value, causing RAs interval of the trickle timer towards a maximum value, causing DIO
to be emitted less frequently, thus reducing network maintenance messages to be emitted less frequently, thus reducing network
overhead and saving energy consumption (which is of utmost maintenance overhead and saving energy consumption.
importance for battery-operated nodes).
o When a node is initialized, it MAY be configured to remain silent o When a node is initialized, it MAY be configured to remain silent
and not multicast any RA messages until it has encountered and and not multicast any DIO messages until it has encountered and
joined a DAG (perhaps initially probing for a nearby DAG with an joined a DAG (perhaps initially probing for a nearby DAG with an
RS message). Alternately, it may choose to root its own floating DIS message). Alternately, it may choose to root its own floating
DAG and begin multicasting RAs using a default trickle DAG and begin multicasting DIO messages using a default trickle
configuration. The second case may be advantageous if it is configuration. The second case may be advantageous if it is
desired for independent nodes to begin aggregating into scattered desired for independent nodes to begin aggregating into scattered
floating DAGs in the absence of a grounded node, for example in floating DAGs in the absence of a grounded node, for example in
support of LLN installation and commissioning. support of LLN installation and commissioning.
Note that if multiple DAG roots are participating in the same DAG, 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 i.e. offering DIO messages with the same DAGID, then they must
coordinate with each other to ensure that their RA-DIO messages are coordinate with each other to ensure that their DIO messages are
consistent when they emit RA-DIO messages. In particular the consistent when they emit DIO messages. In particular the Sequence
Sequence number must be identical from each DAG root, regardless of number must be identical from each DAG root, regardless of which of
which of the multiple DAG roots issues the RA-DIO message, and the multiple DAG roots issues the DIO message, and changes to the
changes to the Sequence number should be issued at the same time. Sequence number should be issued at the same time. The specific
The specific mechanism of this coordination, e.g. along a non-LLN mechanism of this coordination, e.g. along a non-LLN network between
network between DAG roots, is beyond the scope of this specification. DAG roots, is beyond the scope of this specification.
5.3.4. Trickle Timer for RA Transmission 5.4.4. Trickle Timer for DIO Transmission
RPL treats the construction of a DAG as a consistency problem, and RPL treats the construction of a DAG as a consistency problem, and
uses a trickle timer [Levis08] to control the rate of control uses a trickle timer [Levis08] to control the rate of control
broadcasts. broadcasts.
For each DAG that a node is part of, the node must maintain a single For each DAG that a node is part of, the node must maintain a single
trickle timer. The required state contains the following conceptual trickle timer. The required state contains the following conceptual
items: items:
I: The current length of the communication interval I: The current length of the communication interval
T: A timer with a duration set to a random value in the range T: A timer with a duration set to a random value in the range
[I/2, I] [I/2, I]
C: Redundancy Counter C: Redundancy Counter
I_min: The smallest communication interval in milliseconds. This I_min: The smallest communication interval in milliseconds. This
value is learned from the RA-DIO message as value is learned from the DIO message as (2^DIOIntervalMin)ms.
(2^DIOIntervalMin)ms. The default value is The default value is DEFAULT_DIO_INTERVAL_MIN.
DEFAULT_DIO_INTERVAL_MIN.
I_doublings: The number of times I_min should be doubled before I_doublings: The number of times I_min should be doubled before
maintaining a constant rate, i.e. I_max = I_min * maintaining a constant rate, i.e. I_max = I_min *
2^I_doublings. This value is learned from the RA-DIO message 2^I_doublings. This value is learned from the DIO message as
as DIOIntervalDoublings. The default value is DIOIntervalDoublings. The default value is
DEFAULT_DIO_INTERVAL_DOUBLINGS. DEFAULT_DIO_INTERVAL_DOUBLINGS.
5.3.4.1. Resetting the Trickle Timer 5.4.4.1. Resetting the Trickle Timer
The trickle timer for a DAGID is reset by: The trickle timer for a DAGID is reset by:
1. Setting I_min and I_doublings to the values learned from the RA- 1. Setting I_min and I_doublings to the values learned from the DIO
DIO message. message.
2. Setting C to zero. 2. Setting C to zero.
3. Setting I to I_min. 3. Setting I to I_min.
4. Setting T to a random value as described above. 4. Setting T to a random value as described above.
5. Restarting the trickle timer to expire after a duration T 5. Restarting the trickle timer to expire after a duration T
When node learns about a DAG through a RA-DIO message and makes the When node learns about a DAG through a DIO message and makes the
decision to join it, it initializes the state of the trickle timer by decision to join it, it initializes the state of the trickle timer by
resetting the trickle timer and listening. Each time it hears a resetting the trickle timer and listening. Each time it hears a
consistent RA for this DAG from a DAG parent, it MAY increment C. consistent DIO message 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 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 constant, DEFAULT_DIO_REDUNDANCY_CONSTANT. If C is less than that
value, the node generates a new RA and broadcasts it. When the value, the node generates a new DIO message and multicasts it. When
communication interval I expires, the node doubles the interval I so the communication interval I expires, the node doubles the interval I
long as it has previously doubled it fewer than I_doubling times, so long as it has previously doubled it fewer than I_doubling times,
resets C, and chooses a new T value. resets C, and chooses a new T value.
5.3.4.2. Determination of Inconsistency 5.4.4.2. Determination of Inconsistency
The trickle timer is reset whenever an inconsistency is detected The trickle timer is reset whenever an inconsistency is detected
within the DAG, for example: within the DAG, for example:
o The node joins a new DAGID o The node joins a new DAGID
o The node moves within a DAGID o The node moves within a DAGID
o The node receives a modified RA-DIO message from a DAG parent o The node receives a modified DIO message from a DAG parent
o A DAG parent forwards a packet intended to move inwards, o A DAG parent forwards a packet intended to move inwards,
indicating an inconsistency and possible loop. indicating an inconsistency and possible loop.
o A metric communicated in the RA-DIO message is determined to be o A metric communicated in the DIO message is determined to be
inconsistent, as according to a implementation specific path inconsistent, as according to a implementation specific path
metric selection engine. metric selection engine.
o The rank of a DAG parent has changed. o The rank of a DAG parent has changed.
5.4. DAG Heartbeat 5.5. DAG Sequence Number Increment
The DAG root makes the sole determination of when to revise the The DAG root makes the sole determination of when to revise the
DAGSequenceNumber by incrementing it upwards. When the DAGSequenceNumber by incrementing it upwards. When the
DAGSequenceNumber is increased an inconsistency results, causing RA- DAGSequenceNumber is increased an inconsistency results, causing DIO
DIO messages to be sent back outwards along the DAG to convey the messages to be sent back outwards along the DAG to convey the change.
change. The degree to which this mechanism is relied on may be The degree to which this mechanism is relied on may be determined by
determined by the implementation- on one hand it may serve as a the implementation- on one hand it may serve as a periodic heartbeat,
periodic heartbeat, refreshing the DAG states, and on the other hand refreshing the DAG states, and on the other hand it may result in a
it may result in a constant steady-state control cost overhead which constant steady-state control cost overhead which is not desirable.
is not desirable.
Some implementations may provide an administrative interface, such as Some implementations may provide an administrative interface, such as
a command line, at the DAG root whereby the DAGSequenceNumber may be a command line, at the DAG root whereby the DAGSequenceNumber may be
caused to increment in response to some policy outside of the scope caused to increment in response to some policy outside of the scope
of RPL. of RPL.
Other implementations may make use of a periodic timer to Other implementations may make use of a periodic timer to
automatically increment the DAGSequenceNumber, resulting in a automatically increment the DAGSequenceNumber, resulting in a
periodic DAG Heartbeat at a rate appropriate to the application and periodic DAG iteration at a rate appropriate to the application and
implementation. implementation. Other automated mechanisms to determine
DAGSequenceNumber increments are also possible as appropriate to a
deployment.
5.5. DAG Selection 5.6. DAG Selection
The DAG selection is implementation and algorithm dependent. Nodes The DAG selection is implementation and algorithm dependent. Nodes
SHOULD prefer to join DAGs advertising OCPs and destinations SHOULD prefer to join DAGs for InstanceIDs advertising OCPs and
compatible with their implementation specific objectives. In order destinations compatible with their implementation specific
to limit erratic movements, and all metrics being equal, nodes SHOULD objectives. In order to limit erratic movements, and all metrics
keep their previous selection. Also, nodes SHOULD provide a means to being equal, nodes SHOULD keep their previous selection. Also, nodes
filter out a candidate parent whose availability is detected as SHOULD provide a means to filter out a candidate parent whose
fluctuating, at least when more stable choices are available. Nodes availability is detected as fluctuating, at least when more stable
MAY place the failed candidate parent in a Hold Down mode that choices are available.
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 When connection to a fixed network is not possible or preferable for
security or other reasons, scattered DAGs MAY aggregate as much as security or other reasons, scattered DAGs MAY aggregate as much as
possible into larger DAGs in order to allow connectivity within the possible into larger DAGs in order to allow connectivity within the
LLN. LLN.
A node SHOULD verify that bidirectional connectivity and adequate A node SHOULD verify that bidirectional connectivity and adequate
link quality is available with a candidate neighbor before it link quality is available with a candidate neighbor before it
considers that candidate as a DAG parent. considers that candidate as a DAG parent.
5.6. Administrative rank 5.7. Administrative rank
When the DAG is formed under a common administration, or when a node 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 performs a certain role within a community, it might be beneficial to
associate a range of acceptable rank with that node. For instance, a 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 node that has limited battery should be a leaf unless there is no
other choice, and may then augment the rank computation specified by other choice, and may then augment the rank computation specified by
the OCP in order to expose an exaggerated rank. the OF in order to expose an exaggerated rank.
5.7. Candidate DAG Parent States and Stability
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.
5.7.1. Held-Up
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.
5.7.2. Held-Down
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.
5.7.3. Collision
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.
5.7.4. Instability 5.8. Collision
A node is instable when it is prepared to shortly replace a set of A race condition occurs if 2 nodes send DIO messages at the same time
DAG parents in order to jump to a different DAGID. This happens and then attempt to join each other. This might happen, for example,
typically when the node has selected a more preferred candidate between nodes which act as DAG root of their own DAGs. In order to
parent in a different DAG and has to wait for the DAG hop timer to detect the situation, LLN Nodes time stamp the sending of DIO
elapse before adjusting the DAG parent set. Instability may also message. Any DIO message received within a short link-layer-
occur when the entire current DAG parent set is lost and the next dependent period introduces a risk. It is up to the implementation
best candidates are still held up. Instability is resolved when the to define the duration of the risk window.
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 There is risk of a collision when a node receives and processes a DIO
node is unstable, it MUST NOT send RAs with the DIO message. This within the risk window. For example, it may occur that two nodes are
avoids loops when node A decides to attach to node B and node B associated with different DAGs and near-simultaneously send DIO
decides to attach to node A. Unless RAs cross (see Collision messages, which are received and processed by both, and possibly
section), a node receives RA-DIO messages from stable candidate result in both nodes simultaneously deciding to attach to each other.
parents, which do not plan to attach to the node, so the node can As a remedy, in the face of a potential collision, as determined by
safely attach to them. receiving a DIO within the risk window, the DIO message is not
processed. It is expected that subsequent DIOs would not cross.
5.8. Guidelines for Objective Code Points 5.9. Guidelines for Objective Functions
5.8.1. Objective Function 5.9.1. Objective Function
An Objective Function (OF) allows for the selection of a DAG to join, 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 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 compute an ordered list of parents. The OF is also responsible to
guidance. The OF is also responsible to compute the rank of the compute the rank of the device within the DAG.
device within the DAG.
The Objective Function is specified in the RA-DIO message using an The Objective Function is specified in the DIO message within a DAG
objective code point (OCP) and indicates the objective function that Metric Container using an Objective Code Point (OCP), as specified in
has been used to compute the DAG (e.g. "minimize the path cost using [I-D.ietf-roll-routing-metrics], and indicates the method that must
the ETX metric and avoid `Blue' links"). The objective code points be used to compute the DAG (e.g. "minimize the path cost using the
are specified in [I-D.ietf-roll-routing-metrics]. This document ETX metric and avoid `Blue' links"). The Objective Code Points are
specifies the OCP 0, in support of default operation. specified in [I-D.ietf-roll-routing-metrics]. This document
specifies an Objective Function, OF0, in support of default
operation. In the case where the DIO does not include an OCP
specification in the DAG Metric Container, OF0 MAY be presumed.
Most Objective Functions are expected to follow the same abstract Most Objective Functions are expected to follow the same abstract
behavior: behavior:
o The parent selection is triggered each time an event indicates o The parent selection is triggered each time an event indicates
that a potential next_hop information is updated. This might that a potential next hop information is updated. This might
happen upon the reception of a RA-DIO message, a timer elapse, or happen upon the reception of a DIO message, a timer elapse, or a
a trigger indicating that the state of a candidate neighbor has trigger indicating that the state of a candidate neighbor has
changed. changed.
o An OF scans all the interfaces on the device. Although there may o An OF scans all the interfaces on the device. Although there may
typically be only one interface in most application scenarios, typically be only one interface in most application scenarios,
there might be multiple of them and an interface might be there might be multiple of them and an interface might be
configured to be usable or not for RPL operation. An interface configured to be usable or not for RPL operation. An interface
can also be configured with a preference or dynamically learned to can also be configured with a preference or dynamically learned to
be better than another by some heuristics that might be link-layer be better than another by some heuristics that might be link-layer
dependent and are out of scope. Finally an interface might or not dependent and are out of scope. Finally an interface might or not
match a required criterion for an Objective Function, for instance match a required criterion for an Objective Function, for instance
a degree of security. As a result some interfaces might be a degree of security. As a result some interfaces might be
completely excluded from the computation, while others might be completely excluded from the computation, while others might be
more or less preferred. more or less preferred.
o The OF scans all the candidate neighbors on the possible o An OF scans all the candidate neighbors on the possible interfaces
interfaces to check whether they can act as an attachment router to check whether they can act as a router for a DAG. There might
for a DAG. There might be multiple of them and a candidate be multiple of them and a candidate neighbor might need to pass
neighbor might need to pass some validation tests before it can be some validation tests before it can be used. In particular, some
used. In particular, some link layers require experience on the link layers require experience on the activity with a router to
activity with a router to enable the router as a next_hop. enable the router as a next hop.
o The OF computes self's rank by adding the step of rank to that o An OF computes self's rank by adding the step of rank to that
candidate to the rank of that candidate. The step of rank is candidate to the rank of that candidate. The step of rank is
estimated as follows: computed by estimating the link as follows:
* The step of rank might vary from 1 to 16. * The step of rank might vary from 1 to 16.
+ 1 indicates a unusually good link, for instance a link + 1 indicates a unusually good link, for instance a link
between powered devices in a mostly battery operated between powered devices in a mostly battery operated
environment. environment.
+ 4 indicates a `normal'/typical link, as qualified by the + 4 indicates a `normal'/typical link, as qualified by the
implementation. implementation.
skipping to change at page 49, line 41 skipping to change at page 43, line 51
* Candidate neighbors that would cause self's rank to increase * Candidate neighbors that would cause self's rank to increase
are ignored are ignored
o Candidate neighbors that advertise an OF incompatible with the set o Candidate neighbors that advertise an OF incompatible with the set
of OF specified by the policy functions are ignored. of OF specified by the policy functions are ignored.
o As it scans all the candidate neighbors, the OF keeps the current o As it scans all the candidate neighbors, the OF keeps the current
best parent and compares its capabilities with the current best parent and compares its capabilities with the current
candidate neighbor. The OF defines a number of tests that are candidate neighbor. The OF defines a number of tests that are
critical to reach the Objective. A test between the routers critical to reach the objective. A test between the routers
determines an order relation. determines an order relation.
* If the routers are roughly equal for that relation then the * If the routers are roughly equal for that relation then the
next test is attempted between the routers, next test is attempted between the routers,
* Else the best of the 2 becomes the current best parent and the * Else the best of the 2 becomes the current best parent and the
scan continues with the next candidate neighbor scan continues with the next candidate neighbor
* Some OFs may include a test to compare the ranks that would * Some OFs may include a test to compare the ranks that would
result if the node joined either router result if the node joined either router
o When the scan is complete, the preferred parent is elected and o When the scan is complete, the preferred parent is elected and
self's rank is computed as the preferred parent rank plus the step self's rank is computed as the preferred parent rank plus the step
in rank with that parent. in rank with that parent.
o Other rounds of scans might be necessary to elect alternate o Other rounds of scans might be necessary to elect alternate
parents and siblings. In the next rounds: parents and siblings. In the next rounds:
* Candidate neighbors that are not in the same DAG are ignored * Candidate neighbors that are not in the same DAG are ignored
* Candidate neighbors that are of worse rank than self are * Candidate neighbors that are of greater rank than self are
ignored ignored
* Candidate neighbors of a better rank than self (non-siblings) * Candidate neighbors of an equal rank to self (siblings) are
are preferred ignored
5.8.2. Objective Code Point 0 (OCP 0)
Here follows the specification for the default Objective Function * Candidate neighbors of a lesser rank than self (non-siblings)
corresponding to OCP codepoint 0. This is a very simple reference to are preferred
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], 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 5.9.2. Objective Function 0 (OF0)
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.
5.8.2.1. OCP 0 Objective Function (OF0) This document specifies a default objective function, called OF0,
indicated by an OCP value of 0x0000. OF0 is the default objective
function of RPL, and can be used if allowed by the policy of the
processing node when the OF indicated in the DIO message is unknown
to the node. If not allowed, then the DIO message is simply ignored
and not processed by the node. OF0 is notable in that it does not
use physical metrics as described in [I-D.ietf-roll-routing-metrics],
but is only based on abstract information from the DIO message such
as rank and administrative preference.
OF0 favors the connectivity. That is, the Objective Function is OF0 favors connectivity. That is, the Objective Function is designed
designed to find the nearest sink into a 'grounded' topology, and if to find the nearest sink into a 'grounded' topology, and if there is
there is none then join any network per order of administrative none then join any network per order of administrative preference.
preference. The metric in use is the rank. The metric in use is the rank.
OF0 selects a preferred parent and a backup next_hop if one is 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 available. The backup next hop might be a parent or a sibling. All
the traffic is routed via the preferred parent. When the link the traffic is routed via the preferred parent. When the link
conditions do not let a packet through to the preferred parent, the conditions do not let a packet through to the preferred parent, the
packet is passed to the backup next_hop. packet is passed to the backup next hop.
The step of rank is 4 for each hop. The step of rank is 4 for each hop.
5.8.2.2. Selection of the Preferred Parent 5.9.2.1. Selection of the Preferred Parent
As it scans all the candidate neighbors, OF0 keeps the parent that is As it scans all the candidate neighbors, OF0 keeps the parent that is
the best for the following criteria (in order): the best for the following criteria (in order):
1. The interface must be usable and the administrative preference 1. The interface must be usable and any administrative preference
(if any) applies first. associated with the interface applies first.
2. A candidate that would cause the node to augment the rank in the 2. A candidate that would cause the node to augment the rank in the
current DAG is not considered. current DAG is not considered.
3. A router that has been validated as usable, e.g. with a local 3. A router that has been validated as usable, e.g. with a local
confidence that has exceeded some pre-configured threshold, is confidence that has exceeded some pre-configured threshold, is
better. better.
4. If none are grounded then a DAG with a more preferred 4. If none are grounded then a DAG with a more preferred
administrative preference is better. administrative preference (DAGPreference) is better.
5. A router that offers connectivity to a grounded DAG is better. 5. A router that offers connectivity to a grounded DAG is better.
6. A lesser resulting rank is better. 6. A lesser resulting rank is better.
7. A DAG for which there is an alternate parent is better. This 7. A DAG for which there is an alternate parent is better. This
check is optional. It is performed by computing the backup check is optional. It is performed by computing the backup next
next_hop while assuming that this router won. hop while assuming that this router won.
8. The DAG that was in use already is preferred. 8. The DAG that was in use already is preferred.
9. The router with a better router preference wins. 9. The preferred parent that was in use already is better.
10. The preferred parent that was in use already is better.
11. A router that has announced a RA-DIO message more recently is 10. A router that has announced a DIO message more recently is
preferred. preferred.
5.8.2.3. Selection of the Backup next_hop 5.9.2.2. Selection of the Backup Next Hop
o The interface must be usable and the administrative preference (if o The interface must be usable and the administrative preference (if
any) applies first. any) applies first.
o The preferred parent is ignored. o The preferred parent is ignored.
o Candidate neighbors that are not in the same DAG are ignored. o Candidate neighbors that are not in the same DAG are ignored.
o Candidate neighbors with a higher rank are ignored. o Candidate neighbors with a higher rank are ignored.
o Candidate neighbors of a better rank than self (non-siblings) are o Candidate neighbors of a better rank than self (non-siblings) are
preferred. preferred.
o A router that has been validated as usable, e.g. with a local o A router that has been validated as usable, e.g. with a local
confidence that has exceeded some pre-configured threshold, is confidence that has exceeded some pre-configured threshold, is
better. better.
o The router with a better router preference wins. o The router with a better router preference wins.
o The backup next_hop that was in use already is better. o The backup next hop that was in use already is better.
5.9. Establishing Routing State Outward Along the DAG 5.10. Establishing Routing State Outward Along the DAG
The destination advertisement mechanism supports the dissemination of The destination advertisement mechanism supports the dissemination of
routing state required to support traffic flows outward along the routing state required to support traffic flows outward along the
DAG, from the DAG root toward nodes. DAG, from the DAG root toward nodes.
As a result of destination advertisement operation: As a result of destination advertisement operation:
o DAG discovery establishes a DAG oriented toward a DAG root using o DAG discovery establishes a DAG oriented toward a DAG root along
extended Neighbor Discovery RS/RA flows, along which inward routes which inward routes toward the DAG root are set up.
toward the DAG root are set up.
o Destination advertisement extends Neighbor Discovery in order to o Destination advertisement establishes outward routes along the
establish outward routes along the DAG. Such paths consist of: DAG. Such paths consist of:
* Hop-By-Hop routing state within islands of `stateful' nodes. * Hop-By-Hop routing state within islands of `stateful' nodes.
* Source Routing `bridges' across nodes who do not retain state. * Source Routing `bridges' across nodes that do not retain state.
Destinations disseminated with the destination advertisement Destinations disseminated with the destination advertisement
mechanism may be prefixes, individual hosts, or multicast listeners. mechanism may be prefixes, individual hosts, or multicast listeners.
The mechanism supports nodes of varying capabilities as follows: The mechanism supports nodes of varying capabilities as follows:
o When nodes are capable of storing routing state, they may inspect o When nodes are capable of storing routing state, they may inspect
destination advertisements and learn hop-by-hop routing state destination advertisements and learn hop-by-hop routing state
toward destinations by populating their routing tables with the toward destinations by populating their routing tables with the
routes learned from nodes in their sub-DAG. In this process they routes learned from nodes in their sub-DAG. In this process they
may also learn necessary piecewise source routes to traverse may also learn necessary piecewise source routes to traverse
skipping to change at page 53, line 25 skipping to change at page 47, line 27
databases along the DAG, but instead to update them regularly to databases along the DAG, but instead to update them regularly to
recover from the loss of packets. The rationale for that choice is recover from the loss of packets. The rationale for that choice is
time variations in connectivity across unreliable links. If the time variations in connectivity across unreliable links. If the
topology can be expected to change frequently, synchronization might topology can be expected to change frequently, synchronization might
be an excessive goal in terms of exchanges and protocol complexity. be an excessive goal in terms of exchanges and protocol complexity.
The approach used here results in a simple protocol with no real The approach used here results in a simple protocol with no real
peering. The destination advertisement mechanism hence provides for peering. The destination advertisement mechanism hence provides for
periodic updates of the routing state, as cued by occasional RAs and periodic updates of the routing state, as cued by occasional RAs and
other mechanisms, similarly to other protocols such as RIP [RFC2453]. other mechanisms, similarly to other protocols such as RIP [RFC2453].
5.9.1. Destination Advertisement Message Formats 5.10.1. Destination Advertisement Operation
5.9.1.1. DAO Option
RPL extends Neighbor Discovery [RFC4861] and RFC4191 [RFC4191] 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.
5.9.2. Destination Advertisement Operation
5.9.2.1. Overview 5.10.1.1. Overview
According to implementation specific policy, a subset or all of the According to implementation specific policy, a subset or all of the
feasible parents in the DAG may be selected to receive prefix feasible parents in the DAG may be selected to receive prefix
information from the destination advertisement mechanism. This information from the destination advertisement mechanism. This
subset of DAG parents shall be designated the set of DA parents. subset of DAG parents shall be designated the set of DA parents.
As NA-DAO messages for particular destinations move inwards along the As DAO messages for particular destinations move inwards along the
DAG, a sequence counter is used to guarantee their freshness. The DAG, a sequence counter is used to guarantee their freshness. The
sequence counter is incremented by the source of the NA-DAO message sequence counter is incremented by the source of the DAO message (the
(the node that owns the prefix, or learned the prefix via some other 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 means), each time it issues a DAO message for its prefix. Nodes that
who receive the NA-DAO message and, if scope allows, will be receive the DAO message and, if scope allows, will be forwarding a
forwarding a NA-DAO message for the unmodified destination inwards DAO message for the unmodified destination inwards along the DAG,
along the DAG, will leave the sequence number unchanged. will leave the sequence number unchanged. Intermediate nodes will
Intermediate nodes will check the sequence counter before processing check the sequence counter before processing a DAO message, and if
a NA-DAO message, and if the DAO is unchanged (the sequence counter the DAO is unchanged (the sequence counter has not changed), then the
has not changed), then the NA-DAO message will be discarded without DAO message will be discarded without additional processing.
additional processing. Further, if the NA-DAO message appears to be Further, if the DAO message appears to be out of synch (the sequence
out of synch (the sequence counter is 2 or more behind the present counter is 2 or more behind the present value) then the DAO state is
value) then the DAO state is considered to be stale and may be considered to be stale and may be purged, and the DAO message is
purged, and the NA-DAO message is discarded. A depth is also added discarded. A depth is also added for tracking purposes; the depth is
for tracking purposes; the depth is incremented at each hop as the incremented at each hop as the DAO message is propagated up the DAG.
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 Nodes that 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 DIO message as
indicated by the `D' bit, the node sends unicast destination indicated by the `D' bit, the node sends unicast destination
advertisements to its DA parents, and only accepts unicast advertisements to one of its DA parents, that is selected as most
favored for incoming outwards traffic. The node only accepts unicast
destination advertisements from any nodes but those contained in the destination advertisements from any nodes but those contained in the
DA parent subset. DA parent subset.
Every NA to a DA parent MAY contain one or more DAOs. Receiving a Receiving a DIO message with the `D' destination advertisement bit
RA-DIO message with the `D' destination advertisement bit set from a set from a DAG parent stimulates the sending of a delayed destination
DAG parent stimulates the sending of a delayed destination
advertisement back, with the collection of all known prefixes (that advertisement back, with the collection of all known prefixes (that
is the prefixes learned via destination advertisements for nodes is the prefixes learned via destination advertisements for nodes
lower in the DAG, and any connected prefixes). If the Destination lower in the DAG, and any connected prefixes). If the Destination
Advertisement Supported (A) bit is set in the RA-DIO message for the Advertisement Supported (A) bit is set in the DIO message for the
DAG, then a destination advertisement is also sent to a DAG parent 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 once it has been added to the DA parent set after a movement, or when
the list of advertised prefixes has changed. Destination the list of advertised prefixes has changed.
advertisements may also be scheduled for sending when the PathDigest
of the RA-DIO message has changed, indicating that some aspect of the A node that modifies its DAG Parent set may set the `D' bit in
inwards paths along the DAG has been modified. subsequent DIO propagation in order to trigger destination
advertisements to be updated to its DAG Parents and other inward
nodes on the DAG. Additional recommendations and guidelines
regarding the use of this mechanism are still under consideration and
will be elaborated in a future revision of this specification.
Destination advertisements may advertise positive (prefix is present) Destination advertisements may advertise positive (prefix is present)
or negative (removed) NA-DAO messages, termed as no-DAOs. A no-DAO or negative (removed) DAO messages, termed as no-DAOs. A no-DAO is
is stimulated by the disappearance of a prefix below. This is stimulated by the disappearance of a prefix below. This is
discovered by timing out after a request (a RA-DIO message) or by discovered by timing out after a request (a DIO message) or by
receiving a no-DAO. A no-DAO is a conveyed as a NA-DAO message with receiving a no-DAO. A no-DAO is a conveyed as a DAO message with a
a DAO Lifetime of 0. DAO Lifetime of ZERO_LIFETIME.
A node who is capable of recording the state information conveyed in A node that is capable of recording the state information conveyed in
a unicast NA-DAO message will do so upon receiving and processing the a unicast DAO message will do so upon receiving and processing the
NA-DAO message, thus building up routing state concerning DAO message, thus building up routing state concerning destinations
destinations below it in the DAG. If a node capable of recording below it in the DAG. If a node capable of recording state
state information receives a NA-DAO message containing a Reverse information receives a DAO message containing a Reverse Route Stack,
Route Stack, then the node knows that the NA-DAO message has then the node knows that the DAO message has traversed one or more
traversed one or more nodes that did not retain any routing state as nodes that did not retain any routing state as it traversed the path
it traversed the path from the DAO source to the node. The node may from the DAO source to the node. The node may then extract the
then extract the Reverse Route Stack and retain the included state in Reverse Route Stack and retain the included state in order to specify
order to specify Source Routing instructions along the return path Source Routing instructions along the return path towards the
towards the destination. The node MUST set the RRCount back to zero destination. The node MUST set the RRCount back to zero and clear
and clear the Reverse Route Stack prior to passing the NA-DAO message the Reverse Route Stack prior to passing the DAO message information
information on. on.
A node who is unable to record the state information conveyed in the A node that is unable to record the state information conveyed in the
NA-DAO message will append the next-hop address to the Reverse Route DAO message will append the next-hop address to the Reverse Route
Stack, increment the RRCount, and then pass the destination Stack, increment the RRCount, and then pass the destination
advertisement on without recording any additional state. In this way advertisement on without recording any additional state. In this way
the Reverse Route Stack will contain a vector of next hops that must 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 be traversed along the reverse path that the DAO message has
traveled. The vector will be ordered such that the node closest to 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 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, is useful to the implementation to try and build up redundant paths,
the node may choose to convey the destination advertisement to one or the node may choose to convey the destination advertisement to one or
more DAG parents in order of preference as guided by an more DAG parents in order of preference as guided by an
implementation specific policy. implementation specific policy.
In some cases (called hybrid cases), some nodes along the path a In some cases (called hybrid cases), some nodes along the path a
destination advertisement follows inward along the DAG may store destination advertisement follows inward along the DAG may store
state and some may not. The destination advertisement mechanism state and some may not. The destination advertisement mechanism
allows for the provisioning of routing state such that when a packet allows for the provisioning of routing state such that when a packet
is traversing outwards along the DAG, some nodes may be able to 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 directly forward to the next hop, and other nodes may be able to
specify a piecewise source route in order to bridge spans of specify a piecewise source route in order to bridge spans of
stateless nodes within the path on the way to the desired stateless nodes within the path on the way to the desired
destination. destination.
In the case where no node is able to store any routing state as 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- destination advertisements pass by, and the DAG root ends up with DAO
DAO messages that contain a completely specified route back to the messages that contain a completely specified route back to the
originating node in the form of the inverted Reverse Route Stack. A originating node in the form of the inverted Reverse Route Stack. A
DAG root should not request (Destination Advertisement Trigger) nor DAG root should not request (Destination Advertisement Trigger) nor
indicate support (Destination Advertisement Supported) for indicate support (Destination Advertisement Supported) for
destination advertisements if it is not able to store the Reverse destination advertisements if it is not able to store the Reverse
Route Stack information in this case. Route Stack information in this case.
The destination advertisement mechanism requires stateful nodes to The destination advertisement mechanism requires stateful nodes to
maintain lists of known prefixes. A prefix entry contains the maintain lists of known prefixes. A prefix entry contains the
following abstract information: following abstract information:
skipping to change at page 57, line 47 skipping to change at page 50, line 5
o The IPv6 address and interface for the advertising neighbor. o The IPv6 address and interface for the advertising neighbor.
o The logical equivalent of the full destination advertisement o The logical equivalent of the full destination advertisement
information (including the prefixes, depth, and Reverse Route information (including the prefixes, depth, and Reverse Route
Stack, if any). Stack, if any).
o A 'reported' Boolean to keep track whether this prefix was o A 'reported' Boolean to keep track whether this prefix was
reported already, and to which of the DA parents. reported already, and to which of the DA parents.
o A counter of retries to count how many RA-DIO messages were sent o A counter of retries to count how many DIO messages were sent on
on the interface to the advertising neighbor without reachability the interface to the advertising neighbor without reachability
confirmation for the prefix. confirmation for the prefix.
Note that nodes may receive multiple information from different Note that nodes may receive multiple information from different
neighbors for a specific destination, as different paths through the neighbors for a specific destination, as different paths through the
DAG may be propagating information inwards along the DAG for the same DAG may be propagating information inwards along the DAG for the same
destination. A node who is recording routing state will keep track destination. A node that is recording routing state will keep track
of the information from each neighbor independently, and when it of the information from each neighbor independently, and when it
comes time to propagate the NA-DAO message for a particular prefix to comes time to propagate the DAO message for a particular prefix to
the DA parents, then the DAO information will be selected from among the DA parents, then the DAO information will be selected from among
the advertising neighbors who offer the least depth to the the advertising neighbors who offer the least depth to the
destination. destination.
The destination advertisement mechanism stores the prefix entries in The destination advertisement mechanism stores the prefix entries in
one of 3 abstract lists; the Connected, the Reachable and the one of 3 abstract lists; the Connected, the Reachable and the
Unreachable lists. Unreachable lists.
The Connected list corresponds to the prefixes owned and managed by The Connected list corresponds to the prefixes owned and managed by
the local node. the local node.
The Reachable list contains prefixes for which the node keeps The Reachable list contains prefixes for which the node keeps
receiving NA-DAO messages, and for those prefixes which have not yet receiving DAO messages, and for those prefixes which have not yet
timed out. timed out.
The Unreachable list keeps track of prefixes which are no longer 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 valid and in the process of being deleted, in order to send DAO
messages with zero lifetime (also called no-DAO) to the DA parents. messages with zero lifetime (also called no-DAO) to the DA parents.
5.9.2.1.1. Destination Advertisement Timers 5.10.1.1.1. Destination Advertisement Timers
The destination advertisement mechanism requires 2 timers; the The destination advertisement mechanism requires 2 timers; the
DelayNA timer and the RemoveTimer. DelayDAO timer and the RemoveTimer.
o The DelayNA timer is armed upon a stimulation to send a o The DelayDAO timer is armed upon a stimulation to send a
destination advertisement (such as a RA-DIO message from a DA destination advertisement (such as a DIO message from a DA
parent). When the timer is armed, all entries in the Reachable parent). When the timer is armed, all entries in the Reachable
list as well as all entries for Connected list are set to not be list as well as all entries for Connected list are set to not be
reported yet for that particular DA parent. reported yet for that particular DA parent.
o The DelayNA timer has a duration that is DEF_NA_LATENCY divided by o The DelayDAO timer has a duration that is DEF_DAO_LATENCY divided
a multiple of the DAG rank of the node. The intention is that by a multiple of the DAG rank of the node. The intention is that
nodes located deeper in the DAG should have a shorter DelayNA nodes located deeper in the DAG should have a shorter DelayDAO
timer, allowing NA-DAO messages a chance to be reported from timer, allowing DAO messages a chance to be reported from deeper
deeper in the DAG and potentially aggregated along sub-DAGs before in the DAG and potentially aggregated along sub-DAGs before
propagating further inwards. propagating further inwards.
o The RemoveTimer is used to clean up entries for which NA-DAO o The RemoveTimer is used to clean up entries for which DAO messages
messages are no longer being received from the sub-DAG. are no longer being received from the sub-DAG.
* When a RA-DIO message is sent that is requesting destination * When a DIO message is sent that is requesting destination
advertisements, a flag is set for all DAO entries in the advertisements, a flag is set for all DAO entries in the
routing table. routing table.
* If the flag has already been set for a DAO entry, the retry * If the flag has already been set for a DAO entry, the retry
count is incremented. count is incremented.
* If a NA-DAO message is received to confirm the entry, the entry * If a DAO message is received to confirm the entry, the entry is
is refreshed and the flag and count may be cleared. refreshed and the flag and count may be cleared.
* If at least one entry has reached a threshold value and the * If at least one entry has reached a threshold value and the
RemoveTimer is not running, the entry is considered to be RemoveTimer is not running, the entry is considered to be
probably gone and the RemoveTimer is started. probably gone and the RemoveTimer is started.
* When the RemoveTimer elapse, NA-DAO messages with lifetime 0, * When the RemoveTimer elapse, DAO messages with lifetime 0, i.e.
i.e. no-DAOs, are sent to explicitly inform DA parents that the no-DAOs, are sent to explicitly inform DA parents that the
entries who have reached the threshold are no longer available, entries which have reached the threshold are no longer
and the related routing states may be propagated and cleaned available, and the related routing states may be propagated and
up. cleaned up.
o The RemoveTimer has a duration of min (MAX_DESTROY_INTERVAL, o The RemoveTimer has a duration of min (MAX_DESTROY_INTERVAL,
RA_INTERVAL). TBD(DIO Trickle Timer Interval)).
5.9.2.2. Multicast Destination Advertisement messages 5.10.1.2. Multicast Destination Advertisement Messages
It is also possible for a node to multicast a NA-DAO message to the It is also possible for a node to multicast a DAO message to the
link-local scope all-nodes multicast address FF02::1. This message link-local scope all-nodes multicast address FF02::1. This message
will be received by all node listening in range of the emitting node. will be received by all node listening in range of the emitting node.
The objective is to enable direct P2P communication, between The objective is to enable direct P2P communication, between
destinations directly supported by neighboring nodes, without needing destinations directly supported by neighboring nodes, without needing
the RPL routing structure to relay the packets. the RPL routing structure to relay the packets.
A multicast NA-DAO message MUST be used only to advertise information A multicast DAO message MUST be used only to advertise information
about self, i.e. prefixes in the Connected list or addresses owned by 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 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 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 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 multicast DAO message is not used for routing and does not presume
any DAG relationship between the emitter and the receiver; it MUST any DAG relationship between the emitter and the receiver; it MUST
NOT be used to relay information learned (e.g. information in the NOT be used to relay information learned (e.g. information in the
Reachable list) from another node; information obtained from a Reachable list) from another node; information obtained from a
multicast NA-DAO MAY be installed in the routing table and MAY be multicast DAO MAY be installed in the routing table and MAY be
propagated by a router in unicast NA-DAOs. propagated by a router in unicast DAOs.
A node receiving a multicast NA-DAO message addressed to FF02::1 MAY A node receiving a multicast DAO message addressed to FF02::1 MAY
install prefixes contained in the NA-DAO message in the routing table install prefixes contained in the DAO message in the routing table
for local use. Such a node MUST NOT perform any other processing on 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 the DAO message (i.e. such a node does not presume it is a DA
parent). parent).
5.9.2.3. Unicast Destination Advertisement messages from child to 5.10.1.3. Unicast Destination Advertisement Messages from Child to
parent Parent
When sending a destination advertisement to a DA parent, a node When sending a destination advertisement to a DA parent, a node
includes the DAOs for prefix entries not already reported (since the includes the DAOs for prefix entries not already reported (since the
last DA Trigger from an RA-DIO message) in the Reachable and last DA Trigger from an DIO message) in the Reachable and Connected
Connected lists, as well as no-DAOs for all the entries in the lists, as well as no-DAOs for all the entries in the Unreachable
Unreachable list. Depending on its policy and ability to retain list. Depending on its policy and ability to retain routing state,
routing state, the receiving node SHOULD keep a record of the the receiving node SHOULD keep a record of the reported DAO message.
reported NA-DAO message. If the NA-DAO message offers the best route If the DAO message offers the best route to the prefix as determined
to the prefix as determined by policy and other prefix records, the by policy and other prefix records, the node SHOULD install a route
node SHOULD install a route to the prefix reported in the NA-DAO to the prefix reported in the DAO message via the link local address
message via the link local address of the reporting neighbor and it of the reporting neighbor and it SHOULD further propagate the
SHOULD further propagate the information in a NA-DAO message. information in a DAO message.
The RA-DIO message from the DAG root is used to synchronize the whole The DIO message from the DAG root is used to synchronize the whole
DAG, including the periodic reporting of destination advertisements DAG, including the periodic reporting of destination advertisements
back up the DAG. Its period is expected to vary, depending on the back up the DAG. Its period is expected to vary, depending on the
configuration of the trickle timer that governs the RAs. configuration of the trickle timer that governs the RAs.
When a node receives a RA-DIO message over an LLN interface from a DA When a node receives a DIO message over an LLN interface from a DA
parent, the DelayNA is armed to force a full update. parent, the DelayDAO is armed to force a full update.
When the node broadcasts a RA-DIO message on an LLN interface, for When the node broadcasts a DIO message on an LLN interface, for all
all entries on that interface: entries on that interface:
o If the entry is CONFIRMED, it goes PENDING with the retry count o If the entry is CONFIRMED, it goes PENDING with the retry count
set to 0. set to 0.
o If the entry is PENDING, the retry count is incremented. If it o If the entry is PENDING, the retry count is incremented. If it
reaches a maximum threshold, the entry goes ELAPSED If at least reaches a maximum threshold, the entry goes ELAPSED If at least
one entry is ELAPSED at the end of the process: if the Destroy one entry is ELAPSED at the end of the process: if the RemoveTimer
timer is not running then it is armed with a jitter. is not running then it is armed with a jitter.
Since the DelayNA timer has a duration that decreases with the depth, Since the DelayDAO timer has a duration that decreases with the
it is expected to receive all NA-DAO messages from all children depth, it is expected to receive all DAO messages from all children
before the timer elapses and the full update is sent to the DA before the timer elapses and the full update is sent to the DA
parents. parents.
Once the RemoveTimer is elapsed, the prefix entry is scheduled to be 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 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, that need to be informed of the change in status for the prefix,
otherwise the prefix entry is cleaned up right away. 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 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 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, 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 and noting that in some cases parents may have been removed from the
set of DA parents. set of DA parents.
5.9.2.4. Other events 5.10.1.4. Other Events
Finally, the destination advertisement mechanism responds to a series Finally, the destination advertisement mechanism responds to a series
of events, such as: of events, such as:
o Destination advertisement operation stopped: All entries in the o Destination advertisement operation stopped: All entries in the
abstract lists are freed. All the routes learned from NA-DAO abstract lists are freed. All the routes learned from DAO
messages are removed. messages are removed.
o Interface going down: for all entries in the Reachable list on o Interface going down: for all entries in the Reachable list on
that interface, the associated route is removed, and the entry is that interface, the associated route is removed, and the entry is
scheduled to be removed. scheduled to be removed.
o Loss of routing adjacency: When the routing adjacency for a o Loss of routing adjacency: When the routing adjacency for a
neighbor is lost, as per the procedures described in Section 5.11, neighbor is lost, as per the procedures described in Section 5.13,
and if the associated entries are in the Reachable list, the and if the associated entries are in the Reachable list, the
associated routes are removed, and the entries are scheduled to be associated routes are removed, and the entries are scheduled to be
destroyed. destroyed.
o Changes to DA parent set: all entries in the Reachable list are o Changes to DA parent set: all entries in the Reachable list are
set to not 'reported' and DelayNA is armed. set to not 'reported' and DelayDAO is armed.
5.9.2.5. Aggregation of prefixes by a node 5.10.1.5. Aggregation of Prefixes by a Node
There may be number of cases where a aggregation may be shared within 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 a group of nodes. In such a case, it is possible to use aggregation
techniques with destination advertisements and improve scalability. techniques with destination advertisements and improve scalability.
Other cases might occur for which additional support is required: Other cases might occur for which additional support is required:
1. The aggregating node is attached within the sub-DAG of the nodes 1. The aggregating node is attached within the sub-DAG of the nodes
it is aggregating for. it is aggregating for.
2. A node that is to be aggregated for is located somewhere else 2. A node that is to be aggregated for is located somewhere else
within the DAG, not in the sub-DAG of the aggregating node. within the DAG, not in the sub-DAG of the aggregating node.
3. A node that is to be aggregated for is located somewhere else in 3. A node that is to be aggregated for is located somewhere else in
the LLN. the LLN.
Consider a node M who is performing an aggregation, and a node N who Consider a node M that is performing an aggregation, and a node N
is to be a member of the aggregation group. A node Z situated above that is to be a member of the aggregation group. A node Z situated
the node M in the DAG, but not above node N, will see the 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 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 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 node N towards node M, but node M will have no route to the node N
and will fail to forward. and will fail to forward.
Additional protocols may be applied beyond the scope of this Additional protocols may be applied beyond the scope of this
specification to dynamically elect/provision an aggregating node and specification to dynamically elect/provision an aggregating node and
groups of nodes eligible to be aggregated in order to provide route groups of nodes eligible to be aggregated in order to provide route
summarization for a sub-DAG. summarization for a sub-DAG.
5.9.2.6. Default Values 5.11. Loop Detection
DEF_NA_LATENCY = To Be Determined RPL loop avoidance mechanisms are kept simple and designed to
minimize churn and states. Loops may form for a number of reasons,
from control packet loss to sibling forwarding. RPL includes a
reactive loop detection technique that protects from meltdown and
triggers repair of broken paths.
MAX_DESTROY_INTERVAL = To Be Determined RPL loop detection uses information that is placed into the packet in
the flow label. It assumes that the flow label may be overloaded for
this purpose. The flow label is constructed as follows:
5.10. Multicast Operation 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|O|S|R|D| SenderRank | InstanceID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: RPL Flow Label
Outwards 'O' bit: 1-bit flag indicating whether the packet is
expected to progress inwards or outwards. A router sets the
'O' bit when the packet is expect to progress outwards (using
DAO routes), and resets it when forwarding towards the root of
the DAG. A host MUST set the bit to 0.
Sibling 'S' bit: 1-bit flag indicating whether the packet has been
forwarded via a sibling at the present rank, and denotes a risk
of a sibling loop. A host sets the bit to 0.
Rank-Error 'R' bit: 1-bit flag indicating whether a rank error was
detected. A rank error is detected when there is a mismatch in
the relative ranks and the direction as indicated in the 'O'
bit. A host MUST set the bit to 0.
DAO-Error 'D' bit: 1-bit flag indicating whether a DAO error was
detected. An undetected DAO error would have resulted in an
inward to outward transition that is not expected with this
spec. A host MUST set the bit to 0.
SenderRank: 8-bit field indicating the rank of the sender. A host
MUST set the rank to INFINITE_RANK. A router MUST place its
own rank in the flow label when forwarding.
InstanceID: 8-bit field indicating the DAG instance along which the
packet is sent.
5.11.1. Host Basic Operation
It is expected that a host that does not participate to RPL in any
fashion is configured to set the flow label to all zeroes in its
outgoing packets. The host MAY send a packet to any router
regardless of the DAG and RPL operations at large.
A host that participates to RPL SHOULD zero out all the flags, and it
MUST set the sender rank to INFINITE_RANK. If the host can map a
flow to a given InstanceID then it MUST set the flow label
accordingly. Forwarding rules are the same for this host and a
router, and are described in the next section.
5.11.2. Instance Forwarding
Instance IDs is used to avoid loops between DAGs from different
origins. DAGs that constructed for antagonistic constraints might
contain paths that, if mixed together, would yield loops. Those
loops are avoided by forwarding a packet along the DAG that is
associated to a given instance.
The InstanceID is placed by the source in the flow label. It is not
meaningful if the packet has the flow label set to all zeroes.
Otherwise it MUST match the DAG instance onto which the packet is
placed by any node, be it a host or router.
When a router receives a packet that is flagged with a given instance
ID and the node can forward the packet along the DAG associated to
that instance, then the router MUST do so and leave the instance ID
flag unchanged.
If any node can not forward a packet along the DAG associated to the
instance ID in the flow label, then the node MAY either change the
InstanceID to match a DAG that it is using for this packet or discard
the packet. That decision is based on a policy.
The default policy is as follows: if the node can forward along the
DAG associated to the instance RPL_DEFAULT_INSTANCE then it should do
so. Otherwise it should drop the packet.
5.11.3. DAG Inconsistency Loop Detection
The DAG is inconsistent is the direction of a packet does not match
the rank relationship. A receiver detects an inconsistency if it
receives a packet with either:
the 'O' bit set (to outwards) from a node of a higher rank.
the 'O' bit reset (for inwards) from a node of a lesser rank.
the 'S' bit set (to sibling) from a node of a different rank.
The propagation of a new sequence creates local inconsistencies. In
particular, it is possible for a router to forward a packet to a
future parent (same instance, same DAGID, higher sequence) without a
loop, regardless of the rank of that parent. In that case, the
sending router MUST present itself as a host on the future DAG and
use a rank of INFINITE_RANK as it forwards the packets via a future
parent to avoid a false positive.
One inconsistency along the path is not considered as a critical
error and the packet may continue. But a second detection along the
path of a same packet should not occur and the packet is dropped.
This process is controlled by the Rank-Error bit in the Flow Label.
When an inconsistency, is detected on a packet, if the Rank-Error bit
was not set then the Rank-Error bit is set. If it was set the packet
is discarded and the trickle timer is reset.
5.11.4. Sibling Loop Avoidance
When a packet is forwarded along siblings, it cannot be checked for
forward progress and may loop between siblings. Experimental
evidence has shown that one sibling hop can be very useful but is
generally sufficient to avoid loops. Based on that evidence, this
specification enforces the simple rule that a packet may not make 2
sibling hops in a row.
When a host issues a packet or when a router forwards a packet to a
non sibling, the Sibling bit in the packet must be reset. When a
router forwards to a sibling: if the Sibling bit was not set then the
Sibling bit is set. If the Sibling bit was set then the packet is
discarded. This does not denote a graph inconsistency but indicates
that a new graph should probably be formed with a new sequence.
5.11.5. DAO Inconsistency Loop Detection and Recovery
A DAO inconsistency happens when router that has an outwards DAO
route via a child that is a remnant from an obsolete state that is
not matched in the child. With DAO inconsistency loop recovery, a
packet can be used to recursively explore and cleanup the obsolete
DAO states along a sub-DAG.
In a general manner, a packet that goes outwards should never go
inwards again. So rather than routing inwards a packet with the
Outwards bit set, the router MUST discard the packet. If DAO
inconsistency loop recovery is applied, then the router SHOULD send
the packet to the parent that passed it with the DAO-Error bit set.
Upon a packet with a DAO bit set, the parent MUST remove the routing
states that caused forwarding to that child, clear DAO-Error bit and
send the packet again. The packet will make its way either to an
alternate child or inwards to a parent. If that parent still has an
inconsistent DAO state via self, the process will recurse and that
state will be cleaned up as well.
5.12. Multicast Operation
This section describes further the multicast routing operations over This section describes further the multicast routing operations over
an IPv6 RPL network, and specifically how unicast NA-DAOs can be used an IPv6 RPL network, and specifically how unicast DAOs can be used to
to relay group registrations inwards. Wherever the following text relay group registrations inwards. Wherever the following text
mentions MLD, one can read MLDv2 or v3. mentions MLD, one can read MLDv2 or v3.
As is traditional, a listener uses a protocol such as MLD with a As is traditional, a listener uses a protocol such as MLD with a
router to register to a multicast group. router to register to a multicast group.
Along the path between the router and the root of the DAG, MLD 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 requests are mapped and transported as DAO messages within the RPL
protocol; each hop coalesces the multiple requests for a same group 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 as a single DAO message to the parent(s), in a fashion similar to
proxy IGMP, but recursively between child router and parent up to the proxy IGMP, but recursively between child router and parent up to the
root. root.
A router might select to pass a listener registration NA-DAO message A router might select to pass a listener registration DAO message to
to its preferred parent only, in which case multicast packets coming its preferred parent only, in which case multicast packets coming
back might be lost for all of its sub-DAG if the transmission fails back might be lost for all of its sub-DAG if the transmission fails
over that link. Alternatively the router might select to copy over that link. Alternatively the router might select to copy
additional parents as it would do for NA-DAO messages advertising additional parents as it would do for DAO messages advertising
unicast destinations, in which case there might be duplicates that unicast destinations, in which case there might be duplicates that
the router will need to prune. the router will need to prune.
As a result, multicast routing states are installed in each router on 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 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 multicast packet to all its children routers that had issued a DAO
message including a DAO for that multicast group, as well as all the message including a DAO for that multicast group, as well as all the
attached nodes that registered over MLD. attached nodes that registered over MLD.
For unicast traffic, it is expected that the grounded root of an RPL For unicast traffic, it is expected that the grounded root of an RPL
DAG terminates RPL and MAY redistribute the RPL routes over the DAG terminates RPL and MAY redistribute the RPL routes over the
external infrastructure using whatever routing protocol is used external infrastructure using whatever routing protocol is used
there. For multicast traffic, the root MAY proxy MLD for all the there. For multicast traffic, the root MAY proxy MLD for all the
nodes attached to the RPL routers (this would be needed if the nodes attached to the RPL routers (this would be needed if the
multicast source is located in the external infrastructure). For multicast source is located in the external infrastructure). For
such a source, the packet will be replicated as it flows outwards such a source, the packet will be replicated as it flows outwards
along the DAG based on the multicast routing table entries installed along the DAG based on the multicast routing table entries installed
from the NA-DAO message. from the DAO message.
For a source inside the DAG, the packet is passed to the preferred 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 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 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 one that passed the packet. Finally, if there is a listener in the
external infrastructure then the DAG root has to further propagate external infrastructure then the DAG root has to further propagate
the packet into the external infrastructure. the packet into the external infrastructure.
As a result, the DAG Root acts as an automatic proxy Rendez-vous As a result, the DAG Root acts as an automatic proxy Rendezvous Point
Point for the RPL network, and as source towards the Internet for all for the RPL network, and as source towards the Internet for all
multicast flows started in the RPL LLN. So regardless of whether the multicast flows started in the RPL LLN. So regardless of whether the
root is actually attached to the Internet, and regardless of whether root is actually attached to the Internet, and regardless of whether
the DAG is grounded or floating, the root can serve inner multicast the DAG is grounded or floating, the root can serve inner multicast
streams at all times. streams at all times.
5.11. Maintenance of Routing Adjacency 5.13. Maintenance of Routing Adjacency
The selection of successors, along the default paths inward along the The selection of successors, along the default paths inward along the
DAG, or along the paths learned from destination advertisements DAG, or along the paths learned from destination advertisements
outward along the DAG, leads to the formation of routing adjacencies outward along the DAG, leads to the formation of routing adjacencies
that require maintenance. that require maintenance.
In IGPs such as OSPF [RFC4915] or IS-IS [RFC5120], the maintenance of In IGPs such as OSPF [RFC4915] or IS-IS [RFC5120], the maintenance of
a routing adjacency involves the use of Keepalive mechanisms (Hellos) a routing adjacency involves the use of Keepalive mechanisms (Hellos)
or other protocols such as BFD ([I-D.ietf-bfd-base]) and MANET or other protocols such as BFD ([I-D.ietf-bfd-base]) and MANET
Neighborhood Discovery Protocol (NHDP [I-D.ietf-manet-nhdp]). Neighborhood Discovery Protocol (NHDP [I-D.ietf-manet-nhdp]).
skipping to change at page 64, line 5 skipping to change at page 59, line 14
Thus RPL makes use of a different approach consisting of probing the Thus RPL makes use of a different approach consisting of probing the
neighbor using a Neighbor Solicitation message (see [RFC4861]). The neighbor using a Neighbor Solicitation message (see [RFC4861]). The
reception of a Neighbor Advertisement (NA) message with the reception of a Neighbor Advertisement (NA) message with the
"Solicited Flag" set is used to verify the validity of the routing "Solicited Flag" set is used to verify the validity of the routing
adjacency. Such mechanism MAY be used prior to sending a data adjacency. Such mechanism MAY be used prior to sending a data
packet. This allows for detecting whether or not the routing packet. This allows for detecting whether or not the routing
adjacency is still valid, and should it not be the case, select adjacency is still valid, and should it not be the case, select
another feasible successor to forward the packet. another feasible successor to forward the packet.
5.12. Packet Forwarding 5.14. Packet Forwarding
When forwarding a packet to a destination, precedence is given to When forwarding a packet to a destination, precedence is given to
selection of a next-hop successor as follows: selection of a next-hop successor as follows:
1. It is preferred to select a successor from a DAG who is 1. In the scope of this specification, it is preferred to select a
supporting an OCP and related optimization that maps to an successor from a DAG that matches the InstanceID marked in the
objective marked in the IPv6 header of the packet being IPv6 header of the packet being forwarded.
forwarded.
2. If a local administrative preference favors a route that has been 2. If a local administrative preference favors a route that has been
learned from a different routing protocol than RPL, then use that learned from a different routing protocol than RPL, then use that
successor. successor.
3. If there is an entry in the routing table matching the 3. If there is an entry in the routing table matching the
destination that has been learned from a multicast destination destination that has been learned from a multicast destination
advertisement (e.g. the destination is a one-hop neighbor), then advertisement (e.g. the destination is a one-hop neighbor), then
use that successor. use that successor.
4. If there is an entry in the routing table matching the 4. If there is an entry in the routing table matching the
destination that has been learned from a unicast destination destination that has been learned from a unicast destination
advertisement (e.g. the destination is located outwards along the advertisement (e.g. the destination is located outwards along the
sub-DAG), then use that successor. sub-DAG), then use that successor.
5. If there is a DAG offering a route to a prefix matching the 5. If there is a DAG offering a route to a prefix matching the
destination, then select one of those DAG parents as a successor. destination, then select one of those DAG parents as a successor.
6. If there is a DAG offering a default route with a compatible OCP, 6. If there is a DAG parent offering a default route then select
then select one of those DAG parents as a successor. that DAG parent as a successor.
7. If there is a DAG offering a route to a prefix matching the 7. If there is a DAG offering a route to a prefix matching the
destination, but all DAG parents have been tried and are destination, but all DAG parents have been tried and are
temporarily unavailable (as determined by the forwarding temporarily unavailable (as determined by the forwarding
procedure), then select a DAG sibling as a successor. procedure), then select a DAG sibling as a successor.
8. Finally, if no DAG siblings are available, the packet is dropped. 8. Finally, if no DAG siblings are available, the packet is dropped.
ICMP Destination Unreachable may be invoked. An inconsistency is ICMP Destination Unreachable may be invoked. An inconsistency is
detected. detected.
TTL MUST be decremented when forwarding. If the packet is being TTL MUST be decremented when forwarding. If the packet is being
forwarded via a sibling, then the TTL MAY be decremented more forwarded via a sibling, then the TTL MAY be decremented more
aggressively (by more than one) to limit the impact of possible aggressively (by more than one) to limit the impact of possible
loops. loops.
Note that the chosen successor MUST NOT be the neighbor who was the Note that the chosen successor MUST NOT be the neighbor that was the
predecessor of the packet (split horizon), except in the case where 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 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 flow, such as switching from DIO routes to DAO routes as the
destination is neared. destination is neared.
6. RPL Variables 6. RPL Constants and Variables
DIO Timer One instance per DAG that a node is a member of. Expiry ZERO_LIFETIME This is the special value of a lifetime that indicates
triggers RA-DIO message transmission. Trickle timer with immediate death and removal. ZERO_LIFETIME has a value of 0.
variable interval in [0,
DIOIntervalMin..2^DIOIntervalDoublings]. See Section 5.3.4
DAG Hop Timer Up to one instance per candidate DAG parent in the BASE_RANK This is the rank for a virtual root that might be used to
`Held-Up' state per DAG that a node is going to jump to. coordinate multiple roots. BASE_RANK has a value of 0.
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.
Hold-Down Timer Up to one instance per candidate DAG parent in the ROOT_RANK This is the rank for a DAG root. ROOT_RANK has a value of
`Held-Down' state per DAG. Expiry triggers the eviction of the 1.
candidate DAG parent from the candidate DAG parent set. The
interval should be chosen as appropriate to prevent flapping.
See Section 5.7.
DAG Heartbeat Timer Up to one instance per DAG that the node is INFINITE_RANK This is the constant maximum for the rank.
acting as DAG root of. May not be supported in all INFINITE_RANK has a value of 0xFF.
RPL_DEFAULT_INSTANCE This is the instance ID that is used by this
protocol by a node without a policy to know any better.
RPL_DEFAULT_INSTANCE has a value of 0.
DEFAULT_DIO_INTERVAL_MIN To be determined
DEFAULT_DIO_INTERVAL_DOUBLINGS To be determined
DEF_DAO_LATENCY To be determined
MAX_DESTROY_INTERVAL To be determined
DIO Timer One instance per DAG that a node is a member of. Expiry
triggers DIO message transmission. Trickle timer with variable
interval in [0, DIOIntervalMin..2^DIOIntervalDoublings]. See
Section 5.4.4
DAG Sequence Number Increment 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 implementations. Expiry triggers revision of
DAGSequenceNumber, causing a new series of updated RA-DIO DAGSequenceNumber, causing a new series of updated DIO message
message to be sent. Interval should be chosen appropriate to to be sent. Interval should be chosen appropriate to
propagation time of DAG and as appropriate to application propagation time of DAG and as appropriate to application
requirements (e.g. response time vs. overhead). See requirements (e.g. response time vs. overhead). See
Section 5.4 Section 5.5
DelayNA Timer Up to one instance per DA parent (the subset of DAG DelayDAO Timer Up to one instance per DA parent (the subset of DAG
parents chosen to receive destination advertisements) per DAG. parents chosen to receive destination advertisements) per DAG.
Expiry triggers sending of NA-DAO message to the DA parent. Expiry triggers sending of DAO message to the DA parent. The
The interval is to be proportional to DEF_NA_LATENCY/(node interval is to be proportional to DEF_DAO_LATENCY/(node rank),
rank), such that nodes of greater rank (further outward along such that nodes of greater rank (further outward along the DAG)
the DAG) expire first, coordinating the sending of NA-DAO expire first, coordinating the sending of DAO messages to allow
messages to allow for a chance of aggregation. See for a chance of aggregation. See Section 5.10.1.1.1
Section 5.9.2.1.1
DestroyTimer Up to one instance per DA entry per neighbor (i.e. RemoveTimer Up to one instance per DA entry per neighbor (i.e. those
those neighbors who have given NA-DAO messages to this node as neighbors that have given DAO messages to this node as a DAG
a DAG parent) Expiry triggers a change in state for the DA parent) Expiry triggers a change in state for the DA entry,
entry, setting up to do unreachable (No-DAO) advertisements or setting up to do unreachable (No-DAO) advertisements or
immediately deallocating the DA entry if there are no DA immediately deallocating the DA entry if there are no DA
parents. The interval is min(MAX_DESTROY_INTERVAL, parents. The interval is min(MAX_DESTROY_INTERVAL, TBD(DIO
RA_INTERVAL). See Section 5.9.2.1.1 Trickle Timer Interval)). See Section 5.10.1.1.1
7. Manageability Considerations 7. Manageability Considerations
The aim of this section is to give consideration to the manageability 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 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 module. The scope of this section is to consider the following
aspects of manageability: fault management, configuration, accounting aspects of manageability: fault management, configuration, accounting
and performance. and performance.
7.1. Control of Function and Policy 7.1. Control of Function and Policy
7.1.1. Initialization Mode 7.1.1. Initialization Mode
When a node is first powered up, it may either choose to stay silent 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, and not send any multicast DIO message until it has joined a DAG, or
or to immediately root a transient DAG and start sending multicast to immediately root a transient DAG and start sending multicast DIO
RA-DIO messages. A RPL implementation SHOULD allow configuring messages. A RPL implementation SHOULD allow configuring whether the
whether the node should stay silent or should start advertising RA- node should stay silent or should start advertising DIO messages.
DIO messages.
Furthermore, the implementation SHOULD to allow configuring whether Furthermore, the implementation SHOULD to allow configuring whether
or not the node should start sending an RS message as an initial or not the node should start sending an DIS message as an initial
probe for nearby DAGs, or should simply wait until it received RA probe for nearby DAGs, or should simply wait until it received RA
messages from other nodes that are part of existing DAGs. messages from other nodes that are part of existing DAGs.
7.1.2. DIO Base option 7.1.2. DIO Base option
RPL specifies a number of protocol parameters. RPL specifies a number of protocol parameters.
A RPL implementation SHOULD allow configuring the following routing A RPL implementation SHOULD allow configuring the following routing
protocol parameters, which are further described in Section 5.1.1: protocol parameters, which are further described in Section 5.1.3.1:
DAGPreference DAGPreference
InstanceID
NodePreference
DAGDelay
DIOIntervalDoublings
DIOIntervalMin:
DAGObjectiveCodePoint DAGObjectiveCodePoint
PathDigest
DAGID DAGID
Destination Prefixes Destination Prefixes
DIOIntervalDoublings
DIOIntervalMin
DAG Root behavior: In some cases, a node may not want to permanently 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 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 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 root for a long period of time. Thus a RPL implementation MAY
support the ability to configure whether or not a node could support the ability to configure whether or not a node could
act as a DAG root for a configured period of time. 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 DAG Table Entry Suppression A RPL implementation SHOULD provide the
ability to configure a timer after the expiration of which the ability to configure a timer after the expiration of which the
DAG table that contains all the records about a DAG is DAG table that contains all the records about a DAG is
suppressed, to be invoked if the DAG parent set becomes empty. suppressed, to be invoked if the DAG parent set becomes empty.
7.1.3. Trickle Timers 7.1.3. Trickle Timers
A RPL implementation makes use of trickle timer to govern the sending 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 of DIO message. Such an algorithm is determined a by a set of
configurable parameters that are then advertised by the DAG root configurable parameters that are then advertised by the DAG root
along the DAG in RA-DIO messages. along the DAG in DIO messages.
For each DAG, a RPL implementation MUST allow for the monitoring of For each DAG, a RPL implementation MUST allow for the monitoring of
the following parameters, further described in Section 5.3.4: the following parameters, further described in Section 5.4.4:
I I
T T
C C
I_min I_min
I_doublings: I_doublings:
A RPL implementation SHOULD provide a command (for example via API, A RPL implementation SHOULD provide a command (for example via API,
CLI, or SNMP MIB) whereby any procedure that detects an inconsistency CLI, or SNMP MIB) whereby any procedure that detects an inconsistency
may cause the trickle timer to reset. may cause the trickle timer to reset.
7.1.4. DAG Heartbeat 7.1.4. DAG Sequence Number Increment
A RPL implementation may allow by configuration at the DAG root to A RPL implementation may allow by configuration at the DAG root to
refresh the DAG states by updating the DAGSequenceNumber. A RPL refresh the DAG states by updating the DAGSequenceNumber. A RPL
implementation SHOULD allow configuring whether or not periodic or implementation SHOULD allow configuring whether or not periodic or
event triggered mechanism are used by the DAG root to control event triggered mechanism are used by the DAG root to control
DAGSequenceNumber change. DAGSequenceNumber change.
7.1.5. The Destination Advertisement Option 7.1.5. Destination Advertisement Timers
The following set of parameters of the NA-DAO messages SHOULD be The following set of parameters of the DAO messages SHOULD be
configurable: configurable:
o The DelayNA timer o The DelayDAO timer
o The Remove timer o The Remove timer
7.1.6. Policy Control 7.1.6. Policy Control
DAG discovery enables nodes to implement different policies for DAG discovery enables nodes to implement different policies for
selecting their DAG parents. selecting their DAG parents.
A RPL implementation SHOULD allow configuring the set of acceptable A RPL implementation SHOULD allow configuring the set of acceptable
or preferred Objective Functions (OF) referenced by their Objective or preferred Objective Functions (OF) referenced by their Objective
skipping to change at page 68, line 40 skipping to change at page 63, line 40
taken if none of a node's candidate neighbors advertise one of the taken if none of a node's candidate neighbors advertise one of the
configured allowable Objective Functions. configured allowable Objective Functions.
A node in an LLN may learn routing information from different routing A node in an LLN may learn routing information from different routing
protocols including RPL. It is in this case desirable to control via protocols including RPL. It is in this case desirable to control via
administrative preference which route should be favored. An administrative preference which route should be favored. An
implementation SHOULD allow for specifying an administrative implementation SHOULD allow for specifying an administrative
preference for the routing protocol from which the route was learned. preference for the routing protocol from which the route was learned.
A RPL implementation SHOULD allow for the configuration of the "Route 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 Tag" field of the DAO messages according to a set of rules defined by
by policy. policy.
7.1.7. Data Structures 7.1.7. Data Structures
Some RPL implementation may limit the size of the candidate neighbor Some RPL implementation may limit the size of the candidate neighbor
list in order to bound the memory usage, in which case some otherwise list in order to bound the memory usage, in which case some otherwise
viable candidate neighbors may not be considered and simply dropped viable candidate neighbors may not be considered and simply dropped
from the candidate neighbor list. from the candidate neighbor list.
A RPL implementation MAY provide an indicator on the size of the A RPL implementation MAY provide an indicator on the size of the
candidate neighbor list. candidate neighbor list.
skipping to change at page 69, line 22 skipping to change at page 64, line 22
The aim of this section is to describe the various RPL mechanisms The aim of this section is to describe the various RPL mechanisms
specified to monitor the protocol. specified to monitor the protocol.
As specified in Section 5.2, an implementation must maintain a set of As specified in Section 5.2, an implementation must maintain a set of
data structures in support of DAG discovery: data structures in support of DAG discovery:
o The candidate neighbors data structure o The candidate neighbors data structure
o For each DAG: o For each DAG:
* A set of candidate DAG parents * A set of DAG parents
* A set of DAG parents (which are a subset of candidate DAG
parents and may be implemented as such)
7.3.1. Candidate Neighbor Data Structure 7.3.1. Candidate Neighbor Data Structure
A node in the candidate neighbor list is a node discovered by the A node in the candidate neighbor list is a node discovered by the
some means and qualified to potentially become of neighbor or a some means and qualified to potentially become of neighbor or a
sibling (with high enough local confidence). A RPL implementation sibling (with high enough local confidence). A RPL implementation
SHOULD provide a way monitor the candidate neighbors list with some SHOULD provide a way monitor the candidate neighbors list with some
metric reflecting local confidence (the degree of stability of the metric reflecting local confidence (the degree of stability of the
neighbors) measured by some metrics. neighbors) measured by some metrics.
skipping to change at page 69, line 51 skipping to change at page 64, line 48
For each DAG, a RPL implementation MUST keep track of the following For each DAG, a RPL implementation MUST keep track of the following
DAG table values: DAG table values:
o DAGID o DAGID
o DAGObjectiveCodePoint o DAGObjectiveCodePoint
o A set of Destination Prefixes offered inwards along the DAG o A set of Destination Prefixes offered inwards along the DAG
o A set of candidate DAG Parents o A set of DAG Parents
o timer to govern the sending of RA-DIO messages for the DAG
o timer to govern the sending of DIO messages for the DAG
o DAGSequenceNumber o DAGSequenceNumber
The set of candidate DAG parents structure is itself a table with the The set of DAG parents structure is itself a table with the following
following entries: entries:
o A reference to the neighboring device which is the DAG parent o A reference to the neighboring device which is the DAG parent
o A record of most recent information taken from the DAG Information o A record of most recent information taken from the DAG Information
Object last processed from the candidate DAG Parent Object last processed from the DAG Parent
o a state associated with the role of the candidate as a potential o A flag reporting if the Parent is a DA Parent as described in
DAG Parent {Current, Held-Up, Held-Down, Collision}, further Section 5.10
described in Section 5.7
o A DAG Hop Timer, if instantiated 7.3.3. Routing Table
o A Held-Down Timer, if instantiated For each route provisioned by RPL operation, a RPL implementation
MUST keep track of the following:
o A flag reporting if the Parent is a DA Parent as described in o Destination Prefix
Section 5.9
7.3.3. Routing Table o Destination Prefix Length
To be completed. o Lifetime Timer
o Next Hop
o Next Hop Interface
o Flag indicating that the route was provisioned from one of:
* Unicast DAO message
* DIO message
* Multicast DAO message
7.3.4. Other RPL Monitoring Parameters 7.3.4. Other RPL Monitoring Parameters
A RPL implementation SHOULD provide a counter reporting the number of A RPL implementation SHOULD provide a counter reporting the number of
a times the node has detected an inconsistency with respect to a DAG a times the node has detected an inconsistency with respect to a DAG
parent, e.g. if the DAGID has changed. parent, e.g. if the DAGID has changed.
A RPL implementation MAY log the reception of a malformed RA-DIO A RPL implementation MAY log the reception of a malformed DIO message
message along with the neighbor identification if avialable. along with the neighbor identification if avialable.
7.3.5. RPL Trickle Timers 7.3.5. RPL Trickle Timers
A RPL implementation operating on a DAG root MUST allow for the A RPL implementation operating on a DAG root MUST allow for the
configuration of the following trickle parameters: configuration of the following trickle parameters:
o The DIOIntervalMin expressed in ms o The DIOIntervalMin expressed in ms
o The DIOIntervalDoublings o The DIOIntervalDoublings
skipping to change at page 71, line 27 skipping to change at page 66, line 39
To be completed. To be completed.
8. Security Considerations 8. Security Considerations
Security Considerations for RPL are to be developed in accordance Security Considerations for RPL are to be developed in accordance
with recommendations laid out in, for example, with recommendations laid out in, for example,
[I-D.tsao-roll-security-framework]. [I-D.tsao-roll-security-framework].
9. IANA Considerations 9. IANA Considerations
9.1. DAG Information Option (DIO) Base Option 9.1. RPL Control Message
The DAG Information Option is a container option carried within an The RPL Control Message is an ICMP information message type that is
IPv6 Router Advertisement message as defined in [RFC4861], which to be used carry DAG Information Objects, DAG Information
might contain a number of suboptions. The base option regroups the Solicitations, and Destination Advertisement Objects in support of
minimum information set that is mandatory in all cases. RPL operation.
IANA had defined the IPv6 Neighbor Discovery Option Formats registry. IANA has defined a ICMPv6 Type Number Registry. The suggested type
The suggested type value for the DAG Information Option (DIO) Base value for the RPL Control Message is 155, to be confirmed by IANA.
Option is 140, to be confirmed by IANA.
9.2. New Registry for the Flag Field of the DIO Base Option 9.2. New Registry for RPL Control Codes
IANA is requested to create a registry for the Flag field of the DIO IANA is requested to create a registry, RPL Control Codes, for the
Base Option. Code field of the ICMPv6 RPL Control Message.
New codes may be allocated only by an IETF Consensus action. Each
code should be tracked with the following qualities:
o Code
o Description
o Defining RFC
Three codes are currently defined:
+------+----------------------------------+---------------+
| Code | Description | Reference |
+------+----------------------------------+---------------+
| 0x01 | DAG Information Solicitation | This document |
| 0x02 | DAG Information Object | This document |
| 0x04 | Destination Advertisement Object | This document |
+------+----------------------------------+---------------+
RPL Control Codes
9.3. New Registry for the Control Field of the DIO Base Option
IANA is requested to create a registry for the Control field of the
DIO Base Option.
New bit numbers may be allocated only by an IETF Consensus action. New bit numbers may be allocated only by an IETF Consensus action.
Each bit should be tracked with the following qualities: Each bit should be tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit) o Bit number (counting from bit 0 as the most significant bit)
o Capability description o Capability description
o Defining RFC o Defining RFC
Three flags are currently defined: Four groups are currently defined:
+-----+-------------------------------------+---------------+ +-------+-------------------------------------+---------------+
| Bit | Description | Reference | | Bit | Description | Reference |
+-----+-------------------------------------+---------------+ +-------+-------------------------------------+---------------+
| 0 | Grounded DAG | This document | | 0 | Grounded DAG | This document |
| 1 | Destination Advertisement Trigger | This document | | 1 | Destination Advertisement Trigger | This document |
| 2 | Destination Advertisement Supported | This document | | 2 | Destination Advertisement Supported | This document |
+-----+-------------------------------------+---------------+ | 5,6,7 | DAG Preference | This document |
+-------+-------------------------------------+---------------+
DIO Base Option Flags DIO Base Option Flags
9.3. DAG Information Option (DIO) Suboption 9.4. DAG Information Object (DIO) Suboption
IANA is requested to create a registry for the DIO Base Option IANA is requested to create a registry for the DIO Base Option
Suboptions Suboptions
+-------+------------------------------+---------------+ +-------+------------------------------+---------------+
| Value | Meaning | Reference | | Value | Meaning | Reference |
+-------+------------------------------+---------------+ +-------+------------------------------+---------------+
| 0 | Pad1 - DIO Padding | This document | | 0 | Pad1 - DIO Padding | This document |
| 1 | PadN - DIO suboption padding | This document | | 1 | PadN - DIO suboption padding | This document |
| 2 | DAG Metric Container | This Document | | 2 | DAG Metric Container | This Document |
| 3 | Destination Prefix | This Document | | 3 | Destination Prefix | This Document |
| 4 | DAG Timer Configuration | This Document |
+-------+------------------------------+---------------+ +-------+------------------------------+---------------+
DAG Information Option (DIO) Base Option Suboptions DAG Information Option (DIO) Base Option Suboptions
9.4. Destination Advertisement Option (DAO) Option 9.5. Objective Code Point for the Default Objective Function OF0
The RPL protocol extends Neighbor Discovery [RFC4861] and [RFC4191]
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. This specification specifies the Default Objective Function (called
The suggested type value for the Destination Advertisement Option OF0) for which the OCP field of the OF object, as defined in
carried within a Neighbor Advertisement message is 141, to be [I-D.ietf-roll-routing-metrics], is equal to 0x0000
confirmed by IANA.
9.5. Objective Code Point +-------+---------+---------------+
| Value | Meaning | Reference |
+-------+---------+---------------+
| 0 | OF0 | This document |
+-------+---------+---------------+
This specification requests that an Objective Code Point registry, as OCP Allocation
to be specified in [I-D.ietf-roll-routing-metrics], reserve the
Objective Code Point value 0x0000, for the purposes designated as OCP
0 in this document.
10. Acknowledgements 10. Acknowledgements
The ROLL Design Team would like to acknowledge the review, feedback, The authors would like to acknowledge the review, feedback, and
and comments from Dominique Barthel, Yusuf Bashir, Mathilde Durvy, comments from Emmanuel Baccelli, Dominique Barthel, Yusuf Bashir,
Manhar Goindi, Mukul Goyal, Quentin Lampin, Philip Levis, Jerry Mathilde Durvy, Manhar Goindi, Mukul Goyal, Anders Jagd, Quentin
Martocci, Alexandru Petrescu, and Don Sturek. Lampin, Jerry Martocci, Alexandru Petrescu, and Don Sturek.
The ROLL Design Team would like to acknowledge the guidance and input The authors would like to acknowledge the guidance and input provided
provided by the ROLL Chairs, David Culler and JP Vasseur. by the ROLL Chairs, David Culler and JP Vasseur.
The ROLL Design Team would like to acknowledge prior contributions of The authors would like to acknowledge prior contributions of Robert
Robert Assimiti, Mischa Dohler, Julien Abeille, Ryuji Wakikawa, Teco Assimiti, Mischa Dohler, Julien Abeille, Ryuji Wakikawa, Teco Boot,
Boot, Patrick Wetterwald, Bryan Mclaughlin, Carlos J. Bernardos, Patrick Wetterwald, Bryan Mclaughlin, Carlos J. Bernardos, Thomas
Thomas Watteyne, Zach Shelby, Caroline Bontoux, Marco Molteni, Billy Watteyne, Zach Shelby, Caroline Bontoux, Marco Molteni, Billy Moon,
Moon, and Arsalan Tavakoli, which have provided useful design and Arsalan Tavakoli, which have provided useful design
considerations to RPL. considerations to RPL.
11. Contributors 11. Contributors
RPL is the result of the contribution of the following members of the
ROLL Design Team, including the editors, and additional contributors
as listed below:
JP Vasseur JP Vasseur
Cisco Systems, Inc Cisco Systems, Inc
11, Rue Camille Desmoulins 11, Rue Camille Desmoulins
Issy Les Moulineaux, 92782 Issy Les Moulineaux, 92782
France France
Email: jpv@cisco.com Email: jpv@cisco.com
Jonathan W. Hui Jonathan W. Hui
Arch Rock Corporation Arch Rock Corporation
skipping to change at page 74, line 4 skipping to change at page 69, line 44
USA USA
Email: jhui@archrock.com Email: jhui@archrock.com
Thomas Heide Clausen Thomas Heide Clausen
LIX, Ecole Polytechnique, France LIX, Ecole Polytechnique, France
Phone: +33 6 6058 9349 Phone: +33 6 6058 9349
EMail: T.Clausen@computer.org EMail: T.Clausen@computer.org
URI: http://www.ThomasClausen.org/ URI: http://www.ThomasClausen.org/
Richard Kelsey Richard Kelsey
Ember Corporation Ember Corporation
Boston, MA Boston, MA
USA USA
Phone: +1 617 951 1225 Phone: +1 617 951 1225
Email: kelsey@ember.com Email: kelsey@ember.com
Philip Levis
Stanford University
358 Gates Hall, Stanford University
Stanford, CA 94305-9030
USA
Email: pal@cs.stanford.edu
Stephen Dawson-Haggerty Stephen Dawson-Haggerty
UC Berkeley UC Berkeley
Soda Hall, UC Berkeley Soda Hall, UC Berkeley
Berkeley, CA 94720 Berkeley, CA 94720
USA USA
Email: stevedh@cs.berkeley.edu Email: stevedh@cs.berkeley.edu
Kris Pister Kris Pister
Dust Networks Dust Networks
skipping to change at page 75, line 23 skipping to change at page 71, line 29
"Building Automation Routing Requirements in Low Power and "Building Automation Routing Requirements in Low Power and
Lossy Networks", draft-ietf-roll-building-routing-reqs-07 Lossy Networks", draft-ietf-roll-building-routing-reqs-07
(work in progress), September 2009. (work in progress), September 2009.
[I-D.ietf-roll-home-routing-reqs] [I-D.ietf-roll-home-routing-reqs]
Brandt, A., Buron, J., and G. Porcu, "Home Automation Brandt, A., Buron, J., and G. Porcu, "Home Automation
Routing Requirements in Low Power and Lossy Networks", Routing Requirements in Low Power and Lossy Networks",
draft-ietf-roll-home-routing-reqs-08 (work in progress), draft-ietf-roll-home-routing-reqs-08 (work in progress),
September 2009. 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", draft-ietf-roll-indus-routing-reqs-06 (work in
progress), June 2009.
[I-D.ietf-roll-routing-metrics] [I-D.ietf-roll-routing-metrics]
Vasseur, J. and D. Networks, "Routing Metrics used for Vasseur, J. and D. Networks, "Routing Metrics used for
Path Calculation in Low Power and Lossy Networks", Path Calculation in Low Power and Lossy Networks",
draft-ietf-roll-routing-metrics-00 (work in progress), draft-ietf-roll-routing-metrics-01 (work in progress),
April 2009. October 2009.
[I-D.ietf-roll-terminology] [I-D.ietf-roll-terminology]
Vasseur, J., "Terminology in Low power And Lossy Vasseur, J., "Terminology in Low power And Lossy
Networks", draft-ietf-roll-terminology-01 (work in Networks", draft-ietf-roll-terminology-02 (work in
progress), May 2009. progress), October 2009.
[I-D.tsao-roll-security-framework] [I-D.tsao-roll-security-framework]
Tsao, T., Alexander, R., Dohler, M., Daza, V., and A. Tsao, T., Alexander, R., Dohler, M., Daza, V., and A.
Lozano, "A Security Framework for Routing over Low Power Lozano, "A Security Framework for Routing over Low Power
and Lossy Networks", draft-tsao-roll-security-framework-01 and Lossy Networks", draft-tsao-roll-security-framework-01
(work in progress), September 2009. (work in progress), September 2009.
[Levis08] Levis, P., Brewer, E., Culler, D., Gay, D., Madden, S., [Levis08] Levis, P., Brewer, E., Culler, D., Gay, D., Madden, S.,
Patel, N., Polastre, J., Shenker, S., Szewczyk, R., and A. Patel, N., Polastre, J., Shenker, S., Szewczyk, R., and A.
Woo, "The Emergence of a Networking Primitive in Wireless Woo, "The Emergence of a Networking Primitive in Wireless
skipping to change at page 76, line 19 skipping to change at page 72, line 19
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89, Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004. RFC 3819, July 2004.
[RFC4101] Rescorla, E. and IAB, "Writing Protocol Models", RFC 4101, [RFC4101] Rescorla, E. and IAB, "Writing Protocol Models", RFC 4101,
June 2005. June 2005.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005. More-Specific Routes", RFC 4191, November 2005.
[RFC4461] Yasukawa, S., "Signaling Requirements for Point-to- [RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Multipoint Traffic-Engineered MPLS Label Switched Paths Message Protocol (ICMPv6) for the Internet Protocol
(LSPs)", RFC 4461, April 2006. Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman, [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861, "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007. September 2007.
[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.
[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P. [RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF", Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, June 2007. RFC 4915, June 2007.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi [RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120, February 2008. Intermediate Systems (IS-ISs)", RFC 5120, February 2008.
[RFC5548] Dohler, M., Watteyne, T., Winter, T., and D. Barthel, [RFC5548] Dohler, M., Watteyne, T., Winter, T., and D. Barthel,
"Routing Requirements for Urban Low-Power and Lossy "Routing Requirements for Urban Low-Power and Lossy
Networks", RFC 5548, May 2009. Networks", RFC 5548, May 2009.
Appendix A. Deferred Requirements [RFC5673] Pister, K., Thubert, P., Dwars, S., and T. Phinney,
"Industrial Routing Requirements in Low-Power and Lossy
Networks", RFC 5673, October 2009.
Appendix A. Requirements
A.1. Protocol Properties Overview
RPL demonstrates the following properties, consistent with the
requirements specified by the application-specific requirements
documents.
A.1.1. IPv6 Architecture
RPL is strictly compliant with layered IPv6 architecture.