draft-ietf-roll-rpl-04.txt   draft-ietf-roll-rpl-05.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 29, 2010 Cisco Systems Expires: June 10, 2010 Cisco Systems
ROLL Design Team ROLL Design Team
IETF ROLL WG IETF ROLL WG
October 26, 2009 December 7, 2009
RPL: IPv6 Routing Protocol for Low power and Lossy Networks RPL: IPv6 Routing Protocol for Low power and Lossy Networks
draft-ietf-roll-rpl-04 draft-ietf-roll-rpl-05
Abstract
Low power and Lossy Networks (LLNs) are a class of network in which
both the routers and their interconnect are constrained: LLN routers
typically operate with constraints on (any subset of) processing
power, memory and energy (battery), and their interconnects are
characterized by (any subset of) high loss rates, low data rates and
instability. LLNs are comprised of anything from a few dozen and up
to thousands of LLN routers, and support point-to-point traffic
(between devices inside the LLN), point-to-multipoint traffic (from a
central control point to a subset of devices inside the LLN) and
multipoint-to-point traffic (from devices inside the LLN towards a
central control point). This document specifies the IPv6 Routing
Protocol for LLNs (RPL), which provides a mechanism whereby
multipoint-to-point traffic from devices inside the LLN towards a
central control point, as well as point-to-multipoint traffic from
the central control point to the devices inside the LLN, is
supported. Support for point-to-point traffic is also available.
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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Abstract include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
Low power and Lossy Networks (LLNs) are a class of network in which described in the BSD License.
both the routers and their interconnect are constrained: LLN routers
typically operate with constraints on (any subset of) processing
power, memory and energy (battery), and their interconnects are
characterized by (any subset of) high loss rates, low data rates and
instability. LLNs are comprised of anything from a few dozen and up
to thousands of LLN routers, and support point-to- point traffic
(between devices inside the LLN), point-to-multipoint traffic (from a
central control point to a subset of devices inside the LLN) and
multipoint-to- point traffic (from devices inside the LLN towards a
central control point). This document specifies the IPv6 Routing
Protocol for LLNs (RPL), which provides a mechanism whereby
multipoint-to-point traffic from devices inside the LLN towards a
central control point, as well as point-to-multipoint traffic from
the central control point to the devices inside the LLN, is
supported. Support for point-to-point traffic is also available.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.1. Design Principles . . . . . . . . . . . . . . . . . . . . 6 1.1. Design Principles . . . . . . . . . . . . . . . . . . . . 5
1.2. Expectations of Link Layer Type . . . . . . . . . . . . . 7 1.2. Expectations of Link Layer Type . . . . . . . . . . . . . 6
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Protocol Model . . . . . . . . . . . . . . . . . . . . . . . . 9 3. Protocol Model . . . . . . . . . . . . . . . . . . . . . . . . 8
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.1. Instances, DODAGs, and DODAG Iterations . . . . . . . . . 8
3.1.1. Topology Instance and Objectives . . . . . . . . . . . 9 3.2. Traffic Flows . . . . . . . . . . . . . . . . . . . . . . 10
3.1.2. Multipoint-to-Point Traffic Flows and DAGs . . . . . . 11 3.2.1. Multipoint-to-Point Traffic . . . . . . . . . . . . . 10
3.1.3. Point-to-Multipoint Traffic Flows . . . . . . . . . . 11 3.2.2. Point-to-Multipoint Traffic . . . . . . . . . . . . . 10
3.1.4. Point-to-Point Traffic Flows . . . . . . . . . . . . . 12 3.2.3. Point-to-Point Traffic . . . . . . . . . . . . . . . . 10
3.2. Protocol Operation . . . . . . . . . . . . . . . . . . . . 12 3.3. DODAG Construction . . . . . . . . . . . . . . . . . . . . 11
3.2.1. DAG Construction . . . . . . . . . . . . . . . . . . . 12 3.3.1. DAG Information Object (DIO) . . . . . . . . . . . . . 11
3.2.2. Destination Advertisement . . . . . . . . . . . . . . 15 3.3.2. DAG Repair . . . . . . . . . . . . . . . . . . . . . . 11
3.3. Loop Avoidance and Stability . . . . . . . . . . . . . . . 17 3.3.3. Grounded and Floating DODAGs . . . . . . . . . . . . . 12
3.3.1. Greediness and Rank-based Instabilities . . . . . . . 17 3.3.4. Administrative Preference . . . . . . . . . . . . . . 12
3.3.2. DAG Loops . . . . . . . . . . . . . . . . . . . . . . 18 3.3.5. Objective Function (OF) . . . . . . . . . . . . . . . 12
3.3.3. DAO Loops . . . . . . . . . . . . . . . . . . . . . . 18 3.3.6. Distributed Algorithm Operation . . . . . . . . . . . 13
3.3.4. Sibling Loops . . . . . . . . . . . . . . . . . . . . 18 3.4. Destination Advertisement . . . . . . . . . . . . . . . . 13
4. Routing Metrics and Constraints Used By RPL . . . . . . . . . 18 3.4.1. Destination Advertisement Object (DAO) . . . . . . . . 13
5. RPL Protocol Specification . . . . . . . . . . . . . . . . . . 19 4. Routing Metrics and Constraints Used By RPL . . . . . . . . . 14
5.1. RPL Messages . . . . . . . . . . . . . . . . . . . . . . . 19 5. Rank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1.1. ICMPv6 RPL Control Message . . . . . . . . . . . . . . 19 5.1. Loop Avoidance . . . . . . . . . . . . . . . . . . . . . . 15
5.1.2. DAG Information Solicitation (DIS) . . . . . . . . . . 20 5.1.1. Greediness and Rank-based Instabilities . . . . . . . 15
5.1.3. DAG Information Object (DIO) . . . . . . . . . . . . . 20 5.1.2. DODAG Loops . . . . . . . . . . . . . . . . . . . . . 16
5.1.4. Destination Advertisement Object (DAO) . . . . . . . . 27 5.1.3. DAO Loops . . . . . . . . . . . . . . . . . . . . . . 16
5.2. Conceptual Data Structures . . . . . . . . . . . . . . . . 28 5.1.4. Sibling Loops . . . . . . . . . . . . . . . . . . . . 16
5.2.1. Candidate Neighbors Data Structure . . . . . . . . . . 28 5.2. Rank Properties . . . . . . . . . . . . . . . . . . . . . 16
5.2.2. Directed Acyclic Graphs (DAGs) Data Structure . . . . 29 6. RPL Protocol Specification . . . . . . . . . . . . . . . . . . 18
5.3. DAG Rank . . . . . . . . . . . . . . . . . . . . . . . . . 30 6.1. RPL Messages . . . . . . . . . . . . . . . . . . . . . . . 18
5.4. DAG Discovery and Maintenance . . . . . . . . . . . . . . 31 6.1.1. ICMPv6 RPL Control Message . . . . . . . . . . . . . . 18
5.4.1. DAG Discovery Rules . . . . . . . . . . . . . . . . . 32 6.1.2. DAG Information Solicitation (DIS) . . . . . . . . . . 19
5.4.2. Reception and Processing of DIO messages . . . . . . . 36 6.1.3. DAG Information Object (DIO) . . . . . . . . . . . . . 19
5.4.3. DIO Transmission . . . . . . . . . . . . . . . . . . . 38 6.1.4. Destination Advertisement Object (DAO) . . . . . . . . 26
5.4.4. Trickle Timer for DIO Transmission . . . . . . . . . . 39 6.2. Protocol Elements . . . . . . . . . . . . . . . . . . . . 28
5.5. DAG Sequence Number Increment . . . . . . . . . . . . . . 40 6.2.1. Topological Elements . . . . . . . . . . . . . . . . . 28
5.6. DAG Selection . . . . . . . . . . . . . . . . . . . . . . 41 6.2.2. Neighbors, Parents, and Siblings . . . . . . . . . . . 28
5.7. Administrative rank . . . . . . . . . . . . . . . . . . . 41 6.2.3. DODAG Information . . . . . . . . . . . . . . . . . . 29
5.8. Collision . . . . . . . . . . . . . . . . . . . . . . . . 42 6.3. DAG Discovery and Maintenance . . . . . . . . . . . . . . 30
5.9. Guidelines for Objective Functions . . . . . . . . . . . . 42 6.3.1. DAG Discovery Rules . . . . . . . . . . . . . . . . . 31
5.9.1. Objective Function . . . . . . . . . . . . . . . . . . 42 6.3.2. DIO Message Communication . . . . . . . . . . . . . . 35
5.9.2. Objective Function 0 (OF0) . . . . . . . . . . . . . . 44 6.3.3. DIO Transmission . . . . . . . . . . . . . . . . . . . 36
5.10. Establishing Routing State Outward Along the DAG . . . . . 46 6.3.4. Trickle Timer for DIO Transmission . . . . . . . . . . 37
5.10.1. Destination Advertisement Operation . . . . . . . . . 47 6.4. DAG Selection . . . . . . . . . . . . . . . . . . . . . . 38
5.11. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 54 6.5. Operation as a Leaf Node . . . . . . . . . . . . . . . . . 39
5.11.1. Host Basic Operation . . . . . . . . . . . . . . . . . 55 6.6. Administrative rank . . . . . . . . . . . . . . . . . . . 39
5.11.2. Instance Forwarding . . . . . . . . . . . . . . . . . 55 6.7. Collision . . . . . . . . . . . . . . . . . . . . . . . . 39
5.11.3. DAG Inconsistency Loop Detection . . . . . . . . . . . 56 6.8. Establishing Routing State Down the DODAG . . . . . . . . 40
5.11.4. Sibling Loop Avoidance . . . . . . . . . . . . . . . . 56 6.8.1. Destination Advertisement Operation . . . . . . . . . 41
5.11.5. DAO Inconsistency Loop Detection and Recovery . . . . 57 6.9. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 48
5.12. Multicast Operation . . . . . . . . . . . . . . . . . . . 57 6.9.1. Source Node Operation . . . . . . . . . . . . . . . . 49
5.13. Maintenance of Routing Adjacency . . . . . . . . . . . . . 58 6.9.2. Router Operation . . . . . . . . . . . . . . . . . . . 49
5.14. Packet Forwarding . . . . . . . . . . . . . . . . . . . . 59 6.10. Multicast Operation . . . . . . . . . . . . . . . . . . . 51
6. RPL Constants and Variables . . . . . . . . . . . . . . . . . 60 6.11. Maintenance of Routing Adjacency . . . . . . . . . . . . . 52
7. Manageability Considerations . . . . . . . . . . . . . . . . . 61 7. Suggestions for Packet Forwarding . . . . . . . . . . . . . . 53
7.1. Control of Function and Policy . . . . . . . . . . . . . . 61 8. Guidelines for Objective Functions . . . . . . . . . . . . . . 54
7.1.1. Initialization Mode . . . . . . . . . . . . . . . . . 61 9. RPL Constants and Variables . . . . . . . . . . . . . . . . . 56
7.1.2. DIO Base option . . . . . . . . . . . . . . . . . . . 61 10. Manageability Considerations . . . . . . . . . . . . . . . . . 58
7.1.3. Trickle Timers . . . . . . . . . . . . . . . . . . . . 62 10.1. Control of Function and Policy . . . . . . . . . . . . . . 58
7.1.4. DAG Sequence Number Increment . . . . . . . . . . . . 63 10.1.1. Initialization Mode . . . . . . . . . . . . . . . . . 58
7.1.5. Destination Advertisement Timers . . . . . . . . . . . 63 10.1.2. DIO Base option . . . . . . . . . . . . . . . . . . . 58
7.1.6. Policy Control . . . . . . . . . . . . . . . . . . . . 63 10.1.3. Trickle Timers . . . . . . . . . . . . . . . . . . . . 59
7.1.7. Data Structures . . . . . . . . . . . . . . . . . . . 63 10.1.4. DAG Sequence Number Increment . . . . . . . . . . . . 59
7.2. Information and Data Models . . . . . . . . . . . . . . . 64 10.1.5. Destination Advertisement Timers . . . . . . . . . . . 59
7.3. Liveness Detection and Monitoring . . . . . . . . . . . . 64 10.1.6. Policy Control . . . . . . . . . . . . . . . . . . . . 59
7.3.1. Candidate Neighbor Data Structure . . . . . . . . . . 64 10.1.7. Data Structures . . . . . . . . . . . . . . . . . . . 60
7.3.2. Directed Acyclic Graph (DAG) Table . . . . . . . . . . 64 10.2. Information and Data Models . . . . . . . . . . . . . . . 60
7.3.3. Routing Table . . . . . . . . . . . . . . . . . . . . 65 10.3. Liveness Detection and Monitoring . . . . . . . . . . . . 60
7.3.4. Other RPL Monitoring Parameters . . . . . . . . . . . 65 10.3.1. Candidate Neighbor Data Structure . . . . . . . . . . 61
7.3.5. RPL Trickle Timers . . . . . . . . . . . . . . . . . . 66 10.3.2. Directed Acyclic Graph (DAG) Table . . . . . . . . . . 61
7.4. Verifying Correct Operation . . . . . . . . . . . . . . . 66 10.3.3. Routing Table . . . . . . . . . . . . . . . . . . . . 61
7.5. Requirements on Other Protocols and Functional 10.3.4. Other RPL Monitoring Parameters . . . . . . . . . . . 62
Components . . . . . . . . . . . . . . . . . . . . . . . . 66 10.3.5. RPL Trickle Timers . . . . . . . . . . . . . . . . . . 62
7.6. Impact on Network Operation . . . . . . . . . . . . . . . 66 10.4. Verifying Correct Operation . . . . . . . . . . . . . . . 62
8. Security Considerations . . . . . . . . . . . . . . . . . . . 66 10.5. Requirements on Other Protocols and Functional
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 66 Components . . . . . . . . . . . . . . . . . . . . . . . . 63
9.1. RPL Control Message . . . . . . . . . . . . . . . . . . . 66 10.6. Impact on Network Operation . . . . . . . . . . . . . . . 63
9.2. New Registry for RPL Control Codes . . . . . . . . . . . . 67 11. Security Considerations . . . . . . . . . . . . . . . . . . . 63
9.3. New Registry for the Control Field of the DIO Base 12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 63
Option . . . . . . . . . . . . . . . . . . . . . . . . . . 67 12.1. RPL Control Message . . . . . . . . . . . . . . . . . . . 63
9.4. DAG Information Object (DIO) Suboption . . . . . . . . . . 68 12.2. New Registry for RPL Control Codes . . . . . . . . . . . . 63
9.5. Objective Code Point for the Default Objective 12.3. New Registry for the Control Field of the DIO Base . . . . 64
Function OF0 . . . . . . . . . . . . . . . . . . . . . . . 68 12.4. DAG Information Object (DIO) Suboption . . . . . . . . . . 64
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 68 13. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 65
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 69 14. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 65
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 70 15. References . . . . . . . . . . . . . . . . . . . . . . . . . . 67
12.1. Normative References . . . . . . . . . . . . . . . . . . . 70 15.1. Normative References . . . . . . . . . . . . . . . . . . . 67
12.2. Informative References . . . . . . . . . . . . . . . . . . 71 15.2. Informative References . . . . . . . . . . . . . . . . . . 67
Appendix A. Requirements . . . . . . . . . . . . . . . . . . . . 72 Appendix A. Requirements . . . . . . . . . . . . . . . . . . . . 69
A.1. Protocol Properties Overview . . . . . . . . . . . . . . . 72 A.1. Protocol Properties Overview . . . . . . . . . . . . . . . 69
A.1.1. IPv6 Architecture . . . . . . . . . . . . . . . . . . 73 A.1.1. IPv6 Architecture . . . . . . . . . . . . . . . . . . 69
A.1.2. Typical LLN Traffic Patterns . . . . . . . . . . . . . 73 A.1.2. Typical LLN Traffic Patterns . . . . . . . . . . . . . 69
A.1.3. Constraint Based Routing . . . . . . . . . . . . . . . 73 A.1.3. Constraint Based Routing . . . . . . . . . . . . . . . 70
A.2. Deferred Requirements . . . . . . . . . . . . . . . . . . 74 A.2. Deferred Requirements . . . . . . . . . . . . . . . . . . 70
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 74 Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 70
B.1. Destination Advertisement . . . . . . . . . . . . . . . . 76 B.1. Destination Advertisement . . . . . . . . . . . . . . . . 72
B.2. Example: DAG Parent Selection . . . . . . . . . . . . . . 77 B.2. Example: DAG Parent Selection . . . . . . . . . . . . . . 73
B.3. Example: DAG Maintenance . . . . . . . . . . . . . . . . . 78 B.3. Example: DAG Maintenance . . . . . . . . . . . . . . . . . 75
B.4. Example: Greedy Parent Selection and Instability . . . . . 79 B.4. Example: Greedy Parent Selection and Instability . . . . . 76
Appendix C. Outstanding Issues . . . . . . . . . . . . . . . . . 81 Appendix C. Outstanding Issues . . . . . . . . . . . . . . . . . 78
C.1. Additional Support for P2P Routing . . . . . . . . . . . . 81 C.1. Additional Support for P2P Routing . . . . . . . . . . . . 78
C.2. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 81 C.2. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 78
C.3. Destination Advertisement / DAO Fan-out . . . . . . . . . 81 C.3. Destination Advertisement / DAO Fan-out . . . . . . . . . 78
C.4. Source Routing . . . . . . . . . . . . . . . . . . . . . . 82 C.4. Source Routing . . . . . . . . . . . . . . . . . . . . . . 79
C.5. Address / Header Compression . . . . . . . . . . . . . . . 82 C.5. Address / Header Compression . . . . . . . . . . . . . . . 79
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 82 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 79
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, typically time supporting only low data rates, that are lossy links, typically supporting only low data rates, that are
usually unstable with relatively low packet delivery rates. Another usually unstable with relatively low packet delivery rates. Another
characteristic of such networks is that the traffic patterns are not characteristic of such networks is that the traffic patterns are not
simply unicast, but in many cases point-to-multipoint or multipoint- simply unicast, but in many cases point-to-multipoint or multipoint-
to-point. Furthermore such networks may potentially comprise up to to-point. Furthermore such networks may potentially comprise up to
thousands of nodes. These characteristics offer unique challenges to thousands of nodes. These characteristics offer unique challenges to
a routing solution: the IETF ROLL Working Group has defined a routing solution: the IETF ROLL Working Group has defined
application-specific routing requirements for a Low power and Lossy application-specific routing requirements for a Low power and Lossy
Network (LLN) routing protocol, specified in 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], [RFC5673], and [RFC5548]. This [I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548]. This
skipping to change at page 6, line 32 skipping to change at page 6, line 32
Networks (RPL). Networks (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], [RFC5673], and [RFC5548]. Because [I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548]. Because
those requirements are heterogeneous and sometimes incompatible in those requirements are heterogeneous and sometimes incompatible in
nature, the approach is first taken to design a protocol capable of nature, 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. As the RPL design evolves optional
design evolves optional features may be added to address some features may be added to address some application specific
application specific requirements). This is a key protocol design requirements. This is a key protocol design decision providing a
decision providing a granular approach in order to restrict the core granular approach in order to restrict the core of the protocol to a
of the protocol to a minimal set of functionalities, and to allow minimal set of functionalities, and to allow each implementation of
each implementation of the protocol to be optimized in terms of, the protocol to be optimized differently. All "MUST" application
e.g., minimizing required code space and use of limited computation requirements that cannot be satisfied by RPL will be specifically
resources. listed in the Appendix A, accompanied by a justification.
Multiple instances of the protocol can be operated at the same time
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 A network may run multiple instances of RPL concurrently. Each such
applications and is expected to be used in other environments. All instance may serve different and potentially antagonistic constraints
"MUST" application requirements that cannot be satisfied by RPL will or performance criteria. This document defines how a single instance
be specifically listed in the Appendix A, accompanied by a operates.
justification.
The core set of functionalities is to be capable of operating in the RPL is a generic protocol that is to be deployed by instantiating the
most severely constrained environments, with minimal requirements for generic operation described in this document with a specific
memory, energy, processing, communication, and other consumption of objective function (OF) (which ties together metrics, constraints,
limited resources from nodes. Trade-offs inherent in the and an optimization objective) to realize a desired objective in a
provisioning of protocol features will be exposed to the implementer given environment.
in the form of configurable parameters, such that the implementer can
further tweak and optimize the operation of RPL as appropriate to a
specific application and implementation. Finally, RPL is designed to
consult implementation specific policies to determine, for example,
the evaluation of routing metrics.
A set of companion documents to this specification will provide 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 Type 1.2. Expectations of Link Layer Type
This specification does not rely on any particular features of a As RPL is a routing protocol, it of course does not rely on any
specific link layer technologies. It is anticipated that an particular features of a specific link layer technology. RPL should
implementer should be able to operate RPL over a variety of different be able to operate over a variety of different link layers, including
link layers, including but not limited to low power wireless or PLC but not limited to low power wireless or PLC (Power Line
(Power Line Communication) technologies. 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 key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC "OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119]. 2119 [RFC2119].
This document requires readers to be familiar with the terminology This document requires readers to be familiar with the terminology
described in `Terminology in Low power And Lossy Networks' described in `Terminology in Low power And Lossy Networks'
[I-D.ietf-roll-terminology]. [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 All edges are contained in paths oriented toward and
toward and terminating at one or more root nodes (a DAG root, terminating at one or more root nodes.
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.
DAG Instance: A DAG Instance is a set of possibly multiple DAG Instance: A DAG Instance is a set of possibly multiple
Destination Oriented DAGs. A network may have more than one Destination Oriented DAGs. A network may have more than one
DAG Instance, and a RPL router can participate to multiple DAG DAG Instance, and a RPL router can participate to multiple DAG
instances. Each DAG Instance operates independently of other instances. Each DAG Instance operates independently of other
DAG Instances. This document describes operation within a DAG Instances. This document describes operation within a
single DAG instance. single DAG instance.
InstanceID: Unique identifier of a DAG Instance. InstanceID: Unique identifier of a DAG Instance.
Destination Oriented DAG: A DAG rooted at a single destination, Destination Oriented DAG (DODAG): A DAG rooted at a single
which is a node with no outgoing edges. The tuple (InstanceID, destination, which is a node with no outgoing edges. The tuple
DAGID) uniquely identifies a Destination Oriented DAG. In the (InstanceID, DAGID) uniquely identifies a Destination Oriented
RPL context, a router can can belong to at most one Destination DAG (DODAG). In the RPL context, a router can can belong to at
Oriented DAG per DAG Instance. most one DODAG per DAG Instance.
DAGID: The identifier of a DAG root. The DAGID must be unique DAGID: The identifier of a DODAG root. The DAGID must be unique
within the scope of a DAG Instance in the LLN. within the scope of a DAG Instance in the LLN.
DAG Iteration: The DAG that results from the iterative process that DODAG Iteration: A specific sequence number iteration of a DODAG.
reshapes the Destination Oriented DAG upon a stimulation by the
root.
DAGSequenceNumber: A sequential counter that is incremented by the DAGSequenceNumber: A sequential counter that is incremented by the
root to form a new Iteration of a DAG. A DAG Iteration is root to form a new Iteration of a DODAG. A DODAG Iteration is
identified uniquely by the (InstanceID, DAGID, identified uniquely by the (InstanceID, DAGID,
DAGSequenceNumber) tuple. 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. successors of the node on a path towards the DAG root.
DAG sibling: A sibling of a node within a DAG is defined in this 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 within a DAG. Note that siblings defined in this the same rank within a DAG. Note that siblings defined in this
manner do not necessarily share a common parent. manner do not necessarily share a common parent.
skipping to change at page 9, line 10 skipping to change at page 8, line 33
DAG root: A DAG root is a node within the DAG that has no outgoing DAG root: A DAG root is a node within the DAG that has no outgoing
edges. Because the graph is acyclic, by definition all DAGs edges. Because the graph is acyclic, by definition all DAGs
must have at least one DAG root and all paths terminate at a must have at least one DAG root and all paths terminate at a
DAG root. DAG root.
Sub-DAG The sub-DAG of a node is the set of other nodes in the DAG Sub-DAG The sub-DAG of a node is the set of other nodes in the DAG
that might use a path towards the DAG root that contains the that might use a path towards the DAG root that contains the
node. Nodes in the sub-DAG of a node have a greater rank node. Nodes in the sub-DAG of a node have a greater rank
(although not all nodes of greater rank are in the sub-DAG). (although not all nodes of greater rank are in the sub-DAG).
Grounded: A DAG is grounded if it contains a DAG root offering Up: Up refers to the direction from leaf nodes towards DODAG roots,
connectivity to an external routed infrastructure such as the following the orientation of the edges within the DODAG.
public Internet or a private core (non-LLN) IP network.
Floating: A DAG is floating if is not grounded. A floating DAG is
not expected to reach any additional external routed
infrastructure such as the public Internet or a private core
(non-LLN) IP network.
Inward: Inward refers to the direction from leaf nodes towards DAG
roots, following the orientation of the edges within the DAG.
Outward: Outward refers to the direction from DAG roots towards leaf Down: Down refers to the direction from DODAG roots towards leaf
nodes, going against the orientation of the edges within the nodes, going against the orientation of the edges within the
DAG. DODAG.
OCP: Objective Code Point. The Objective Code Point is used to OCP: Objective Code Point. The Objective Code Point is used to
indicate which Objective Function is in use in a DAG. The indicate which Objective Function is in use in a DODAG. The
Objective Code Point is further described in Objective Code Point is further described in
[I-D.ietf-roll-routing-metrics]. [I-D.ietf-roll-routing-metrics].
OF: Objective Function. The Objective Function (OF) defines which OF: Objective Function. The Objective Function (OF) defines which
routing metrics, optimization objectives, and related functions routing metrics, optimization objectives, and related functions
are in use in a DAG. The Objective Function is further are in use in a DODAG. The Objective Function is further
described in [I-D.ietf-roll-routing-metrics]. described in [I-D.ietf-roll-routing-metrics].
Note that in this document, the terms `node' and `LLN router' are Goal: The Goal is a host or set of hosts that satisfy a particular
used interchangeably. application objective / OF. Whether or not a DODAG can provide
connectivity to a goal is a property of the DODAG. For
example, a goal might be a host serving as a data collection
point, or a gateway providing connectivity to an external
infrastructure.
Grounded: A DAG is grounded when the root can reach the Goal of the
objective function.
Floating: A DAG is floating if is not Grounded. A floating DAG is
not expected to reach the Goal defined for the OF.
As they form networks, LLN devices often mix the roles of `host' and
`router' when compared to traditional IP networks. In this document,
`host' refers to an LLN device that can generate but does not forward
RPL traffic, `router' refers to an LLN device that can forward as
well as generate RPL traffic, and `node' refers to any RPL device,
either a host or a router.
3. Protocol Model 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. Overview 3.1. Instances, DODAGs, and DODAG Iterations
3.1.1. Topology Instance and Objectives Each DAG instance constructs a routing topology optimized for a
certain Objective Function (OF). A DAG instance may provide routes
to certain destination prefixes. A single DAG instance contains one
or more Destination Oriented DAG (DODAG) roots. These roots may
operate independently, or may coordinate over a non-LLN backchannel.
A topology instance of RPL exists over the scope of an LLN in support Each root has a unique identifier, the DAGID, such that nodes can
of a particular application, or service, and is optimized according identify the DODAG root.
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.
A single topology instance may comprise: A DAG instance may comprise:
o a single Destination Oriented DAG with a single DAG root o a single DODAG with a single root
* For example, a DAG optimized to minimize latency rooted at a * For example, a DODAG optimized to minimize latency rooted at a
single centralized lighting controller in a home automation single centralized lighting controller in a home automation
application. application.
o multiple uncoordinated Destination Oriented DAGs with independent o multiple uncoordinated DODAGs with independent roots (differing
DAG roots (differing DAGIDs) DAGIDs)
* For example, multiple data collection points in an urban data * For example, multiple data collection points in an urban data
collection application that do not have an always-on backbone collection application that do not have an always-on backbone
suitable to coordinate to form a single DAG, and further use suitable to coordinate to form a single DODAG, and further use
the formation of multiple DAGs as a means to dynamically and the formation of multiple DODAGs as a means to dynamically and
autonomously partition the network. autonomously partition the network.
o a single Destination Oriented DAG with multiple DAG roots o a single DODAG with a single virtual root coordinating LLN sinks
coordinating over some backbone (with the same DAGID) over some non-LLN backbone
* For example, multiple border routers operating with a reliable * For example, multiple border routers operating with a reliable
backbone, e.g. in support of a 6LowPAN application, that are backbone, e.g. in support of a 6LowPAN application, that are
capable to act as logically equivalent sinks to the same DAG. capable to act as logically equivalent sinks to the same DODAG.
o a combination of one of the above as suited to some application o a combination of one of the above as suited to some application
scenario scenario.
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.
Traffic is bound to a specific DAG Instance by a marking in the flow Traffic is bound to a specific DODAG Instance by a marking in the
label of the IPv6 header. Traffic originating in support of a flow label of the IPv6 header. Traffic originating in support of a
particular application may be tagged to follow an appropriate particular application may be tagged to follow an appropriate DAG
instance, for example to follow paths optimized for low latency or instance, for example to follow paths optimized for low latency or
low energy. The provisioning or automated discovery of a mapping low energy. The provisioning or automated discovery of a mapping
between an InstanceID and a type or service of application traffic is between an InstanceID and a type or service of application traffic is
beyond the scope of this specification. beyond the scope of this specification.
Conceptually a node running RPL may capable to maintain a membership An example of a DAG Instance comprising a number of DODAGs is
in multiple DAG Instances in support of different application depicted in Figure 1. A DODAG Iteration is depicted in Figure 2.
services and/or optimization objectives. For example, one instance
may optimize for minimizing latency and a separate orthogonal
instance may optimize for minimizing energy. This scenario does
introduce some additional considerations, for example loop avoidance
and default routing behavior. These considerations are beyond the
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 +----------------------------------------------------------------+
| |
| +--------------+ |
| | | |
| | (R1) | (R2) (Rn) |
| | / \ | /| \ / | \ |
| | / \ | / | \ / | \ |
| | (A) (B) | (C) | (D) ... (F) (G) (H) |
| | /|\ |\ | / | |\ | | | |
| | : : : : : | : (E) : : : : : |
| | | / \ |
| +--------------+ : : |
| DODAG |
| |
+----------------------------------------------------------------+
DAG Instance
Many of the dominant traffic flows in support of the LLN application Figure 1: DAG Instance
scenarios are MP2P flows ([I-D.ietf-roll-building-routing-reqs],
[I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548]). These
flows are rooted at designated nodes that have some application
significance, such as providing connectivity to an external routed
infrastructure. The term "external" is used to refer to the public
Internet or a core private (non-LLN) IP network.
LLN nodes running RPL will construct Directed Acyclic Graphs (DAGs) +----------------+ +----------------+
rooted at DAG roots, which may be naturally designated according to | | | |
their application significance. This structure provides the routing | (R1) | | (R1) |
solution for the dominant MP2P traffic flows. The DAG structure | / \ | | / |
further provides each node potentially multiple successors for MP2P | / \ | | / |
flows, which may be used for, e.g., local route repair or load | (A) (B) | \ | (A) |
balancing. | /|\ |\ | ------\ | /|\ |
| : : (C) : : | \ | : : (C) |
| | / | \ |
| | ------/ | \ |
| | / | (B) |
| | | |\ |
| | | : : |
| | | |
+----------------+ +----------------+
Sequence N Sequence N+1
Nodes running RPL are able to further restrict the scope of the Figure 2: DODAG Iteration
routing problem by using the DAG as a reference topology. By
referencing a rank property that is related to the positions in the
DAG, nodes are able to determine their positions in a DAG relative to
each other. This information is used by RPL in part to construct
rules for movement relative to the DAG that endeavor to avoid loops.
It is important to note that the rank property is derived from
metrics, and not directly from the position in the DAG (Section 5.3).
3.1.3. Point-to-Multipoint Traffic Flows 3.2. Traffic Flows
As DAGs are organized, RPL will use a destination advertisement 3.2.1. Multipoint-to-Point Traffic
mechanism to build up routing tables in support of outward P2MP
traffic flows. This mechanism, using the DAG as a reference,
distributes routing information across intermediate nodes (between
the DAG leaves and the root), guided along the DAG, such that the
routes toward destination prefixes in the outward direction may be
set up. As the DAG undergoes modification during DAG maintenance,
the destination advertisement mechanism can be triggered to update
the outward routing state.
3.1.4. Point-to-Point Traffic Flows Multipoint-to-Point (MP2P) is a dominant traffic flow in many LLN
applications ([I-D.ietf-roll-building-routing-reqs],
[I-D.ietf-roll-home-routing-reqs], [RFC5673], [RFC5548]). The
destinations of MP2P flows are designated nodes that have some
application significance, such as providing connectivity to the
larger Internet or core private IP network. RPL supports MP2P
traffic by allowing MP2P destinations to be reached via DODAG roots.
A baseline support for P2P traffic in RPL is provided by the DAG, as 3.2.2. Point-to-Multipoint Traffic
P2P traffic may flow inward along the DAG until a common parent is
reached that has stored an entry for the destination in its routing
table and is capable of directing the traffic outward along the
correct outward path. RPL also provides support for the trivial case
where a P2P destination may be a `one-hop' neighbor. In the present
document RPL does not specify nor preclude any additional mechanisms
that may be capable to compute and install more optimal routes into
LLN nodes in support of arbitrary P2P traffic according to some
routing metric.
3.2. Protocol Operation Point-to-multipoint (P2MP) is a traffic pattern required by several
LLN applications ([I-D.ietf-roll-building-routing-reqs],
[I-D.ietf-roll-home-routing-reqs], [RFC5673], [RFC5548]). RPL
supports P2MP traffic by using a destination advertisement mechanism
that provisions routes toward destination prefixes and away from
roots. Destination advertisements can update routing tables as the
underlying DODAG topology changes.
3.2.1. DAG Construction 3.2.3. Point-to-Point Traffic
3.2.1.1. DAG Information Object (DIO) RPL DODAGs provide a basic structure for point-to-point (P2P)
traffic. For a RPL network to support P2P traffic, a root must be
able to route packets to a destination. Nodes within the network may
also have routing tables to destinations. A packet flows towards a
root until it reaches an ancestor that has a known route to the
destination.
A DAG Information Object is defined and used by RPL in order to build RPL also supports the case where a P2P destination is a `one-hop'
and maintain a DAG. This document defines an ICMPv6 Message Type RPL neighbor.
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. RPL neither specifies nor precludes additional mechanisms for
The DAGID must be unique to a single DAG in the scope of the LLN. computing and installing more optimal routes to support arbitrary P2P
traffic.
o Objective Function identified by an Objective Code Point (OCP) as 3.3. DODAG Construction
described below.
o Rank information used by nodes to determine their positions in the RPL provisions routes up towards DODAG roots, forming a DODAG
DAG relative to each other. optimized according to the Objective Function (OF) in use. RPL nodes
construct and maintain these DODAGs through exchange of DAG
Information Object (DIO) messages. Undirected links between siblings
are also identified during this process, which are used to provide
additional diversity.
o Sequence number originated from the DAG root, used to aid in 3.3.1. DAG Information Object (DIO)
identification of dependent sub-DAGs and coordinate topology
changes in a manner so as to avoid loops.
o Indications and configuration for the DAG, e.g. grounded or A DIO identifies the DAG Instance, the DAGID, the values used to
floating, administrative preference, ... compute the DAG Instance's objective function, and the present DODAG
Sequence Number. It can also include additional routing and
configuration information. The DIO includes a measure derived from
the position of the node within the DODAG, the rank, which is used
for nodes to determine their positions relative to each other and to
inform loop avoidance/detection procedures. RPL exchanges DIO
messages to establish and maintain routes.
o A set of path metrics and constraints, as further described in RPL adapts the rate at which nodes send DIO messages. When a DODAG
[I-D.ietf-roll-routing-metrics]. is detected to be inconsistent or needs repair, RPL sends DIO
messages more frequently. As the DODAG stabilizes, the DIO message
rate tapers off, reducing the maintenance cost of a steady and well-
working DODAG.
o List of additional destination prefixes reachable inwards along This document defines an ICMPv6 Message Type RPL Control Message,
the DAG. which is capable of carrying a DIO.
The DIO messages are issued whenever a change is detected to the DAG 3.3.2. DAG Repair
such that a node is able to determine that a region of the DAG has
become inconsistent. As the DAG stabilizes the period at which RA
messages occur is configured to taper off, reducing the steady-state
overhead of DAG maintenance. The periodic issue of DIO messages,
along with the triggered DIO messages in response to inconsistency,
is one feature that enables RPL to operate in the presence of
unreliable links.
3.2.1.2. Grounded and Floating DAGs RPL supports global repair over the DODAG. A DODAG Root may
increment the DODAG Sequence Number to institute a global repair,
revising the DODAG and allowing nodes to choose an arbitrary new
position within the new DODAG iteration.
Certain LLN nodes may offer connectivity to an external routed RPL may support mechanisms for local repair within the DODAG
infrastructure in support of an application scenario. These nodes iteration. The DIO message will specify the necessary parameters as
are designated `grounded', and may serve as the DAG roots of a configured from the DODAG root. Local repair options include the
grounded DAG. DAGs that do not have a grounded DAG root are floating allowing a node, upon detecting a loss of connectivity to a DODAG it
DAGs. In either case routes may be provisioned toward the DAG root, is a member of, to:
although in the floating case there is no expectation to reach an
external infrastructure. Some applications will include permanent
floating DAGs.
3.2.1.3. Administrative Preference o Poison its sub-DAG by advertising an effective rank of INFINITY,
OR detach and form a floating DODAG in order to preserve inner
connectivity within its sub-DAG.
An administrative preference may be associated with each DAG root, o Move down the DODAG iteration in a limited manner, no further than
and thereby each DAG, in order that some DAGs in the LLN may be more a bound configured via the DIO so as not to count all the way to
preferred over other DAGs. For example, a DAG root that is sinking infinity. Such a move may be undertaken after waiting an
traffic in support of a data collection application may be configured appropriate poisoning interval, and should allow the node to
by the application to be very preferred. A transient DAG, e.g. a DAG restore connectivity to the DODAG Iteration if possible.
that is only existing until a permanent DAG is found, may be
configured to be less preferred. The administrative preference
offers a way to engineer the formation of the DAG in support of the
application.
3.2.1.4. Objective Function (OF) 3.3.3. Grounded and Floating DODAGs
The Objective Function (OF) conveys and controls the optimization DODAGs can be grounded or floating. A grounded DODAG offers
objectives in use within the DAG. The Objective Function is connectivity to to a goal. A floating DODAG offers no such
indicated by an Objective Code Point (OCP), and is further specified connectivity, and provides routes only to nodes within the DODAG.
in [I-D.ietf-roll-routing-metrics]. Each instance of an allocated OF Floating DODAGs may be used, for example, to preserve inner
indicates: connectivity during repair.
o The set of metrics used within the DAG 3.3.4. Administrative Preference
o The method used for least cost path determination. An implementation/deployment may specify that some DODAG roots should
be used over others through an administrative preference.
Administrative preference offers a way to control traffic and
engineer DODAG formation in order to better support application
requirements or needs.
o The method used to compute DAG Rank 3.3.5. Objective Function (OF)
o The methods used to prepare metrics for propagation within a DIO The Objective Function (OF) implements the optimization objectives of
message route selection within the DAG Instance. The OF is identified by an
Objective Code Point (OCP) within the DIO, and its specification also
indicates the metrics and constraints in use. The OF also specifies
the procedure used to compute rank within a DODAG iteration. Further
details may be found in [I-D.ietf-roll-routing-metrics] and related
companion specifications.
By using defined OCPs that are understood by all nodes in a By using defined OFs that are understood by all nodes in a particular
particular implementation, and by conveying them in the DIO message, implementation, and by referencing them in the DIO message, RPL nodes
RPL nodes may work to build optimized LLN using a variety of may work to build optimized LLN routes using a variety of application
application and implementation specific metrics and goals. and implementation specific metrics and goals.
A default OF, OF0 (designated by OCP value of 0x0000), is specified In the case where a node is unable to encounter a suitable DAG
with a well-defined default behavior. OF0 may be used to define RPL Instance using a known Objective Function, it may be configured to
behaviors in the case where a node encounters a DIO message join DAG Instance using and unknown Objective Function but only
containing a code point that it does not support, if allowed by acting as a leaf node.
policy.
3.2.1.5. Distributed Algorithm Operation 3.3.6. Distributed Algorithm Operation
A high level overview of the distributed algorithm which constructs A high level overview of the distributed algorithm which constructs
the DAG is as follows: the DODAG is as follows:
o Some nodes may be initially provisioned to act as DAG roots,
either permanent or transient, with associated preferences.
o Nodes will maintain a data structure containing their candidate o Some nodes are configured to be DODAG roots, with associated DODAG
(viable) neighbors, as determined by the implementation. This configuration.
data structure will also track DAG information as learned from and
associated with each neighbor.
o Nodes that are members of a DAG, including DAG roots, will o Nodes advertise their presence, affiliation with a DODAG, routing
multicast DIO messages as needed (when inconsistency is detected), cost, and related metrics by sending link-local multicast DIO
to their link-local neighbors. Nodes will also respond to DIS
messages. messages.
o Nodes that receive DIO messages may either discard the DIO based o Nodes may adjust the rate at which DIO messages are sent in
on several criteria, including rank-based loop avoidance rules, or response to stability or detection of routing inconsistencies.
process the DIO to maintain a position in an existing DAG or
improve a position as according to an Objective Function (OF) and
current path cost.
o Nodes manage a set of DAG Parents according to the rules specified
by RPL. This set is also augmented to include DAG siblings.
o DIO messages may be emitted more or less frequently as a function
of DAG consistency.
o As less preferred DAGs encounter more preferred DAGs that offer o Nodes listen for DIOs and use their information to join a new
equivalent or better optimization objectives for the same DODAG, or to maintain an existing DODAG, as according to the
InstanceID, the nodes in the less preferred DAGs may leave to join specified Objective Function and rank-based loop avoidance rules.
the more preferred DAGs, finally leaving only the more preferred
DAGs. This is an illustration of the mechanism by which an
application may engineer DAG construction.
o The nodes provision routing table entries for the destinations o The nodes provision routing table entries for the destinations
specified by the DIO towards their DAG Parents. Nodes may specified by the DIO towards their parents in the DODAG iteration.
provision a DAG Parent as a default gateway. Nodes may provision a parent as a default gateway.
3.2.2. Destination Advertisement
As RPL constructs DAGs, nodes may provision routes toward
destinations advertised through DIO messages through their selected
parents, and are thus able to send traffic inward along the DAG by
forwarding to their selected parents. However, this mechanism alone
is not sufficient to support P2MP traffic flowing outward along the
DAG from the DAG root toward nodes. A destination advertisement
mechanism is employed by RPL to build up routing state in support of
these outward flows. The destination advertisement mechanism may not
be supported in all implementations, as appropriate to the
application requirements. A DAG root that supports using the
destination advertisement mechanism to build up routing state will
indicate such in the DIO message. A DAG root that supports using the
destination advertisement mechanism must be capable of allocating
enough state to store the routing state received from the LLN.
3.2.2.1. Destination Advertisement Object (DAO)
A Destination Advertisement Object is defined and used by RPL in o Nodes may identify siblings within the DODAG iteration to increase
order to convey the destination information inward along the DAG path diversity.
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 Using both DIOs and possibly information in data packets, RPL
destination advertisement. nodes detect possible routing loops. When a RPL node detects a
possible routing loop, it may adapt its DIO transmission rate to
apply a local repair to the topology. This process and its
limitations is discussed in greater detail in 3.4.
o Depth information used by nodes to determine how far away the 3.4. Destination Advertisement
destination (the source of the destination advertisement) is
o Prefix information to identify the destination, which may be a As RPL constructs and maintains DODAGs with DIO messages to establish
prefix, an individual host, or multicast listeners upward routes, it may use Destination Advertisement Object (DAO)
messages to establish downward routes along the DODAG. DAO messages
and support for downward routes are an optional feature for
applications that require P2MP or P2P traffic. DIO messages
advertise whether the destination advertisement mechanism is enabled.
o Reverse Route information to record the nodes visited (along the 3.4.1. Destination Advertisement Object (DAO)
outward path) when the intermediate nodes along the path cannot
support storing state for Hop-By-Hop routing.
3.2.2.2. Destination Advertisement Operation A Destination Advertisement Object (DAO) conveys destination
information upwards along the DODAG so that a DODAG root (an other
intermediate nodes) can provision downward routes. A DAO message
includes prefix information to identify destinations, a capability to
record routes in support of source routing, and information to
determine the freshness of a particular advertisement.
As the DAG is constructed and maintained, nodes are capable to emit Nodes that are capable of maintaining routing state may aggregate
DAO messages to a subset of their DAG parents. routes from DAO messages that they receive before transmitting a DAO
message. Nodes that are not capable to maintain routing state may
attach a next-hop address to the Reverse Route Stack contained within
the DAO message. The Reverse Route Stack is subsequently used to
generate piecewise source routes over regions of the LLN that are
incapable of storing downward routing state.
3.2.2.2.1. `One-Hop' Neighbors A special case of the DAO message, termed a no-DAO, is used to clear
downward routing state that has been provisioned through DAO
operation.
As a special case, a node may periodically emit a link-local This document defines an ICMPv6 Message Type RPL Control Message,
multicast IPv6 DAO message advertising its locally available which is capable to carry the DAO.
destination prefixes. This mechanism allows for the one-hop
neighbors of a node to learn explicitly of the prefixes on the node,
and in some application specific scenarios this is desirable in
support of provisioning a trivial `one-hop' route. In this case,
nodes that receive the multicast destination advertisement may use it
to provision the one-hop route only, and not engage in any additional
processing (so as not to engage the mechanisms used by a DAG parent).
3.2.2.2.2. Operation in Support of Stateful Nodes 3.4.1.1. `One-Hop' Neighbors
When a (unicast) DAO message reaches a node capable of storing In addition to sending DAOs toward DODAG roots, RPL nodes may
routing state, the node extracts information from the DAO message and occasionally emit a link-local multicast DAO message advertising
updates its local database with a record of the DAO message and the available destination prefixes. This mechanism allow provisioning a
neighbor that it was received from. When the node later propagates trivial `one-hop' route to local neighbors.
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
may perform route aggregation if it is able, thus reducing the
overall number of DAO messages.
3.2.2.2.3. Operation in Support of Stateless Nodes 4. Routing Metrics and Constraints Used By RPL
When a (unicast) DAO message reaches a node incapable of storing Routing metrics are used by routing protocols to compute the shortest
additional state, the node must append the next-hop address (from paths. Interior Gateway Protocols (IGPs) such as IS-IS ([RFC5120])
which neighbor the DAO message was received) to a Reverse Route Stack and OSPF ([RFC4915]) use static link metrics. Such link metrics can
carried within the DAO message. The node then passes the DAO message simply reflect the bandwidth or can also be computed according to a
on to one or more of its DAG parents without storing any additional polynomial function of several metrics defining different link
state. 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.
When a node that is capable of storing routing state encounters a In contrast, LLNs do require the support of both static and dynamic
(unicast) DAO message with a Reverse Route Stack that has been metrics. Furthermore, both link and node metrics are required. In
populated, the node knows that the DAO message has traversed a region the case of RPL, it is virtually impossible to define one metric, or
of nodes that did not record any routing state. The node is able to even a composite, that will satisfy all use cases.
detach and store the Reverse Route State and associate it with the
destination described by the DAO message. Subsequently the node may
use this information to construct a source route in order to bridge
the region of nodes that are unable to support Hop-By-Hop routing to
reach the destination.
3.2.2.2.4. Additional Considerations In addition, RPL supports constrained-based routing where constraints
may be applied to link and nodes. If a link or a node does not
satisfy a required constraint, it is `pruned' from the candidate list
thus leading to a constrained shortest path.
Further aggregations of DAO messages prefix reachability information The set of supported link/node constraints and metrics is specified
by destinations are possible in order to support additional in [I-D.ietf-roll-routing-metrics].
scalability.
A special case of an DAO message, termed a `no-DAO', may be used to The role of the Objective Function is to advertise routing metrics
tear down the routing state that has been established by the and constraints in addition to the objectives used to compute the
destination advertisement mechanism in case of, e.g., unreachability (constrained) shortest path.
or other events that affect the outward routing state.
A further example of the operation of the destination advertisement Example 1: Shortest path: path offering the shortest end-to-end delay
mechanism is available in Appendix B.1
3.3. Loop Avoidance and Stability Example 2: Constrained shortest path: the path that does not traverse
any battery-operated node and that optimizes the path
reliability
The goal of a guaranteed consistent and loop free global routing 5. Rank
solution for an LLN may not be practically achieved given the real
behavior and volatility of the underlying metrics. The trade offs to
achieve a stable approximation of global convergence may be too
restrictive with respect to the need of the LLN to react quickly in
response to the lossy environment. Globally the LLN may be able to
achieve a weak convergence, in particular as link changes are able to
be handled locally and result in minimal changes to global topology.
RPL does not aim to guarantee loop free path selection, or strong 5.1. Loop Avoidance
global convergence. In order to reduce control overhead, in
particular the expense of mechanisms such as count-to-infinity, RPL
does try to avoid the creation of loops when undergoing topology
changes.
RPL includes rank-based mechanisms for detecting loops to ensure that RPL guarantees neither loop free path selection nor strong global
packets make forward progress within the DAG and trigger DAG repair convergence. In order to reduce control overhead, however, such as
if necessary. the cost of the count-to-infinity problem, RPL avoids creating loops
when undergoing topology changes. Furthermore, RPL includes rank-
based mechanisms for detecting loops when they do occur. RPL uses
this loop detection to ensure that packets make forward progress
within the DODAG iteration and trigger repairs when necessary.
3.3.1. Greediness and Rank-based Instabilities 5.1.1. Greediness and Rank-based Instabilities
Once a node has joined a DAG, RPL disallows certain behaviors, Once a node has joined a DODAG, RPL disallows certain behaviors,
including greediness, in order to prevent resulting instabilities in including greediness, in order to prevent resulting instabilities in
the DAG. the DODAG.
If a node is allowed to be greedy and attempts to move deeper in the If a node is allowed to be greedy and attempts to move deeper in the
DAG, beyond its most preferred parent, in order to increase the size DODAG, beyond its most preferred parent, in order to increase the
of the DAG parent set, then an instability can result. This is size of the parent set, then an instability can result. This is
illustrated in Figure 14. illustrated in Figure 16.
Suppose a node is willing to receive and process a DIO messages from 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 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 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 more nodes continue to try and move in the DODAG in order to optimize
against each other. In some cases this will result in an 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 limits the cases where a
never receive and process DIO messages from deeper nodes. This rule node may process DIO messages from deeper nodes to some forms of
creates an `event horizon', whereby a node cannot be influenced into local repair. This approach creates an `event horizon', whereby a
an instability by the action of nodes that may be in its own sub-DAG. node cannot be influenced beyond some limit into an instability by
the action of nodes that may be in its own sub-DAG.
A further example of the consequences of greedy operation, and A further example of the consequences of greedy operation, and
instability related to processing DIO messages from nodes of greater instability related to processing DIO messages from nodes of greater
rank, may be found in Appendix B.4 rank, may be found in Appendix B.4
3.3.2. DAG Loops 5.1.2. DODAG Loops
A DAG loop may occur when a node detaches from the DAG and reattaches A DODAG loop may occur when a node detaches from the DODAG and
to a device in its prior sub-DAG. This may happen in particular when reattaches to a device in its prior sub-DAG. This may happen in
DIO messages are missed. Strict use of the DAG sequence number can particular when DIO messages are missed. Strict use of the DAG
eliminate this type of loop. sequence number can eliminate this type of loop, but this type of
loop may possibly be encountered when using some local repair
mechanisms.
3.3.3. DAO Loops 5.1.3. 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 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. RPL includes DAO was missed until a heartbeat cleans up all states. RPL includes
loop detection mechanisms that may mitigate the impact of DAO loops loop detection mechanisms that may mitigate the impact of DAO loops
and trigger their repair. and trigger their repair.
In the case where stateless DAO operation is used, i.e. source In the case where stateless DAO operation is used, i.e. source
routing specifies the outwards routes, then DAO Loops should not routing specifies the down routes, then DAO Loops should not occur on
occur on the stateless portions of the path. the stateless portions of the path.
3.3.4. Sibling Loops 5.1.4. Sibling Loops
Sibling loops could occur if a group of siblings kept choosing Sibling loops could occur if a group of siblings kept choosing
amongst themselves as successors such that a packet does not make amongst themselves as successors such that a packet does not make
forward progress. This specification limits the number of times that forward progress. This specification limits the number of times that
sibling forwarding may be used at a given rank to prevent sibling sibling forwarding may be used at a given rank to prevent sibling
loops. loops.
4. Routing Metrics and Constraints Used By RPL 5.2. Rank Properties
Routing metrics are used by routing protocols to compute the shortest The rank of a node is a scalar representation of the location of that
paths. Interior Gateway Protocols (IGPs) such as IS-IS ([RFC5120]) node within a DODAG iteration. The rank is used to avoid and detect
and OSPF ([RFC4915]) use static link metrics. Such link metrics can loops, and as such must demonstrate certain properties. The exact
simply reflect the bandwidth or can also be computed according to a calculation of the rank is left to the Objective Function, and may
polynomial function of several metrics defining different link depend on parents, link metrics, and the node configuration and
characteristics; in all cases they are static metrics. Some routing policies.
protocols support more than one metric: in the vast majority of the
cases, one metric is used per (sub)topology. Less often, a second
metric may be used as a tie-breaker in the presence of Equal Cost
Multiple Paths (ECMP). The optimization of multiple metrics is known
as an NP complete problem and is sometimes supported by some
centralized path computation engine.
In contrast, LLNs do require the support of both static and dynamic The rank is not a cost metric, although its value can be derived from
metrics. Furthermore, both link and node metrics are required. In and influenced by metrics. The rank has properties of its own that
the case of RPL, it is virtually impossible to define one metric, or are not necessarily that of all metrics:
even a composite, that will satisfy all use cases.
In addition, RPL supports constrained-based routing where constraints Type: Rank is an abstract scalar. Some metrics are boolean (e.g.
may be applied to link and nodes. If a link or a node does not grounded), others are statistical and better expressed as a
satisfy a required constraint, it is `pruned' from the candidate list tuple like an expected value and a variance. Some OCPs use
thus leading to a constrained shortest path. not one but a set of metrics bound by a piece of logic.
The set of supported link/node constraints and metrics is specified Function: Rank is the expression of a relative position within a
in [I-D.ietf-roll-routing-metrics]. DODAG iteration with regard to neighbors and, not necessarily
a good indication or a proper expression of a distance or a
cost to the root.
The role of the Objective Function is to advertise routing metrics Stability: The stability of the rank determines that of the routing
and constraints in addition to the objectives used to compute the topology. Some dampening or filtering might be applied to
(constrained) shortest path. keep the topology stable and the rank does not necessarily
change as fast as some physical metrics would. A new
iteration is a good opportunity to reconcile the
discrepancies that might form over time between the metrics
and the ranks.
Example 1: Shortest path: path offering the shortest end-to-end delay Granularity: Rank is coarse grained. A fine granularity would
prevent the selection of siblings.
Example 2: Constrained shortest path: the path that does traverse any Properties: Rank is strictly monotonic and can be used to validate a
battery-operated node and that optimizes the path progression from or towards the root. A metric like
reliability bandwidth or jitter does not necessarily exhibit such
property.
5. RPL Protocol Specification Abstract: Rank does not have a physical unit, but rather a range of
increment per hop that varies from 1 (best) to 16 (worst),
where the assignment of each value is to be determined by the
implementation.
5.1. RPL Messages The rank value feeds back into the DAG parent selection according to
the RPL loop-avoidance strategy. Once a parent has been added, and a
rank value for the node within the DODAG has been advertised, the
nodes further options with regard to DAG parent selection and
movement within the DODAG are restricted in favor of loop avoidance.
5.1.1. ICMPv6 RPL Control Message The computation of the DAG rank MUST be done in such a way so as to
maintain the following properties for any nodes M and N that are
neighbors in the LLN:
DAGRank(M) is less than DAGRank(N): In this case, M is probably
located in a more preferred position than N in the DODAG with
respect to the metrics and optimizations defined by the
objective code point. In any fashion, Node M may safely be a
DAG parent for Node N without risk of creating a loop.
Further, for a node N, all parents in the DAG parent set must
be of rank less than self's DAGRank(N). In other words, the
rank presented by a node N MUST be greater (deeper) than that
presented by any of its parents.
DAGRank(M) equals DAGRank(N): In this case M and N are located
positions of relatively the same optimality within the DODAG.
In some cases, Node M may be used as a successor by Node N,
but with related chance of creating a loop that must be
detected and broken by some other means.
DAGRank(M) is greater than DAGRank(N): In this case, then node M is
located in a less preferred position than N in the DODAG with
respect to the metrics and optimizations defined by the
objective code point. Further, Node (M) may in fact be in
Node (N)'s sub-DAG. There is a higher risk to Node (N)
selecting Node (M) as a DAG parent, as such a selection may
create a loop.
As an example, the DAG rank could be computed in such a way so as to
closely track ETX when the objective function is to minimize ETX, or
latency when the objective function is to minimize latency, or in a
more complicated way as appropriate to the objective code point being
used within the DODAG.
6. RPL Protocol Specification
6.1. RPL Messages
6.1.1. ICMPv6 RPL Control Message
This document defines the RPL Control Message, a new ICMPv6 message. This document defines the RPL Control Message, a new ICMPv6 message.
The RPL Control Message has the following general format, in The RPL Control Message has the following general format, in
accordance with [RFC4443]: accordance with [RFC4443]:
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 | Code | Checksum | | Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
+ Message Body + + Message Body +
| | | |
Figure 1: RPL Control Message Figure 3: RPL Control Message
The RPL Control message is an ICMPv6 information message with a The RPL Control message is an ICMPv6 information message with a
requested Type of 155. requested Type of 155.
The Code will be used to identify RPL Control Messages as follows: The Code will be used to identify RPL Control Messages as follows:
o 0x01: DAG Information Solicitation (Section 5.1.2) o 0x01: DAG Information Solicitation (Section 6.1.2)
o 0x02: DAG Information Object (Section 5.1.3) o 0x02: DAG Information Object (Section 6.1.3)
o 0x04: Destination Advertisement Object (Section 5.1.4) o 0x04: Destination Advertisement Object (Section 6.1.4)
5.1.2. DAG Information Solicitation (DIS) 6.1.2. DAG Information Solicitation (DIS)
The DAG Information Solicitation (DIS) message may be used to solicit The DAG Information Solicitation (DIS) message may be used to solicit
a DAG Information Object from a RPL node. Its use is analogous to 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 that of a Router Solicitation; a node may use DIS to probe its
neighborhood for nearby DAGs. The DAG Information Solicitation neighborhood for nearby DAGs. The DAG Information Solicitation
carries no additional message body. carries no additional message body.
5.1.3. DAG Information Object (DIO) 6.1.3. DAG Information Object (DIO)
The DAG Information Object 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 Instance, select its
parents, and identify its siblings while employing loop avoidance DAG parents, and identify its siblings while employing loop avoidance
strategies. strategies.
5.1.3.1. DIO Base Option 6.1.3.1. DIO Base
The DIO Base Option is a container option, which is always present, The DIO Base is a container option, which is always present, and
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|G|D|A|0|0| Prf | Sequence | InstanceID | DAGRank | |G|D|A|0|0| Prf | Sequence | InstanceID | DAGRank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
+ + + +
| DAGID | | DAGID |
+ + + +
| | | |
+ + + +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-option(s)... | sub-option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: DIO Base Option Figure 4: DIO Base
Control Field: The DAG Control Field is currently allocated as Control Field: The DAG Control Field is currently allocated as
follows: follows:
Grounded (G): The Grounded (G) flag is set when the DAG root Grounded (G): The Grounded (G) flag is set when the DODAG root
is offering connectivity to an external routed is a Goal for the OF.
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 DODAG 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 down direction along the
DAG, as further detailed in Section 5.10. Note that the DODAG, as further detailed in Section 6.8. 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 DODAG root is capable
support the collection of destination advertisement to 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 DODAG.
DAGPreference (Prf): 3-bit unsigned integer set by the DAG DAGPreference (Prf): 3-bit unsigned integer set by the DODAG
root to its preference and unchanged at propagation. root to its preference and unchanged at propagation.
DAGPreference ranges from 0x00 (least preferred) to 0x07 DAGPreference ranges from 0x00 (least preferred) to 0x07
(most preferred). The default is 0 (least preferred). (most preferred). The default is 0 (least preferred).
The DAG preference provides an administrative mechanism The DAG preference provides an administrative mechanism
to engineer the self-organization of the LLN, for example to engineer the self-organization of the LLN, for example
indicating the most preferred LBR. If a node has the indicating the most preferred LBR. If a node has the
option to join a more preferred DAG while still meeting option to join a more preferred DODAG while still meeting
other optimization objectives, then the node will other optimization objectives, then the node will
generally seek to join the more preferred DAG as generally seek to join the more preferred DODAG as
determined by the OF. determined by the OF.
Unassigned bits of the Control Field are considered as Unassigned bits of the Control Field are considered as
reserved. They MUST be set to zero on transmission and MUST be reserved. They MUST be set to zero on transmission and MUST be
ignored on receipt. ignored on receipt.
Sequence Number: 8-bit unsigned integer set by the DAG root, Sequence Number: 8-bit unsigned integer set by the DODAG root,
incremented according to a policy provisioned at the DAG root, incremented according to a policy provisioned at the DODAG
and propagated with no change outwards along the DAG. Each root, and propagated with no change down the DODAG. 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.
InstanceID: 8-bit field indicating the topology instance associated InstanceID: 8-bit field indicating the topology instance associated
with the DAG, as provisioned at the DAG root. with the DODAG, as provisioned at the DODAG root.
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 DIO message. The DAGRank of the DAG root is sending the DIO message. The DAGRank of the DODAG root is
ROOT_RANK. DAGRank is further described in Section 5.4. ROOT_RANK. DAGRank is further described in Section 6.3.
DAGID: 128-bit unsigned integer which uniquely identify a DAG. This DAGID: 128-bit unsigned integer which uniquely identify a DODAG.
value is set by the DAG root. The global IPv6 address of the This value is set by the DODAG root. The global IPv6 address
DAG root can be used, however. the DAGID MUST be unique per DAG of the DODAG root can be used. the DAGID MUST be unique per DAG
within the scope of the LLN. In the case where a DAG root is Instance within the scope of the LLN.
rooting multiple DAGs the DAGID MUST be unique for each DAG
rooted at a specific DAG root.
The following values MUST NOT change during the propagation of DIO The following values MUST NOT change during the propagation of DIO
messages outwards along the DAG: messages down the DAG:
Grounded (G) Grounded (G)
Destination Advertisement Supported (A) Destination Advertisement Supported (A)
DAGPreference (Prf) DAGPreference (Prf)
Sequence Sequence
InstanceID InstanceID
DAGID DAGID
All other fields of the DIO message may be updated at each hop of the All other fields of the DIO message may be updated at each hop of the
propagation. propagation.
5.1.3.1.1. DIO Suboptions 6.1.3.1.1. 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 DIO message: several suboptions are defined for the DIO message:
5.1.3.1.1.1. Format 6.1.3.1.1.1. Format
0 1 2 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 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Subopt. Type | Subopt Length | Subopt Data | Subopt. Type | Subopt Length | Subopt Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
Figure 3: DIO Suboption Generic Format Figure 5: 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 DIO message containing a suboption for which the processing a DIO message containing a suboption for which 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: 16-bit unsigned integer, representing the length Suboption Length: 16-bit unsigned integer, representing the length
in octets of the suboption, not including the suboption Type in octets of the suboption, not including the suboption Type
skipping to change at page 23, line 39 skipping to change at page 23, line 39
Implementations MUST silently ignore any DIO message suboptions Implementations MUST silently ignore any DIO message suboptions
options that they do not understand. options that they do not understand.
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).
5.1.3.1.1.2. Pad1 6.1.3.1.1.2. 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 4: Pad 1 Figure 6: 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 or two octets of padding in the The Pad1 option is used to insert one or two octets of padding in the
DIO message to enable suboptions alignment. If more than two octets DIO message to enable suboptions alignment. If more than two octets
of padding is required, the PadN option, described next, should be of padding is required, the PadN option, described next, should be
used rather than multiple Pad1 options. used rather than multiple Pad1 options.
5.1.3.1.1.3. PadN 6.1.3.1.1.3. 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 2 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 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 5: Pad N Figure 7: Pad N
The PadN option is used to insert three or more octets of padding in The PadN option is used to insert three or more octets of padding in
the DIO message to enable suboptions alignment. For N (N > 2) octets the DIO message to enable suboptions alignment. For N (N > 2) octets
of padding, the Option Length field contains the value N-3, and the of padding, the Option Length field contains the value N-3, and the
Option Data consists of N-3 zero-valued octets. PadN Option data Option Data consists of N-3 zero-valued octets. PadN Option data
MUST be ignored by the receiver. MUST be ignored by the receiver.
5.1.3.1.1.4. DAG Metric Container 6.1.3.1.1.4. 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 2 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 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Type = 2 | Container Length | DAG Metric Data | Type = 2 | Container Length | DAG Metric Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
Figure 6: DAG Metric Container Figure 8: 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 DODAG. 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 DAG Metric Container MUST include the value for the DAG Objective
Code Point.
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.
5.1.3.1.1.5. Destination Prefix 6.1.3.1.1.5. Destination Prefix
The Destination Prefix suboption does not have any alignment The Destination Prefix suboption does not have any alignment
requirements. 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 |Resvd|Prf|Resvd| | Type = 3 | Length |Resvd|Prf|Resvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Lifetime | | Prefix Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | | | Prefix Length | |
+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+ |
| Destination Prefix (Variable Length) | | Destination Prefix (Variable Length) |
. . . .
. . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: DAG Destination Prefix Figure 9: DAG Destination Prefix
The Destination Prefix suboption is used when the DAG root, or The Destination Prefix suboption is used when the DODAG root, or
another node located inwards along the DAG on the path to the DAG another node located upwards along the DODAG on the path to the DODAG
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 DODAG, a node MAY decide to join
multiple DAGs in support of a particular application. multiple DODAGs 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.
Prf is the Route Preference as in [RFC4191]. The reserved fields Prf is the Route Preference as in [RFC4191]. The reserved fields
MUST be set to zero on transmission and MUST be ignored on receipt. 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. The
value of all one bits (0xFFFFFFFF) represents infinity. A value of lifetime is initially set by the node that owns the prefix and
all zero bits (0x00000000) indicates a loss of reachability. denotes the valid lifetime for that prefix (similar to
AdvValidLifetime [RFC4861]). The value might be reduced by the
originator and/or en-route nodes that will not provide connectivity
for the whole valid lifetime. A value of all one bits (0xFFFFFFFF)
represents infinity. A value of all zero bits (0x00000000) indicates
a loss of reachability.
The Prefix Length is an 16-bit unsigned integer that indicates the The Prefix Length is an 8-bit unsigned integer that indicates the
number of leading bits in the destination prefix. 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 DIO message may need to specify connectivity to In the event that a DIO message may need to specify connectivity 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 6.1.3.1.1.6. DAG Configuration
The DAG Timer Configuration suboption does not have any alignment The DAG Configuration suboption does not have any alignment
requirements. 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 = 4 | Length | DIOIntDoubl. | | Type = 4 | Length | DIOIntDoubl. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DIOIntMin. | | DIOIntMin. | DIORedun. | MaxRankInc |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: DAG Timer Configuration Figure 10: DAG Configuration
The DAG Timer Configuration suboption is used to distribute The DAG Configuration suboption is used to distribute configuration
configuration information for DAG Timer Operation through the DAG. information for DAG Operation through the DODAG. The information
The information communicated in this suboption is generally static communicated in this suboption is generally static and unchanging
and unchanging within the DAG, therefore it is not necessary to within the DODAG, therefore it is not necessary to include in every
include in every DIO. This suboption MAY be included periodically by DIO. This suboption MAY be included occasionally by the DODAG Root,
the DAG Root, and SHOULD be included in response to a unicast and MUST be included in response to a unicast request, e.g. a DAG
request, e.g. a DAG Information Solicitation (DIS) message. Information Solicitation (DIS) message.
The Length is coded as 2. The Length is coded as 5.
DIOIntervalDoublings is an 8-bit unsigned integer. Configured on the DIOIntervalDoublings is an 8-bit unsigned integer, configured on the
DAG root and used to configure the trickle timer governing when DIO DODAG root and used to configure the trickle timer governing when DIO
message should be sent within the DAG. DIOIntervalDoublings is the message should be sent within the DODAG. DIOIntervalDoublings is the
number of times that the DIOIntervalMin is allowed to be doubled number of times that the DIOIntervalMin is allowed to be doubled
during the trickle timer operation. during the trickle timer operation.
DIOIntervalMin is an 8-bit unsigned integer. Configured on the DAG DIOIntervalMin is an 8-bit unsigned integer, configured on the DODAG
root and used to configure the trickle timer governing when DIO root and used to configure the trickle timer governing when DIO
message should be sent within the DAG. The minimum configured message should be sent within the DODAG. The minimum configured
interval for the DIO trickle timer in units of ms is interval for the DIO trickle timer in units of ms is
2^DIOIntervalMin. For example, a DIOIntervalMin value of 16ms is 2^DIOIntervalMin. For example, a DIOIntervalMin value of 16ms is
expressed as 4. expressed as 4.
5.1.4. Destination Advertisement Object (DAO) DIORedundancyConstant is an 8-bit unsigned integer used to configure
suppression of DIO transmissions. DIORedundancyConstant is the
minimum number of relevant incoming DIOs required to suppress a DIO
transmission. If the value is 0xFF then the suppression mechanism is
disabled.
MaxRankInc, 8-bit unsigned integer, is the DAGMaxRankIncrease. This
is the allowable increase in rank in support of local repair. If
DAGMaxRankIncrease is 0 then this mechanism is disabled.
6.1.4. Destination Advertisement Object (DAO)
The Destination Advertisement Object (DAO) is used to propagate The Destination Advertisement Object (DAO) is used to propagate
destination information inwards along the DAG. The RPL use of the destination information upwards along the DODAG. The RPL use of the
DAO allows the nodes in the DAG to build up routing state for nodes DAO allows the nodes in the DODAG to provision routing state for
contained in the sub-DAG in support of traffic flowing outward along nodes contained in the sub-DAG in support of traffic flowing down
the DAG. along the DODAG.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Sequence | InstanceID | DAO Rank | | DAO Sequence | InstanceID | DAO Rank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Lifetime | | DAO Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Tag | | Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at page 27, line 34 skipping to change at page 28, line 25
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Prefix (Variable Length) | | Prefix (Variable Length) |
. . . .
. . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse Route Stack (Variable Length) | | Reverse Route Stack (Variable Length) |
. . . .
. . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: The Destination Advertisement Object (DAO) Figure 11: The Destination Advertisement Object (DAO)
DAO Sequence: Incremented by the node that owns the prefix for each DAO Sequence: Incremented by the node that owns the prefix for each
new DAO message for that prefix. new DAO message for that prefix.
InstanceID: 8-bit field indicating the topology instance associated InstanceID: 8-bit field indicating the topology instance associated
with the DAG, as learned from the DIO. with the DODAG, as learned from the DIO.
DAO Rank: Set by the node that owns the prefix and first issues the DAO Rank: Set by the node that owns the prefix and first issues the
DAO message to its rank. DAO message to its rank.
DAO Lifetime: 32-bit unsigned integer. The length of time in DAO Lifetime: 32-bit unsigned integer. The length of time in
seconds (relative to the time the packet is sent) that the seconds (relative to the time the packet is sent) that the
prefix is valid for route determination. A value of all one prefix is valid for route determination. A value of all one
bits (0xFFFFFFFF) represents infinity. A value of all zero bits (0xFFFFFFFF) represents infinity. A value of all zero
bits (0x00000000) indicates a loss of reachability. bits (0x00000000) indicates a loss of reachability.
Route Tag: 32-bit unsigned integer. The Route Tag may be used to 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 give a priority to prefixes that should be stored. This may be
useful in cases where intermediate nodes are capable of storing useful in cases where intermediate nodes are capable of storing
a limited amount of routing state. The further specification a limited amount of routing state. The further specification
of this field and its use is under investigation. of this field and its use is under investigation.
Prefix Length: Number of valid leading bits in the IPv6 Prefix. Prefix Length: 8-bit unsigned integer. Number of valid leading bits
in the IPv6 Prefix.
RRCount: 8-bit unsigned integer. This counter is used to count the RRCount: 8-bit unsigned integer. This counter is used to count the
number of entries in the Reverse Route Stack. A value of `0' number of entries in the Reverse Route Stack. A value of `0'
indicates that no Reverse Route Stack is present. indicates that no Reverse Route Stack is present.
Prefix: Variable-length field containing an IPv6 address or a prefix Prefix: Variable-length field containing an IPv6 address or a prefix
of an IPv6 address. The Prefix Length field contains the of an IPv6 address. The Prefix Length field contains the
number of valid leading bits in the prefix. The bits in the number of valid leading bits in the prefix. The bits in the
prefix after the prefix length (if any) are reserved and MUST prefix after the prefix length (if any) are reserved and MUST
be set to zero on transmission and MUST be ignored on receipt. be set to zero on transmission and MUST be ignored on receipt.
Reverse Route Stack: Variable-length field containing a sequence of Reverse Route Stack: Variable-length field containing a sequence of
RRCount (possibly compressed) IPv6 addresses. A node that adds RRCount (possibly compressed) IPv6 addresses. A node that adds
on to the Reverse Route Stack will append to the list and on to the Reverse Route Stack will append to the list and
increment the RRCount. increment the RRCount.
5.2. Conceptual Data Structures 6.2. Protocol Elements
The RPL implementation MUST maintain the following conceptual data 6.2.1. Topological Elements
structures in support of DAG discovery:
o A set of candidate neighbors RPL uses four identifiers to track and control the routing topology
o For each DAG: o The first is an InstanceID. An InstanceID defines what OF a DAG
uses and may also indicate what destinations are offered. A
network may have multiple InstanceIDs, each of which defines an
independent DAG optimized for a different OF and/or application.
The DAG defined by an InstanceID is called a DAG Instance.
* A set of DAG parents and siblings o The second is a DAGID. The scope of a DAGID is a DAG Instance. A
combination of InstanceID and DAGID defines a DODAG. A DAG
Instance may have multiple DODAGs.
5.2.1. Candidate Neighbors Data Structure o The third value is a DAG Sequence Number. The scope of a DAG
Sequence Number is a DODAG. A DODAG is sometimes reconstructed
from the root, by incrementing the DAGSequenceNumber. A
combination of InstanceID, DAGID, and DAG Sequence Number defines
a DODAG Iteration.
The set of candidate neighbors is to be populated by neighbors that o The fourth value is rank. The scope of rank is a DODAG Iteration.
are discovered by the neighbor discovery mechanism and further Rank establishes a partial order over a DODAG Iteration, defining
qualified as statistically stable as per the mechanisms discussed in individual node positions.
[I-D.ietf-roll-routing-metrics]. The candidate neighbors, and
related metrics, should demonstrate stability/reliability beyond a
certain threshold, and it is recommended that a local confidence
value be maintained with respect to the neighbor in order to track
this. Implementations MAY choose to bound the maximum size of the
candidate neighbor set, in which case a local confidence value will
assist in ordering neighbors to determine which ones should remain in
the candidate neighbor set and which should be evicted.
If Neighbor Unreachability Detection (NUD) determines that a 6.2.2. Neighbors, Parents, and Siblings
candidate neighbor is no longer reachable, then it shall be removed
from the candidate neighbor set. In the case that the candidate
neighbor has associated states in the DAG parent set or active DA
entries, then the removal of the candidate neighbor shall be
coordinated with tearing down these states. All provisioned routes
associated with the candidate neighbor should be removed.
5.2.2. Directed Acyclic Graphs (DAGs) Data Structure 1. A node that is not a DODAG root MAY maintain multiple DAG parents
for a single DAG Instance.
At a given point of time, a DAG Iteration is uniquely identified by 2. The set of DAG parents MUST be a conceptual subset of the set of
the tuple (DagID, InstanceID, DAGSequenceNumber) where a change in candidate neighbors. (This does not dictate implementation,
the sequence denotes the iteration of a given DAG over time. When a e.g., to use a certain data structure).
single device is capable to root multiple DAGs in support of an
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 3. If Neighbor Unreachability Detection (NUD), or an equivalent
implementation MUST keep a DAG table with the following entries: mechanism, determines that a neighbor is no longer reachable,
then a RPL node MUST NOT consider this node in the neighbor set
when calculating and advertising routes until the node determines
it is reachable again.
4. Routes via that unreachable neighbor MUST be eliminated from the
routing table, and the node SHOULD poison using no-DAO all DAO
routes that it has advertised via DAO and that it can reach only
via that neighbor.
A node's neighbor set is an unconstrained subset of the nodes that it
can reach with a link-local multicast.
The OF guides in the selection and maintains a number of neighbors to
interact with, which neighbors being qualified as statistically
stable and presenting adequate properties as per the the OF logic,
for instance following mechanisms discussed in
[I-D.ietf-roll-routing-metrics]. Those neighbors are referred to as
candidate neighbors.
Candidate neighbors may take the role of Parent or Siblings, in part
as determined by rank.
For the purpose of inheriting metrics and computing rank, the OF
might select one preferred parent. In that case, the rank of this
node is computed as the rank of the preferred parent plus a rank
increment as determined by the OF.
6.2.3. DODAG Information
For each DODAG that a node is, or may become, a member of, the
implementation should conceptually keep track of the following
information for each DODAG. The data structures described in this
section are intended to illustrate a possible implementation to aid
in the description of the protocol, but are not intended to be
normative.
o InstanceID o InstanceID
o DAGID o DAGID
o DAGSequenceNumber o DAGSequenceNumber
o DAG Metric Container, including DAGObjectiveCodePoint o DAG Metric Container, including DAGObjectiveCodePoint
o A set of Destination Prefixes offered upwards along the DODAG
o A set of Destination Prefixes offered inwards along the DAG o A set of DAG parents
o A set of DAG parents and siblings
o A timer to govern the sending of DIO messages for the DAG o A set of DAG siblings
When a DAG is discovered for which no DAG data structure is o A timer to govern the sending of DIO messages
instantiated, and the node wants to join, then the DAG data structure
is instantiated.
When the DAG parent set is depleted (i.e. the last DAG is removed), When the DAG parent set is depleted on a node that is not a root,
then the DAG data structure SHOULD be suppressed after the expiration (i.e. the last parent is removed), then the DAG information should
of an implementation-specific local timer. An implementation SHOULD not be suppressed until after the expiration of an implementation-
delay before deallocating the DAG data structure in order to observe specific local timer in order to observe that the DAGSequenceNumber
that the DAGSequenceNumber has incremented should any new DAG parents has incremented should any new parents appear for the DODAG.
appear for the DAG.
5.2.2.1. DAG Parents/Siblings Structure 6.2.3.1. DAG Parents/Siblings Structure
When the DAG is self-rooted, the set of DAG parents/siblings is When the DODAG is self-rooted, the set of DAG parents is empty.
empty.
In all other cases, for each node in the set, the implementation MUST For each node in a DAG parent/sibling set, the implementation should
keep a record of: conceptually keep track of:
o a reference to the neighboring device which is the DAG parent or o a reference to the neighboring device which is the DAG parent/
sibling 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 DAG parent Object last processed in the case where the neighboring device is
a DAG parent
DAG parents may be ordered, according to the OF. When ordering DAG DAG parents may be ordered, according to the OF. When ordering DAG
parents, in consultation with the OF, the most preferred DAG parent parents, in consultation with the OF, the most preferred DAG parent
may be identified. All current DAG parents must have a rank less 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. than self. All current DAG siblings must have a rank equal to self.
When nodes are added to or removed from the DAG set the most When nodes are added to or removed from the DAG parent/sibling sets
preferred DAG parent may have changed. The role of all the nodes in the most preferred DAG parent may have changed. The role of all the
the list should be reevaluated. In particular, any nodes having a nodes in the list should be reevaluated. In particular, any nodes
rank greater than self after such a change must be evicted from the having a rank greater than self after such a change must be evicted
set. from the set.
An implementation may choose to keep these records as an extension of
the Default Router List (DRL).
5.3. DAG Rank
Based on the selection of DAG Parents, the metrics conveyed by the
most preferred DAG parent, the nodes own metrics and configuration,
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.
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. The
only aim of the rank is to inform loop avoidance and detection.
The computation of the DAG Rank MUST be done in such a way so as to
maintain the following properties for any nodes M and N that are
neighbors in the LLN:
DAGRank(M) is less than DAGRank(N): In this case, M is probably
located in a more preferred position than N in the 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.
DAGRank(M) equals DAGRank(N): In this case 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.
DAGRank(M) is greater than DAGRank(N): In this case, then node M is 6.3. DAG Discovery and Maintenance
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 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 DAG discovery allows a node to join a DODAG rooted at a DODAG root by
closely track ETX when the objective function is to minimize ETX, or discovering neighbors that are members of the DODAG, and identifying
latency when the objective function is to minimize latency, or in a a set of parents. DAG discovery also identifies siblings, which may
more complicated way as appropriate to the objective code point being be used later to provide additional path diversity towards the DODAG
used within the DAG. root.
5.4. DAG Discovery and Maintenance DODAG discovery may avoid loops by constraining how and when nodes
can increase their rank, and by statistically poisoning the nodes
that present the highest risk.
DAG discovery locates the nearest sink (aka root), as determined DAG discovery enables nodes to implement different policies for
according to some metrics and constraints, and forms a Directed selecting their DAG parents in the DODAG by using implementation
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 specific policy functions. DAG discovery specifies a set of rules to
be followed by all implementations in order to ensure interoperation. be followed by all implementations to enable interoperation.
DAG discovery also standardizes the format that is used to advertise
the most common information that is used in order to select DAG
parents.
One of these information, the DAG rank, is used by DAG discovery to
provide loop avoidance even if nodes implement different policies.
The DAG Rank is computed as specified by the OF in use by the DAG,
demonstrating the properties described in Section 5.3. The rank
should be computed in such a way so as to provide a comparable basis
with other nodes which may not use the same metric at all.
The DAG discovery procedures take into account a number of factors,
including:
o RPL rules for loop avoidance based on DAGs and ranks
o The Objective Function 6.3.1. DAG Discovery Rules
o The advertised metrics The following rules define the RPL DAG Discovery procedures:
o Local policy functions (e.g. a bounded number of candidate 6.3.1.1. DODAG Iteration
neighbors).
5.4.1. DAG Discovery Rules 1. An InstanceID SHOULD be administratively provisioned on a DODAG
root that is significant RPL objective. The InstanceID MUST be
unique to that purpose across the scope of the LLN.
In order to organize and maintain loopless structure, the DAG 2. A DAGID MUST be unique within the scope of the InstanceID. It
discovery implementation in the nodes MUST obey to the following MAY be derived from the IPv6 address of the DODAG root.
rules and definitions:
5.4.1.1. DAGs 3. A node MAY belong to multiple DAG instances. The related
details of operation are outside the scope of this
specification.
1. DAG discovery instantiates LLN topologies that are each optimized 4. DODAG roots MAY increment the DAGSequenceNumber that they
for specific constraints and goals. A topology assumes the shape advertise.
of a DAG, and a DAG Instance is uniquely identified by its
instanceID.
2. For reasons of scalability and operations of the protocol, a DAG 5. When a DODAG root increments its DAGSequenceNumber, it MUST
Instance is partitioned into a set of DAGs rooted at a follow the conventions of Serial Number Arithmetic as described
destination, aka Destination Oriented DAGs. A destination is in [RFC1982].
uniquely identified by a DAGID so a DAG rooted at a destination
is uniquely identified by the pair (InstanceID, DAGID).
3. A Destination Oriented DAG is periodically reconstructed from the 6. The tuple (InstanceID, DAGID, DAGSequenceNumber) uniquely
root, by incrementing a DAGSequenceNumber. An Iteration of a defines a DODAG Iteration. All of a node's parents within a
Destination Oriented DAG is thus uniquely identified by the tuple DODAG MUST belong to the same DODAG iteration, as conveyed by
(InstanceID, DAGID, DAGSequenceNumber). Through this document, the last heard DIO from each parent.
the graph formed by this iterative process is referred to as the
DAG Iteration, or in short, the DAG.
4. The rank is defined within the scope of a DAG Iteration as an 7. A node MUST NOT propagate DIOs for a DODAG Iteration unless it
abstract coordinate to compare the relative position of nodes and is the DODAG root of the DODAG iteration or has selected DODAG
ensure forward progress of the traffic. parents in that DODAG iteration.
5. A node MUST belong at most to one DAG Iteration per InstanceID 8. A node acting as a leaf SHOULD NOT propagate DIOs for a DODAG
and MUST select all its parents and siblings within that same DAG Iteration.
Iteration.
5.4.1.2. DAG Sequence Number 9. A node MUST belong at most to one DODAG Iteration per
InstanceID.
1. The DAGSequenceNumber is incremented by the root and flooded 10. Within a given DODAG, a node that is a not a root MUST NOT
through DIOs. advertise a DAGSequenceNumber higher than the highest
DAGSequenceNumber it has heard.
2. The root floods a new DAGSequenceNumber periodically, at a rate Within a particular implementation, a DODAG root may increment the
that depends on the deployment. This rate can be set to 0 if DAGSequenceNumber periodically, at a rate that depends on the
other methods such as loop detection are considered sufficient to deployment. In other implementations loop detection may be
solve the routing issues in that deployment. considered sufficient to solve the routing issues, and the DODAG root
may increment the DAGSequenceNumber only upon administrative
intervention. Another possibility is that nodes within the LLN have
some means to signal the DODAG root in order to request an on-demand
increment when routing issues are detected.
3. The root MAY also flood a new DAGSequenceNumber on-demand. The As the DAGSequenceNumber is incremented, a new DODAG Iteration
details of the mechanism to signal the root to do so are to be spreads outward from the DODAG root. Thus a parent that advertises
specified in a future revision of this document. the new DAGSequenceNumber can not possibly belong to the sub-DAG of a
node that still advertises an older DAGSequenceNumber. A node may
safely add such a parent, without risk of forming a loop, without
regard to its relative rank in the prior DODAG Iteration. This is
equivalent to jumping to a different DODAG.
4. A parent that advertises the new DAGSequenceNumber can not As a node transitions to new DODAG Iterations as a consequence of
possibly belong to the sub-DAG of a node that still advertises an following these rules, the node will be unable to advertise the
older DAGSequenceNumber. The node MAY thus attach to that parent previous DODAG Iteration (prior DAGSequenceNumber) once it has
regardless of the relative rank, and this situation is equivalent committed to advertising the new DODAG Iteration.
to jumping onto a different Destination Oriented DAG.
5. Thus, as a new DAGSequenceNumber spreads, a new DAG Iteration During a transition to a new DODAG Iteration, a node may decide to
forms that supersedes the previous one. During a forward packets via 'future parents' that belong to the same DODAG
DAGSequenceNumber transition, a node MAY decide to forward (same InstanceID and DAGID), but are observed to advertise a more
packets via 'future parents' that belong to the same Destination recent (incremented) DAGSequenceNumber.
Oriented DAG (same InstanceID and DagID), but a more recent
(incremented) DAGSequenceNumber.
5.4.1.3. DAG Root 6.3.1.2. DODAG Roots
1. A node that does not have any DAG parent MAY become the root of 1. A DODAG root that does not have connectivity to a network outside
its own floating DAG. It's rank is ROOT_RANK. of the LLN MUST NOT set the Grounded bit.
2. A (non-LLN) router is considered connected to a grounded 2. A DODAG root MUST advertise a rank of ROOT_RANK.
infrastructure at rank BASE_RANK. A LLN node that is attached to
such an infrastructure router is the DAG root of its own grounded
DAG. It's rank is ROOT_RANK.
3. In a deployment that uses a backbone link to federate a number of 3. A node that does not have any DODAG parent MAY become the DODAG
LLN roots, it is possible to run RPL over the backbone and use root of a floating DODAG. It MAY also set its DAGPreference such
one router as a backbone root. The backbone root exposes a rank that it is less preferred. This behavior may be a desired
of BASE_RANK over the backbone. All the LLN roots that are alternate to poisoning.
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.
4. The DAG root exposes the DAG in the DIO message and LLN nodes An LLN node that is a Goal for the Objective Function is the root of
propagate the DIO message outwards along the DAG. its own grounded DODAG, at rank ROOT_RANK.
5.4.1.4. Moving Inside a DAG In a deployment that uses a backbone link to federate a number of LLN
roots, it is possible to run RPL over the backbone and use one router
as a backbone root. The backbone root is the virtual root of the
DODAG and exposes a rank of BASE_RANK over the backbone. All the LLN
roots that are parented to that backbone root, including the backbone
root if it also serves as LLN root, expose a rank of ROOT_RANK over
the LLN and are part of the same DODAG, coordinated with the virtual
root over the backbone.
1. A node moves when it changes its parent selection within the same 6.3.1.3. Rank and Movement within a DODAG Iteration
DAG Iteration. When a node moves (within its DAG) in a fashion
that cause its rank to decrease, the node MUST abandon all
parents and siblings with a rank larger than self, and MAY adopt
as siblings nodes with the same rank.
2. A node MAY move at any time, with no delay, within its DAG when 1. A node MUST NOT advertise a rank less than or equal to any member
the move does not cause the node to increase its own DAG rank, as of its parent set within the DODAG Iteration.
per the rank calculation indicated by the OF.
3. A node MUST NOT move outwards along a DAG that it is attached to, 2. A node MAY advertise a rank lower than its prior advertisement
causing the DAG rank to increase. If a node cannot stay within within the DODAG Iteration. (This corresponds to a node moving
the DAG without a rank increase, then it MUST poison its routes up within the DODAG Iteration).
as described in Section 5.4.1.6.
4. When DIO messages are received from other routers located at 3. Let L be the lowest rank within a DODAG iteration that a given
lesser rank in the same DAG, those routers are eligible for node has advertised. Within a DODAG Iteration, that node MUST
consideration as DAG parents. DIO messages received from other NOT advertise an effective rank deeper than L +
routers located at the same rank in the same DAG may be DAGMaxRankIncrease. INFINITE_RANK is an exception to this rule:
considered as coming from siblings. DIO messages that are a node MAY advertise an INFINITE_RANK at any time. (This
received from other routers located at greater rank within the corresponds to a limited rank increase for the purpose of local
same DAG might cause greedy behaviors and loops; such a DIO is repair within the DODAG Iteration.)
ignored unless:
1. The DIO comes from an existing parent or sibling; in which 4. A node MAY, at any time, choose to join a different DODAG within
case that parent must be removed. a DAG Instance. Such a join has no rank restrictions, unless
that different DODAG is a DODAG Iteration that the node has been
a prior member of, in which case the rule of the previous bullet
(3) must be observed. Until a node transmits a DIO indicating
its new DODAG membership, it MUST forward packets along the
previous DODAG.
2. The DIO comes from a node that has better OF ratings than any 5. A node MAY, at any time after hearing the next DAGSequenceNumber
parent known at this point; in that case, this potential Iteration advertised from suitable parents, choose to migrate up
parent MAY be remembered in order to jump at a better to the next DODAG Iteration within the DODAG.
position when the next sequence is flooded.
5.4.1.5. Jumping Onto Another DAG Conceptually, an implementation is maintaining a parent set within
the DODAG Iteration. Movement entails changes to the parent set.
Moving up does not present the risk to create a loop but moving down
might, so that operation is subject to additional constraints.
1. A node jumps when it performs a new parent selection whereby its When a node migrates into the next DODAG Iteration, the parent and
DAG Iteration changes within the same DAG Instance. When a node sibling sets need to be rebuilt for the new iteration. An
jumps onto a new DAG Iteration, it MUST abandon all parents and implementation could defer to migrate until for some reasonable time
siblings from its previous position. to see if some other neighbors with potentially better metrics but
higher rank announce themselves. Similarly, when a node jumps into a
new DODAG it needs to construct new parent/sibling sets for the new
DODAG.
2. A node MAY jump from its current DAG onto any other DAG that When a node moves to improve its position, it must conceptually
provides service for the same InstanceID if it is preferred by abandon all parents and siblings with a rank larger than itself. As
the OF, for example for reasons such as connectivity, configured a consequence of the movement it may also add new siblings. Such a
preference, free medium time, size, security, bandwidth, DAG movement may occur at any time to decrease the rank, as per the
rank, or whatever metrics the LLN uses. This is allowed calculation indicated by the OF. Maintenance of the parent and
regardless of the rank that the node reaches in the new DAG. sibling sets occurs as the rank of candidate neighbors is observed as
reported in their DIOs.
3. A node that jumps should attempt to transmit all the packets If a node needs to move down a DODAG that it is attached to, causing
received as part of the previous DAG along the previous DAG. In the DAG rank to increase, then it MAY poison its routes and delay
other words, it should switch the parent set only after the before moving as described in Section 6.3.1.4.
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 6.3.1.4. Poisoning a Broken Path
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 MAY poison, in order to avoid being used as an ancestor by
the nodes in its sub-DAG, by advertising an effective rank of
INFINITE_RANK and resetting the associated DIO trickle timer to
cause the INFINITE_RANK to be announced promptly.
1. A node SHOULD poison its inwards routes when it looses all of its 2. The node MAY advertise an effective rank of INFINITE_RANK for an
current feasible parents, i.e. the set of DAG parents becomes arbitrary number of DIO timer events before announcing a new
depleted, and it can not jump onto an alternate DAG. rank.
2. In order to poison its inwards routes, a node MAY stay at its 3. As per Section 6.3.1.3, the node MUST advertise INFINITE_RANK
position within its DAG (that is maintain its InstanceID, DagID, within the DODAG iteration if its revised rank would exceed the
DAGSequenceNumber and Rank) but it SHOULD immediately advertise a maximum DAG rank increase.
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 An implementation may choose to employ this poisoning mechanism when
detaches becomes root of its own floating DAG and MUST a node that loses all of its current parents, i.e. the set of DAG
immediately advertise its new situation in a DIO. parents becomes depleted, and it can not jump onto an alternate DODAG
An alternate mechanism is to form a floating DODAG.
4. Either way, the route poisoning will recursively be flooded The motivation for delaying announcement of the revised route through
throughout the impacted sub-DAG as children lose their last multiple DIO events is to (i) increase tolerance to DIO loss, (ii)
parent in the original DAG. allow time for the poisoning action to propagate, and (iii) to
develop an accurate assessment of its new rank. Such gains are
obtained at the expense of potentially increasing the delay before
lower portions of the network are able to re-establish up routes.
Path redundancy in the DAG reduces the significance of either effect,
since children with alternate parents should be able to utilize those
alternates and retain rank while the detached parent re-establishes
its rank.
5. The loss of a DIO message may interrupt the flooding. This can Although an implementation may advertise INFINITE_RANK for the
be compensated by cheer repetition through the trickle algorithm. purposes of poisoning, it is not expected to be equivalent to setting
If that also fails, packet loops will be prevented by the the rank to INFINITE_RANK, and an implementation would likely retain
detection mechanism described in Section 5.11. its rank value prior to the poisoning in some form, for purpose of
maintaining its effective position within (L + DAGMaxRankIncrease).
5.4.1.7. Following a Parent 6.3.1.5. Detaching
1. If a node that receives a DIO from one of its DAG parents 1. A node that does not have a solution to stay connected to a DODAG
indicating that the parent has left the DAG, it may either follow within a given iteration MAY detach from its current DODAG
that parent or stay in its current DAG through an alternate DAG iteration. A node that detaches becomes root of its own floating
parent if that is possible. DODAG and SHOULD immediately advertise its new situation in a DIO
as an alternate to poisoning.
2. If a DAG parent increases its rank such that the node rank would 6.3.1.6. Following a Parent
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. If a node receives a DIO from one of its parents indicating that
the parent has left the DODAG, it SHOULD stay in its current
DODAG through an alternate DAG parent if that is possible. It
MAY follow that parent.
1. When a node detects or causes a DAG inconsistency, as described A DAG parent may have moved, migrated forward into the next DODAG
in Section 5.4.4.2, then the node SHOULD send an unsolicited DIO Iteration, or jumped to a different DODAG. A node should give some
message to its one-hop neighbors. The DIO is updated to preference to remaining in the current DODAG if possible, but ought
propagate the new DAG information. Such an event MUST also cause to follow the parent if there are no other options.
the trickle timer governing the periodic sending of DIO messages
to be reset.
5.4.2. Reception and Processing of DIO messages 6.3.2. DIO Message Communication
When an DIO message is received from a source device named SRC, the When an DIO message is received from a source device named SRC, the
receiving node must first determine whether or not the DIO message receiving node must first determine whether or not the DIO message
should be accepted for further processing, and subsequently present should be accepted for further processing, and subsequently present
the DIO message for further processing if eligible. the DIO message for further processing if eligible.
1. If the DIO message is malformed, then the DIO message is not 1. If the DIO message is malformed, then the DIO message is not
eligible for further processing and is silently discarded. A RPL eligible for further processing and is silently discarded. A RPL
implementation MAY log the reception of a malformed DIO message. implementation MAY log the reception of a malformed DIO message.
2. If SRC is not a member of the candidate neighbor set, then the 2. If SRC is a member of the candidate neighbor set, then the DIO is
DIO is not eligible for further processing. (Further evaluation/ eligible for further processing.
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
node is a member of, and if the new/alternate DAG is the same
InstanceID as the other DAG, then the DAG parent is known to
have jumped.
Remove SRC as a DAG parent from the other DAG
If the other DAG is now empty of candidate parents, then
prepare to directly follow SRC into the new DAG by adding it
as a DAG parent for the new DAG, else ignore the DIO message
(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 6.3.2.1. DIO Message Processing
optimization objectives, then jump: copy the DIO information
place the neighbor into the DAG parent set.
If the DIO message is for a known/existing DAG: If the node has sent an DIO message within the risk window as
described in Section 6.7 then a collision has occurred; do not
process the DIO message any further.
Process the DIO message as per the rules in Section 5.4 Process the DIO message as per the rules in Section 6.3
As DIO messages are received from candidate neighbors, the neighbors As DIO messages are received from candidate neighbors, the neighbors
may be promoted to DAG parents by following the rules of DAG may be promoted to DAG parents by following the rules of DAG
discovery as described in Section 5.4. When a node places a neighbor discovery as described in Section 6.3. When a node places a neighbor
into the DAG Parent set, the node becomes attached to the DAG through into the DAG Parent set, the node becomes attached to the DODAG
the new parent node. through 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).
5.4.3. DIO Transmission 6.3.3. DIO Transmission
Each node maintains a timer that governs when to multicast DIO 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 a trickle timer, as detailed in
over a variable interval. Trickle timers are further detailed in Section 6.3.4. The DIO Configuration Option includes the
Section 5.4.4. The governing parameters for the timer should be configuration of a DAG Instance's trickle timer.
configured consistently across the DAG, and are provided by the DAG
root in the DIO message. In addition to periodic DIO messages, each
node may respond to a DIS message with a DIO message.
o When a node detects an inconsistency, it SHOULD reset the interval o When a node detects or causes an inconsistency, it MUST reset the
of the trickle timer to a minimum value, causing DIO messages to interval of the trickle timer to a minimum value.
be emitted more frequently as part of a strategy to quickly
correct the inconsistency. Such inconsistencies may be, for
example, an update to a key parameter (e.g. sequence number) in
the DIO message or a loop detected when a node located inwards
along the DAG forwards traffic outwards. Inconsistencies are
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 migrates to a new DODAG Iteration it MUST reset the
i.e. DIO messages from its DAG parents are consistent and no trickle timer to its minimum value
other inconsistencies are detected, it may begin to open up the
interval of the trickle timer towards a maximum value, causing DIO o When a node detects an inconsistency when forwarding a packet, as
messages to be emitted less frequently, thus reducing network detailed in Section 6.9, the node MUST reset the trickle timer to
maintenance overhead and saving energy consumption. its minimum value.
o When a node receives a multicast DIS message, it MUST reset the
trickle timer to the minimum value.
o When a node receives a unicast DIS message, it MUST unicast a DIO
message in response, and include the DAG Configuration Object. In
this case the node SHOULD NOT reset the trickle timer.
o If a node is not a member of a DODAG, it MUST suppress
transmitting DIO messages.
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 DIO 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 DODAG (perhaps initially probing for a nearby DODAG with
DIS message). Alternately, it may choose to root its own floating an DIS message). Alternately, it may choose to root its own
DAG and begin multicasting DIO messages using a default trickle floating DODAG and begin multicasting DIO messages using a default
configuration. The second case may be advantageous if it is trickle configuration. The second case may be advantageous if it
desired for independent nodes to begin aggregating into scattered is desired for independent nodes to begin aggregating into
floating DAGs in the absence of a grounded node, for example in scattered floating DODAGs in the absence of a grounded node, for
support of LLN installation and commissioning. example in support of LLN installation and commissioning.
Note that if multiple DAG roots are participating in the same DAG,
i.e. offering DIO messages with the same DAGID, then they must
coordinate with each other to ensure that their DIO messages are
consistent when they emit DIO messages. In particular the Sequence
number must be identical from each DAG root, regardless of which of
the multiple DAG roots issues the DIO message, and changes to the
Sequence number should be issued at the same time. The specific
mechanism of this coordination, e.g. along a non-LLN network between
DAG roots, is beyond the scope of this specification.
5.4.4. Trickle Timer for DIO Transmission 6.3.4. Trickle Timer for DIO Transmission
RPL treats the construction of a DAG as a consistency problem, and RPL treats the construction of a DODAG 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 DODAG that a node is part of, the node must maintain a
trickle timer. The required state contains the following conceptual single trickle timer. The required state contains the following
items: conceptual 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 DIO message as (2^DIOIntervalMin)ms. value is learned from the DIO message as (2^DIOIntervalMin)ms.
The default value is DEFAULT_DIO_INTERVAL_MIN. The default value is 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 DIO message as 2^I_doublings. This value is learned from the DIO message as
DIOIntervalDoublings. The default value is DIOIntervalDoublings. The default value is
DEFAULT_DIO_INTERVAL_DOUBLINGS. DEFAULT_DIO_INTERVAL_DOUBLINGS.
5.4.4.1. Resetting the Trickle Timer 6.3.4.1. Resetting the Trickle Timer
The trickle timer for a DAGID is reset by: The trickle timer for a DODAG is reset by:
1. Setting I_min and I_doublings to the values learned from the DIO 1. Setting I_min and I_doublings to the values learned from the 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 DIO message and makes the When a node learns about a DODAG through a DIO message and makes the
decision to join it, it initializes the state of the trickle timer by 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 DIO message for this DAG from a DAG parent, it MAY redundant DIO message for this DODAG, it MAY increment C. The exact
increment C. determination of redundant is left to an implementation; it could
include DIOs that advertise the same rank.
When the timer fires at time T, the node compares C to the redundancy 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, DIORedundancyConstant. If C is less than that value, or if
value, the node generates a new DIO message and multicasts it. When the DIORedundancyConstant value is 0xFF, the node generates a new DIO
the communication interval I expires, the node doubles the interval I message and multicasts it. When the communication interval I
so long as it has previously doubled it fewer than I_doubling times, expires, the node doubles the interval I so long as it has previously
resets C, and chooses a new T value. doubled it fewer than I_doubling times, resets C, and chooses a new T
value.
5.4.4.2. Determination of Inconsistency 6.3.4.2. 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 DODAG, for example:
o The node joins a new DAGID o The node joins a new DODAG
o The node moves within a DAGID o The node moves within a DODAG
o The node receives a modified 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 up, indicating an
indicating an inconsistency and possible loop. inconsistency and possible loop.
o A metric communicated in the 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.
5.5. DAG Sequence Number Increment 6.4. DAG Selection
The DAG root makes the sole determination of when to revise the
DAGSequenceNumber by incrementing it upwards. When the
DAGSequenceNumber is increased an inconsistency results, causing DIO
messages to be sent back outwards along the DAG to convey the change.
The degree to which this mechanism is relied on may be determined by
the implementation- on one hand it may serve as a periodic heartbeat,
refreshing the DAG states, and on the other hand it may result in a
constant steady-state control cost overhead which is not desirable.
Some implementations may provide an administrative interface, such as
a command line, at the DAG root whereby the DAGSequenceNumber may be
caused to increment in response to some policy outside of the scope
of RPL.
Other implementations may make use of a periodic timer to
automatically increment the DAGSequenceNumber, resulting in a
periodic DAG iteration at a rate appropriate to the application and
implementation. Other automated mechanisms to determine
DAGSequenceNumber increments are also possible as appropriate to a
deployment.
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 for InstanceIDs advertising OCPs and SHOULD prefer to join DODAGs for InstanceIDs advertising OCPs and
destinations compatible with their implementation specific destinations compatible with their implementation specific
objectives. In order to limit erratic movements, and all metrics objectives. In order to limit erratic movements, and all metrics
being equal, nodes SHOULD keep their previous selection. Also, nodes being equal, nodes SHOULD keep their previous selection. Also, nodes
SHOULD provide a means to filter out a candidate parent whose SHOULD provide a means to filter out a parent whose availability is
availability is detected as fluctuating, at least when more stable detected as fluctuating, at least when more stable choices are
choices are available. available.
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 DODAGs MAY aggregate as much as
possible into larger DAGs in order to allow connectivity within the possible into larger DODAGs 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.
5.7. Administrative rank 6.5. Operation as a Leaf Node
When the DAG is formed under a common administration, or when a node In some cases it a RPL node may attach to a DODAG for DAG Instance as
performs a certain role within a community, it might be beneficial to a leaf node only; the node in this case is not to extend connectivity
associate a range of acceptable rank with that node. For instance, a to the DODAG to other nodes under any circumstances. Such a case may
node that has limited battery should be a leaf unless there is no occur, for example, when a node is attaching to a DODAG that is using
other choice, and may then augment the rank computation specified by an unknown Objective Function. When operating as a leaf node, a
the OF in order to expose an exaggerated rank. node:
5.8. Collision 1. MAY receive and process DIOs for that DODAG
2. SHOULD NOT transmit DIOs for that DODAG
3. MUST NOT transmit DIOs containing the DAG Metric Container for
that DODAG
4. MAY transmit unicast DAOs to the chosen parents for that DODAG
5. MAY transmit multicast DAOs to the `1 hop' neighborhood.
6.6. Administrative rank
When the DODAG is formed under a common administration, or when a
node performs a certain role within a community, it might be
beneficial to associate a range of acceptable rank with that node.
For instance, a node that has limited battery should be a leaf unless
there is no other choice, and may then augment the rank computation
specified by the OF in order to expose an exaggerated rank.
6.7. Collision
A race condition occurs if 2 nodes send DIO messages at the same time A race condition occurs if 2 nodes send DIO messages at the same time
and then attempt to join each other. This might happen, for example, 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 between nodes which act as DAG root of their own DODAGs. In order to
detect the situation, LLN Nodes time stamp the sending of DIO detect the situation, LLN Nodes time stamp the sending of DIO
message. Any DIO message received within a short link-layer- message. Any DIO message received within a short link-layer-
dependent period introduces a risk. It is up to the implementation dependent period introduces a risk. It left to the implementation to
to define the duration of the risk window. define the duration of the risk window.
There is risk of a collision when a node receives and processes a DIO There is risk of a collision when a node receives and processes a DIO
within the risk window. For example, it may occur that two nodes are within the risk window. For example, it may occur that two nodes are
associated with different DAGs and near-simultaneously send DIO associated with different DODAGs and near-simultaneously send DIO
messages, which are received and processed by both, and possibly messages, which are received and processed by both, and possibly
result in both nodes simultaneously deciding to attach to each other. result in both nodes simultaneously deciding to attach to each other.
As a remedy, in the face of a potential collision, as determined by As a remedy, in the face of a potential collision, as determined by
receiving a DIO within the risk window, the DIO message is not receiving a DIO within the risk window, the DIO message is not
processed. It is expected that subsequent DIOs would not cross. processed. It is expected that subsequent DIOs would not cross.
5.9. Guidelines for Objective Functions 6.8. Establishing Routing State Down the DODAG
5.9.1. Objective Function
An Objective Function (OF) allows for the selection of a DAG to join,
and a number of peers in that DAG as parents. The OF is used to
compute an ordered list of parents. The OF is also responsible to
compute the rank of the device within the DAG.
The Objective Function is specified in the DIO message within a DAG
Metric Container using an Objective Code Point (OCP), as specified in
[I-D.ietf-roll-routing-metrics], and indicates the method that must
be used to compute the DAG (e.g. "minimize the path cost using the
ETX metric and avoid `Blue' links"). The Objective Code Points are
specified in [I-D.ietf-roll-routing-metrics]. 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
behavior:
o The parent selection is triggered each time an event indicates
that a potential next hop information is updated. This might
happen upon the reception of a DIO message, a timer elapse, or a
trigger indicating that the state of a candidate neighbor has
changed.
o An OF scans all the interfaces on the device. Although there may
typically be only one interface in most application scenarios,
there might be multiple of them and an interface might be
configured to be usable or not for RPL operation. An interface
can also be configured with a preference or dynamically learned to
be better than another by some heuristics that might be link-layer
dependent and are out of scope. Finally an interface might or not
match a required criterion for an Objective Function, for instance
a degree of security. As a result some interfaces might be
completely excluded from the computation, while others might be
more or less preferred.
o An OF scans all the candidate neighbors on the possible interfaces
to check whether they can act as a router for a DAG. There might
be multiple of them and a candidate neighbor might need to pass
some validation tests before it can be used. In particular, some
link layers require experience on the activity with a router to
enable the router as a next hop.
o An OF computes self's rank by adding the step of rank to that
candidate to the rank of that candidate. The step of rank is
computed by estimating the link as follows:
* The step of rank might vary from 1 to 16.
+ 1 indicates a unusually good link, for instance a link
between powered devices in a mostly battery operated
environment.
+ 4 indicates a `normal'/typical link, as qualified by the
implementation.
+ 16 indicates a link that can hardly be used to forward any
packet, for instance a radio link with quality indicator or
expected transmission count that is close to the acceptable
threshold.
* Candidate neighbors that would cause self's rank to increase
are ignored
o Candidate neighbors that advertise an OF incompatible with the set
of OF specified by the policy functions are ignored.
o As it scans all the candidate neighbors, the OF keeps the current
best parent and compares its capabilities with the current
candidate neighbor. The OF defines a number of tests that are
critical to reach the objective. A test between the routers
determines an order relation.
* If the routers are roughly equal for that relation then the
next test is attempted between the routers,
* Else the best of the 2 becomes the current best parent and the
scan continues with the next candidate neighbor
* Some OFs may include a test to compare the ranks that would
result if the node joined either router
o When the scan is complete, the preferred parent is elected and
self's rank is computed as the preferred parent rank plus the step
in rank with that parent.
o Other rounds of scans might be necessary to elect alternate
parents and siblings. In the next rounds:
* Candidate neighbors that are not in the same DAG are ignored
* Candidate neighbors that are of greater rank than self are
ignored
* Candidate neighbors of an equal rank to self (siblings) are
ignored
* Candidate neighbors of a lesser rank than self (non-siblings)
are preferred
5.9.2. Objective Function 0 (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 connectivity. That is, the Objective Function is designed
to find the nearest sink into a 'grounded' topology, and if there is
none then join any network per order of administrative preference.
The metric in use is the rank.
OF0 selects a preferred parent and a backup next hop if one is
available. The backup next hop might be a parent or a sibling. All
the traffic is routed via the preferred parent. When the link
conditions do not let a packet through to the preferred parent, the
packet is passed to the backup next hop.
The step of rank is 4 for each hop.
5.9.2.1. Selection of the Preferred Parent
As it scans all the candidate neighbors, OF0 keeps the parent that is
the best for the following criteria (in order):
1. The interface must be usable and any administrative preference
associated with the interface applies first.
2. A candidate that would cause the node to augment the rank in the
current DAG is not considered.
3. A router that has been validated as usable, e.g. with a local
confidence that has exceeded some pre-configured threshold, is
better.
4. If none are grounded then a DAG with a more preferred
administrative preference (DAGPreference) is better.
5. A router that offers connectivity to a grounded DAG is better.
6. A lesser resulting rank is better.
7. A DAG for which there is an alternate parent is better. This
check is optional. It is performed by computing the backup next
hop while assuming that this router won.
8. The DAG that was in use already is preferred.
9. The preferred parent that was in use already is better.
10. A router that has announced a DIO message more recently is
preferred.
5.9.2.2. Selection of the Backup Next Hop
o The interface must be usable and the administrative preference (if
any) applies first.
o The preferred parent is 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 of a better rank than self (non-siblings) are
preferred.
o A router that has been validated as usable, e.g. with a local
confidence that has exceeded some pre-configured threshold, is
better.
o The router with a better router preference wins.
o The backup next hop that was in use already is better.
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 down along the DODAG,
DAG, from the DAG root toward nodes. from the DODAG 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 along o Destination advertisement establishes down routes along the DODAG.
which inward routes toward the DAG root are set up. Such paths consist of:
o Destination advertisement establishes outward routes along the
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 that 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
skipping to change at page 46, line 50 skipping to change at page 41, line 36
may also learn necessary piecewise source routes to traverse may also learn necessary piecewise source routes to traverse
regions of the LLN that do not maintain routing state. They may regions of the LLN that do not maintain routing state. They may
perform route aggregation on known destinations before emitting perform route aggregation on known destinations before emitting
Destination Advertisements. Destination Advertisements.
o When nodes are incapable of storing routing state, they may o When nodes are incapable of storing routing state, they may
forward destination advertisements, recording the reverse route as forward destination advertisements, recording the reverse route as
the go in order to support the construction of piecewise source the go in order to support the construction of piecewise source
routes. routes.
Nodes that are capable of storing routing state, and finally the DAG Nodes that are capable of storing routing state, and finally the
roots, are able to learn which destinations are contained in the sub- DODAG roots, are able to learn which destinations are contained in
DAG below the node, and via which next-hop neighbors. The the sub-DAG below the node, and via which next-hop neighbors. The
dissemination and installation of this routing state into nodes dissemination and installation of this routing state into nodes
allows for Hop-By-Hop routing from the DAG root outwards along the allows for Hop-By-Hop routing from the DODAG root down the DODAG.
DAG. The mechanism is further enhance by supporting the construction The mechanism is further enhance by supporting the construction of
of source routes across stateless `gaps' in the DAG, where nodes are source routes across stateless `gaps' in the DODAG, where nodes are
incapable of storing additional routing state. An adaptation of this incapable of storing additional routing state. An adaptation of this
mechanism allows for the implementation of loose-source routing. mechanism allows for the implementation of loose-source routing.
A special case, the reception of a destination advertisement A special case, the reception of a destination advertisement
addressed to a link-local multicast address, allows for a node to addressed to a link-local multicast address, allows for a node to
learn destinations directly available from its one-hop neighbors. learn destinations directly available from its one-hop neighbors.
A design choice behind advertising routes via destination A design choice behind advertising routes via destination
advertisements is not to synchronize the parent and children advertisements is not to synchronize the parent and children
databases along the DAG, but instead to update them regularly to databases along the DODAG, 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, similarly to other protocols
other mechanisms, similarly to other protocols such as RIP [RFC2453]. such as RIP [RFC2453].
5.10.1. Destination Advertisement Operation 6.8.1. Destination Advertisement Operation
5.10.1.1. Overview 6.8.1.1. 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 DODAG 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 DAO messages for particular destinations move inwards along the As DAO messages for particular destinations move up the DODAG, a
DAG, a sequence counter is used to guarantee their freshness. The sequence counter is used to guarantee their freshness. The sequence
sequence counter is incremented by the source of the DAO message (the counter is incremented by the source of the DAO message (the node
node that owns the prefix, or learned the prefix via some other that owns the prefix, or learned the prefix via some other means),
means), each time it issues a DAO message for its prefix. Nodes that each time it issues a DAO message for its prefix. Nodes that receive
receive the DAO message and, if scope allows, will be forwarding a the DAO message and, if scope allows, will be forwarding a DAO
DAO message for the unmodified destination inwards along the DAG, message for the unmodified destination up the DODAG, will leave the
will leave the sequence number unchanged. Intermediate nodes will sequence number unchanged. Intermediate nodes will check the
check the sequence counter before processing a DAO message, and if sequence counter before processing a DAO message, and if the DAO is
the DAO is unchanged (the sequence counter has not changed), then the unchanged (the sequence counter has not changed), then the DAO
DAO message will be discarded without additional processing. message will be discarded without additional processing. Further, if
Further, if the DAO message appears to be out of synch (the sequence the DAO message appears to be out of synch (the sequence counter is 2
counter is 2 or more behind the present value) then the DAO state is or more behind the present value) then the DAO state is considered to
considered to be stale and may be purged, and the DAO message is be stale and may be purged, and the DAO message is discarded. The
discarded. A depth is also added for tracking purposes; the depth is rank is also added for tracking purposes; nodes that are storing
incremented at each hop as the DAO message is propagated up the DAG. routing state may use it to determine which possible next-hops for
the destination are more optimal.
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 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 one of its DA parents, that is selected as most advertisements to one of its DA parents, that is selected as most
favored for incoming outwards traffic. The node only accepts unicast favored for incoming down 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.
Receiving a DIO message with the `D' destination advertisement bit Receiving a DIO message with the `D' destination advertisement bit
set from a DAG parent stimulates the sending of a delayed destination set from a 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 DODAG, and any connected prefixes). If the Destination
Advertisement Supported (A) bit is set in the 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 DODAG, then a destination advertisement is also sent to a DAG parent
once it has been added to the DA parent set after a movement, or when once it has been added to the DA parent set after a movement, or when
the list of advertised prefixes has changed. the list of advertised prefixes has changed.
A node that modifies its DAG Parent set may set the `D' bit in A node that modifies its DAG Parent set may set the `D' bit in
subsequent DIO propagation in order to trigger destination subsequent DIO propagation in order to trigger destination
advertisements to be updated to its DAG Parents and other inward advertisements to be updated to its DAG Parents and other ancestors
nodes on the DAG. Additional recommendations and guidelines on the DODAG. Additional recommendations and guidelines regarding
regarding the use of this mechanism are still under consideration and the use of this mechanism are still under consideration and will be
will be elaborated in a future revision of this specification. 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) DAO messages, termed as no-DAOs. A no-DAO is or negative (removed) DAO messages, termed as no-DAOs. A no-DAO 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 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 DAO message with a receiving a no-DAO. A no-DAO is a conveyed as a DAO message with a
DAO Lifetime of ZERO_LIFETIME. DAO Lifetime of ZERO_LIFETIME.
A node that is capable of recording the state information conveyed in A node that is capable of recording the state information conveyed in
a unicast DAO message will do so upon receiving and processing the a unicast DAO message will do so upon receiving and processing the
DAO message, thus building up routing state concerning destinations DAO message, thus provisioning routing state concerning destinations
below it in the DAG. If a node capable of recording state located downwards along the DODAG. If a node capable of recording
information receives a DAO message containing a Reverse Route Stack, state information receives a DAO message containing a Reverse Route
then the node knows that the DAO message has traversed one or more Stack, then the node knows that the DAO message has traversed one or
nodes that did not retain any routing state as it traversed the path more nodes that did not retain any routing state as it traversed the
from the DAO source to the node. The node may then extract the path from the DAO source to the node. The node may then extract the
Reverse Route Stack and retain the included state in order to specify Reverse Route Stack and retain the included state in order to specify
Source Routing instructions along the return path towards the Source Routing instructions along the return path towards the
destination. The node MUST set the RRCount back to zero and clear destination. The node MUST set the RRCount back to zero and clear
the Reverse Route Stack prior to passing the DAO message information the Reverse Route Stack prior to passing the DAO message information
on. on.
A node that is unable to record the state information conveyed in the A node that is unable to record the state information conveyed in the
DAO message will append the next-hop address to the Reverse Route 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 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 provision 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 certain cases (called hybrid cases), some nodes along the path a
destination advertisement follows inward along the DAG may store destination advertisement follows up the DODAG may store state and
state and some may not. The destination advertisement mechanism some may not. The destination advertisement mechanism allows for the
allows for the provisioning of routing state such that when a packet provisioning of routing state such that when a packet is traversing
is traversing outwards along the DAG, some nodes may be able to down the DODAG, some nodes may be able to directly forward to the
directly forward to the next hop, and other nodes may be able to next hop, and other nodes may be able to specify a piecewise source
specify a piecewise source route in order to bridge spans of route in order to bridge spans of stateless nodes within the path on
stateless nodes within the path on the way to the desired 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 DAO destination advertisements pass by, and the DAG root ends up with 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.
skipping to change at page 50, line 11 skipping to change at page 44, line 42
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 DIO messages were sent on o A counter of retries to count how many DIO messages were sent 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 DODAG may be propagating information up the DODAG for the same
destination. A node that 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 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.
When a node loses connectivity to a child that is used as next hop
for a route learned from a DAO, the node should cleanup all routes
and DAO states that are related to that child. If the lost child was
the only adjacency leading to the DAO prefix, the node should poison
the route by sending no-DAOs to the parents to which it has
advertised the DAO prefixes.
The destination advertisement mechanism stores the prefix entries in 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 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 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.
5.10.1.1.1. Destination Advertisement Timers 6.8.1.1.1. Destination Advertisement Timers
The destination advertisement mechanism requires 2 timers; the The destination advertisement mechanism requires 2 timers; the
DelayDAO timer and the RemoveTimer. DelayDAO timer and the RemoveTimer.
o The DelayDAO 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 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 DelayDAO timer has a duration that is DEF_DAO_LATENCY divided o For a root, the DIO timer has a duration of DEF_DAO_LATENCY. For
by a multiple of the DAG rank of the node. The intention is that a node in a DODAG iteration, the DelayDAO timer has a duration
nodes located deeper in the DAG should have a shorter DelayDAO that is randomized between (DEF_DAO_LATENCY divided by the Rank of
timer, allowing DAO messages a chance to be reported from deeper the node) and (DEF_DAO_LATENCY divided by the Rank of the parent).
in the DAG and potentially aggregated along sub-DAGs before The intention is that nodes located deeper in the DODAG iteration
propagating further inwards. should have a shorter DelayDAO timer, allowing DAO messages a
chance to be reported from deeper in the DODAG and potentially
aggregated along sub-DAGs before propagating further up.
o The RemoveTimer is used to clean up entries for which DAO messages o The RemoveTimer is used to clean up entries for which DAO messages
are no longer being received from the sub-DAG. are no longer being received from the sub-DAG.
* When a 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.
skipping to change at page 51, line 31 skipping to change at page 46, line 24
* When the RemoveTimer elapse, DAO messages with lifetime 0, i.e. * When the RemoveTimer elapse, DAO messages with lifetime 0, 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 which have reached the threshold are no longer entries which have reached the threshold are no longer
available, and the related routing states may be propagated and available, and the related routing states may be propagated and
cleaned 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,
TBD(DIO Trickle Timer Interval)). TBD(DIO Trickle Timer Interval)).
5.10.1.2. Multicast Destination Advertisement Messages 6.8.1.2. Multicast Destination Advertisement Messages
It is also possible for a node to multicast a 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 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 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 DODAG 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 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 DAOs. propagated by a router in unicast DAOs.
A node receiving a multicast DAO message addressed to FF02::1 MAY A node receiving a multicast DAO message addressed to FF02::1 MAY
install prefixes contained in the 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 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).
5.10.1.3. Unicast Destination Advertisement Messages from Child to 6.8.1.3. 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 DIO message) in the Reachable and Connected last DA Trigger from an DIO message) in the Reachable and Connected
lists, as well as no-DAOs for all the entries in the Unreachable lists, as well as no-DAOs for all the entries in the Unreachable
list. Depending on its policy and ability to retain routing state, list. Depending on its policy and ability to retain routing state,
the receiving node SHOULD keep a record of the reported DAO message. the receiving node SHOULD keep a record of the reported DAO message.
If the DAO message offers the best route to the prefix as determined If the DAO message offers the best route to the prefix as determined
by policy and other prefix records, the node SHOULD install a route by policy and other prefix records, the node SHOULD install a route
to the prefix reported in the DAO message via the link local address to the prefix reported in the DAO message via the link local address
of the reporting neighbor and it SHOULD further propagate the of the reporting neighbor and it SHOULD further propagate the
information in a DAO message. information in a DAO message.
The DIO message from the DAG root is used to synchronize the whole The DIO message from the DODAG root is used to synchronize the whole
DAG, including the periodic reporting of destination advertisements DODAG iteration, including the periodic reporting of destination
back up the DAG. Its period is expected to vary, depending on the advertisements back up the DODAG. Its period is expected to vary,
configuration of the trickle timer that governs the RAs. depending on the configuration of the DIO trickle timer.
When a node receives a 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 DelayDAO is armed to force a full update. parent, the DelayDAO is armed to force a full update.
When the node broadcasts a DIO message on an LLN interface, for all When the node broadcasts a DIO message on an LLN interface, for 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.
skipping to change at page 53, line 10 skipping to change at page 48, line 5
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.
5.10.1.4. Other Events 6.8.1.4. 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 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 5.13, neighbor is lost, as per the procedures described in Section 6.11,
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 DelayDAO is armed. set to not 'reported' and DelayDAO is armed.
5.10.1.5. Aggregation of Prefixes by a Node 6.8.1.5. 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 DODAG iteration, not in the sub-DAG of the aggregating
node.
3. A node that is to be aggregated for is located somewhere else in 3. A node that is to be aggregated for is located somewhere else in
the LLN. the LLN.
Consider a node M that is performing an aggregation, and a node N Consider a node M that is performing an aggregation, and a node N
that is to be a member of the aggregation group. A node Z situated that is to be a member of the aggregation group. A node Z situated
above the node M in the DAG, but not above node N, will see the above the node M in the DODAG, but not above node N, will see the
advertisements for the aggregation owned by M but not that of the 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.
5.11. Loop Detection 6.9. Loop Detection
RPL loop avoidance mechanisms are kept simple and designed to RPL loop avoidance mechanisms are kept simple and designed to
minimize churn and states. Loops may form for a number of reasons, minimize churn and states. Loops may form for a number of reasons,
from control packet loss to sibling forwarding. RPL includes a from control packet loss to sibling forwarding. RPL includes a
reactive loop detection technique that protects from meltdown and reactive loop detection technique that protects from meltdown and
triggers repair of broken paths. triggers repair of broken paths.
RPL loop detection uses information that is placed into the packet in 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 the IPv6 flow label. The IPv6 flow label is defined in [RFC2460] and
this purpose. The flow label is constructed as follows: its operation is further specified in [RFC3697]. For the purpose of
RPL operations, the flow label is constructed as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|O|S|R|D| SenderRank | InstanceID | |O|S|R|F| SenderRank | InstanceID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: RPL Flow Label Figure 12: RPL Flow Label
Outwards 'O' bit: 1-bit flag indicating whether the packet is Down 'O' bit: 1-bit flag indicating whether the packet is expected
expected to progress inwards or outwards. A router sets the to progress up or down. A router sets the 'O' bit when the
'O' bit when the packet is expect to progress outwards (using packet is expect to progress down (using DAO routes), and
DAO routes), and resets it when forwarding towards the root of resets it when forwarding towards the root of the DODAG
the DAG. A host MUST set the bit to 0. iteration. A host MUST set the bit to 0.
Sibling 'S' bit: 1-bit flag indicating whether the packet has been Sibling 'S' bit: 1-bit flag indicating whether the packet has been
forwarded via a sibling at the present rank, and denotes a risk forwarded via a sibling at the present rank, and denotes a risk
of a sibling loop. A host sets the bit to 0. of a sibling loop. A host sets the bit to 0.
Rank-Error 'R' bit: 1-bit flag indicating whether a rank error was 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 detected. A rank error is detected when there is a mismatch in
the relative ranks and the direction as indicated in the 'O' the relative ranks and the direction as indicated in the 'O'
bit. A host MUST set the bit to 0. bit. A host MUST set the bit to 0.
DAO-Error 'D' bit: 1-bit flag indicating whether a DAO error was Forwarding-Error 'F' bit: 1-bit flag indicating that this node can
detected. An undetected DAO error would have resulted in an not forward the packet further towards the destination. The
inward to outward transition that is not expected with this 'F' bit might be set by sibling that can not forward to a
spec. A host MUST set the bit to 0. parent a packet with the Sibling 'S' bit set, or by a child
node that does not have a route to destination for a packet
with the down 'O' bit set. A host MUST set the bit to 0.
SenderRank: 8-bit field indicating the rank of the sender. A host SenderRank: 8-bit field set to zero by the source and to its rank by
MUST set the rank to INFINITE_RANK. A router MUST place its a router that forwards inside the RPL network.
own rank in the flow label when forwarding.
InstanceID: 8-bit field indicating the DAG instance along which the InstanceID: 8-bit field indicating the DODAG instance along which
packet is sent. the packet is sent.
5.11.1. Host Basic Operation 6.9.1. Source Node Operation
It is expected that a host that does not participate to RPL in any A packet that is sourced at a node connected to a RPL network or
fashion is configured to set the flow label to all zeroes in its destined to a node connected to a RPL network MUST be issued with the
outgoing packets. The host MAY send a packet to any router flow label zeroed out, but for the InstanceID field.
regardless of the DAG and RPL operations at large.
A host that participates to RPL SHOULD zero out all the flags, and it If the source is aware of the InstanceID that is preferred for the
MUST set the sender rank to INFINITE_RANK. If the host can map a flow, then it MUST set the InstanceID field in the flow label
flow to a given InstanceID then it MUST set the flow label accordingly, otherwise it MUST set it to the RPL_DEFAULT_INSTANCE.
accordingly. Forwarding rules are the same for this host and a
router, and are described in the next section.
5.11.2. Instance Forwarding If a compression mechanism such as 6LoWPAN is applied to the packet,
the flow label MUST NOT be compressed even if it is set to all
zeroes.
Instance IDs is used to avoid loops between DAGs from different 6.9.2. Router Operation
origins. DAGs that constructed for antagonistic constraints might
6.9.2.1. Conformance to RFC 3697
[RFC3697] mandates that the Flow Label value set by the source MUST
be delivered unchanged to the destination node(s).
In order to restore the flow label to its original value, an RPL
router that delivers a packet to a destination connected to a RPL
network or that routes a packet outside the RPL network MUST zero out
all the fields but the InstanceID field that must be delivered
without a change.
6.9.2.2. Instance Forwarding
Instance IDs are used to avoid loops between DODAGs from different
origins. DODAGs that constructed for antagonistic constraints might
contain paths that, if mixed together, would yield loops. Those contain paths that, if mixed together, would yield loops. Those
loops are avoided by forwarding a packet along the DAG that is loops are avoided by forwarding a packet along the DODAG that is
associated to a given instance. associated to a given instance.
The InstanceID is placed by the source in the flow label. It is not The InstanceID is placed by the source in the flow label. This
meaningful if the packet has the flow label set to all zeroes. InstanceID MUST match the DODAG instance onto which the packet is
Otherwise it MUST match the DAG instance onto which the packet is
placed by any node, be it a host or router. placed by any node, be it a host or router.
When a router receives a packet that is flagged with a given instance When a router receives a packet that is flagged with a given
ID and the node can forward the packet along the DAG associated to InstanceID and the node can forward the packet along the DODAG
that instance, then the router MUST do so and leave the instance ID associated to that instance, then the router MUST do so and leave the
flag unchanged. InstanceID 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 If any node can not forward a packet along the DODAG associated to
DAG associated to the instance RPL_DEFAULT_INSTANCE then it should do the InstanceID in the flow label, then the node SHOULD discard the
so. Otherwise it should drop the packet. packet.
5.11.3. DAG Inconsistency Loop Detection 6.9.2.3. DAG Inconsistency Loop Detection
The DAG is inconsistent is the direction of a packet does not match The DODAG is inconsistent if the direction of a packet does not match
the rank relationship. A receiver detects an inconsistency if it the rank relationship. A receiver detects an inconsistency if it
receives a packet with either: receives a packet with either:
the 'O' bit set (to outwards) from a node of a higher rank. the 'O' bit set (to down) from a node of a higher rank.
the 'O' bit reset (for inwards) from a node of a lesser rank. the 'O' bit reset (for up) from a node of a lesser rank.
the 'S' bit set (to sibling) from a node of a different rank. the 'S' bit set (to sibling) from a node of a different rank.
The propagation of a new sequence creates local inconsistencies. In When the DODAG root increments the DAG Sequence Number a temporary
particular, it is possible for a router to forward a packet to a rank discontinuity may form between the next iteration and the prior
future parent (same instance, same DAGID, higher sequence) without a iteration, in particular if nodes are adjusting their rank in the
loop, regardless of the rank of that parent. In that case, the next iteration and deferring their migration into the next iteration.
sending router MUST present itself as a host on the future DAG and A router that is still a member of the prior iteration may choose to
use a rank of INFINITE_RANK as it forwards the packets via a future forward a packet to a (future) parent that is in the next iteration.
parent to avoid a false positive. In some cases this could cause the parent to detect an inconsistency
because the rank-ordering in the prior iteration is not necessarily
the same as in the next iteration and the packet may be judged to not
be making forward progress. If the sending router is aware that the
chosen successor has already joined the next iteration, then the
sending router MUST update the SenderRank to INFINITE_RANK as it
forwards the packets across the discontinuity into the next DODAG
iteration in order to avoid a false detection of rank inconsistency.
One inconsistency along the path is not considered as a critical One inconsistency along the path is not considered as a critical
error and the packet may continue. But a second detection along the 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. 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. 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 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 was not set then the Rank-Error bit is set. If it was set the packet
is discarded and the trickle timer is reset. is discarded and the trickle timer is reset.
5.11.4. Sibling Loop Avoidance 6.9.2.4. Sibling Loop Avoidance
When a packet is forwarded along siblings, it cannot be checked for When a packet is forwarded along siblings, it cannot be checked for
forward progress and may loop between siblings. Experimental forward progress and may loop between siblings. Experimental
evidence has shown that one sibling hop can be very useful but is evidence has shown that one sibling hop can be very useful but is
generally sufficient to avoid loops. Based on that evidence, this generally sufficient to avoid loops. Based on that evidence, this
specification enforces the simple rule that a packet may not make 2 specification enforces the simple rule that a packet may not make 2
sibling hops in a row. sibling hops in a row.
When a host issues a packet or when a router forwards a packet to a 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 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 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 Sibling bit is set. If the Sibling bit was set then then the router
discarded. This does not denote a graph inconsistency but indicates SHOULD return the packet to the sibling that that passed it with the
that a new graph should probably be formed with a new sequence. Forwarding-Error 'F' bit set.
5.11.5. DAO Inconsistency Loop Detection and Recovery 6.9.2.5. DAO Inconsistency Loop Detection and Recovery
A DAO inconsistency happens when router that has an outwards DAO A DAO inconsistency happens when router that has an down DAO route
route via a child that is a remnant from an obsolete state that is via a child that is a remnant from an obsolete state that is not
not matched in the child. With DAO inconsistency loop recovery, a matched in the child. With DAO inconsistency loop recovery, a packet
packet can be used to recursively explore and cleanup the obsolete can be used to recursively explore and cleanup the obsolete DAO
DAO states along a sub-DAG. states along a sub-DAG.
In a general manner, a packet that goes outwards should never go In a general manner, a packet that goes down should never go up
inwards again. So rather than routing inwards a packet with the again. So rather than routing up a packet with the down bit set, the
Outwards bit set, the router MUST discard the packet. If DAO router MUST discard the packet. If DAO inconsistency loop recovery
inconsistency loop recovery is applied, then the router SHOULD send is applied, then the router SHOULD send the packet to the parent that
the packet to the parent that passed it with the DAO-Error bit set. passed it with the Forwarding-Error 'F' bit set.
Upon a packet with a DAO bit set, the parent MUST remove the routing 6.9.2.6. Forward Path Recovery
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 Upon receiving a packet with a Forwarding-Error bit set, the node
MUST remove the routing states that caused forwarding to that
neighbor, clear the Forwarding-Error bit and attempt to send the
packet again. The packet may its way to an alternate neighbor. If
that alternate neighbor still has an inconsistent DAO state via this
node, the process will recurse, this node will set the Forwarding-
Error 'F' bit and the routing state in the alternate neighbor will be
cleaned up as well.
6.10. Multicast Operation
This section describes further the multicast routing operations over This section describes further the multicast routing operations over
an IPv6 RPL network, and specifically how unicast DAOs can be used to an IPv6 RPL network, and specifically how unicast DAOs can be used to
relay group registrations inwards. Wherever the following text relay group registrations up. Wherever the following text mentions
mentions MLD, one can read MLDv2 or v3. 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 DODAG root, MLD requests
requests are mapped and transported as DAO messages within the RPL are mapped and transported as DAO messages within the RPL protocol;
protocol; each hop coalesces the multiple requests for a same group each hop coalesces the multiple requests for a same group as a single
as a single DAO message to the parent(s), in a fashion similar to DAO message to the parent(s), in a fashion similar to proxy IGMP, but
proxy IGMP, but recursively between child router and parent up to the recursively between child router and parent up to the root.
root.
A router might select to pass a listener registration DAO message to A router might select to pass a listener registration DAO message 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 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 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
DAG terminates RPL and MAY redistribute the RPL routes over the DODAG 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 down the
along the DAG based on the multicast routing table entries installed DODAG based on the multicast routing table entries installed from the
from the DAO message. DAO message.
For a source inside the DAG, the packet is passed to the preferred For a source inside the DODAG, 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 DODAG. 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 DODAG 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 Rendezvous Point As a result, the DODAG Root acts as an automatic proxy Rendezvous
for the RPL network, and as source towards the Internet for all Point for the RPL network, and as source towards the Internet for all
multicast flows started in the RPL LLN. So regardless of whether the 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 DODAG is grounded or floating, the root can serve inner multicast
streams at all times. streams at all times.
5.13. Maintenance of Routing Adjacency 6.11. Maintenance of Routing Adjacency
The selection of successors, along the default paths inward along the The selection of successors, along the default paths up along the
DAG, or along the paths learned from destination advertisements DODAG, or along the paths learned from destination advertisements
outward along the DAG, leads to the formation of routing adjacencies down along the DODAG, 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]).
Unfortunately, such an approach is not desirable in constrained Unfortunately, such an approach is not desirable in constrained
environments such as LLN and would lead to excessive control traffic environments such as LLN and would lead to excessive control traffic
in light of the data traffic with a negative impact on both link in light of the data traffic with a negative impact on both link
loads and nodes resources. Overhead to maintain the routing loads and nodes resources. Overhead to maintain the routing
skipping to change at page 59, line 14 skipping to change at page 54, line 27
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.
5.14. Packet Forwarding 7. Suggestions for 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. In the scope of this specification, it is preferred to select a 1. In the scope of this specification, it is preferred to select a
successor from a DAG that matches the InstanceID marked in the successor from a DODAG iteration that matches the InstanceID
IPv6 header of the packet being forwarded. marked in the IPv6 header of the packet being 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 down the sub-DAG),
sub-DAG), then use that successor. then use that successor.
5. If there is a DAG offering a route to a prefix matching the 5. If there is a DODAG iteration offering a route to a prefix
destination, then select one of those DAG parents as a successor. matching the destination, then select one of those DODAG parents
as a successor.
6. If there is a DAG parent offering a default route then select 6. If there is a DAG parent offering a default route then select
that DAG parent 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 DODAG iteration offering a route to a prefix
destination, but all DAG parents have been tried and are matching the destination, but all DAG parents have been tried and
temporarily unavailable (as determined by the forwarding are 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 that 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 up to an down flow,
flow, such as switching from DIO routes to DAO routes as the such as switching from DIO routes to DAO routes as the destination is
destination is neared. neared.
6. RPL Constants and Variables 8. Guidelines for Objective Functions
An Objective Function (OF) allows for the selection of a DODAG to
join, and a number of peers in that DAG as parents. The OF is used
to compute an ordered list of parents. The OF is also responsible to
compute the rank of the device within the DODAG iteration.
The Objective Function is indicated in the DIO message using an
Objective Code Point (OCP), as specified in
[I-D.ietf-roll-routing-metrics], and indicates the method that must
be used to compute the DODAG (e.g. "minimize the path cost using the
ETX metric and avoid `Blue' links"). The Objective Code Points are
specified in [I-D.ietf-roll-routing-metrics] and related companion
specifications.
Most Objective Functions are expected to follow the same abstract
behavior:
o The parent selection is triggered each time an event indicates
that a potential next hop information is updated. This might
happen upon the reception of a DIO message, a timer elapse, or a
trigger indicating that the state of a candidate neighbor has
changed.
o An OF scans all the interfaces on the device. Although there may
typically be only one interface in most application scenarios,
there might be multiple of them and an interface might be
configured to be usable or not for RPL operation. An interface
can also be configured with a preference or dynamically learned to
be better than another by some heuristics that might be link-layer
dependent and are out of scope. Finally an interface might or not
match a required criterion for an Objective Function, for instance
a degree of security. As a result some interfaces might be
completely excluded from the computation, while others might be
more or less preferred.
o An OF scans all the candidate neighbors on the possible interfaces
to check whether they can act as a router for a DODAG. There
might be multiple of them and a candidate neighbor might need to
pass some validation tests before it can be used. In particular,
some link layers require experience on the activity with a router
to enable the router as a next hop.
o An OF computes self's rank by adding the step of rank to that
candidate to the rank of that candidate. The step of rank is
computed by estimating the link as follows:
* The step of rank might vary from 1 to 16.
+ 1 indicates a unusually good link, for instance a link
between powered devices in a mostly battery operated
environment.
+ 4 indicates a `normal'/typical link, as qualified by the
implementation.
+ 16 indicates a link that can hardly be used to forward any
packet, for instance a radio link with quality indicator or
expected transmission count that is close to the acceptable
threshold.
* Candidate neighbors that would cause self's rank to increase
are ignored
o Candidate neighbors that advertise an OF incompatible with the set
of OF specified by the policy functions are ignored.
o As it scans all the candidate neighbors, the OF keeps the current
best parent and compares its capabilities with the current
candidate neighbor. The OF defines a number of tests that are
critical to reach the objective. A test between the routers
determines an order relation.
* If the routers are roughly equal for that relation then the
next test is attempted between the routers,
* Else the best of the 2 becomes the current best parent and the
scan continues with the next candidate neighbor
* Some OFs may include a test to compare the ranks that would
result if the node joined either router
o When the scan is complete, the preferred parent is elected and
self's rank is computed as the preferred parent rank plus the step
in rank with that parent.
o Other rounds of scans might be necessary to elect alternate
parents and siblings. In the next rounds:
* Candidate neighbors that are not in the same DODAG are ignored
* Candidate neighbors that are of greater rank than self are
ignored
* Candidate neighbors of an equal rank to self (siblings) are
ignored
* Candidate neighbors of a lesser rank than self (non-siblings)
are preferred
9. RPL Constants and Variables
Following is a summary of RPL constants and variables. Some default
values are to be determined in companion applicability statements.
ZERO_LIFETIME This is the special value of a lifetime that indicates ZERO_LIFETIME This is the special value of a lifetime that indicates
immediate death and removal. ZERO_LIFETIME has a value of 0. immediate death and removal. ZERO_LIFETIME has a value of 0.
BASE_RANK This is the rank for a virtual root that might be used to BASE_RANK This is the rank for a virtual root that might be used to
coordinate multiple roots. BASE_RANK has a value of 0. coordinate multiple roots. BASE_RANK has a value of 0.
ROOT_RANK This is the rank for a DAG root. ROOT_RANK has a value of ROOT_RANK This is the rank for a DAG root. ROOT_RANK has a value of
1. 1.
INFINITE_RANK This is the constant maximum for the rank. INFINITE_RANK This is the constant maximum for the rank.
INFINITE_RANK has a value of 0xFF. INFINITE_RANK has a value of 0xFF.
RPL_DEFAULT_INSTANCE This is the instance ID that is used by this RPL_DEFAULT_INSTANCE This is the InstanceID that is used by this
protocol by a node without a policy to know any better. protocol by a node without any overriding policy.
RPL_DEFAULT_INSTANCE has a value of 0. RPL_DEFAULT_INSTANCE has a value of 0.
DEFAULT_DIO_INTERVAL_MIN To be determined DEFAULT_DIO_INTERVAL_MIN To be determined
DEFAULT_DIO_INTERVAL_DOUBLINGS To be determined DEFAULT_DIO_INTERVAL_DOUBLINGS To be determined
DEFAULT_DIO_REDUNDANCY_CONSTANT To be determined
DEF_DAO_LATENCY To be determined DEF_DAO_LATENCY To be determined
MAX_DESTROY_INTERVAL To be determined MAX_DESTROY_INTERVAL To be determined
DIO Timer One instance per DAG that a node is a member of. Expiry DIO Timer One instance per DODAG that a node is a member of. Expiry
triggers DIO message transmission. Trickle timer with variable triggers DIO message transmission. Trickle timer with variable
interval in [0, DIOIntervalMin..2^DIOIntervalDoublings]. See interval in [0, DIOIntervalMin..2^DIOIntervalDoublings]. See
Section 5.4.4 Section 6.3.4
DAG Sequence Number Increment Timer Up to one instance per DAG that DAG Sequence Number Increment Timer Up to one instance per DODAG
the node is acting as DAG root of. May not be supported in all that the node is acting as DAG root of. May not be supported
implementations. Expiry triggers revision of in all implementations. Expiry triggers revision of
DAGSequenceNumber, causing a new series of updated DIO message DAGSequenceNumber, causing a new series of updated DIO 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 DODAG and as appropriate to application
requirements (e.g. response time vs. overhead). See requirements (e.g. response time vs. overhead).
Section 5.5
DelayDAO 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
Expiry triggers sending of DAO message to the DA parent. The DODAG. Expiry triggers sending of DAO message to the DA
interval is to be proportional to DEF_DAO_LATENCY/(node rank), parent. The interval is to be proportional to DEF_DAO_LATENCY/
such that nodes of greater rank (further outward along the DAG) (node rank), such that nodes of greater rank (further down
expire first, coordinating the sending of DAO messages to allow along the DODAG) expire first, coordinating the sending of DAO
for a chance of aggregation. See Section 5.10.1.1.1 messages to allow for a chance of aggregation. See
Section 6.8.1.1.1
RemoveTimer Up to one instance per DA entry per neighbor (i.e. those RemoveTimer Up to one instance per DA entry per neighbor (i.e. those
neighbors that have given DAO messages to this node as a DAG neighbors that have given DAO messages to this node as a DAG
parent) Expiry triggers a change in state for the DA entry, parent) Expiry triggers a change in state for the DA 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, TBD(DIO parents. The interval is min(MAX_DESTROY_INTERVAL, TBD(DIO
Trickle Timer Interval)). See Section 5.10.1.1.1 Trickle Timer Interval)). See Section 6.8.1.1.1
7. Manageability Considerations 10. 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 10.1. Control of Function and Policy
7.1.1. Initialization Mode 10.1.1. 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 DIO message until it has joined a DAG, or and not send any multicast DIO message until it has joined a DODAG,
to immediately root a transient DAG and start sending multicast DIO or to immediately root a transient DODAG and start sending multicast
messages. A RPL implementation SHOULD allow configuring whether the DIO messages. A RPL implementation SHOULD allow configuring whether
node should stay silent or should start advertising DIO messages. the node should stay silent or should start advertising 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 DIS 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 DODAGs, or should simply wait until it received DIO
messages from other nodes that are part of existing DAGs. messages from other nodes that are part of existing DODAGs.
7.1.2. DIO Base option 10.1.2. 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 5.1.3.1: protocol parameters, which are further described in Section 6.1.3.1:
DAGPreference DAGPreference
InstanceID InstanceID
DAGObjectiveCodePoint DAGObjectiveCodePoint
DAGID DAGID
Destination Prefixes Destination Prefixes
DIOIntervalDoublings DIOIntervalDoublings
DIOIntervalMin DIOIntervalMin
DIORedundancyConstant
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 DODAG. 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 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 10.1.3. 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 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 DIO messages. along the DODAG in DIO messages.
For each DAG, a RPL implementation MUST allow for the monitoring of For each DODAG, a RPL implementation MUST allow for the monitoring of
the following parameters, further described in Section 5.4.4: the following parameters, further described in Section 6.3.4:
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 Sequence Number Increment 10.1.4. 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 DODAG 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. Destination Advertisement Timers 10.1.5. Destination Advertisement Timers
The following set of parameters of the DAO messages SHOULD be The following set of parameters of the DAO messages SHOULD be
configurable: configurable:
o The DelayDAO timer o The DelayDAO timer
o The Remove timer o The Remove timer
7.1.6. Policy Control 10.1.6. 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
Codepoints (OCPs) for a node to join a DAG, and what action should be Codepoints (OCPs) for a node to join a DODAG, and what action should
taken if none of a node's candidate neighbors advertise one of the be 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 DAO messages according to a set of rules defined by Tag" field of the DAO messages according to a set of rules defined by
policy. policy.
7.1.7. Data Structures 10.1.7. 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.
7.2. Information and Data Models 10.2. Information and Data Models
The information and data models necessary for the operation of RPL The information and data models necessary for the operation of RPL
will be defined in a separate document specifying the RPL SNMP MIB. will be defined in a separate document specifying the RPL SNMP MIB.
7.3. Liveness Detection and Monitoring 10.3. Liveness Detection and Monitoring
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 6.2, an implementation is expected to
data structures in support of DAG discovery: maintain a set of 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 DODAG:
* A set of DAG parents * A set of DAG parents
7.3.1. Candidate Neighbor Data Structure 10.3.1. 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.
A RPL implementation MAY provide a counter reporting the number of A RPL implementation MAY provide a counter reporting the number of
times a candidate neighbor has been ignored, should the number of times a candidate neighbor has been ignored, should the number of
candidate neighbors exceeds the maximum authorized value. candidate neighbors exceeds the maximum authorized value.
7.3.2. Directed Acyclic Graph (DAG) Table 10.3.2. Directed Acyclic Graph (DAG) Table
For each DAG, a RPL implementation MUST keep track of the following For each DAG, a RPL implementation is expected to keep track of the
DAG table values: following DODAG 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 upwards along the DODAG
o A set of DAG Parents o A set of DAG Parents
o timer to govern the sending of DIO messages for the DAG o timer to govern the sending of DIO messages for the DODAG
o DAGSequenceNumber o DAGSequenceNumber
The set of DAG parents structure is itself a table with the following The set of DAG parents structure is itself a table with the 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 DAG Parent Object last processed from the DAG Parent
o A flag reporting if the Parent is a DA Parent as described in o A flag reporting if the Parent is a DA Parent as described in
Section 5.10 Section 6.8
7.3.3. Routing Table 10.3.3. Routing Table
For each route provisioned by RPL operation, a RPL implementation For each route provisioned by RPL operation, a RPL implementation
MUST keep track of the following: MUST keep track of the following:
o Destination Prefix o Destination Prefix
o Destination Prefix Length o Destination Prefix Length
o Lifetime Timer o Lifetime Timer
skipping to change at page 65, line 40 skipping to change at page 63, line 23
o Next Hop Interface o Next Hop Interface
o Flag indicating that the route was provisioned from one of: o Flag indicating that the route was provisioned from one of:
* Unicast DAO message * Unicast DAO message
* DIO message * DIO message
* Multicast DAO message * Multicast DAO message
7.3.4. Other RPL Monitoring Parameters 10.3.4. 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 DIO message A RPL implementation MAY log the reception of a malformed DIO message
along with the neighbor identification if avialable. along with the neighbor identification if avialable.
7.3.5. RPL Trickle Timers 10.3.5. 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
o The DIORedundancyConstant
A RPL implementation MAY provide a counter reporting the number of A RPL implementation MAY provide a counter reporting the number of
times an inconsistency (and thus the trickle timer has been reset). times an inconsistency (and thus the trickle timer has been reset).
7.4. Verifying Correct Operation 10.4. Verifying Correct Operation
This section has to be completed in further revision of this document This section has to be completed in further revision of this document
to list potential Operations and Management (OAM) tools that could be to list potential Operations and Management (OAM) tools that could be
used for verifying the correct operation of RPL. used for verifying the correct operation of RPL.
7.5. Requirements on Other Protocols and Functional Components 10.5. Requirements on Other Protocols and Functional Components
RPL does not have any impact on the operation of existing protocols. RPL does not have any impact on the operation of existing protocols.
7.6. Impact on Network Operation 10.6. Impact on Network Operation
To be completed. To be completed.
8. Security Considerations 11. 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 12. IANA Considerations
9.1. RPL Control Message 12.1. RPL Control Message
The RPL Control Message is an ICMP information message type that is The RPL Control Message is an ICMP information message type that is
to be used carry DAG Information Objects, DAG Information to be used carry DAG Information Objects, DAG Information
Solicitations, and Destination Advertisement Objects in support of Solicitations, and Destination Advertisement Objects in support of
RPL operation. RPL operation.
IANA has defined a ICMPv6 Type Number Registry. The suggested type IANA has defined a ICMPv6 Type Number Registry. The suggested type
value for the RPL Control Message is 155, to be confirmed by IANA. value for the RPL Control Message is 155, to be confirmed by IANA.
9.2. New Registry for RPL Control Codes 12.2. New Registry for RPL Control Codes
IANA is requested to create a registry, RPL Control Codes, for the IANA is requested to create a registry, RPL Control Codes, for the
Code field of the ICMPv6 RPL Control Message. Code field of the ICMPv6 RPL Control Message.
New codes may be allocated only by an IETF Consensus action. Each New codes may be allocated only by an IETF Consensus action. Each
code should be tracked with the following qualities: code should be tracked with the following qualities:
o Code o Code
o Description o Description
skipping to change at page 67, line 31 skipping to change at page 65, line 15
+------+----------------------------------+---------------+ +------+----------------------------------+---------------+
| Code | Description | Reference | | Code | Description | Reference |
+------+----------------------------------+---------------+ +------+----------------------------------+---------------+
| 0x01 | DAG Information Solicitation | This document | | 0x01 | DAG Information Solicitation | This document |
| 0x02 | DAG Information Object | This document | | 0x02 | DAG Information Object | This document |
| 0x04 | Destination Advertisement Object | This document | | 0x04 | Destination Advertisement Object | This document |
+------+----------------------------------+---------------+ +------+----------------------------------+---------------+
RPL Control Codes RPL Control Codes
9.3. New Registry for the Control Field of the DIO Base Option 12.3. New Registry for the Control Field of the DIO Base
IANA is requested to create a registry for the Control field of the IANA is requested to create a registry for the Control field of the
DIO Base Option. DIO Base.
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
Four groups are currently defined: Four groups are currently defined:
+-------+-------------------------------------+---------------+ +-------+-------------------------------------+---------------+
| Bit | Description | Reference | | Bit | Description | Reference |
+-------+-------------------------------------+---------------+ +-------+-------------------------------------+---------------+
| 0 | Grounded DAG | This document | | 0 | Grounded DODAG | 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 | | 5,6,7 | DAG Preference | This document |
+-------+-------------------------------------+---------------+ +-------+-------------------------------------+---------------+
DIO Base Option Flags DIO Base Flags
9.4. DAG Information Object (DIO) Suboption
IANA is requested to create a registry for the DIO Base Option 12.4. DAG Information Object (DIO) Suboption
Suboptions
IANA is requested to create a registry for the DIO Base 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 | | 4 | DAG Timer Configuration | This Document |
+-------+------------------------------+---------------+ +-------+------------------------------+---------------+
DAG Information Option (DIO) Base Option Suboptions DAG Information Option (DIO) Base Suboptions
9.5. Objective Code Point for the Default Objective Function OF0
This specification specifies the Default Objective Function (called
OF0) for which the OCP field of the OF object, as defined in
[I-D.ietf-roll-routing-metrics], is equal to 0x0000
+-------+---------+---------------+
| Value | Meaning | Reference |
+-------+---------+---------------+
| 0 | OF0 | This document |
+-------+---------+---------------+
OCP Allocation
10. Acknowledgements 13. Acknowledgements
The authors would like to acknowledge the review, feedback, and The authors would like to acknowledge the review, feedback, and
comments from Emmanuel Baccelli, Dominique Barthel, Yusuf Bashir, comments from Emmanuel Baccelli, Dominique Barthel, Yusuf Bashir,
Mathilde Durvy, Manhar Goindi, Mukul Goyal, Anders Jagd, Quentin Mathilde Durvy, Manhar Goindi, Mukul Goyal, Anders Jagd, Quentin
Lampin, Jerry Martocci, Alexandru Petrescu, and Don Sturek. Lampin, Jerry Martocci, Alexandru Petrescu, and Don Sturek.
The authors would like to acknowledge the guidance and input provided The authors would like to acknowledge the guidance and input provided
by the ROLL Chairs, David Culler and JP Vasseur. by the ROLL Chairs, David Culler and JP Vasseur.
The authors would like to acknowledge prior contributions of Robert The authors would like to acknowledge prior contributions of Robert
Assimiti, Mischa Dohler, Julien Abeille, Ryuji Wakikawa, Teco Boot, Assimiti, Mischa Dohler, Julien Abeille, Ryuji Wakikawa, Teco Boot,
Patrick Wetterwald, Bryan Mclaughlin, Carlos J. Bernardos, Thomas Patrick Wetterwald, Bryan Mclaughlin, Carlos J. Bernardos, Thomas
Watteyne, Zach Shelby, Caroline Bontoux, Marco Molteni, Billy Moon, Watteyne, Zach Shelby, Caroline Bontoux, Marco Molteni, Billy 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 14. Contributors
RPL is the result of the contribution of the following members of the RPL is the result of the contribution of the following members of the
ROLL Design Team, including the editors, and additional contributors ROLL Design Team, including the editors, and additional contributors
as listed below: 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
skipping to change at page 69, line 45 skipping to change at page 67, line 17
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
Ember Corporation
Boston, MA
USA
Phone: +1 617 951 1225
Email: kelsey@ember.com
Philip Levis Philip Levis
Stanford University Stanford University
358 Gates Hall, Stanford University 358 Gates Hall, Stanford University
Stanford, CA 94305-9030 Stanford, CA 94305-9030
USA USA
Email: pal@cs.stanford.edu Email: pal@cs.stanford.edu
Richard Kelsey
Ember Corporation
Boston, MA
USA
Phone: +1 617 951 1225
Email: kelsey@ember.com
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 70, line 41 skipping to change at page 68, line 14
Email: kpister@dustnetworks.com Email: kpister@dustnetworks.com
Anders Brandt Anders Brandt
Zensys, Inc. Zensys, Inc.
Emdrupvej 26 Emdrupvej 26
Copenhagen, DK-2100 Copenhagen, DK-2100
Denmark Denmark
Email: abr@zen-sys.com Email: abr@zen-sys.com
12. References 15. References
12.1. Normative References 15.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
12.2. Informative References [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
15.2. Informative References
[I-D.ietf-bfd-base] [I-D.ietf-bfd-base]
Katz, D. and D. Ward, "Bidirectional Forwarding Katz, D. and D. Ward, "Bidirectional Forwarding
Detection", draft-ietf-bfd-base-09 (work in progress), Detection", draft-ietf-bfd-base-09 (work in progress),
February 2009. February 2009.
[I-D.ietf-manet-nhdp] [I-D.ietf-manet-nhdp]
Clausen, T., Dearlove, C., and J. Dean, "MANET Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc
Neighborhood Discovery Protocol (NHDP)", Network (MANET) Neighborhood Discovery Protocol (NHDP)",
draft-ietf-manet-nhdp-10 (work in progress), July 2009. draft-ietf-manet-nhdp-11 (work in progress), October 2009.
[I-D.ietf-roll-building-routing-reqs] [I-D.ietf-roll-building-routing-reqs]
Martocci, J., Riou, N., Mil, P., and W. Vermeylen, Martocci, J., Riou, N., Mil, P., and W. Vermeylen,
"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-08
(work in progress), September 2009. (work in progress), December 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. and J. Buron, "Home Automation Routing
Routing Requirements in Low Power and Lossy Networks", Requirements in Low Power and Lossy Networks",
draft-ietf-roll-home-routing-reqs-08 (work in progress), draft-ietf-roll-home-routing-reqs-09 (work in progress),
September 2009. November 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-01 (work in progress), draft-ietf-roll-routing-metrics-04 (work in progress),
October 2009. December 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-02 (work in Networks", draft-ietf-roll-terminology-02 (work in
progress), October 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
Sensor Networks", Communications of the ACM, v.51 n.7, Sensor Networks", Communications of the ACM, v.51 n.7,
July 2008, July 2008,
<http://portal.acm.org/citation.cfm?id=1364804>. <http://portal.acm.org/citation.cfm?id=1364804>.
[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
August 1996.
[RFC2453] Malkin, G., "RIP Version 2", STD 56, RFC 2453, [RFC2453] Malkin, G., "RIP Version 2", STD 56, RFC 2453,
November 1998. November 1998.
[RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering,
"IPv6 Flow Label Specification", RFC 3697, March 2004.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., [RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
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.
skipping to change at page 74, line 13 skipping to change at page 71, line 39
path computation algorithm. path computation algorithm.
A.2. Deferred Requirements A.2. Deferred Requirements
NOTE: RPL is still a work in progress. At this time there remain NOTE: RPL is still a work in progress. At this time there remain
several unsatisfied application requirements, but these are to be several unsatisfied application requirements, but these are to be
addressed as RPL is further specified. addressed as RPL is further specified.
Appendix B. Examples Appendix B. Examples
Consider the example LLN physical topology in Figure 11. In this Consider the example LLN physical topology in Figure 13. In this
example the links depicted are all usable L2 links. Suppose that all example the links depicted are all usable L2 links. Suppose that all
links are equally usable, and that the implementation specific policy links are equally usable, and that the implementation specific policy
function is simply to minimize hops. This LLN physical topology then function is simply to minimize hops. This LLN physical topology then
yields the DAG depicted in Figure 12, where the links depicted are yields the DAG depicted in Figure 14, where the links depicted are
the edges toward DAG parents. This topology includes one DAG, rooted the edges toward DAG parents. This topology includes one DAG, rooted
by an LBR node (LBR) at rank 1. The LBR node will issue DIO by an LBR node (LBR) at rank 1. The LBR node will issue DIO
messages, as governed by a trickle timer. Nodes (11), (12), (13), messages, as governed by a trickle timer. Nodes (11), (12), (13),
have selected (LBR) as their only parent, attached to the DAG at rank have selected (LBR) as their only parent, attached to the DAG at rank
2, and periodically multicast DIOs. Node (22) has selected (11) and 2, and periodically multicast DIOs. Node (22) has selected (11) and
(12) in its DAG parent set, and advertises itself at rank 3. Node (12) in its DAG parent set, and advertises itself at rank 3. Node
(22) thus has a set of DAG parents {(11), (12)} and siblings {((21), (22) thus has a set of DAG parents {(11), (12)} and siblings {((21),
(23)}. (23)}.
(LBR) (LBR)
skipping to change at page 75, line 10 skipping to change at page 72, line 34
/ / / | \| \ / / / | \| \
(51)------(52)------(53)------(54)------(55)------(56) (51)------(52)------(53)------(54)------(55)------(56)
Note that the links depicted represent the usable L2 connectivity Note that the links depicted represent the usable L2 connectivity
available in the LLN. For example, Node (31) can communicate available in the LLN. For example, Node (31) can communicate
directly with its neighbors, Nodes (21), (22), (32), and (41). Node directly with its neighbors, Nodes (21), (22), (32), and (41). Node
(31) cannot communicate directly with any other nodes, e.g. (33), (31) cannot communicate directly with any other nodes, e.g. (33),
(23), (42). In this example these links offer bidirectional (23), (42). In this example these links offer bidirectional
communication, and `bad' links are not depicted. communication, and `bad' links are not depicted.
Figure 11: Example LLN Topology Figure 13: Example LLN Topology
(LBR) (LBR)
/ | \ / | \
.---` | `----. .---` | `----.
/ | \ / | \
(11) (12) (13) (11) (12) (13)
| \ | \ | \ | \ | \ | \
| `----. | `----. | `----. | `----. | `----. | `----.
| \| \| \ | \| \| \
(21) (22) (23) (24) (21) (22) (23) (24)
| /| /| | | /| /| |
skipping to change at page 75, line 35 skipping to change at page 73, line 27
| /| \ | \ | \ | /| \ | \ | \
| .----` | `----. | `----. | `----. | .----` | `----. | `----. | `----.
| / | \| \| \ | / | \| \| \
.--------(41) (42) (43) (44) (45) .--------(41) (42) (43) (44) (45)
/ / /| \ | \ / / /| \ | \
.----` .----` .----` | `----. | `----. .----` .----` .----` | `----. | `----.
/ / / | \| \ / / / | \| \
(51) (52) (53) (54) (55) (56) (51) (52) (53) (54) (55) (56)
Note that the links depicted represent directed links in the DAG Note that the links depicted represent directed links in the DAG
overlaid on top of the physical topology depicted in Figure 11. As overlaid on top of the physical topology depicted in Figure 13. As
such, the depicted edges represent the relationship between nodes and such, the depicted edges represent the relationship between nodes and
their DAG parents, wherein all depicted edges are directed and their DAG parents, wherein all depicted edges are directed and
oriented `up' on the page toward the DAG root (LBR). The DAG may oriented `up' on the page toward the DAG root (LBR). The DAG may
provide default routes within the LLN, and serves as the foundation provide default routes within the LLN, and serves as the foundation
on which RPL builds further routing structure, e.g. through the on which RPL builds further routing structure, e.g. through the
destination advertisement mechanism. destination advertisement mechanism.
Figure 12: Example DAG Figure 14: Example DAG
B.1. Destination Advertisement B.1. Destination Advertisement
Consider the example DAG depicted in Figure 12. Suppose that Nodes Consider the example DAG depicted in Figure 14. Suppose that Nodes
(22) and (32) are unable to record routing state. Suppose that Node (22) and (32) are unable to record routing state. Suppose that Node
(42) is able to perform prefix aggregation on behalf of Nodes (53), (42) is able to perform prefix aggregation on behalf of Nodes (53),
(54), and (55). (54), and (55).
o Node (53) would send a DAO message to Node (42), indicating the o Node (53) would send a DAO message to Node (42), indicating the
availability of destination (53). availability of destination (53).
o Node (54) and Node (55) would similarly send DAO messages to Node o Node (54) and Node (55) would similarly send DAO messages to Node
(42) indicating their own destinations. (42) indicating their own destinations.
skipping to change at page 77, line 33 skipping to change at page 75, line 20
o Similarly, Node (N) hears from Node (A) at rank 9, Node (C) at o Similarly, Node (N) hears from Node (A) at rank 9, Node (C) at
rank 5, and Node (E) at rank 4. rank 5, and Node (E) at rank 4.
o Node (D) responds. Node (D) has a DIO message that indicates that o Node (D) responds. Node (D) has a DIO message that indicates that
it is a member of DAGID 1 at rank 2, but it carries the attribute it is a member of DAGID 1 at rank 2, but it carries the attribute
`Blue'. Node (N)'s policy function rejects Node (D), and no `Blue'. Node (N)'s policy function rejects Node (D), and no
further consideration is given. further consideration is given.
o This process continues until Node (N), based on implementation o This process continues until Node (N), based on implementation
specific policy, builds up enough confidence to trigger a decision specific policy, builds enough confidence to trigger a decision to
to join DAGID 1. Let Node (N) determine its most preferred parent join DAGID 1. Let Node (N) determine its most preferred parent to
to be Node (E). be Node (E).
o Node (N) adds Node (E) (rank 4) to its set of DAG parents for o Node (N) adds Node (E) (rank 4) to its set of DAG parents for
DAGID 1. Following the mechanisms specified by the OCP, and given DAGID 1. Following the mechanisms specified by the OCP, and given
that the ETX is 1 for the link between (N) and (E), Node (N) is that the ETX is 1 for the link between (N) and (E), Node (N) is
now at rank 5 in DAGID 1. now at rank 5 in DAGID 1.
o Node (N) adds Node (B) (rank 4) to its set of DAG parents for o Node (N) adds Node (B) (rank 4) to its set of DAG parents for
DAGID 1. DAGID 1.
o Node (N) is a sibling of Node (C), both are at rank 5. o Node (N) is a sibling of Node (C), both are at rank 5.
o Node (N) may now forward traffic intended for the default o Node (N) may now forward traffic intended for the default
destination inward along DAGID 1 via nodes (B) and (E). In some destination upwards along DAGID 1 via nodes (B) and (E). In some
cases, e.g. if nodes (B) and (E) are tried and fail, node (N) may cases, e.g. if nodes (B) and (E) are tried and fail, node (N) may
also choose to forward traffic to its sibling node (C), without also choose to forward traffic to its sibling node (C), without
making inward progress but with the intention that node (C) or a making upwards progress but with the intention that node (C) or a
following successor can make inward progress. Should Node (C) not following successor can make upwards progress. Should Node (C)
have a viable parent, it should never send the packet back to Node not have a viable parent, it should never send the packet back to
(N) (to avoid a 2-node loop). Node (N) (to avoid a 2-node loop).
B.3. Example: DAG Maintenance B.3. Example: DAG Maintenance
: : : : : :
: : : : : :
(A) (A) (A) (A) (A) (A)
|\ | | |\ | |
| `-----. | | | `-----. | |
| \ | | | \ | |
(B) (C) (B) (C) (B) (B) (C) (B) (C) (B)
skipping to change at page 78, line 27 skipping to change at page 76, line 25
| | `-----. | | `-----.
| | \ | | \
(D) (D) (C) (D) (D) (C)
| |
| |
| |
(D) (D)
-1- -2- -3- -1- -2- -3-
Figure 13: DAG Maintenance Figure 15: DAG Maintenance
Consider the example depicted in Figure 13-1. In this example, Node Consider the example depicted in Figure 15-1. In this example, Node
(A) is attached to a DAG at some rank d. Node (A) is a DAG parent of (A) is attached to a DAG at some rank d. Node (A) is a DAG parent of
Nodes (B) and (C). Node (C) is a DAG parent of Node (D). There is Nodes (B) and (C). Node (C) is a DAG parent of Node (D). There is
also an undirected sibling link between Nodes (B) and (C). also an undirected sibling link between Nodes (B) and (C).
In this example, Node (C) may safely forward to Node (A) without In this example, Node (C) may safely forward to Node (A) without
creating a loop. Node (C) may not safely forward to Node (D), creating a loop. Node (C) may not safely forward to Node (D),
contained within it's own sub-DAG, without creating a loop. Node (C) contained within it's own sub-DAG, without creating a loop. Node (C)
may forward to Node (B) in some cases, e.g. the link (C)->(A) is may forward to Node (B) in some cases, e.g. the link (C)->(A) is
temporarily unavailable, but with some chance of creating a loop temporarily unavailable, but with some chance of creating a loop
(e.g. if multiple nodes in a set of siblings start forwarding (e.g. if multiple nodes in a set of siblings start forwarding
skipping to change at page 79, line 11 skipping to change at page 77, line 10
loop, because its rank will decrease. loop, because its rank will decrease.
Now consider the case where the link (C)->(A) becomes nonviable, and Now consider the case where the link (C)->(A) becomes nonviable, and
node (C) must move to a deeper rank within the DAG: node (C) must move to a deeper rank within the DAG:
o Node (C) must first detach from the DAG by removing Node (A) from o Node (C) must first detach from the DAG by removing Node (A) from
its DAG parent set, leaving an empty DAG parent set. Node (C) may its DAG parent set, leaving an empty DAG parent set. Node (C) may
become the root of its own floating, less preferred, DAG. become the root of its own floating, less preferred, DAG.
o Node (D), hearing a modified DIO message from Node (C), follows o Node (D), hearing a modified DIO message from Node (C), follows
Node (C) into the floating DAG. This is depicted in Figure 13-2. Node (C) into the floating DAG. This is depicted in Figure 15-2.
In general, any node with no other options in the sub-DAG of Node In general, any node with no other options in the sub-DAG of Node
(C) will follow Node (C) into the floating DAG, maintaining the (C) will follow Node (C) into the floating DAG, maintaining the
structure of the sub-DAG. structure of the sub-DAG.
o Node (C) hears a DIO message with an incremented DAGSequenceNumber o Node (C) hears a DIO message with an incremented DAGSequenceNumber
from Node (B) and determines it is able to rejoin the grounded DAG from Node (B) and determines it is able to rejoin the grounded DAG
by reattaching at a deeper rank to Node (B). Node (C) adds Node by reattaching at a deeper rank to Node (B). Node (C) adds Node
(B) to its DAG parent set. Node (C) has now safely moved deeper (B) to its DAG parent set. Node (C) has now safely moved deeper
within the grounded DAG without creating any loops. within the grounded DAG without creating any loops.
o Node (D), and any other sub-DAG of Node (C), will hear the o Node (D), and any other sub-DAG of Node (C), will hear the
modified DIO message sourced from Node (C) and follow Node (C) in modified DIO message sourced from Node (C) and follow Node (C) in
a coordinated manner to reattach to the grounded DAG. The final a coordinated manner to reattach to the grounded DAG. The final
DAG is depicted in Figure 13-3 DAG is depicted in Figure 15-3
B.4. Example: Greedy Parent Selection and Instability B.4. Example: Greedy Parent Selection and Instability
(A) (A) (A) (A) (A) (A)
|\ |\ |\ |\ |\ |\
| `-----. | `-----. | `-----. | `-----. | `-----. | `-----.
| \ | \ | \ | \ | \ | \
(B) (C) (B) \ | (C) (B) (C) (B) \ | (C)
\ | | / \ | | /
`-----. | | .-----` `-----. | | .-----`
\| |/ \| |/
(C) (B) (C) (B)
-1- -2- -3- -1- -2- -3-
Figure 14: Greedy DAG Parent Selection Figure 16: Greedy DAG Parent Selection
Consider the example depicted in Figure 14. A DAG is depicted in 3 Consider the example depicted in Figure 16. A DAG is depicted in 3
different configurations. A usable link between (B) and (C) exists different configurations. A usable link between (B) and (C) exists
in all 3 configurations. In Figure 14-1, Node (A) is a DAG parent in all 3 configurations. In Figure 16-1, Node (A) is a DAG parent
for Nodes (B) and (C), and (B)--(C) is a sibling link. In for Nodes (B) and (C), and (B)--(C) is a sibling link. In
Figure 14-2, Node (A) is a DAG parent for Nodes (B) and (C), and Node Figure 16-2, Node (A) is a DAG parent for Nodes (B) and (C), and Node
(B) is also a DAG parent for Node (C). In Figure 14-3, Node (A) is a (B) is also a DAG parent for Node (C). In Figure 16-3, Node (A) is a
DAG parent for Nodes (B) and (C), and Node (C) is also a DAG parent DAG parent for Nodes (B) and (C), and Node (C) is also a DAG parent
for Node (B). for Node (B).
If a RPL node is too greedy, in that it attempts to optimize for an If a RPL node is too greedy, in that it attempts to optimize for an
additional number of parents beyond its preferred parent, then an additional number of parents beyond its preferred parent, then an
instability can result. Consider the DAG illustrated in Figure 14-1. instability can result. Consider the DAG illustrated in Figure 16-1.
In this example, Nodes (B) and (C) may most prefer Node (A) as a DAG In this example, Nodes (B) and (C) may most prefer Node (A) as a DAG
parent, but are operating under the greedy condition that will try to parent, but are operating under the greedy condition that will try to
optimize for 2 parents. optimize for 2 parents.
When the preferred parent selection causes a node to have only one When the preferred parent selection causes a node to have only one
parent and no siblings, the node may decide to insert itself at a parent and no siblings, the node may decide to insert itself at a
slightly higher rank in order to have at least one sibling and thus slightly higher rank in order to have at least one sibling and thus
an alternate forwarding solution. This does not deprive other nodes an alternate forwarding solution. This does not deprive other nodes
of a forwarding solution and this is considered acceptable of a forwarding solution and this is considered acceptable
greediness. greediness.
o Let Figure 14-1 be the initial condition. o Let Figure 16-1 be the initial condition.
o Suppose Node (C) first is able to leave the DAG and rejoin at a o Suppose Node (C) first is able to leave the DAG and rejoin at a
lower rank, taking both Nodes (A) and (B) as DAG parents as lower rank, taking both Nodes (A) and (B) as DAG parents as
depicted in Figure 14-2. Now Node (C) is deeper than both Nodes depicted in Figure 16-2. Now Node (C) is deeper than both Nodes
(A) and (B), and Node (C) is satisfied to have 2 DAG parents. (A) and (B), and Node (C) is satisfied to have 2 DAG parents.
o Suppose Node (B), in its greediness, is willing to receive and o Suppose Node (B), in its greediness, is willing to receive and
process a DIO message from Node (C) (against the rules of RPL), process a DIO message from Node (C) (against the rules of RPL),
and then Node (B) leaves the DAG and rejoins at a lower rank, and then Node (B) leaves the DAG and rejoins at a lower rank,
taking both Nodes (A) and (C) as DAG parents. Now Node (B) is taking both Nodes (A) and (C) as DAG parents. Now Node (B) is
deeper than both Nodes (A) and (C) and is satisfied with 2 DAG deeper than both Nodes (A) and (C) and is satisfied with 2 DAG
parents. parents.
o Then Node (C), because it is also greedy, will leave and rejoin o Then Node (C), because it is also greedy, will leave and rejoin
deeper, to again get 2 parents and have a lower rank then both of deeper, to again get 2 parents and have a lower rank then both of
them. them.
o Next Node (B) will again leave and rejoin deeper, to again get 2 o Next Node (B) will again leave and rejoin deeper, to again get 2
parents parents
o And again Node (C) leaves and rejoins deeper... o And again Node (C) leaves and rejoins deeper...
o The process will repeat, and the DAG will oscillate between o The process will repeat, and the DAG will oscillate between
Figure 14-2 and Figure 14-3 until the nodes count to infinity and Figure 16-2 and Figure 16-3 until the nodes count to infinity and
restart the cycle again. restart the cycle again.
o This cycle can be averted through mechanisms in RPL: o This cycle can be averted through mechanisms in RPL:
* Nodes (B) and (C) stay at a rank sufficient to attach to their * Nodes (B) and (C) stay at a rank sufficient to attach to their
most preferred parent (A) and don't go for any deeper (worse) most preferred parent (A) and don't go for any deeper (worse)
alternate parents (Nodes are not greedy) alternate parents (Nodes are not greedy)
* Nodes (B) and (C) do not process DIO messages from nodes deeper * Nodes (B) and (C) do not process DIO messages from nodes deeper
than themselves (because such nodes are possibly in their own than themselves (because such nodes are possibly in their own
sub-DAGs) sub-DAGs)
Appendix C. Outstanding Issues Appendix C. Outstanding Issues
This section enumerates some outstanding issues that are to be This section enumerates some outstanding issues that are to be
addressed in future revisions of the RPL specification. addressed in future revisions of the RPL specification.
C.1. Additional Support for P2P Routing C.1. Additional Support for P2P Routing
In some situations the baseline mechanism to support arbitrary P2P In some situations the baseline mechanism to support arbitrary P2P
traffic, by flowing inward along the DAG until a common parent is traffic, by flowing upwards along the DAG until a common ancestor is
reached and then flowing outward, may not be suitable for all reached and then flowing down, may not be suitable for all
application scenarios. A related scenario may occur when the outward application scenarios. A related scenario may occur when the down
paths setup along the DAG by the destination advertisement mechanism paths setup along the DAG by the destination advertisement mechanism
are not be the most desirable outward paths for the specific are not be the most desirable downward paths for the specific
application scenario (in part because the DAG links may not be application scenario (in part because the DAG links may not be
symmetric). It may be desired to support within RPL the discovery symmetric). It may be desired to support within RPL the discovery
and installation of more direct routes `across' the DAG. Such and installation of more direct routes `across' the DAG. Such
mechanisms need to be investigated. mechanisms need to be investigated.
C.2. Loop Detection C.2. Loop Detection
It is under investigation to complement the loop avoidance strategies It is under investigation to complement the loop avoidance strategies
provided by RPL with a loop detection mechanism that may be employed provided by RPL with a loop detection mechanism that may be employed
when traffic is forwarded. when traffic is forwarded.
C.3. Destination Advertisement / DAO Fan-out C.3. Destination Advertisement / DAO Fan-out
When DAO messages are relayed to more than one DAG parent, in some When DAO messages are relayed to more than one DAG parent, in some
cases a situation may be created where a large number of DAO messages cases a situation may be created where a large number of DAO messages
conveying information about the same destination flow inward along conveying information about the same destination flow upwards along
the DAG. It is desirable to bound/limit the multiplication/fan-out the DAG. It is desirable to bound/limit the multiplication/fan-out
of DAO messages in this manner. Some aspects of the Destination of DAO messages in this manner. Some aspects of the Destination
Advertisement mechanism remain under investigation, such as behavior Advertisement mechanism remain under investigation, such as behavior
in the face of links that may not be symmetric. in the face of links that may not be symmetric.
In general, the utility of providing redundancy along outwards routes In general, the utility of providing redundancy along downwards
by sending DAO messages to more than one parent is under routes by sending DAO messages to more than one parent is under
investigation. investigation.
The use of suitable triggers, such as the `D' bit, to trigger DA The use of suitable triggers, such as the `D' bit, to trigger DA
operation within an affected sub-DAG, is under investigation. operation within an affected sub-DAG, is under investigation.
Further, the ability to limit scope of the affected depth within the Further, the ability to limit scope of the affected depth within the
sub-DAG is under investigation (e.g. if a stateful node can proxy for sub-DAG is under investigation (e.g. if a stateful node can proxy for
all nodes `behind' it, then there may be no need to propagate the all nodes `behind' it, then there may be no need to propagate the
triggered `D' bit further). triggered `D' bit further).
C.4. Source Routing C.4. Source Routing
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