draft-ietf-roll-rpl-01.txt   draft-ietf-roll-rpl-02.txt 
Networking Working Group T. Winter, Ed. Networking Working Group T. Winter, Ed.
Internet-Draft Internet-Draft
Intended status: Standards Track ROLL Design Team Intended status: Standards Track P. Thubert, Ed.
Expires: March 18, 2010 IETF ROLL WG Expires: March 27, 2010 Cisco Systems
September 14, 2009 ROLL Design Team
IETF ROLL WG
September 23, 2009
RPL: Routing Protocol for Low Power and Lossy Networks RPL: Routing Protocol for Low Power and Lossy Networks
draft-ietf-roll-rpl-01 draft-ietf-roll-rpl-02
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
skipping to change at page 1, line 33 skipping to change at page 1, line 35
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on March 18, 2010. This Internet-Draft will expire on March 27, 2010.
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
Provisions Relating to IETF Documents in effect on the date of Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info). publication of this document (http://trustee.ietf.org/license-info).
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. and restrictions with respect to this document.
Abstract Abstract
This document specifies the Routing Protocol for Low Power and Lossy Low Power and Lossy Networks (LLNs) are made largely of constrained
Networks (RPL), in accordance with the requirements described in nodes (with limited processing power, memory, and sometimes energy
when they are battery operated). These routers are interconnected by
[I-D.ietf-roll-building-routing-reqs], lossy links, most of the time supporting only low data rates, that
[I-D.ietf-roll-home-routing-reqs], are usually fairly unstable with relatively low packet delivery
[I-D.ietf-roll-indus-routing-reqs], and [RFC5548]. rates. Another characteristic of such networks is that the traffic
patterns are not simply unicast, but in many cases point-to-
multipoint or multipoint-to-point. Furthermore such networks may
potentially comprise a large number of nodes, up to several dozens or
hundreds or more nodes in the network. These characteristics offer
unique challenges to a routing solution: the IETF ROLL Working Group
has defined application-specific routing requirements for a Low Power
and Lossy Network (LLN) routing protocol. This document specifies
the Routing Protocol for Low Power and Lossy Networks (RPL).
Requirements Language Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1. Design Principles . . . . . . . . . . . . . . . . . . . . 4 1.1. Design Principles . . . . . . . . . . . . . . . . . . . . 6
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2. Expectations of Link Layer Behavior . . . . . . . . . . . 7
3. Protocol Model . . . . . . . . . . . . . . . . . . . . . . . . 6 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Problem . . . . . . . . . . . . . . . . . . . . . . . . . 7 3. Protocol Model . . . . . . . . . . . . . . . . . . . . . . . . 9
3.2. Protocol Properties Overview . . . . . . . . . . . . . . . 7 3.1. Protocol Properties Overview . . . . . . . . . . . . . . . 9
3.2.1. IPv6 Architecture . . . . . . . . . . . . . . . . . . 7 3.1.1. IPv6 Architecture . . . . . . . . . . . . . . . . . . 9
3.2.2. Path Properties for LLN Traffic Flows . . . . . . . . 7 3.1.2. Typical LLN Traffic Patterns . . . . . . . . . . . . . 10
3.2.3. Constraint Based Routing . . . . . . . . . . . . . . . 8 3.1.3. Constraint Based Routing . . . . . . . . . . . . . . . 10
3.2.4. Autonomous Operation . . . . . . . . . . . . . . . . . 8 3.2. Protocol Operation . . . . . . . . . . . . . . . . . . . . 10
3.3. Protocol Operation . . . . . . . . . . . . . . . . . . . . 8 3.2.1. DAG Construction . . . . . . . . . . . . . . . . . . . 11
3.3.1. DAG Construction . . . . . . . . . . . . . . . . . . . 9 3.2.2. Destination Advertisement . . . . . . . . . . . . . . 21
3.3.2. Source Routing . . . . . . . . . . . . . . . . . . . . 19 3.3. Other Considerations . . . . . . . . . . . . . . . . . . . 23
3.3.3. Destination Advertisement . . . . . . . . . . . . . . 19 3.3.1. DAG Rank and Loop Avoidance . . . . . . . . . . . . . 23
3.4. Other Considerations . . . . . . . . . . . . . . . . . . . 21 3.3.2. DAG Parent Selection, Stability, and Greediness . . . 27
3.4.1. DAG Rank and Loop Avoidance . . . . . . . . . . . . . 21 3.3.3. Merging DAGs . . . . . . . . . . . . . . . . . . . . . 29
3.4.2. DAG Parent Selection, Stability, and Greediness . . . 25 3.4. Local and Temporary Routing Decision . . . . . . . . . . . 32
3.4.3. Merging DAGs . . . . . . . . . . . . . . . . . . . . . 27 3.5. Maintenance of Routing Adjacency . . . . . . . . . . . . . 32
3.4.4. Local and Temporary Routing Decision . . . . . . . . . 29 4. Constraint Based Routing in LLNs . . . . . . . . . . . . . . . 33
3.4.5. Scalability . . . . . . . . . . . . . . . . . . . . . 30 4.1. Routing Metrics . . . . . . . . . . . . . . . . . . . . . 33
3.4.6. Maintenance of Routing Adjacency . . . . . . . . . . . 30 4.2. Routing Constraints . . . . . . . . . . . . . . . . . . . 34
4. Constraint Based Routing in LLNs . . . . . . . . . . . . . . . 30 4.3. Constraint Based Routing . . . . . . . . . . . . . . . . . 34
4.1. Routing Metrics . . . . . . . . . . . . . . . . . . . . . 30 5. RPL Protocol Specification . . . . . . . . . . . . . . . . . . 35
4.2. Routing Constraints . . . . . . . . . . . . . . . . . . . 32 5.1. DAG Information Option . . . . . . . . . . . . . . . . . . 35
4.3. Constraint Based Routing . . . . . . . . . . . . . . . . . 32 5.1.1. DAG Information Option (DIO) base option . . . . . . . 35
5. Specification of Core Protocol . . . . . . . . . . . . . . . . 32 5.2. Conceptual Data Structures . . . . . . . . . . . . . . . . 42
5.1. DAG Information Option . . . . . . . . . . . . . . . . . . 33 5.2.1. Candidate Neighbors Data Structure . . . . . . . . . . 42
5.1.1. DIO base option . . . . . . . . . . . . . . . . . . . 33 5.2.2. Directed Acyclic Graphs (DAGs) Data Structure . . . . 43
5.2. Conceptual Data Structures . . . . . . . . . . . . . . . . 39 5.3. DAG Discovery and Maintenance . . . . . . . . . . . . . . 44
5.2.1. Candidate Neighbors . . . . . . . . . . . . . . . . . 39 5.3.1. DAG Discovery Rules . . . . . . . . . . . . . . . . . 45
5.2.2. DAGs . . . . . . . . . . . . . . . . . . . . . . . . . 40 5.3.2. Reception and Processing of RA-DIO messages . . . . . 47
5.3. Initialization and Configuration . . . . . . . . . . . . . 41 5.3.3. RA-DIO Transmission . . . . . . . . . . . . . . . . . 49
5.4. DAG Discovery . . . . . . . . . . . . . . . . . . . . . . 42 5.3.4. Trickle Timer for RA Transmission . . . . . . . . . . 50
5.4.1. RA-DIO Reception . . . . . . . . . . . . . . . . . . . 45 5.4. DAG Heartbeat . . . . . . . . . . . . . . . . . . . . . . 52
5.4.2. RA-DIO Transmission . . . . . . . . . . . . . . . . . 47 5.5. DAG Selection . . . . . . . . . . . . . . . . . . . . . . 52
5.4.3. Trickle Timer for RA Transmission . . . . . . . . . . 48 5.6. Administrative rank . . . . . . . . . . . . . . . . . . . 53
5.5. DAG Heartbeat . . . . . . . . . . . . . . . . . . . . . . 49 5.7. Candidate DAG Parent States and Stability . . . . . . . . 53
5.6. DAG Selection . . . . . . . . . . . . . . . . . . . . . . 50 5.7.1. Held-Up . . . . . . . . . . . . . . . . . . . . . . . 53
5.7. Administrative rank . . . . . . . . . . . . . . . . . . . 50 5.7.2. Held-Down . . . . . . . . . . . . . . . . . . . . . . 54
5.8. Candidate DAG Parent States and Stability . . . . . . . . 51 5.7.3. Collision . . . . . . . . . . . . . . . . . . . . . . 54
5.8.1. Held-Up . . . . . . . . . . . . . . . . . . . . . . . 51 5.7.4. Instability . . . . . . . . . . . . . . . . . . . . . 55
5.8.2. Held-Down . . . . . . . . . . . . . . . . . . . . . . 52 5.8. Guidelines for Objective Code Points . . . . . . . . . . . 56
5.8.3. Collision . . . . . . . . . . . . . . . . . . . . . . 52 5.8.1. Objective Function . . . . . . . . . . . . . . . . . . 56
5.8.4. Instability . . . . . . . . . . . . . . . . . . . . . 53 5.8.2. Objective Code Point 0 (OCP 0) . . . . . . . . . . . . 58
5.9. Guidelines for Objective Code Points . . . . . . . . . . . 53 5.9. Establishing Routing State Outward Along the DAG . . . . . 60
5.9.1. Objective Function . . . . . . . . . . . . . . . . . . 53 5.9.1. Destination Advertisement Message Formats . . . . . . 61
5.9.2. Objective Code Point 0 (OCP 0) . . . . . . . . . . . . 55 5.9.2. Destination Advertisement Operation . . . . . . . . . 63
5.10. Establishing Routing State Outward Along the DAG . . . . . 57 5.10. Multicast Operation . . . . . . . . . . . . . . . . . . . 70
5.10.1. Destination Advertisement Message Formats . . . . . . 58 5.11. Maintenance of Routing Adjacency . . . . . . . . . . . . . 71
5.10.2. Destination Advertisement Operation . . . . . . . . . 60 5.12. Packet Forwarding . . . . . . . . . . . . . . . . . . . . 72
5.11. Maintenance of Routing Adjacency . . . . . . . . . . . . . 67 5.12.1. Loop Taxonomy . . . . . . . . . . . . . . . . . . . . 73
5.12. Packet Forwarding . . . . . . . . . . . . . . . . . . . . 67 6. RPL Variables . . . . . . . . . . . . . . . . . . . . . . . . 74
5.12.1. Loop Taxonomy . . . . . . . . . . . . . . . . . . . . 68 7. Manageability Considerations . . . . . . . . . . . . . . . . . 75
5.13. Expectations of Link Layer Behavior . . . . . . . . . . . 70 7.1. Control of Function and Policy . . . . . . . . . . . . . . 75
6. Summary of RPL Timers . . . . . . . . . . . . . . . . . . . . 70 7.1.1. Initialization Mode . . . . . . . . . . . . . . . . . 75
7. Protocol Extensions . . . . . . . . . . . . . . . . . . . . . 71 7.1.2. DIO Base option . . . . . . . . . . . . . . . . . . . 76
8. Manageability Considerations . . . . . . . . . . . . . . . . . 71 7.1.3. Trickle Timers . . . . . . . . . . . . . . . . . . . . 77
9. Security Considerations . . . . . . . . . . . . . . . . . . . 71 7.1.4. DAG Heartbeat . . . . . . . . . . . . . . . . . . . . 77
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 72 7.1.5. The Destination Advertisement Option . . . . . . . . . 78
10.1. DAG Information Option . . . . . . . . . . . . . . . . . . 72 7.1.6. Policy Control . . . . . . . . . . . . . . . . . . . . 78
10.2. Objective Code Point . . . . . . . . . . . . . . . . . . . 72 7.1.7. Data Structures . . . . . . . . . . . . . . . . . . . 78
10.3. Destination Advertisement Option . . . . . . . . . . . . . 72 7.2. Information and Data Models . . . . . . . . . . . . . . . 78
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 72 7.3. Liveness Detection and Monitoring . . . . . . . . . . . . 79
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 72 7.3.1. Candidate Neighbor Data Structure . . . . . . . . . . 79
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 74 7.3.2. Directed Acyclic Graph (DAG) Table . . . . . . . . . . 79
13.1. Normative References . . . . . . . . . . . . . . . . . . . 74 7.3.3. Routing Table . . . . . . . . . . . . . . . . . . . . 80
13.2. Informative References . . . . . . . . . . . . . . . . . . 74 7.3.4. Other RPL Monitoring Parameters . . . . . . . . . . . 80
Appendix A. Deferred Requirements . . . . . . . . . . . . . . . . 76 7.3.5. RPL Trickle Timers . . . . . . . . . . . . . . . . . . 80
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 76 7.4. Verifying Correct Operation . . . . . . . . . . . . . . . 80
B.1. Moving Down a DAG . . . . . . . . . . . . . . . . . . . . 78 7.5. Requirements on Other Protocols and Functional
B.2. Link Removed . . . . . . . . . . . . . . . . . . . . . . . 79 Components . . . . . . . . . . . . . . . . . . . . . . . . 81
B.3. Link Added . . . . . . . . . . . . . . . . . . . . . . . . 79 7.6. Impact on Network Operation . . . . . . . . . . . . . . . 81
B.4. Node Removed . . . . . . . . . . . . . . . . . . . . . . . 80 8. Security Considerations . . . . . . . . . . . . . . . . . . . 81
B.5. New LBR Added . . . . . . . . . . . . . . . . . . . . . . 80 9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 81
B.6. Destination Advertisement . . . . . . . . . . . . . . . . 81 9.1. DAG Information Option (DIO) Base Option . . . . . . . . . 81
Appendix C. Additional Examples . . . . . . . . . . . . . . . . . 82 9.2. New Registry for the Flag Field of the DIO Base Option . . 81
Appendix D. Outstanding Issues . . . . . . . . . . . . . . . . . 86 9.3. DAG Information Option (DIO) Suboption . . . . . . . . . . 82
D.1. Additional Support for P2P Routing . . . . . . . . . . . . 86 9.4. Destination Advertisement Option (DAO) Option . . . . . . 82
D.2. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 86 9.5. Objective Code Point . . . . . . . . . . . . . . . . . . . 82
D.3. DAO Fan-out . . . . . . . . . . . . . . . . . . . . . . . 86 10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 83
D.4. Source Routing . . . . . . . . . . . . . . . . . . . . . . 86 11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 83
D.5. Address / Header Compression . . . . . . . . . . . . . . . 86 12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 87 12.1. Normative References . . . . . . . . . . . . . . . . . . . 84
12.2. Informative References . . . . . . . . . . . . . . . . . . 84
Appendix A. Deferred Requirements . . . . . . . . . . . . . . . . 86
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 87
B.1. Moving Down a DAG . . . . . . . . . . . . . . . . . . . . 88
B.2. Link Removed . . . . . . . . . . . . . . . . . . . . . . . 89
B.3. Link Added . . . . . . . . . . . . . . . . . . . . . . . . 89
B.4. Node Removed . . . . . . . . . . . . . . . . . . . . . . . 90
B.5. New LBR Added . . . . . . . . . . . . . . . . . . . . . . 90
B.6. Destination Advertisement . . . . . . . . . . . . . . . . 91
Appendix C. Additional Examples . . . . . . . . . . . . . . . . . 92
Appendix D. Outstanding Issues . . . . . . . . . . . . . . . . . 96
D.1. Additional Support for P2P Routing . . . . . . . . . . . . 96
D.2. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 96
D.3. Destination Advertisement / DAO Fan-out . . . . . . . . . 96
D.4. Source Routing . . . . . . . . . . . . . . . . . . . . . . 96
D.5. Address / Header Compression . . . . . . . . . . . . . . . 97
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 97
1. Introduction 1. Introduction
The defining characteristics of Low Power and Lossy Networks (LLNs) Low Power and Lossy Networks (LLNs) are made largely of constrained
offer unique challenges to a routing solution. The IETF ROLL Working nodes (with limited processing power, memory, and sometimes energy
Group has defined application-specific routing requirements for a Low when they are battery operated). These routers are interconnected by
Power and Lossy Network (LLN) routing protocol lossy links, most of the time supporting only low data rates, that
[I-D.ietf-roll-building-routing-reqs] are usually fairly unstable with relatively low packet delivery
[I-D.ietf-roll-home-routing-reqs] [I-D.ietf-roll-indus-routing-reqs] rates. Another characteristic of such networks is that the traffic
[RFC5548]. RPL is a new routing protocol designed to meet these patterns are not simply unicast, but in many cases point-to-
requirements. multipoint or multipoint-to-point. Furthermore such networks may
potentially comprise a large number of nodes, up to several dozens or
hundreds or more nodes in the network. These characteristics offer
unique challenges to a routing solution: the IETF ROLL Working Group
has defined application-specific routing requirements for a Low Power
and Lossy Network (LLN) routing protocol, specified in
[I-D.ietf-roll-building-routing-reqs],
[I-D.ietf-roll-home-routing-reqs],
[I-D.ietf-roll-indus-routing-reqs], and [RFC5548]. This document
specifies the Routing Protocol for Low Power and Lossy 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], [I-D.ietf-roll-home-routing-reqs],
[I-D.ietf-roll-indus-routing-reqs], and [RFC5548]. Because those [I-D.ietf-roll-indus-routing-reqs], and [RFC5548]. Because those
requirements are heterogeneous and sometimes incompatible in nature, requirements are heterogeneous and sometimes incompatible in nature,
the approach is first taken to design a protocol capable of the approach is first taken to design a protocol capable of
supporting a core set of functionalities corresponding to the supporting a core set of functionalities corresponding to the
intersection of the requirements. (Note: it is intended that as this intersection of the requirements. (Note: it is intended that as this
design evolves optional features may be added to address some design evolves optional features may be added to address some
application specific requirements). All "MUST" application application specific requirements). This is a key protocol design
requirements that cannot be satisfied by RPL will be specifically decision providing a granular approach in order to restrict the core
listed in the Appendix A, accompanied by a justification. of the protocol to a minimal set of functionalities, and to allow
each instantiation of the protocol to be optimized in terms of
required code space. It must be noted that RPL is not restricted to
the aforementioned applications and is expected to be used in other
environments. All "MUST" application requirements that cannot be
satisfied by RPL will be specifically listed in the Appendix A,
accompanied by a justification.
The core set of functionalities is to be capable of operating in the The core set of functionalities is to be capable of operating in the
most severely constrained environments, with minimal requirements for most severely constrained environments, with minimal requirements for
memory, energy, processing, communication, and other consumption of memory, energy, processing, communication, and other consumption of
limited resources from nodes. Trade-offs inherent in the limited resources from nodes. Trade-offs inherent in the
provisioning of protocol features will be exposed to the implementer provisioning of protocol features will be exposed to the implementer
in the form of configurable parameters, such that the implementer can in the form of configurable parameters, such that the implementer can
further tweak and optimize the operation of RPL as appropriate to a further tweak and optimize the operation of RPL as appropriate to a
specific application and implementation. Finally, RPL is designed to specific application and implementation. Finally, RPL is designed to
consult implementation specific policies to determine, for example, consult implementation specific policies to determine, for example,
the evaluation of routing metrics. the evaluation of routing metrics.
A set of companion documents to this specification will provide A set of companion documents to this specification will provide
further guidance in the form of applicability statements specifying a further guidance in the form of applicability statements specifying a
set of operating points appropriate to the Building Automation, Home set of operating points appropriate to the Building Automation, Home
Automation, Industrial, and Urban application scenarios. Automation, Industrial, and Urban application scenarios.
1.2. Expectations of Link Layer Behavior
This specification does not rely on any particular features of a
specific link layer technologies. It is anticipated that an
implementer should be able to operate RPL over a variety of different
low power wireless or PLC (Power Line Communication) link layer
technologies.
Implementers may find RFC 3819 [RFC3819] a useful reference when
designing a link layer interface between RPL and a particular link
layer technology.
2. Terminology 2. Terminology
The terminology used in this document is consistent with and The terminology used in this document is consistent with and
incorporates that described in `Terminology in Low power And Lossy incorporates that described in `Terminology in Low power And Lossy
Networks' [I-D.ietf-roll-terminology]. The terminology is extended Networks' [I-D.ietf-roll-terminology]. The terminology is extended
in this document as follows: in this document as follows:
Autonomous: Refers to the ability of a routing protocol to Autonomous: The ability of a routing protocol to independently
independently function without requiring any external influence function without relying on any external influence or guidance.
or guidance. Includes self-organization capabilities. Includes self-organization capabilities.
DAG: Directed Acyclic Graph- A directed graph having the property DAG: Directed Acyclic Graph. A directed graph having the property
that all edges are oriented in such a way that no cycles exist. that all edges are oriented in such a way that no cycles exist.
In the RPL context, all edges are contained in paths oriented In the RPL context, all edges are contained in paths oriented
toward and terminating at a root node (a DAG root, or sink- toward and terminating at a root node (a DAG root, or sink-
typically a LBR). typically a Low Power and Lossy Network Border Router (LBR)).
DAGID: DAG Identifier- A globally unique identifier for a DAG. All DAGID: DAG Identifier. A globally unique identifier for a DAG. All
nodes who are members of a DAG have knowledge of the DAGID. nodes who are part of a given DAG have knowledge of the DAGID.
This knowledge is used to identify peer nodes within the DAG in This knowledge is used to identify peer nodes within the DAG in
order to coordinate DAG Maintenance while avoiding loops. order to coordinate DAG maintenance while avoiding loops.
DAG Parent: A parent of a node within a DAG is one of the immediate DAG parent: A parent of a node within a DAG is one of the immediate
successors of the node on a path towards the DAG root. For successors of the node on a path towards the DAG root. For
each DAGID that a node is a member of, the node will maintain a each DAGID that a node is a member of, the node will maintain a
set containing one or more DAG Parents. If a node is a member set containing one or more DAG parents. If a node is a member
of multiple DAGs then it must conceptually maintain a set of of multiple DAGs then it must conceptually maintain a set of
DAG Parents for each DAGID. DAG parents for each DAGID.
DAG Sibling: A sibling of a node within a DAG is defined in this DAG sibling: A sibling of a node within a DAG is defined in this
specification to be any neighboring node which is located at specification to be any neighboring node which is located at
the same rank (depth) within a DAG. Note that siblings defined the same rank (depth) within a DAG. Note that siblings defined
in this manner do not necessarily share a common parent. For in this manner do not necessarily share a common parent. For
each DAG that a node is a member of, the node will maintain a each DAG that a node is a member of, the node will maintain a
set of DAG Siblings. If a node is a member of multiple DAGs set of DAG siblings. If a node is a member of multiple DAGs
then it must conceptually maintain a set of DAG Siblings for then it must conceptually maintain a set of DAG siblings for
each DAG. each DAG.
DAG Root: A DAG root is a sink within the DAG graph. All paths in DAG root: A DAG root is a sink within the DAG. All paths in the DAG
the DAG terminate at a DAG root, and all DAG edges contained in terminate at a DAG root, and all DAG edges contained in the
the paths terminating at a DAG root are oriented toward the DAG paths terminating at a DAG root are oriented toward the DAG
root. There must be at least one DAG Root per DAG, and in some root. There must be at least one DAG root per DAG, and in some
cases there may be more than one. In many use cases, source- cases there may be more than one. In many use cases, source-
sink represents a dominant traffic flow, where the sink is a sink represents a dominant traffic flow, where the sink is a
DAG root. Maintaining default routing towards DAG roots is DAG root or is located behind the DAG root. Maintaining routes
therefore a prominent functionality for RPL. towards DAG roots is therefore a prominent functionality for
RPL.
Grounded: A DAG is grounded if it contains a DAG Root offering a Grounded: A DAG is grounded if it contains a DAG root offering
default route to an external routed infrastructure such as the connectivity to an external routed infrastructure such as the
Internet. public Internet or a private core (non-LLN) IP network.
Floating: A DAG is floating if is not Grounded. A floating DAG may Floating: A DAG is floating if is not grounded. A floating DAG is
install a default route, although it is not expected to reach not expected to reach any additional external routed
any additional external routed infrastructure such as the infrastructure such as the public Internet or a private core
Internet. (non-LLN) IP network.
Inward: In the context of RPL, inward refers to the direction from Inward: Inward refers to the direction from leaf nodes towards DAG
leaf nodes towards DAG roots, following the orientation of the roots, following the orientation of the edges within the DAG.
edges within the DAG.
Outward: In the context of RPL, outward refers to the direction from Outward: Outward refers to the direction from DAG roots towards leaf
DAG roots towards leaf nodes, going against the orientation of nodes, going against the orientation of the edges within the
the edges within the DAG. DAG.
P2P: Point-to-point. This refers to traffic exchanged between two P2P: Point-to-point. This refers to traffic exchanged between two
nodes. nodes.
P2MP: Point-to-Multipoint. This refers to traffic between one node P2MP: Point-to-Multipoint. This refers to traffic between one node
and a set of nodes. This is similar to the P2MP concept in and a set of nodes. This is similar to the P2MP concept in
Multicast or MPLS Traffic Engineering ([RFC4461] and Multicast or MPLS Traffic Engineering ([RFC4461] and
[RFC4875]). A common RPL use case involves P2MP flows from or [RFC4875]). A common RPL use case involves P2MP flows from or
through a DAG Root outward towards other nodes contained in the through a DAG root outward towards other nodes contained in the
DAG. DAG.
MP2P: Multipoint-to-Point; used to describe a particular traffic MP2P: Multipoint-to-Point; used to describe a particular traffic
pattern. A common RPL use case involves MP2P flows collecting pattern. A common RPL use case involves MP2P flows collecting
information from many nodes in the DAG, flowing inwards towards information from many nodes in the DAG, flowing inwards towards
DAG roots. Note that a DAG root may not be the ultimate DAG roots. Note that a DAG root may not be the ultimate
destination of the information, but it is a common transit destination of the information, but it is a common transit
node. node.
OCP: Objective Code Point. In RPL, the Objective Code Point (OCP) OCP: Objective Code Point. In RPL, the Objective Code Point (OCP)
skipping to change at page 6, line 47 skipping to change at page 9, line 31
related functions are in use in a DAG. Instances of the related functions are in use in a DAG. Instances of the
Objective Code Point are further described in Objective Code Point are further described in
[I-D.ietf-roll-routing-metrics]. [I-D.ietf-roll-routing-metrics].
Note that in this document, the terms `node' and `LLN router' are Note that in this document, the terms `node' and `LLN router' are
used interchangeably. used interchangeably.
3. Protocol Model 3. Protocol Model
The aim of this section is to describe RPL in the spirit of The aim of this section is to describe RPL in the spirit of
[RFC4101]. An architectural protocol overview (the big picture of [RFC4101]. Protocol details can be found in further sections.
the protocol) is provided in this section. Protocol details can be
found in further sections.
3.1. Problem
Some well-defined LLN application-specific scenarios are Building
Automation, Home Automation, Industrial, and Urban; for which the
unique routing requirements have been detailed respectively in
[I-D.ietf-roll-building-routing-reqs],
[I-D.ietf-roll-home-routing-reqs],
[I-D.ietf-roll-indus-routing-reqs], and [RFC5548]. Within these
application-specific scenarios there are some common elements
required of routing. RPL intends to address the requirements of
these application-specific scenarios, and it is further intended to
be flexible enough to extend to other application scenarios.
3.2. Protocol Properties Overview 3.1. Protocol Properties Overview
RPL demonstrates the following properties, consistent with the RPL demonstrates the following properties, consistent with the
requirements specified by the requirements documents. requirements specified by the application-specific requirements
documents.
3.2.1. IPv6 Architecture 3.1.1. IPv6 Architecture
RPL is strictly compliant with layered IPv6 architecture. RPL is strictly compliant with layered IPv6 architecture.
Further, RPL is designed with consideration to the practical support Further, RPL is designed with consideration to the practical support
and implementation of IPv6 architecture on devices which may operate and implementation of IPv6 architecture on devices which may operate
under severe resource constraints, including but not limited to under severe resource constraints, including but not limited to
memory, processing power, energy, and communication. The RPL design memory, processing power, energy, and communication. The RPL design
does not presume high quality reliable links, and should be able to does not presume high quality reliable links, and operates over lossy
operate over lossy links (usually low bandwidth with low packet links (usually low bandwidth with low packet delivery success rate).
delivery success rate).
3.2.2. Path Properties for LLN Traffic Flows 3.1.2. Typical LLN Traffic Patterns
Multipoint-to-point (MP2P) and Point-to-multipoint (P2MP) traffic Multipoint-to-point (MP2P) and Point-to-multipoint (P2MP) traffic
flows from nodes within the LLN from and to egress points are very flows from nodes within the LLN from and to egress points are very
common in LLNs. Low power and lossy network Border Router (LBR) common in LLNs. Low power and lossy network Border Router (LBR)
nodes may typically be at the root of such flows, although such flows nodes may typically be at the root of such flows, although such flows
are not exclusively rooted at LBRs as determined on an application- are not exclusively rooted at LBRs as determined on an application-
specific basis. In particular, several applications such as building specific basis. In particular, several applications such as building
or home automation do require P2P (Point-to-Point) communication. or home automation do require P2P (Point-to-Point) communication.
As required by the aforementioned routing requirements documents, RPL As required by the aforementioned routing requirements documents, RPL
supports the installation of multiple paths. The use of multiple supports the installation of multiple paths. The use of multiple
paths include sending duplicated traffic along diverse paths, as well paths include sending duplicated traffic along diverse paths, as well
as to support advanced features such as Class of Service (CoS) based as to support advanced features such as Class of Service (CoS) based
routing, or simple load balancing among a set of paths (which could routing, or simple load balancing among a set of paths (which could
be useful for the LLN to spread traffic load and avoid fast energy be useful for the LLN to spread traffic load and avoid fast energy
depletion on some nodes). depletion on some, e.g. battery powered, nodes).
3.2.3. Constraint Based Routing 3.1.3. Constraint Based Routing
The RPL design supports constraint based routing, based on a set of The RPL design supports constraint based routing, based on a set of
routing metrics. The routing metrics supported by RPL are specified routing metrics. The routing metrics for links and nodes with
in a companion document to this specification, capabilities supported by RPL are specified in a companion document
[I-D.ietf-roll-routing-metrics]. RPL signals the metrics and related to this specification, [I-D.ietf-roll-routing-metrics]. RPL signals
objective functions in use in a particular implementation by means of the metrics and related objective functions in use in a particular
an Objective Code Point (OCP). Both the routing metrics and the OCP implementation by means of an Objective Code Point (OCP). Both the
help determine the construction of the Directed Acyclic Graphs (DAG) routing metrics and the OCP help determine the construction of the
using a distributed path computation algorithm. Directed Acyclic Graphs (DAG) using a distributed path computation
algorithm.
RPL supports the computation and installation of different paths in RPL supports the computation and installation of different paths in
support of and optimized for a set of application and implementation support of and optimized for a set of application and implementation
specific constraints, as guided by an OCP. Traffic may subsequently specific constraints, as guided by an OCP. Traffic may subsequently
be directed along the appropriate constrained path based on traffic be directed along the appropriate constrained path based on traffic
marking within the IPv6 header. For more details on the approach marking within the IPv6 header. For more details on the approach
towards constraint-based routing, see Section 4. towards constraint-based routing, see Section 4.
3.2.4. Autonomous Operation 3.2. Protocol Operation
Nodes running RPL are able to independently and autonomously discover
a network topology and compute and install routes, without requiring
further administrative interaction.
3.3. Protocol Operation
LLN nodes running RPL will construct Directed Acyclic Graphs (DAGs) LLN nodes running RPL will construct Directed Acyclic Graphs (DAGs)
rooted at designated nodes that generally have some application rooted at designated nodes that generally have some application
significance, such as providing a default route to an external routed significance, such as providing connectivity to an external routed
infrastructure. The DAG is sufficient to support inward MP2P traffic infrastructure. The term "external" is used top refer to the public
flows, flowing inward along the LLN towards a sink (DAG Root), which Internet or a core private (non-LLN) IP network. The DAG is
is one of the dominant traffic flows described in the requirements sufficient to support inward MP2P traffic flows, flowing inward along
documents ([I-D.ietf-roll-building-routing-reqs], the LLN towards a sink (DAG root), which is one of the dominant
traffic flows described in the requirements documents
([I-D.ietf-roll-building-routing-reqs],
[I-D.ietf-roll-home-routing-reqs], [I-D.ietf-roll-home-routing-reqs],
[I-D.ietf-roll-indus-routing-reqs], and [RFC5548]). [I-D.ietf-roll-indus-routing-reqs], and [RFC5548]).
By utilizing a DAG for dominant MP2P flows, RPL allows each node to By utilizing a DAG for dominant MP2P flows, RPL allows each node to
select and maintain potentially multiple successors capable of select and maintain potentially multiple successors capable of
forwarding traffic inwards towards the root. The DAG does not forwarding traffic inwards towards the root. The DAG does not
present as many single points of failure as a tree, and in addition present as many single points of failure as a tree, and in addition
can offer a node a set of pre-computed successors in support of, e.g. can offer a node a set of pre-computed successors in support of, e.g.
local route repair in case of a temporary failure, load balancing, or local route repair in case of a temporary failure, load balancing, or
short term fluctuations in link characteristics. short term fluctuations in link characteristics.
skipping to change at page 9, line 4 skipping to change at page 11, line 20
forwarding traffic inwards towards the root. The DAG does not forwarding traffic inwards towards the root. The DAG does not
present as many single points of failure as a tree, and in addition present as many single points of failure as a tree, and in addition
can offer a node a set of pre-computed successors in support of, e.g. can offer a node a set of pre-computed successors in support of, e.g.
local route repair in case of a temporary failure, load balancing, or local route repair in case of a temporary failure, load balancing, or
short term fluctuations in link characteristics. short term fluctuations in link characteristics.
A DAG also serves to restrict the routing problem on the nodes when A DAG also serves to restrict the routing problem on the nodes when
it is used as a reference topology. This allows nodes to determine it is used as a reference topology. This allows nodes to determine
their positions in a DAG relative to each other and provides a means their positions in a DAG relative to each other and provides a means
to coordinate route repair in a way that endeavors to avoid loops. to coordinate route repair in a way that endeavors to avoid loops.
These mechanisms will be described in more detail later in this These mechanisms will be described in more detail later in this
specification. specification.
As DAGs are organized, RPL will use a Destination Advertisement As DAGs are organized, RPL will use a destination advertisement
mechanism to build up routing state in support of outward P2MP mechanism to build up routing tables in support of outward P2MP
traffic flows. This mechanism, using the DAG as a reference, traffic flows. This mechanism, using the DAG as a reference,
`paints' the underlying LLN graph, guided along the DAG, such that distributes routing information across intermediate nodes (between
the routes toward destination prefixes in the outward direction may the DAG leaves and the root), guided along the DAG, such that the
be set up. As the DAG undergoes modification during DAG maintenance, routes toward destination prefixes in the outward direction may be
the Destination Advertisement mechanism can be triggered to update set up. As the DAG undergoes modification during DAG maintenance,
the destination advertisement mechanism can be triggered to update
the outward routing state. the outward routing state.
Arbitrary P2P traffic MAY flow inward along the DAG until a common Arbitrary P2P traffic may flow inward along the DAG until a common
parent is reached who has stored routing state and is capable of parent is reached who has stored an entry for the destination in its
directing the traffic outward along the correct outward path. In the routing table and is capable of directing the traffic outward along
present specification RPL does not specify nor preclude any the correct outward path. In the present specification RPL does not
additional mechanisms that may be capable to compute and install more specify nor preclude any additional mechanisms that may be capable to
optimal routes into LLN nodes in support of arbitrary P2P traffic. compute and install more optimal routes into LLN nodes in support of
(Note that in some application scenarios it may be important to arbitrary P2P traffic according to some routing metric.
support arbitrary P2P traffic along more optimal paths `across' the
DAG). This functionality is to be investigated further in a future
revision.
This section further describes the high level operation of RPL.
3.3.1. DAG Construction 3.2.1. DAG Construction
3.3.1.1. Overview of a Typical Case 3.2.1.1. Overview of a Typical Case
RPL constructs one or more base routing topologies, in the form of RPL constructs one or more DAGs, over gradients defined by optimizing
DAGs, over gradients defined by optimizing cost metrics along paths cost metrics along paths rooted at designated nodes.
rooted at designated nodes.
DAGs may be grounded, in which case the DAG Root (e.g. an LBR) is DAGs may be grounded, in which case the DAG root (e.g. an LBR) is
offering a default route to an external routed infrastructure such as offering connectivity to an external routed infrastructure such as
the Internet. A typical goal for a node participating in DAG the public Internet or a private core (non-LLN) IP network. A
Construction may be to find and join a grounded DAG. Any DAG which typical goal for a node participating in DAG construction may be to
is not grounded is floating, and default routes may still be find and join a grounded DAG. Any DAG which is not grounded is
provisioned toward the DAG root although with no expectations of floating, and routes may still be provisioned toward the DAG root
reaching an external infrastructure. although with no expectations of reaching an external infrastructure.
In the context of a particular LLN application one or more nodes will In the context of a particular LLN application one or more nodes will
be capable of, e.g. serving as an LBR or acting as a data collection be capable of, e.g. serving as an LBR or acting as a data collection
point, and thus be provisioned to act as the most preferred DAG point, and thus be provisioned to act as the most preferred DAG
roots. These nodes will begin the process of constructing a DAG by roots. These nodes will initiate and continue the process of
occasionally emitting Router Advertisements containing the necessary constructing a DAG by occasionally emitting IPv6 Router Advertisement
information for neighboring nodes to evaluate the DAG Root as a (RA) messages containing the necessary information for neighboring
potential DAG parent. This information will include a DAGID, a nodes to evaluate the DAG root as a potential DAG parent. This
DAGPreference, and an Objective Code Point (OCP). The DAGID is an information will include at least a DAGID, an administrative
identifier unique to the DAG. The DAGPreference offers a way to preference, and an Objective Code Point (OCP). The DAGID is an
engineer the formation of the DAG in support of the application, by identifier unique to the DAG. The administrative preference offers a
providing a mechanism by which the DAG may look attractive for other way to engineer the formation of the DAG in support of the
nodes to join. The OCP provides information as to which metrics and application, by providing a mechanism by which the DAG may look more
optimization goals are being employed across the DAG. Note that a or less attractive for other nodes to join. The OCP provides
single DAG Root may conceptually root different DAGs with different information as to which metrics and optimization goals are being
OCPs as required to support different sets of routing constraints. employed across the DAG.
In this case the DAG Root must provision each different DAG with a
different DAGID. Note that if multiple nodes acting as DAG roots are
rooting the same DAG, i.e. presenting the same DAGID, then they must
have some means of coordinating with each other when emitting Router
Advertisements (This may be the case, for example, when the DAG is
provisioned with a `virtual root' through some backbone mechanism).
This is described further below.
Nodes who hear Router Advertisements, advertising a specific DAGID, Nodes who hear RA messages, advertising a specific DAGID, will take
will take into consideration several criteria when processing the into consideration several criteria when processing the extracted DAG
extracted DAG information. A node may seek a DAG advertising a information. A node may seek a DAG advertising a specific OCP,
specific OCP, reflecting the implementation specific routing reflecting the implementation specific routing constraints understood
constraints understood by the node. In particular, a node will be by the node. In particular, a node will be seeking to find a least
seeking to find a least cost path satisfying some objective function cost path satisfying some objective function as indicated by the OCP
as indicated by the OCP according to some routing metrics defined in according to some routing metrics defined in
[I-D.ietf-roll-routing-metrics]. For example, the least cost path [I-D.ietf-roll-routing-metrics]. For example, the least cost path
may be determined in part by minimizing energy along a path, or may be determined in part by minimizing energy along a path, or
latency, or avoiding the use of battery powered nodes. A node may be latency, or avoiding the use of battery powered nodes. A node may be
seeking to explicitly join a grounded DAG. Further, a node may seek seeking to explicitly join a grounded DAG. Further, a node may seek
the minimum DAGPreference when selecting a DAG, all else being equal. the most desirable administrative preference when selecting a DAG,
Based on the evaluation of such criteria, a node may determine if the all else being equal. Based on the evaluation of such criteria, a
node who emitted the Router Advertisement should be considered as a node may determine if the node who emitted the RA message should be
potential DAG parent. If so, then the node may add the advertising considered as a potential DAG parent. If so, then the node may add
node to its set of candidate DAG parents for the advertised DAGID, the advertising node to its set of candidate DAG parents for the
and after waiting for a designated delay, the node may follow the advertised DAGID, and after waiting for a designated delay, the node
procedures to activate the advertising node as a DAG parent and may may follow the procedures to activate the advertising node as a DAG
then be considered to have joined the DAG designated by DAGID. parent and may then be considered to have joined the DAG designated
by DAGID.
When a node adds the first DAG parent to the set of DAG parents for a When a node adds the first DAG parent to the set of DAG parents for a
particular DAGID, the node is said to have joined, or attached to, particular DAGID, the node is said to have joined, or attached to,
the DAG designated by DAGID. Adding additional DAG parents beyond the DAG designated by DAGID. Adding additional DAG parents beyond
the first simply increases path diversity inwards toward the DAG the first simply increases path diversity inwards toward the DAG
root. When a node removes the last DAG Parent from the set of DAG root. When a node removes the last DAG parent from the set of DAG
parents for a particular DAGID, the node is said to have left, or parents for a particular DAGID, the node is said to have left, or
detached from, the DAG designated by DAGID. RPL will coordinate the detached from, the DAG designated by DAGID. RPL will coordinate the
joining, leaving, and movement of nodes within a DAGID in such a way joining, leaving, and movement of nodes within a DAGID in such a way
so as to avoid the formation of loops, as described further below. so as to avoid the formation of loops, as described further below.
As nodes join the DAG they are able advertise the fact by beginning As nodes join the DAG they are able advertise the fact by
to multicast the DAG information in Router Advertisements (to multicasting the DAG information in RA messages (to neighbors with a
neighbors with a link-local scope). In this way, nodes are able to link-local scope). In this way, nodes are able to join the DAG at
join the DAG at ever-increasing rank outward from the DAG root. As ever-increasing rank outward from the DAG root. As nodes continue to
nodes continue to receive DAG multicasts they may continue to expand receive DAG multicasts they may continue to expand their set of DAG
their set of DAG parents, while employing loop avoidance strategies parents, while employing loop avoidance strategies as described
as describe below, in order to build path diversity inwards toward below, in order to build path diversity inwards toward the DAG root.
the DAG root.
Using the information conveyed in the metrics of its most preferred Using the information conveyed in the metrics of its most preferred
DAG parent, its own metrics, and the conventions and functions DAG parent, its own metrics, and the conventions and functions
indicated by the OCP, a node is able to compute a rank value within indicated by the OCP, a node is able to compute a rank value within
the DAG which it will use to coordinate its DAG maintenance. the DAG which it will use to coordinate its DAG maintenance.
In addition to identifying DAG parents, a node also may hear the Once a preferred parent is selected, the node can compute its own
Router Advertisements of other neighboring nodes at the same rank rank in the DAG and determine alternate parents. Any node inwards
within the DAG. In this way a node can discover DAG Siblings. from this node, that is with a lower rank than this node, can be used
as an alternate parent for forwarding.
In addition to identifying DAG parents, a node also may hear the RA
messages of other neighboring nodes at the same rank within the DAG.
In this way a node can discover DAG siblings. As it selects its
initial position within a DAG, a node MAY increment its rank it order
to have at least one sibling but it SHOULD NOT increase it as to
obtain more parents.
A node may order its set of DAG parents according to some A node may order its set of DAG parents according to some
implementation specific preference. To this list the node may also implementation specific preference, and it SHOULD install a DAG
append a similarly ordered set of DAG siblings. By forwarding parent as a default gateway. To this list the node may also append a
traffic intended for the default destination towards the DAG parents, similarly ordered set of DAG siblings. By forwarding traffic
the node is able to support the main Multipoint-to-point (MP2P) intended for the default destination towards the DAG parents, the
traffic flows required by a typical LLN application. By using the node is able to support the main Multipoint-to-point (MP2P) traffic
ordered set of DAG parents and DAG siblings the node is able to take flows required by a typical LLN application. By using the ordered
set of DAG parents and DAG siblings the node is able to take
advantage of path diversity. For example, preferring to forward advantage of path diversity. For example, preferring to forward
traffic towards parents guarantees to get the traffic inwards, closer traffic towards parents guarantees to get the traffic inwards, closer
to the DAG root, by definition, regardless of which parent is to the DAG root, by definition, regardless of which parent is
selected. In this example, if forwarding towards parents is not selected. In this example, if forwarding towards parents is not
possible, perhaps due to a transient phenomena, then a node may then possible, perhaps due to a transient phenomena, then a node may then
choose to forward traffic towards siblings, moving across the DAG at choose to forward traffic towards siblings, moving across the DAG at
the same level (neither inwards or outwards). When receiving traffic the same level (neither inwards or outwards). When receiving traffic
forwarded from a sibling, the traffic should not be forwarded back to forwarded from a sibling, the traffic should not be forwarded back to
the same sibling in order to avoid a 2-node loop. In a further the same sibling in order to avoid a 2-node loop.
example, a forwarding implementation may choose to decrease the hop
limit more quickly when forwarding along sibling paths than along
parent paths. A forwarding engine may behave in a manner similar to
these examples, however the specific implementation of a forwarding
engine and related path diversity strategies is beyond the scope of
this specification. Various related techniques are currently under
investigation to be added in a later revision of this specification.
Note that the further interaction of the routing solution and the
forwarding engine, in particular how they utilize and react to
changes in metrics, and how the forwarding engine may use the
constrained set of successors provided by the routing engine based on
L2 triggers and metrics, is under investigation.
By employing this procedure, the LLN is able to set up a path- By employing this procedure, the LLN is able to set up a path-
constrained DAG, rooted at designated nodes, with other nodes constrained DAG, rooted at designated nodes, with other nodes
organized along paths leading inward toward the DAG root. MP2P organized along paths leading inward toward the DAG root. MP2P
traffic intended for the destinations available to or through the DAG traffic intended for the destinations available to or through the DAG
root, e.g. the default destination or other advertised prefixes, root, e.g. the default destination or other advertised prefixes,
flows inward along the DAG towards the root, and nodes forwarding flows inward along the DAG towards the root, and nodes forwarding
traffic are able to leverage the path diversity of the DAG as traffic are able to leverage the path diversity of the DAG as
necessary. necessary.
The DAG is then used by RPL as a reference topology, constraining the Further mechanisms described below will populate the routing tables
LLN routing problem, on which to build additional routing mechanisms. along the DAG in support of P2MP and P2P traffic.
3.3.1.2. Further Operation 3.2.1.2. Further Operation
The sub-DAG of a node is the set of other nodes of greater rank in The sub-DAG of a node is the set of other nodes of greater rank in
the DAG that might use a path towards the DAG root that contains this the DAG that might use a path towards the DAG root that contains this
node. Rank in the DAG is defined such that nodes contained in the node. Rank in the DAG is defined such that nodes contained in the
sub-DAG of a specific node should have a greater rank than the node. sub-DAG of a specific node should have a greater rank than the node.
This is an important property that is leveraged for loop avoidance- This is an important property that is leveraged for loop avoidance-
if a node has lesser rank then it is NOT in the sub-DAG. (An if a node has lesser rank then it is not in the sub-DAG. (An
arbitrary node with greater rank may or may not be contained in the arbitrary node with greater rank may or may not be contained in the
sub-DAG). Paths through siblings are not contained in this set. sub-DAG). Paths through siblings are not contained in this set.
As a further illustration, consider the DAG examples in Appendix B. As a further illustration, consider the DAG examples in Appendix B.
Consider Node (24) in the DAG Example depicted in Figure 12. In this Consider Node (24) in the DAG Example depicted in Figure 12. In this
example, the sub-DAG of Node (24) is comprised of Nodes (34), (44), example, the sub-DAG of Node (24) is comprised of Nodes (34), (44),
and (45). and (45).
A DAG may also be floating. Floating DAGs may be encountered, for A DAG may also be floating. Floating DAGs may be encountered, for
example, during coordinated reconfigurations of the network topology example, during coordinated reconfigurations of the network topology
wherein a node and its sub-DAG breaks off the DAG, temporarily wherein a node and its sub-DAG breaks off the DAG, temporarily
becomes a floating DAG, and reattaches to a grounded DAG at a becomes a floating DAG, and reattaches to a grounded DAG at a
different (more optimal) location. (Such coordination endeavors to different (more optimal) location. (Such coordination endeavors to
avoid the construction of transient loops in the LLN). A DAG, or a avoid the construction of transient loops in the LLN). A DAG, or a
sub-DAG, may also become floating because of a network element sub-DAG, may also become floating because of a network element
failure. Note that in the case where a floating DAG exists as a failure. Note that in the case where a floating DAG exists as a
consequence of DAG repair, the floating DAG is also intended to be consequence of DAG repair, the floating DAG is also intended to be
transient and carries a marking to make it less attractive. Some transient and carries a marking to make it less attractive. Some
specific application scenarios may employ permanent floating DAGS, specific application scenarios may employ permanent floating DAGs,
e.g. DAGs without connectivity to an external routed infrastructure, e.g. DAGs without connectivity to an external routed infrastructure,
as a matter of normal operation. In such cases the floating DAG is as a matter of normal operation. In such cases the floating DAG is
likely to have been provisioned by the application with a marking to likely to have been provisioned by the application with an
make it more attractive. DAGPreference, a configurable property that administrative preference which will make it more attractive.
may be used to engineer the attractiveness of a DAG, is further
described below.
A node will generally join at least one DAG, typically (but not A node will generally join at least one DAG, typically (but not
necessarily) to or through a grounded DAG rooted at an LBR. In some necessarily) to or through a grounded DAG rooted at an LBR. In some
cases, as suitable to the application scenario, a DAG may still cases, as suitable to the application scenario, a DAG may still
provision the default route toward DAG Parents and not be connected provision the DAG parents as default gateways and not be connected to
to a backbone network or the Internet. a non-LLN infrastructure such as the public Internet or a private IP
network.
This specification does not preclude a node from joining multiple This specification does not preclude a node from joining multiple
DAGs. In one such case, a particular application may require the DAGs. In one such case, a particular application may require the
node to maintain membership in multiple DAGs in order to satisfy node to maintain membership in multiple DAGs in order to satisfy
competing constraints, for example to support different types of competing constraints, for example to support different types of
traffic, similar to the concept of MTR (Multi-topology routing) as traffic, similar to the concept of MTR (Multi-topology routing) as
supported by other routing protocols such as IS-IS [RFC5120] or OSPF supported by other routing protocols such as IS-IS [RFC5120] or OSPF
[RFC4915], although the RPL mechanisms will significantly differ from [RFC4915], although the RPL mechanisms will significantly differ from
the ones specified for these protocols. (Note that not all the ones specified for these protocols. (Note that not all
constrained traffic cases may require multiple DAGs). In support of constrained traffic cases may require multiple DAGs). In support of
such cases the RPL implementation must independently maintain such cases the RPL implementation must independently maintain
requisite information and state for each DAG in parallel. In cases requisite information and state for each DAG in parallel. In cases
where a competing constraints must be satisfied toward the same DAG where a competing constraints must be satisfied toward the same DAG
root, the OCP should differ by definition and may serve to coordinate root, the OCP should differ by definition and may serve to coordinate
the maintenance of the multiple DAGs. Further, additional the maintenance of the multiple DAGs. Further, additional
recommendations for the operation of loop avoidance/loop detection recommendations for the operation of loop avoidance/loop detection
mechanisms in the presence of multiple DAGs are under investigation. mechanisms in the presence of multiple DAGs are under investigation.
An administered preference (DAGPreference) shall be associated with An administrative preference, the DAG preference, shall be associated
each DAG. In cases where a RPL node has a choice of joining more with each DAG. In cases where a RPL node has a choice of joining
than one DAG to satisfy a particular constraint, and all else being more than one DAG to satisfy a particular constraint, and all else
equal, the node will seek to join the DAG with the lowest preference being equal, the node will seek to join the most preferred DAG as
value. In practice this mechanism may be assist in engineering the indicated by the administrative preference. In practice this
construction of a DAG as appropriate to an application. For example, mechanism may be assist in engineering the construction of a DAG as
nodes that are to become DAG roots in support of a particular appropriate to an application. For example, nodes that are to become
application role, e.g. as a data sink or a controller, may be DAG roots in support of a particular application role, e.g. as a data
provisioned with a low DAG preference, e.g. 0x00. Nodes who are sink or a controller, may be provisioned such that they have are more
serving as the DAG root of a transient DAG, e.g. for DAG repair, may preferred. Nodes who are serving as the DAG root of a transient DAG,
take on a high DAG preference, e.g. 0xFF. Nodes will then be able to e.g. for DAG repair, may take on a less desirable preference value.
yield their transient DAGs to join the DAGs with lower DAGPreference. Nodes will then be able to yield their transient DAGs to join the
DAGs that are more preferred.
3.3.1.3. Router Advertisement - DAG Information Option (RA-DIO) 3.2.1.3. IP Router Advertisement - DAG Information Option (RA-DIO)
The IPv6 Router Advertisement mechanism (as specified in [RFC4861]) The IPv6 Router Advertisement (RA) mechanism (as specified in
is used by RPL in order to build and maintain a DAG. [RFC4861]) is used by RPL in order to build and maintain a DAG.
The IPv6 Router Advertisement message is augmented with a DAG The IPv6 RA message is augmented with a DAG Information Option (DIO),
Information Option (DIO) in order to facilitate the formation and forming an RA-DIO message, in order to facilitate the formation and
maintenance of DAGs. The information conveyed in the DIO includes maintenance of DAGs. The information conveyed in the RA-DIO message
the following: includes the following:
o A DAGID used to identify the DAG as sourced from the DAG Root. o A DAGID used to identify the DAG as sourced from the DAG root.
Typically the (potentially compressed) IPv6 address of the DAG Typically the (potentially compressed) IPv6 address of the DAG
Root. May be tested for equality. The DAGID MUST be unique to a root. The DAGID must be unique to a single DAG in the scope of
single DAG in the scope of the LLN. If the DAG Root is rooting the LLN. If the DAG root is rooting multiple DAGs, each DAG must
multiple DAGs, each must be provisioned with their own IPv6 be provisioned with their own IPv6 address and thus derive unique
address and thus derive unique DAGIDs. DAGIDs.
o Objective Code Point (OCP) as described below. o Objective Code Point (OCP) as described below.
o Rank information used by nodes to determine their relationships in o Rank information used by nodes to determine their relationships in
the DAG relative to each other, i.e. parents, siblings, or the DAG relative to each other, i.e. parents, siblings, or
children. This is not a metric, although its derivation is children. This is not a metric, although its derivation is
typically closely related to one or more metrics as specified by typically closely related to one or more metrics as specified by
the OCP. Used to support loop avoidance strategies and in support the OCP. The rank information is used to support loop avoidance
of ordering alternate successors when engaged in path maintenance. strategies and in support of ordering alternate successors when
engaged in path maintenance.
o Sequence number originated from the DAG root, used to aid in o Sequence number originated from the DAG root, used to aid in
identification of dependent sub-DAGs and coordinate topology identification of dependent sub-DAGs and coordinate topology
changes in a manner so as to avoid loops. changes in a manner so as to avoid loops.
o Indications for the DAG, e.g. grounded or floating. o Indications for the DAG, e.g. grounded or floating.
o DAG configuration parameters. o DAG configuration parameters.
o A vector of path metrics. As discussed in o A vector of path metrics. As discussed in
[I-D.ietf-roll-routing-metrics] such metrics may be cumulative, [I-D.ietf-roll-routing-metrics] such metrics may be cumulative,
may report a maximum, minimum, or average scalar value, or a link may report a maximum, minimum, or average scalar value, or a link
property. property.
o List of additional destination prefixes reachable via the DAG o List of additional destination prefixes reachable via the DAG
root. root.
The Router Advertisements are issued whenever a change is detected to The RA messages are issued whenever a change is detected to the DAG
the DAG such that a node is able to determine that a region of the such that a node is able to determine that a region of the DAG has
DAG has become inconsistent. As the DAG stabilizes the period at become inconsistent. As the DAG stabilizes the period at which RA
which Router Advertisements occur is configured to taper off, messages occur is configured to taper off, reducing the steady-state
reducing the steady-state overhead of DAG maintenance. The periodic overhead of DAG maintenance. The periodic issue of RA messages,
issue of Router Advertisements, along with the triggered Router along with the triggered RA messages in response to inconsistency, is
Advertisements in response to inconsistency, is one feature that one feature that enables RPL to operate in the presence of unreliable
enables RPL to operate in the presence of unreliable links. links.
The RA-DIO and related mechanisms are described in more detail in The RA-DIO and related mechanisms are described in more detail in
Section 5. Section 5.
3.3.1.4. Objective Code Point (OCP) 3.2.1.4. Objective Code Point (OCP)
The OCP is seeded by the DAG Root and serves to convey and control The OCP is seeded by the DAG root and serves to convey and control
the optimization functions used within the DAG. The OCP is further the optimization functions used within the DAG. The OCP is further
specified in [I-D.ietf-roll-routing-metrics]. Each instance of an specified in [I-D.ietf-roll-routing-metrics]. Each instance of an
allocated OCP indicates: allocated OCP indicates:
o The set of metrics used within the DAG o The set of metrics used within the DAG
o The objective functions used to determine the least cost o The objective functions used to determine the least cost
constrained paths in order to optimize the DAG constrained paths in order to optimize the DAG
o The function used to compute DAG Rank o The function used to compute DAG Rank
o The functions used to construct derived metrics for propagation o The functions used to construct derived metrics for propagation
within a DIO within a RA-DIO message
For example, an objective code point might indicate that the DAG is For example, an objective code point might indicate that the DAG is
using ETX as a metric, that the optimization goal is to minimize ETX, using the Expected Number of Transmissions (ETX) as a metric, that
that DAG Rank is equivalent to ETX, and that DIO propagation entails the optimization goal is to minimize ETX, that DAG Rank is equivalent
adding the advertised ETX of the most preferred parent to the ETX of to ETX, and that RA-DIO propagation entails adding the advertised ETX
the link to the most preferred parent. of the most preferred parent to the ETX of the link to the most
preferred parent.
By using defined OCPs that are understood by all nodes in a By using defined OCPs that are understood by all nodes in a
particular implementation, and by conveying them in the DIO, RPL particular implementation, and by conveying them in the RA-DIO
nodes may work to build optimized LLN using a variety of application message, RPL nodes may work to build optimized LLN using a variety of
and implementation specific metrics and goals. application and implementation specific metrics and goals.
A default OCP, OCP 0, is specified with a well-defined default A default OCP, OCP 0, is specified with a well-defined default
behavior. OCP 0 is used to define RPL behaviors in the case where a behavior. OCP 0 is used to define RPL behaviors in the case where a
node encounters a DIO containing a code point that it does not node encounters a RA-DIO message containing a code point that it does
support. not support.
3.3.1.5. Selection of Feasible DAG Parents 3.2.1.5. Selection of Feasible DAG Parents
The decision for a node to join a DAG may be optimized according to The decision for a node to join a DAG may be optimized according to
implementation specific policy functions on the node as indicated by implementation specific policy functions on the node as indicated by
one or more specific OCP values. For example, a node may be one or more specific OCP values. For example, a node may be
configured for one goal to optimize a bandwidth metric (OCP-1), and configured for one goal to optimize a bandwidth metric (OCP-1), and
with a parallel goal to optimize for a reliability metric (OCP-2). with a parallel goal to optimize for a reliability metric (OCP-2).
Thus two DAGs, with two unique DAGIDs, may be constructed and Thus two DAGs, with two unique DAGIDs, may be constructed and
maintained in the LLN: DAG-1 would be optimized according to OCP-1, maintained in the LLN: DAG-1 would be optimized according to OCP-1,
whereas DAG-2 would be optimized according to OCP-2. A node may then whereas DAG-2 would be optimized according to OCP-2. A node may then
maintain two parallel sets of DAG parents and related data maintain two parallel sets of DAG parents and related data
structures. Note that in such a case traffic may directed along the structures. Note that in such a case traffic may directed along the
appropriate constrained DAG based on traffic marking within the IPv6 appropriate constrained DAG based on traffic marking within the IPv6
header. header.
As a node hears RAs from its neighbors it may process their DIOs. At As a node hears RA messages from its neighbors it may process their
this time the node may be able to take into consideration, for attached DIO messages. At this time the node may be able to take
example, the following: into consideration, for example, the following:
o Is the neighboring node heard reliably enough, and are the metrics o Is the neighboring node heard reliably enough, and are the metrics
stable enough, that a local degree of confidence may be stable enough, that a local degree of confidence may be
established with respect to the neighboring node? Should the established with respect to the neighboring node? Should the
neighboring node be considered in the set of candidate neighbors? neighboring node be considered in the set of candidate neighbors?
o In consultation with implementation specific policy (OCP goal), is o In consultation with implementation specific policy (OCP goal), is
the neighboring node a feasible parent from a constrained-path the neighboring node a feasible parent from a constrained-path
perspective? perspective?
o According to the implementation specific policy (OCP), does the o According to the implementation specific policy (OCP), does the
neighboring node offer a better optimized position into the DAG? neighboring node offer a better optimized position into the DAG?
o Does the neighboring node offer a DAG with a better DAGPreference o Does the neighboring node offer a DAG with a more desirable
for an otherwise currently satisfied optimization objective, all administrative preference for an otherwise currently satisfied
else being equal? optimization objective, all else being equal?
o Is the neighboring node a peer (sibling) within the DAG? o Is the neighboring node a peer (sibling) within the DAG?
Based on such considerations, the node may incorporate the Based on such considerations, the node may incorporate the
neighboring node into the set of DAG parents according to neighboring node into the set of DAG parents. When the node inserts
implementation specific algorithms that are outside the scope of this the first DAG parent into the empty DAG parent set, it is able to
document. join the DAG. As the DAG parent set is updated, the node will
consult a rank computation function indicated by the OCP for the DAG
in order to determine its own rank value, which it will subsequently
advertise when it emits its own RA-DIO messages.
When the node inserts the first DAG parent into the empty DAG parent Following is an overview of the rules used to select a parent (the
set, it is able to join the DAG. After the DAG parent set is detailed mode of operation for the selection of the candidate DAG
updated, the node will consult a rank computation function indicated parent(s) is described in Section 5.3. First, it is important to
by the OCP for the DAG in order to determine its rank value, which it note that the rank of the node is not directly used as a selection
will subsequently advertise when it emits its own DIOs. A general criteria. The metric of choice as indicated by the OCP advertised by
property of the rank value presented by the node is that it should be the candidate parents is used to select the parent, although the use
greater than that presented by any of its DAG parents. A node must of a cumulative metric to reflect the rank is not precluded.
maintain its DAG Parent set such that its most preferred parent from
the OCP goals also has the greatest rank value in the DAG parent set. Consider an example where a node N receives two RAs from node A and B
All reliable neighboring nodes of a lesser rank than the node may with (rank, metric) equal to (2,4) and (5,3) respectively. Node N
then be considered as potential DAG parents (Note that as a may chose B as its parent (higher rank but smaller metric). Once the
consequence of satisfying a particular OCP goal, the most preferred parent, B, is selected, the node computes its own rank according to
parent may not necessarily be the potential parent of least rank, for the OCP.
example a potential parent of lesser rank may also be an energy
constrained device that is to generally be avoided and thus not the If the node receives other RA messages it cannot attach to other
most preferred). No nodes of greater rank than the most preferred parents if choosing that parent would cause the nodes own rank to
parent may be in the DAG Parent set; to allow such nodes will increase. Back to the previous example, suppose that a node C
introduce a possibility to create loops (by potentially allowing a appears with a (rank, metric) equal to (5,1). By selecting C as the
packet to make backwards progress as it is forwarded in the DAG). new parent, N would have a rank of 6 (making the assumption that the
All neighboring nodes of equal rank may be considered as siblings rank is increased by a value of 1 according to the OCP). Although
within the DAG (even though they may not have parents in common, they the path metric would be lower, this may lead to a DAG Loop should C
may still provide path diversity towards the DAG root). belong to the sub-DAG of N as further discussed in Section 3.3.1.
All reliable neighboring nodes of a lesser rank than the node may be
considered as potential DAG parents (Note that, as in the above
example, as a consequence of satisfying a particular OCP goal, the
most preferred parent may not necessarily be the potential parent of
least rank, for example a potential parent of lesser rank may also be
an energy constrained device that is to generally be avoided and thus
not the most preferred). No nodes of greater rank than self may be
in the DAG parent set; to allow such nodes will introduce a
possibility to create loops (by potentially allowing a packet to make
backwards progress as it is forwarded in the DAG). All neighboring
nodes of equal rank may be considered as siblings within the DAG
(even though they may not have parents in common, they may still
provide path diversity towards the DAG root).
The computation of rank, and related properties, are further The computation of rank, and related properties, are further
described in Section 3.4.1. described in Section 3.3.1.
3.3.1.5.1. Example 3.2.1.5.1. Example
For example, suppose that a node (N) is not attached to any DAG, and For example, suppose that a node (N) is not attached to any DAG, and
that it is in range of nodes (A), (B), (C), (D), and (E). Let all that it is in range of nodes (A), (B), (C), (D), and (E). Let all
nodes be configured to use an OCP which defines a policy such that nodes be configured to use an OCP which defines a policy such that
ETX is to be minimized and paths with the attribute `Blue' should be ETX is to be minimized and paths with the attribute `Blue' should be
avoided. Let the rank computation indicated by the OCP simply avoided. Let the rank computation indicated by the OCP simply
reflect the ETX aggregated along the path. Let the links between reflect the ETX aggregated along the path. Let the links between
node (N) and its neighbors (A-E) all have an ETX of 1 (which is node (N) and its neighbors (A-E) all have an ETX of 1 (which is
learned by node (N) through some implementation specific method). learned by node (N) through some implementation specific method).
Let node (N) be configured to send Router Solicitations to probe for Let node (N) be configured to send IPv6 Router Solicitation (RS)
nearby DAGs. messages to probe for nearby DAGs.
o Node (N) transmits a Router Solicitation. o Node (N) transmits a Router Solicitation.
o Node (B) responds. Node (N) investigates the DIO, and learns that o Node (B) responds. Node (N) investigates the RA-DIO message, and
Node (B) is a member of DAGID 1 at rank 4, and not `Blue'. Node learns that Node (B) is a member of DAGID 1 at rank 4, and not
(N) takes note of this, but is not yet confident. `Blue'. Node (N) takes note of this, but is not yet confident.
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 that indicates that it is a o Node (D) responds. Node (D) has a RA-DIO message that indicates
member of DAGID 1 at rank 2, but it carries the attribute `Blue'. that it is a member of DAGID 1 at rank 2, but it carries the
Node (N)'s policy function rejects Node (D), and no further attribute `Blue'. Node (N)'s policy function rejects Node (D),
consideration is given. and no 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 up enough confidence to trigger a decision
to join DAGID 1. Let Node (N) determine its most preferred parent to join DAGID 1. Let Node (N) determine its most preferred parent
to be Node (E). to 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 inward 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 inward progress but with the intention that node (C) or a
following successor can make inward progress. Should Node (C) not following successor can make inward progress. Should Node (C) not
have a viable parent, it should never send the packet back to Node have a viable parent, it should never send the packet back to Node
(N) (to avoid a 2-node loop). (N) (to avoid a 2-node loop).
3.3.1.6. DAG Maintenance 3.2.1.6. DAG Maintenance
When a node moves within a DAG, the move is defined as updating the When a node moves within a DAG, the move is defined as updating the
set of DAG Parents for a particular DAGID, i.e. adding or deleting set of DAG parents for a particular DAGID, i.e. adding or deleting
DAG Parents. Not all moves entail changes in rank. DAG parents. Not all moves entail changes in rank.
A jump in the context of a DAG is attaching to a new DAGID, in such a A jump from one DAG to another DAG is attaching to a new DAGID, in
way that an old DAGID is replaced by the new DAGID. In particular, such a way that an old DAGID is replaced by the new DAGID. In
when an old DAGID is left, all associated parents are no longer particular, when an old DAGID is left, all associated parents are no
feasible, and a new DAGID is joined. longer feasible, and a new DAGID is joined.
When a node in a DAG follows a DAG parent, it means that the DAG When a node in a DAG follows a DAG parent, it means that the DAG
parent has changed its DAGID (e.g. by joining a new DAG) and that the parent has changed its DAGID (e.g. by joining a new DAG) and that the
node updates its own DAGID in order to keep the DAG parent. node updates its own DAGID in order to keep the DAG parent.
A frozen sub-DAG is a subset of nodes in the sub-DAG of a node who A frozen sub-DAG is a subset of nodes in the sub-DAG of a node who
have been informed of a change to the node, and choose to follow the have been informed of a change to the node, and choose to follow the
node in a manner consistent with the change, for example in node in a manner consistent with the change, for example in
preparation for a coordinated move. Nodes in the sub-DAG who hear of preparation for a coordinated move. Nodes in the sub-DAG who hear of
a change and have other options than to follow the node do not have a change and have other options than to follow the node do not have
to become part of the frozen sub-DAG, for example such a node may be to become part of the frozen sub-DAG, for example such a node may be
able to remain attached to the original DAG through a different DAG able to remain attached to the original DAG through a different DAG
parent. A further example may be found in Section 3.4.1.1. parent. A further example may be found in Section 3.3.1.1.
When the node encounters new candidate neighbors that offer higher When the node encounters new candidate neighbors that offer higher
positions in the DAG, it may incorporate them directly into its set positions in the DAG, it may incorporate them directly into its set
of DAG parents. In this case the node may update its choice of most of DAG parents. In this case the node may update its choice of most
preferred parent, possibly causing its own advertised rank to preferred parent, possibly causing its own advertised rank to
decrease, and discarding any former parents now of a deeper rank. decrease, and discarding any former parents now of a deeper rank.
This case is `moving inwards along the DAG' and does not require any This case is `moving inwards along the DAG' and does not require any
additional coordination for loop avoidance. additional coordination for loop avoidance.
If the DAG parent set of the node becomes completely depleted, the If the DAG parent set of the node becomes completely depleted, the
node will have detached from the DAG, and may, if so configured, node will have detached from the DAG, and may, if so configured,
become the root of its own transient floating DAG with a high become the root of its own transient floating DAG with a less
DAGPreference (0xFF) (thus beginning the process of establishing the desirable administrative preference (thus beginning the process of
frozen sub-DAG), and then may reattach to the original DAG at a lower establishing the frozen sub-DAG), and then may reattach to the
point if it is able (after hearing RA-DIOs from alternate attachment original DAG at a lower point if it is able (after hearing RA-DIO
points). messages from alternate attachment points).
When the node encounters candidate parents that are in a different When the node encounters candidate parents that are in a different
DAG, and decides to leave the current DAG in order to join the DAG, and decides to leave the current DAG in order to join the
different DAG, it may do so safely without regard to loop avoidance. different DAG (thus doing a jump), it may do so safely without regard
However, it may not return immediately to the current DAG as such to loop avoidance. However, it may not return immediately to the
movement may result in the creation of loops. current DAG as such movement may result in the creation of a DAG
Loop, in particular if it reattaches back into its own former sub-DAG
in an uncoordinated manner.
When a node, and perhaps a related frozen sub-DAG, jumps to a When a node, and perhaps a related frozen sub-DAG, jumps to a
different DAG, the move is coordinated by a DAG Hop timer. The DAG different DAG, the move is coordinated by a DAG Hop timer. The DAG
Hop timer allows the nodes who will attach closer to the sink of the Hop timer allows the nodes who will attach closer to the sink of the
new DAG to `jump' first, and then drag dependent nodes behind them, new DAG to `jump' first, and then drag dependent nodes behind them,
thus endeavoring to efficiently coordinate the attachment of the thus endeavoring to efficiently coordinate the attachment of the
frozen sub-DAG into the new DAG. A further illustration of this frozen sub-DAG into the new DAG. A further illustration of this
mechanism may be found in Section 3.4.3. mechanism may be found in Section 3.3.3.
Section 5 contains more detail on the processes and rules used for
DAG discovery and maintenance.
Appendix B provides additional examples of DAG discovery and Appendix B provides additional examples of DAG discovery and
maintenance. maintenance.
3.3.2. Source Routing 3.2.2. Destination Advertisement
A Source Routing mechanism for RPL is currently under investigation.
3.3.3. Destination Advertisement
As RPL constructs DAGs, nodes are able to learn a set of default As RPL constructs DAGs, nodes may provision routes toward
routes in order to send traffic to the sink. However, this mechanism destinations advertised through RA-DIO messages through their
selected parents, and are thus able to send traffic inward along the
DAG by forwarding to their selected parents. However, this mechanism
alone is not sufficient to support P2MP traffic flowing outward along alone is not sufficient to support P2MP traffic flowing outward along
the DAG from the DAG root toward nodes. A Destination Advertisement the DAG from the DAG root toward nodes. A destination advertisement
mechanism is employed by RPL to build up routing state in support of mechanism is employed by RPL to build up routing state in support of
these outward flows. The Destination Advertisement mechanism may not these outward flows. The destination advertisement mechanism may not
be supported in all implementations, as appropriate to the be supported in all implementations, as appropriate to the
application requirements. A DAG Root that supports using the application requirements. A DAG root that supports using the
Destination Advertisement mechanism to build up routing state will destination advertisement mechanism to build up routing state will
indicate such in the DIO. A DAG Root that supports using the indicate such in the RA-DIO message. A DAG root that supports using
Destination Advertisement mechanism MUST be capable of allocating the destination advertisement mechanism must be capable of allocating
enough state to store the routing state received from the LLN. enough state to store the routing state received from the LLN.
3.3.3.1. Destination Advertisement Option (DAO) 3.2.2.1. IPv6 Neighbor Advertisement - Destination Advertisement Option
(NA-DAO)
A Destination Advertisement Option (DAO) is used to convey the An IPv6 Neighbor Advertisement Message with Destination Advertisement
Destination information inward along the DAG toward the DAG root. Options (NA-DAO) is used to convey the destination information inward
along the DAG toward the DAG root.
The information conveyed in the DAO includes the following: The information conveyed in the NA-DAO message includes the
following:
o A lifetime and sequence counter to determine the freshness of the o A lifetime and sequence counter to determine the freshness of the
Destination Advertisement. destination advertisement.
o Depth information used by nodes to determine how far away the o Depth information used by nodes to determine how far away the
destination (the source of the Destination Advertisement) is destination (the source of the destination advertisement) is
o Prefix information to identify the destination, which may be a o Prefix information to identify the destination, which may be a
prefix, an individual host, or multicast listeners prefix, an individual host, or multicast listeners
o Reverse Route information to record the nodes visited (along the o Reverse Route information to record the nodes visited (along the
outward path) when the intermediate nodes along the path cannot outward path) when the intermediate nodes along the path cannot
support storing state for Hop-By-Hop routing. support storing state for Hop-By-Hop routing.
3.3.3.2. Destination Advertisement Operation 3.2.2.2. Destination Advertisement Operation
As the DAG is constructed and maintained, nodes are capable to emit As the DAG is constructed and maintained, nodes are capable to emit
messages containing Destination Advertisement Options to a subset of NA-DAO messages to a subset, or all, of their DAG parents. The
their DAG Parents. The selection of this subset is according to an selection of this subset is according to an implementation specific
implementation specific policy. policy.
As a special case, a node may periodically emit a link-local As a special case, a node may periodically emit a link-local
multicast message containing a Destination Advertisement Options multicast IPv6 NA-DAO message advertising its locally available
advertising its locally available destination prefixes. This destination prefixes. This mechanism allows for the one-hop
mechanism allows for the one-hop neighbors of a node to learn neighbors of a node to learn explicitly of the prefixes on the node,
explicitly of the prefixes on the node, and in some application and in some application specific scenarios this is desirable in
specific scenarios this is desirable in support of provisioning a support of provisioning a trivial `one-hop' route. In this case,
trivial `one-hop' route. In this case, nodes who receive the nodes who receive the multicast destination advertisement may use it
multicast Destination Advertisement may use it to provision the one- to provision the one-hop route only, and not engage in any additional
hop route only, and not engage in any additional processing (so as processing (so as not to engage the mechanisms used by a DAG parent).
not to engage the mechanisms used by a DAG Parent).
When a (unicast) DAO reaches a node capable of storing routing state, When a (unicast) NA-DAO message reaches a node capable of storing
the node extracts information from the DAO and updates its local routing state, the node extracts information from the NA-DAO message
database with a record of the DAO and who it was received from. When and updates its local database with a record of the NA-DAO message
the node later propagates DAOs, it selects the best (least depth) and who it was received from. When the node later propagates NA-DAO
information for each destination and conveys this information again messages, it selects the best (least depth) information for each
in the form of DAOs to a subset of its own DAG parents. At this time destination and conveys this information again in the form of NA-DAO
the node may perform route aggregation if it is able, thus reducing messages to a subset of its own DAG parents. At this time the node
the overall number of DAOs. may perform route aggregation if it is able, thus reducing the
overall number of NA-DAO messages.
When a (unicast) DAO reaches a node incapable of storing additional When a (unicast) NA-DAO message reaches a node incapable of storing
state, the node MUST append the next-hop address (from which neighbor additional state, the node must append the next-hop address (from
the DAO was received) to a Reverse Route Stack carried within the which neighbor the NA-DAO message was received) to a Reverse Route
DAO. The node then passes the DAO on to one or more of its DAG Stack carried within the NA-DAO message. The node then passes the
parents without storing any additional state. NA-DAO message on to one or more of its DAG parents without storing
any additional state.
When a node that is capable of storing routing state encounters a When a node that is capable of storing routing state encounters a
(unicast) DAO with a Reverse Route Stack that has been populated, the (unicast) NA-DAO message with a Reverse Route Stack that has been
node knows that the DAO has traversed a region of nodes that did not populated, the node knows that the NA-DAO message has traversed a
record any routing state. The node is able to detach and store the region of nodes that did not record any routing state. The node is
Reverse Route State and associate it with the destination described able to detach and store the Reverse Route State and associate it
by the DAO. Subsequently the node may use this information to with the destination described by the NA-DAO message. Subsequently
construct a source route in order to bridge the region of nodes that the node may use this information to construct a source route in
are unable to support Hop-By-Hop routing to reach the destination. order to bridge the region of nodes that are unable to support Hop-
By-Hop routing to reach the destination.
In this way the Destination Advertisement mechanism is able to In this way the destination advertisement mechanism is able to
provision routing state in support of P2MP traffic flows outward provision routing state in support of P2MP traffic flows outward
along the DAG, and as according to the available resources in the along the DAG, and as according to the available resources in the
network. network.
Further aggregations of DAOs by destinations are possible in order to Further aggregations of NA-DAO messages prefix reachability
support additional scalability. information by destinations are possible in order to support
additional scalability.
A further example of the operation of the Destination Advertisement A further example of the operation of the destination advertisement
mechanism is available in Appendix B.6 mechanism is available in Appendix B.6
3.4. Other Considerations 3.3. Other Considerations
3.4.1. DAG Rank and Loop Avoidance 3.3.1. DAG Rank and Loop Avoidance
When nodes select DAG Parents, they should select the most preferred When nodes select DAG parents, they should select the most preferred
parent according to their implementation specific objectives, using parent according to their implementation specific objectives, using
the cost metrics conveyed in the DIOs along the DAG in conjunction the cost metrics conveyed in the RA-DIO messages along the DAG in
with the related objective functions as specified by the OCP. conjunction with the related objective functions as specified by the
OCP.
Based on this selection, the metrics conveyed by the most preferred Based on this selection, the metrics conveyed by the most preferred
DAG parent, the nodes own metrics and configuration, and a related DAG parent, the nodes own metrics and configuration, and a related
function defined by the objective code point, a node will be able to function defined by the objective code point, a node will be able to
compute a value for its rank as a consequence of selecting a most compute a value for its rank as a consequence of selecting a most
preferred DAG parent. preferred DAG parent.
It is important to note that the DAG Rank is not itself a metric, 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 although its value is derived from and influenced by the use of
metrics to select DAG parents and take up a position in the DAG. In metrics to select DAG parents and take up a position in the DAG. In
other words, routing metrics and OCP (not rank directly) are used to other words, routing metrics and OCP (not rank directly) are used to
determine the DAG structure and consequently the path cost. The only determine the DAG structure and consequently the path cost. The only
aim of the rank is to inform loop avoidance as explained hereafter. aim of the rank is to inform loop avoidance as explained hereafter.
The computation of the DAG Rank MUST be done in such a way so as to The computation of the DAG Rank MUST be done in such a way so as to
maintain the following properties for any nodes M and N who are maintain the following properties for any nodes M and N who are
neighbors in the LLN: neighbors in the LLN:
For a node N, and its most preferred parent M, DAGRank(N) > For a node N, and its most preferred parent M, DAGRank(N) >
DAGRank(M) must hold. Further, all parents in the DAG parent set DAGRank(M) must hold. Further, all parents in the DAG parent set
must be of a rank less than or equal to DAGRank(M). In other must be of a rank less than self's DAGRank(N). In other words,
words, the rank presented by a node N MUST be greater (deeper) the rank presented by a node N MUST be greater (deeper) than that
than that presented by any of its parents. (This mechanism serves presented by any of its parents. (This mechanism serves to avoid
to avoid loops in the case where an alternate parent is used- if loops in the case where an alternate parent is used- if no
no alternate parent is deeper than the preferred parent then loops alternate parent is deeper than the preferred parent then loops
are avoided. The risk of loops occurs if there is a chance for an are avoided. The risk of loops occurs if there is a chance for an
alternate parent to forward traffic to a deeper successor, which alternate parent to forward traffic to a deeper successor, which
may be in the sub-DAG, and traffic then makes backwards progress may be in the sub-DAG, and traffic then makes backwards progress
and comes back to the node again). and comes back to the node again).
If DAGRank(M) < DAGRank(N), then M is located in a more optimum If DAGRank(M) < DAGRank(N), then M is probably located in a more
position than N in the DAG with respect to the metrics and optimum position than N in the DAG with respect to the metrics and
optimizations defined by the objective code point. Node M may optimizations defined by the objective code point. In any
safely be a DAG Parent for Node N without risk of creating a loop. fashion, Node M may safely be a DAG parent for Node N without risk
For example, a Node M of rank 3 is located in a more optimum of creating a loop. For example, a Node M of rank 3 is located in
position than a Node N of rank 5. A packet directed inwards and a more optimum position than a Node N of rank 5. A packet
forwarded from Node N to Node M will always make forward progress directed inwards and forwarded from Node N to Node M will always
with respect to the DAG organization on that link; there is no make forward progress with respect to the DAG organization on that
risk of Node M at rank 3 forwarding the packet back into Node N's link; there is no risk of Node M at rank 3 forwarding the packet
sub-DAG at rank of 5 or greater (which would be a sufficient back into Node N's sub-DAG at rank of 5 or greater (which would be
condition for a loop to occur). a sufficient condition for a loop to occur).
If DAGRank(M) == DAGRank(N), then M and N are located positions of If DAGRank(M) == DAGRank(N), then M and N are located positions of
relatively the same optimality within the DAG. In some cases, relatively the same optimality within the DAG. In some cases,
Node M may be used as a successor by Node N, but with related 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 chance of creating a loop that must be detected and broken by some
other means. If Node M is at rank 3 and node N is at rank 3, then other means. If Node M is at rank 3 and node N is at rank 3, then
they are siblings; by definition Node M and N cannot be in each they are siblings; by definition Node M and N cannot be in each
others sub-DAG. They may then forward to each other failing others sub-DAG. They may then forward to each other failing
serviceable parents, making `sideways' progress (but not reverse serviceable parents, making `sideways' progress (but not reverse
progress). If another sibling or more gets involved there may progress). If another sibling or more gets involved there may
then be some chance for 3 or more way loops, which is the risk of then be some chance for 3 or more way loops, which is the risk of
sibling forwarding. sibling forwarding.
If DAGRank(M) > DAGRank(N), then node M is located in a less If DAGRank(M) > DAGRank(N), then node M is located in a less
optimum position than N in the DAG with respect to the metrics and optimum position than N in the DAG with respect to the metrics and
optimizations defined by the objective code point. Further, Node optimizations defined by the objective code point. Further, Node
(M) may in fact be in Node (N)'s sub-DAG. There is no advantage (M) may in fact be in Node (N)'s sub-DAG. There is no advantage
to Node (N) selecting Node (M) as a DAG Parent, and such a to Node (N) selecting Node (M) as a DAG parent, and such a
selection may create a loop. For example, if Node M is of rank 3 selection may create a loop. For example, if Node M is of rank 3
and Node N is of rank 5, then by definition Node N is in a less and Node N is of rank 5, then by definition Node N is in a less
optimum position than Node N. Further, Node N at rank 5 may in optimum position than Node N. Further, Node N at rank 5 may in
fact be in Node M's own sub-DAG, and forwarding a packet directed fact be in Node M's own sub-DAG, and forwarding a packet directed
inwards towards the DAG root from M to N will result in backwards inwards towards the DAG root from M to N will result in backwards
progress and possibly a loop. progress and possibly a loop.
For example, the DAG Rank could be computed in such a way so as to For 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 closely track ETX when the objective function is to minimize ETX, or
latency when the objective function is to minimize latency, or in a latency when the objective function is to minimize latency, or in a
skipping to change at page 23, line 14 skipping to change at page 25, line 37
connectivity to the DAG, it must first leave the DAG before it may connectivity to the DAG, it must first leave the DAG before it may
then rejoin at a deeper point. This allows for the node to then rejoin at a deeper point. This allows for the node to
coordinate moving down, freezing its own sub-DAG and poisoning stale coordinate moving down, freezing its own sub-DAG and poisoning stale
routes to the DAG, and minimizing the chances of re-attaching to its routes to the DAG, and minimizing the chances of re-attaching to its
own sub-DAG thinking that it has found the original DAG again. If a own sub-DAG thinking that it has found the original DAG again. If a
node where allowed to re-attach into its own sub-DAG a loop would node where allowed to re-attach into its own sub-DAG a loop would
most certainly occur, and may not be broken until a count-to-infinity most certainly occur, and may not be broken until a count-to-infinity
process elapses. The procedure of detaching before moving down process elapses. The procedure of detaching before moving down
eliminates the need to count-to-infinity. eliminates the need to count-to-infinity.
Any neighboring nodes of lesser or equal rank to the current most Any neighboring nodes of lesser rank than self are eligible to be
preferred DAG parent are eligible to be considered as alternate DAG considered as alternate DAG parents for forwarding. But this node
parents. may only adopt such a parent as new preferred parent if that does not
cause the resulting rank for this node to increase.
The goal of a guaranteed consistent and loop free global routing The goal of a guaranteed consistent and loop free global routing
solution for an LLN may not be practically achieved given the real solution for an LLN may not be practically achieved given the real
behavior and volatility of the underlying metrics. The trade offs to behavior and volatility of the underlying metrics. The trade offs to
achieve a stable approximation of global convergence may be too achieve a stable approximation of global convergence may be too
restrictive with respect to the need of the LLN to react quickly in restrictive with respect to the need of the LLN to react quickly in
response to the lossy environment. Globally the LLN may be able to response to the lossy environment. Globally the LLN may be able to
achieve a weak convergence, in particular as link changes are able to achieve a weak convergence, in particular as link changes are able to
be handled locally and result in minimal changes to global topology. be handled locally and result in minimal changes to global topology.
RPL does not aim to guarantee loop free path selection, or strong RPL does not aim to guarantee loop free path selection, or strong
global convergence. In order to reduce control overhead, in global convergence. In order to reduce control overhead, in
particular the expense of mechanisms such as count-to-infinity, RPL particular the expense of mechanisms such as count-to-infinity, RPL
does try to avoid the creation of loops when undergoing topology does try to avoid the creation of loops when undergoing topology
changes. Further mechanisms to mitigate the impact of loops, such as changes. Further mechanisms to mitigate the impact of loops, such as
loop detection when forwarding, are under investigation. loop detection when forwarding, are under investigation.
3.4.1.1. Example 3.3.1.1. Example
: : : : : :
: : : : : :
(A) (A) (A) (A) (A) (A)
|\ | | |\ | |
| `-----. | | | `-----. | |
| \ | | | \ | |
(B) (C) (B) (C) (B) (B) (C) (B) (C) (B)
| | \ | | \
| | `-----. | | `-----.
skipping to change at page 24, line 28 skipping to change at page 26, line 33
| |
| |
| |
(D) (D)
-1- -2- -3- -1- -2- -3-
Figure 1: DAG Maintenance Figure 1: DAG Maintenance
Consider the example depicted in Figure 1-1. In this example, Node Consider the example depicted in Figure 1-1. 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
`sideways' in a cycle) and requiring the intervention of additional `sideways' in a cycle) and requiring the intervention of additional
mechanisms to detect and break the loop. mechanisms to detect and break the loop.
Consider the case where Node (C) hears a DIO from a Node (Z) at a Consider the case where Node (C) hears a RA-DIO message from a Node
lesser rank and superior position in the DAG than node (A). Node (C) (Z) at a lesser rank and superior position in the DAG than node (A).
may safely undergo the process to evict node (A) from its DAG Parent Node (C) may safely undergo the process to evict node (A) from its
set and attach directly to Node (Z) without creating a loop, because DAG parent set and attach directly to Node (Z) without creating a
its rank will decrease. loop, because its rank will decrease.
Consider the case where the link (C)->(A) becomes nonviable, and node Now consider the case where the link (C)->(A) becomes nonviable, and
(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) its DAG parent set, leaving an empty DAG parent set. Node (C)
becomes the root of its own floating, less preferred, DAG. becomes the root of its own floating, less preferred, DAG.
o Node (D), hearing a modified RA-DIO from Node (C), follows Node o Node (D), hearing a modified RA-DIO message from Node (C), follows
(C) into the floating DAG. This is depicted in Figure 1-2. In Node (C) into the floating DAG. This is depicted in Figure 1-2.
general, any node with no other options in the sub-DAG of Node (C) In general, any node with no other options in the sub-DAG of Node
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 RA-DIO from Node (B) and determines it is able to o Node (C) hears a RA-DIO message from Node (B) and determines it is
rejoin the grounded DAG by reattaching at a deeper rank to Node able to rejoin the grounded DAG by reattaching at a deeper rank to
(B). Node (C) starts a DAG Hop timer to coordinate this move. Node (B). Node (C) starts a DAG Hop timer to coordinate this
move.
o The timer expires and Node (C) adds Node (B) to its DAG Parent o The timer expires and Node (C) adds Node (B) to its DAG parent
set. Node (C) has now safely moved deeper within the grounded DAG set. Node (C) has now safely moved deeper within the grounded DAG
without creating any loops. Node (D), and any other sub-DAG of without creating any loops. Node (D), and any other sub-DAG of
Node (C), will hear the modified RA-DIO sourced from Node (C) and Node (C), will hear the modified RA-DIO message sourced from Node
follow Node (C) in a coordinated manner to reattach to the (C) and follow Node (C) in a coordinated manner to reattach to the
grounded DAG. The final DAG is depicted in Figure 1-3 grounded DAG. The final DAG is depicted in Figure 1-3
3.4.2. DAG Parent Selection, Stability, and Greediness 3.3.2. DAG Parent Selection, Stability, and Greediness
If a node is greedy and attempts to move deeper in the DAG, beyond If a node is greedy and attempts to move deeper in the DAG, beyond
its most preferred parent, in order to increase the size of the DAG its most preferred parent, in order to increase the size of the DAG
Parent set, then an instability can result. This is illustrated in parent set, then an instability can result. This is illustrated in
Figure 2. Figure 2.
Suppose a node is willing to receive and process a RA-DIOs from a Suppose a node is willing to receive and process a RA-DIO messages
node in its own sub-DAG, and in general a node deeper than it. In from a node in its own sub-DAG, and in general a node deeper than it.
such cases a chance exists to create a feedback loop, wherein two or In such cases a chance exists to create a feedback loop, wherein two
more nodes continue to try and move in the DAG in order to optimize or more nodes continue to try and move in the DAG in order to
against each other. In some cases this will result in an optimize against each other. In some cases this will result in an
instability. It is for this reason that RPL mandates that a node instability. It is for this reason that RPL mandates that a node
MUST NOT receive and process RA-DIOs from deeper nodes. This rule never receive and process RA-DIO messages from deeper nodes. This
creates an `event horizon', whereby a node cannot be influenced into rule creates an `event horizon', whereby a node cannot be influenced
an instability by the action of nodes that may be in its own sub-DAG. into an instability by the action of nodes that may be in its own
sub-DAG.
3.4.2.1. Example 3.3.2.1. Example
(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 2: Greedy DAG Parent Selection Figure 2: Greedy DAG Parent Selection
Consider the example depicted in Figure 2. A DAG is depicted in 3 Consider the example depicted in Figure 2. 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 2-1, Node (A) is a DAG Parent for in all 3 configurations. In Figure 2-1, Node (A) is a DAG parent for
Nodes (B) and (C), and (B)--(C) is a sibling link. In Figure 2-2, Nodes (B) and (C), and (B)--(C) is a sibling link. In Figure 2-2,
Node (A) is a DAG Parent for Nodes (B) and (C), and Node (B) is also Node (A) is a DAG parent for Nodes (B) and (C), and Node (B) is also
a DAG Parent for Node (C). In Figure 2-3, Node (A) is a DAG Parent a DAG parent for Node (C). In Figure 2-3, Node (A) is a DAG parent
for Nodes (B) and (C), and Node (C) is also a DAG Parent for Node for Nodes (B) and (C), and Node (C) is also a DAG parent for Node
(B). (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 2-1. instability can result. Consider the DAG illustrated in Figure 2-1.
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
parent and no siblings, the node may decide to insert itself at a
slightly higher rank in order to have at least one sibling and thus
an alternate forwarding solution. This does not deprive other nodes
of a forwarding solution and this is considered acceptable
greediness.
o Let Figure 2-1 be the initial condition. o Let Figure 2-1 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 2-2. Now Node (C) is deeper than both Nodes depicted in Figure 2-2. 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 from Node (C) (against the rules of RPL), and then process a RA-DIO message from Node (C) (against the rules of RPL),
Node (B) leaves the DAG and rejoins at a lower rank, taking both and then Node (B) leaves the DAG and rejoins at a lower rank,
Nodes (A) and (C) as DAG Parents. Now Node (B) is deeper than taking both Nodes (A) and (C) as DAG parents. Now Node (B) is
both Nodes (A) and (C) and is satisfied with 2 DAG parents. deeper than both Nodes (A) and (C) and is satisfied with 2 DAG
parents.
o Then Node (C) will leave and rejoin deeper, to again get 2 parents 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
them.
o Then Node (B) will leave and rejoin deeper, to again get 2 parents o Next Node (B) will again leave and rejoin deeper, to again get 2
parents
o ... 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 2-2 and Figure 2-3 until the nodes count to infinity and Figure 2-2 and Figure 2-3 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) stick 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) don't process DIOs from nodes deeper than * Nodes (B) and (C) do not process RA-DIO messages from nodes
themselves (possibly in their own sub-DAGs) deeper than themselves (because such nodes are possibly in
their own sub-DAGs)
3.4.3. Merging DAGs 3.3.3. Merging DAGs
The merging of DAGs is coordinated in a way such as to try and merge The merging of DAGs is coordinated in a way such as to try and merge
two DAGs cleanly, preserving as much DAG structure as possible, and two DAGs cleanly, preserving as much DAG structure as possible, and
in the process effecting a clean merge with minimal likelihood of in the process effecting a clean merge with minimal likelihood of
forming transient loops forming transient DAG loops. The coordinated merge is also intended
to minimize the related control cost.
3.4.3.1. Example 3.3.3.1. Example
: :
: :
(A) (D) (A) (D)
| | | |
| | | |
| | | |
(B) (E) (B) (E)
| | | |
| | | |
skipping to change at page 28, line 16 skipping to change at page 30, line 37
detach from a grounded DAG and (E) and (F) followed. All nodes are detach from a grounded DAG and (E) and (F) followed. All nodes are
using compatible objective code points. using compatible objective code points.
Nodes (D), (E), and (F) would rather join the more preferred grounded Nodes (D), (E), and (F) would rather join the more preferred grounded
DAG if they are able than to remain in the less preferred floating DAG if they are able than to remain in the less preferred floating
DAG. DAG.
Next, let links (C)--(D) and (A)--(E) become viable. The following Next, let links (C)--(D) and (A)--(E) become viable. The following
sequence of events may then occur in a typical case: sequence of events may then occur in a typical case:
o Node (D) will receive and process a RA-DIO from Node (C) on link o Node (D) will receive and process a RA-DIO message from Node (C)
(C)--(D). Node (D) will consider Node (C) a candidate neighbor, on link (C)--(D). Node (D) will consider Node (C) a candidate
will note that Node (C) is in a grounded DAG at rank d+2, and will neighbor and process the RA-DIO message since Node (C) belongs to
a different DAG (different DAGID) than Node (D). Node (D) will
note that Node (C) is in a grounded DAG at rank d+2, and will
begin the process to join the grounded DAG at rank d+3. Node (D) begin the process to join the grounded DAG at rank d+3. Node (D)
will start a DAG Hop timer, logically associated with the grounded will start a DAG Hop timer, logically associated with the grounded
DAG at Node (C), to coordinate the jump. The DAG Hop timer will DAG at Node (C), to coordinate the jump. The DAG Hop timer will
have a duration proportional to d+2. have a duration proportional to d+2.
o Similarly, Node (E) will receive and process a RA-DIO from Node o Similarly, Node (E) will receive and process a RA-DIO message from
(A) on link (A)--(E). Node (E) will consider Node (A) a candidate Node (A) on link (A)--(E). Node (E) will consider Node (A) a
neighbor, will note that Node (A) is in a grounded DAG at rank d, candidate neighbor, will note that Node (A) is in a grounded DAG
and will begin the process to join the grounded DAG at rank d+1. at rank d, and will begin the process to join the grounded DAG at
Node (E) will start a DAG Hop timer, logically associated with the rank d+1. Node (E) will start a DAG Hop timer, logically
grounded DAG at Node (A), to coordinate the jump. The DAG Hop associated with the grounded DAG at Node (A), to coordinate the
timer will have a duration proportional to d. jump. The DAG Hop timer will have a duration proportional to d.
o Node (F) takes no action, for Node (F) has observed nothing new to o Node (F) takes no action, for Node (F) has observed nothing new to
act on. act on.
o Node (E)'s DAG Hop timer for the grounded DAG at Node (A) expires o Node (E)'s DAG Hop timer for the grounded DAG at Node (A) expires
first. Node (E), upon the DAG Hop timer expiry, is removes Node first. Node (E), upon the DAG Hop timer expiry, removes Node (D)
(D), thus emptying the DAG parent set for the floating DAG and as its parent, thus emptying the DAG parent set for the floating
leaving the floating DAG. Node (E) then jumps to the grounded DAG DAG, and leaving the floating DAG. Node (E) then jumps to the
by entering Node (A) into the set of DAG Parents for the grounded grounded DAG by entering Node (A) into the set of DAG parents for
DAG. Node (E) is now in the grounded DAG at rank d+1. Node (E), the grounded DAG. Node (E) is now in the grounded DAG at rank
by jumping into the grounded DAG, has created an inconsistency and d+1. Node (E), by jumping into the grounded DAG, has created an
will begin to emit RA-DIOs more frequently. inconsistency by changing its DAGID, and will begin to emit RA-DIO
messages more frequently.
o Node (F) will receive and process a RA-DIO from Node (E). Node o Node (F) will receive and process a RA-DIO message from Node (E).
(F) will observe that Node (E) has changed its DAGID and will Node (F) will observe that Node (E) has changed its DAGID and will
directly follow Node (E) into the grounded DAG. Node (F) is now a directly follow Node (E) into the grounded DAG. Node (F) is now a
member of the grounded DAG at rank d+2. Note that any additional member of the grounded DAG at rank d+2. Note that any additional
sub-DAG of Node (E) would continue to join into the grounded DAG sub-DAG of Node (E) would continue to join into the grounded DAG
in this coordinated manner. in this coordinated manner.
o Node (D) will receive a RA-DIO from Node (E). Since Node (E) is o Node (D) will receive a RA-DIO message from Node (E). Since Node
now in a different DAG, Node (D) may process the RA-DIO from Node (E) is now in a different DAG, Node (D) may process the RA-DIO
(E). Node (D) will observe that, via node (E), it could attach to message from Node (E). Node (D) will observe that, via node (E),
the grounded DAG at rank d+2. Node (D) will start another DAG Hop it could attach to the grounded DAG at rank d+2. Node (D) will
timer, logically associated with the grounded DAG at Node (E), start another DAG Hop timer, logically associated with the
with a duration proportional to d+1. Node (D) now is running two grounded DAG at Node (E), with a duration proportional to d+1.
DAG hop timers, one which was started with duration proportional Node (D) now is running two DAG hop timers, one which was started
to d+1 and one proportional to d+2. with duration proportional to d+1 and one proportional to d+2.
o Generally, Node (D) will expire the timer associated with the jump o Generally, Node (D) will expire the timer associated with the jump
to the grounded DAG at node (E) first. Node (D) may then jump to to the grounded DAG at node (E) first. Node (D) may then jump to
the grounded DAG by entering Node (E) into its DAG Parent set for the grounded DAG by entering Node (E) into its DAG parent set for
the grounded DAG. Node (D) is now in the grounded DAG at rank the grounded DAG. Node (D) is now in the grounded DAG at rank
d+2. d+2.
o In this way RPL has coordinated a merge between the more preferred o In this way RPL has coordinated a merge between the more preferred
grounded DAG and the less preferred floating DAG, such that the grounded DAG and the less preferred floating DAG, such that the
nodes within the two DAGs come together in a generally ordered nodes within the two DAGs come together in a generally ordered
manner, avoiding the formation of loops in the process. manner, avoiding the formation of loops in the process.
3.4.4. Local and Temporary Routing Decision 3.4. Local and Temporary Routing Decision
Although implementation specific, it is worth noting that a node may Although implementation specific, it is worth noting that a node may
decide to implement some local routing decision based on some decide to implement some local routing decision based on some
metrics, as observed locally or reported in the DIO. For example, metrics, as observed locally or reported in the RA-DIO message. For
the routing may reflect a set of successors (next-hop), along with example, the routing may reflect a set of successors (next-hop),
various aggregated metrics used to load balance the traffic according along with various aggregated metrics used to load balance the
to some local policy. Such decisions are local and implementation traffic according to some local policy. Such decisions are local and
specific. implementation specific.
Routing stability is crucial in a LLN: in the presence of unstable Routing stability is crucial in a LLN: in the presence of unstable
links, the first option consists of removing the link from the DAG links, the first option consists of removing the link from the DAG
and triggering a DAG recomputation across all of the nodes affected and triggering a DAG recomputation across all of the nodes affected
by the removed link. Such a naive approach could unavoidably lead to by the removed link. Such a naive approach could unavoidably lead to
frequent and undesirable changes of the DAG, routing instability, and frequent and undesirable changes of the DAG, routing instability, and
high-energy consumption. The alternative approach adopted by RPL high-energy consumption. The alternative approach adopted by RPL
relies on the ability to temporarily not use a link toward a relies on the ability to temporarily not use a link toward a
successor marked as valid, with no change on the DAG structure. If successor marked as valid, with no change on the DAG structure. If
the link is perceived as non-usable for some period of time (locally the link is perceived as non-usable for some period of time (locally
configurable), this triggers a DAG recomputation, through the DAG configurable), this triggers a DAG recomputation, through the DAG
Discovery mechanism further detailed in Section 5.4, after reporting discovery mechanism further detailed in Section 5.3, after reporting
the link failure. Note that this concept may be extended to take the link failure. Note that this concept may be extended to take
into account other link characteristics: for the sake of into account other link characteristics: for the sake of
illustration, a node may decide to send a fixed number of packets to illustration, a node may decide to send a fixed number of packets to
a particular successor (because of limited buffering capability of a particular successor (because of limited buffering capability of
the successor) before starting to send traffic to another successor. the successor) before starting to send traffic to another successor.
According to the local policy function, it is possible for the node According to the local policy function, it is possible for the node
to order the DAG parent set from `most preferred' to `least to order the DAG parent set from `most preferred' to `least
preferred'. By constructing such an ordered set, and by appending preferred'. By constructing such an ordered set, and by appending
the set with siblings, the node is able to construct an ordered list the set with siblings, the node is able to construct an ordered list
skipping to change at page 30, line 19 skipping to change at page 32, line 48
loosely constrained, and may take into account the dynamics of the loosely constrained, and may take into account the dynamics of the
LLN. Further, a forwarding engine implementation may decide to LLN. Further, a forwarding engine implementation may decide to
perform load balancing functions using hash-based mechanisms to avoid perform load balancing functions using hash-based mechanisms to avoid
packet re-ordering. Note however, that specific details of a packet re-ordering. Note however, that specific details of a
forwarding engine implementation are beyond the scope of this forwarding engine implementation are beyond the scope of this
document. document.
These decisions may be local and/or temporary with the objective to These decisions may be local and/or temporary with the objective to
maintain the DAG shape while preserving routing stability. maintain the DAG shape while preserving routing stability.
3.4.5. Scalability 3.5. Maintenance of Routing Adjacency
As each node selects DAG Parents according to implementation specific
objectives, RPL is able to dynamically partition an LLN network into
different regions, each anchored by a DAG root. Multiple DAG roots
may be deployed in accordance with an implementation specific policy
designed to limit the size of a partition, e.g. for performance or
other reasons.
A further example is illustrated in Appendix C.
3.4.6. Maintenance of Routing Adjacency
In order to relieve the LLN of the overhead of periodic keepalives, In order to relieve the LLN of the overhead of periodic keepalives,
RPL MAY employ an as-needed mechanism of NS/NA in order to verify RPL may employ an as-needed mechanism of NS/NA in order to verify
routing adjacencies just prior to forwarding data. Pending the routing adjacencies just prior to forwarding data. Pending the
outcome of verifying the routing adjacency, the packet may either be outcome of verifying the routing adjacency, the packet may either be
forwarded or an alternate next-hop may be selected. forwarded or an alternate next-hop may be selected.
4. Constraint Based Routing in LLNs 4. Constraint Based Routing in LLNs
This aim of this section is to make a clear distinction between This aim of this section is to make a clear distinction between
routing metrics and constraints and define the term constraint based routing metrics and constraints and define the term constraint based
routing as used in this document. routing as used in this document.
skipping to change at page 31, line 21 skipping to change at page 33, line 37
Some routing protocols support more than one metric: in the vast Some routing protocols support more than one metric: in the vast
majority of the cases, one metric is used per (sub)topology. Less 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 often, a second metric may be used as a tie breaker in the presence
of ECMP (Equal Cost Multiple Paths). The optimization of multiple of ECMP (Equal Cost Multiple Paths). The optimization of multiple
metrics is known as an NP complete problem and is sometimes supported metrics is known as an NP complete problem and is sometimes supported
by some centralized path computation engine. by some centralized path computation engine.
In the case of RPL, it is virtually impossible to define *the* In the case of RPL, it is virtually impossible to define *the*
metric, or even a composite, that will fit it all: metric, or even a composite, that will fit it all:
o Some information apply to path setup time, other apply to packet o Some information apply when determining routes, other information
forwarding time. may apply only when forwarding packets along provisioned routes.
o Some values are aggregated hop-by-hop, others are triggers from o Some values are aggregated hop-by-hop, others are triggers from
L2. L2.
o Some properties are very stable, others vary rapidly. o Some properties are very stable, others vary rapidly.
o Some data are useful in a given scenario and useless in another. o Some data are useful in a given scenario and useless in another.
o Some arguments are scalar, others statistical. o Some arguments are scalar, others statistical.
For that reason, the RPL protocol core is agnostic to the logic that For that reason, the RPL protocol core is agnostic to the logic that
handles metrics. A node will be configured with some external logic handles metrics. A node will be configured with some external logic
to use and prioritize certain metrics for a specific scenario. As to use and prioritize certain metrics for a specific scenario. As
new heterogeneous devices are installed to support the evolution of a new heterogeneous devices are installed to support the evolution of a
network, or as networks form in a totally ad-hoc fashion, it will network, or as networks form in a totally ad-hoc fashion, it will
happen that nodes that are programmed with antagonistic logics and happen that nodes that are programmed with antagonistic logics and
conflicting or orthogonal priorities end up participating in the same conflicting or orthogonal priorities end up participating in the same
network. It is thus RECOMMENDED to use consistent parent selection network. It is thus recommended to use consistent parent selection
policy, as per Objective Code Points (OCP), to ensure consistent policy, as per Objective Code Points (OCP), to ensure consistent
optimized paths. optimized paths.
RPL is designed to survive and still operate, though in a somewhat RPL is designed to survive and still operate, though in a somewhat
degraded fashion, when confronted to such heterogeneity. The key degraded fashion, when confronted to such heterogeneity. The key
design point is that each node is solely responsible for setting the design point is that each node is solely responsible for setting the
vector of metrics that it sources in the DAG, derived in part from vector of metrics that it sources in the DAG, derived in part from
the metrics sourced from its preferred parent. As a result, the DAG the metrics sourced from its preferred parent. As a result, the DAG
is not broken if another node makes its decisions in as antagonistic is not broken if another node makes its decisions in as antagonistic
fashion, though an end-to-end path might not fully achieve any of the fashion, though an end-to-end path might not fully achieve any of the
skipping to change at page 32, line 45 skipping to change at page 35, line 15
Example 2: Example 2:
Link Metric: Delay Link Metric: Delay
Link Constraint: Bandwidth Link Constraint: Bandwidth
Objective function 2: Objective function 2:
"Find the shortest path (path with lowest cost where the path "Find the shortest path (path with lowest cost where the path
cost is the sum of all link costs (Delay)) along the path such cost is the sum of all link costs (Delay)) along the path such
that all links provide at least X Bit/s of reservable that all links provide at least X Bit/s of reservable
bandwidth." bandwidth."
5. Specification of Core Protocol 5. RPL Protocol Specification
5.1. DAG Information Option 5.1. DAG Information Option
The DAG Information Option carries a number of metrics and other The DAG Information Option carries a number of metrics and other
information that allows a node to discover a DAG, select its DAG information that allows a node to discover a DAG, select its DAG
parents, and identify its siblings while employing loop avoidance parents, and identify its siblings while employing loop avoidance
strategies. strategies.
5.1.1. DIO base option 5.1.1. DAG Information Option (DIO) base option
The DAG Information Option is a container option, which might contain The DAG Information Option is a container option carried within an
a number of suboptions. The base option regroups the minimum IPv6 Router Advertisement message as defined in [RFC4861], which
information set that is mandatory in all cases. might contain a number of suboptions. The base option regroups the
minimum information set that is mandatory in all cases.
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |G|D|A| Rsvd | Sequence | | Type | Length |G|D|A| 00000 | Sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAGPreference | BootTimeRandom | | DAGPreference | BootTimeRandom |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NodePref. | DAGRank | DAGDelay | | NodePref. | DAGRank | DAGDelay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DIOIntDoubl. | DIOIntMin. | DAGObjectiveCodePoint | | DIOIntDoubl. | DIOIntMin. | DAGObjectiveCodePoint |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PathDigest | | PathDigest |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
skipping to change at page 33, line 43 skipping to change at page 36, line 31
+ + + +
| | | |
+ + + +
| | | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-option(s)... | sub-option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: DIO Base Option Figure 4: DIO Base Option
Type: 8-bit unsigned identifying the DIO base option. The value is Type: 8-bit unsigned identifying the DIO base option. The suggested
to be assigned by the IANA. value is 140 to be confirmed by the IANA.
Length: 8-bit unsigned integer set to 4 when there is no suboption. Length: 8-bit unsigned integer set to 4 when there is no suboption.
The length of the option (including the type and length fields The length of the option (including the type and length fields
and the suboptions) in units of 8 octets. and the suboptions) in units of 8 octets.
Grounded (G): The Grounded (G) flag is set when the DAG root is Flag Field: Three flags are currently defined:
offering connectivity to an external routed infrastructure such
as the Internet.
Destination Advertisement Trigger (D): The Destination Advertisement Grounded (G): The Grounded (G) flag is set when the DAG root
Trigger (D) flag is set when the DAG root or another node in is offering connectivity to an external routed
the successor chain decides to trigger the sending of infrastructure such as the Internet.
Destination Advertisements in order to update routing state for
the outward direction along the DAG, as further detailed in
Section 5.10. Note that the use and semantics of this flag are
still under investigation.
Destination Advertisement Supported (A) : The Destination Supported Destination Advertisement Trigger (D): The Destination
(A) bit is set when the DAG root is capable to support the Advertisement Trigger (D) flag is set when the DAG root
collection of Destination Advertisement related routing state or another node in the successor chain decides to trigger
and enables the Destination Advertisement mechanism within the the sending of destination advertisements in order to
DAG. update routing state for the outward direction along the
DAG, as further detailed in Section 5.9. Note that the
use and semantics of this flag are still under
investigation.
Reserved: 5-bit unsigned integer set to 0 by the DAG root and left Destination Advertisement Supported (A) : The Destination
unchanged by nodes propagating the DIO. Supported (A) bit is set when the DAG root is capable to
support the collection of destination advertisement
related routing state and enables the operation of the
destination advertisement mechanism within the DAG.
Unassigned bits of the Flag Field are considered as reserved.
They MUST be set to zero on transmission and MUST be ignored on
receipt.
Sequence Number: 8-bit unsigned integer set by the DAG root, Sequence Number: 8-bit unsigned integer set by the DAG root,
incremented with each new DIO it sends on a link, and incremented according to a policy provisioned at the DAG root,
propagated with no change outwards along the DAG. and propagated with no change outwards along the DAG. Each
increment SHOULD have a value of 1 and may cause a wrap back to
zero.
DAGPreference: 8-bit unsigned integer set by the DAG root to its DAGPreference: 8-bit unsigned integer set by the DAG root to its
preference and unchanged at propagation. Default is 0 (lowest preference and unchanged at propagation. DAGPreference ranges
preference). The DAG preference provides an administrative from 0x00 (least preferred) to 0xFF (most preferred). The
mechanism to engineer the self-organization of the LLN, for default is 0 (least preferred). The DAG preference provides an
example indicating the most preferred LBR. If a node has the administrative mechanism to engineer the self-organization of
option to join a DAG of lower preference while still meeting the LLN, for example indicating the most preferred LBR. If a
other optimization objectives, then the node will seek the node has the option to join a more preferred DAG while still
minimum available preference. meeting other optimization objectives, then the node will seek
to join the more preferred DAG.
BootTimeRandom: A random value computed at boot time and recomputed BootTimeRandom: A random value computed at boot time and recomputed
in case of a duplication with another node. The concatenation in case of a duplication with another node. The concatenation
of the NodePreference and the BootTimeRandom is a 32-bit of the NodePreference and the BootTimeRandom is a 32-bit
extended preference that is used to resolve collisions. It is extended preference that is used to resolve collisions. It is
set by each node at propagation time. set by each node at propagation time.
NodePreference: The administrative preference of that LLN Node. NodePreference: The administrative preference of that LLN Node.
Default is 0. 255 is the highest possible preference. Set by Default is 0. 255 is the highest possible preference. Set by
each LLN Node at propagation time. Forms a collision each LLN Node at propagation time. Forms a collision
tiebreaker in combination with BootTimeRandom. tiebreaker in combination with BootTimeRandom.
DAGRank: 8-bit unsigned integer. The DAG rank of the DAG root is 0. DAGRank: 8-bit unsigned integer indicating the DAG rank of the node
The DAG Rank of a node attached to the DAG should be greater sending the RA-DIO message. The DAGRank of the DAG root is
than rank of its deepest DAG parent, as computed by an typically 1. DAGRank is further described in Section 5.3.
implementation specific routine. All nodes in the DAG
advertise their DAG rank in the DAG Information Options that
they append to the RA messages over their LLN interfaces as
part of the propagation process.
DAGDelay: 16-bit unsigned integer set by the DAG root indicating the DAGDelay: 16-bit unsigned integer set by the DAG root indicating the
delay before changing the DAG configuration, in TBD-units. A delay before changing the DAG configuration, in TBD-units. A
default value is TBD. It is expected to be an order of default value is TBD. It is expected to be an order of
magnitude smaller than the RA-interval. It is also expected to magnitude smaller than the RA-interval. It is also expected to
be an order of magnitude longer than the typical propagation be an order of magnitude longer than the typical propagation
delay inside the LLN. delay inside the LLN.
DIOIntervalDoublings: 8-bit unsigned integer. Used to configure the DIOIntervalDoublings: 8-bit unsigned integer. Configured on the DAG
trickle timer governing when RA-DIO should be sent within the root and used to configure the trickle timer governing when RA-
DAG. DIOIntervalDoublings is the number of times that the DIO message should be sent within the DAG.
DIOIntervalDoublings is the number of times that the
DIOIntervalMin is allowed to be doubled during the trickle DIOIntervalMin is allowed to be doubled during the trickle
timer operation, i.e. DIOIntervalMax = DIOIntervalMin * timer operation.
2^(DIOIntervalDoublings).
DIOIntervalMin: 8-bit unsigned integer. Used to configure the DIOIntervalMin: 8-bit unsigned integer. Configured on the DAG root
trickle timer governing when RA-DIO should be sent within the and used to configure the trickle timer governing when RA-DIO
DAG. The minimum configured interval for the RA-DIO trickle message should be sent within the DAG. The minimum configured
timer in units of ms is 2^DIOIntervalMin. For example, a interval for the RA-DIO trickle timer in units of ms is
DIOIntervalMin value of 16ms is expressed as 4. 2^DIOIntervalMin. For example, a DIOIntervalMin value of 16ms
is expressed as 4.
DAGObjectiveCodePoint: The DAG Objective Code Point is used to DAGObjectiveCodePoint: The DAG Objective Code Point is used to
indicate the cost metrics, objective functions, and methods of indicate the cost metrics, objective functions, and methods of
computation and comparison for DAGRank in use in the DAG. The computation and comparison for DAGRank in use in the DAG. The
DAG OCP is set by the DAG Root. (Objective Code Points are to DAG OCP is set by the DAG root. (Objective Code Points are to
be further defined in [I-D.ietf-roll-routing-metrics]. be further defined in [I-D.ietf-roll-routing-metrics].
PathDigest: 32-bit unsigned integer CRC, updated by each LLN Node. PathDigest: 32-bit unsigned integer CRC, updated by each LLN Node.
This is the result of a CRC-32c computation on a bit string This is the result of a CRC-32c computation on a bit string
obtained by appending the received value and the ordered set of obtained by appending the received value and the ordered set of
DAG parents at the LLN Node. DAG roots use a 'previous value' DAG parents at the LLN Node. DAG roots use a 'previous value'
of zeroes to initially set the PathDigest. Used to determine of zeroes to initially set the PathDigest. Used to determine
when something in the set of successor paths has changed. when something in the set of successor paths has changed.
DAGID: 128-bit unsigned integer which uniquely identify a DAG. This DAGID: 128-bit unsigned integer which uniquely identify a DAG. This
value is set by the DAG root. The global IPv6 address of the value is set by the DAG root. The global IPv6 address of the
DAG root can be used. DAG root can be used, however. the DAGID MUST be unique per DAG
within the scope of the LLN. In the case where a DAG root is
rooting multiple DAGs the DAGID MUST be unique for each DAG
rooted at a specific DAG root.
The following values MUST NOT change during the propagation of the The following values MUST NOT change during the propagation of RA-DIO
DIO outwards along the DAG: Type, Length, G, DAGPreference, DAGDelay messages outwards along the DAG: Type, Length, G, DAGPreference,
and DAGID. All other fields of the DIO are updated at each hop of DAGDelay and DAGID. All other fields of the RA-DIO message are
the propagation. updated at each hop of the propagation.
5.1.1.1. DIO Suboptions 5.1.1.1. DAG Information Option (DIO) Suboptions
In addition to the minimum options presented in the base option, a In addition to the minimum options presented in the base option,
number of suboptions are defined for the DIO: several suboptions are defined for the RA-DIO message:
5.1.1.1.1. Format 5.1.1.1.1. Format
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subopt. Type | Subopt Length | Suboption Data... | Subopt. Type | Subopt Length | Suboption Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: 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 containing a suboption for which the Suboption processing a RA-DIO message containing a suboption for which
Type value is not recognized by the receiver, the receiver MUST the Suboption Type value is not recognized by the receiver, the
silently ignore and skip over the suboption, correctly handling receiver MUST silently ignore the unrecognized option, continue
any remaining options in the message. to process the following suboption, correctly handling any
remaining options in the message.
Suboption Length: 8-bit unsigned integer, representing the length in Suboption Length: 8-bit unsigned integer, representing the length in
octets of the suboption, not including the suboption Type and octets of the suboption, not including the suboption Type and
Length fields. Length fields.
Suboption Data: A variable length field that contains data specific Suboption Data: A variable length field that contains data specific
to the option. to the option.
The following subsections specify the DIO suboptions which are The following subsections specify the RA-DIO message suboptions which
currently defined for use in the DAG Information Option. are currently defined for use in the DAG Information Option.
Implementations MUST silently ignore any DIO suboptions options that Implementations MUST silently ignore any RA-DIO message suboptions
they do not understand. options that they do not understand.
DIO suboptions may have alignment requirements. Following the RA-DIO message suboptions may have alignment requirements. Following
convention in IPv6, these options are aligned in a packet such that the convention in IPv6, these options are aligned in a packet such
multi-octet values within the Option Data field of each option fall that multi-octet values within the Option Data field of each option
on natural boundaries (i.e., fields of width n octets are placed at fall on natural boundaries (i.e., fields of width n octets are placed
an integer multiple of n octets from the start of the header, for n = at an integer multiple of n octets from the start of the header, for
1, 2, 4, or 8). n = 1, 2, 4, or 8).
5.1.1.1.2. Pad1 5.1.1.1.2. 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 6: 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 octet of padding in the DIO to The Pad1 option is used to insert one octet of padding in the RA-DIO
enable suboptions alignment. If more than one octet of padding is message to enable suboptions alignment. If more than one octet of
required, the PadN option, described next, should be used rather than padding is required, the PadN option, described next, should be used
multiple Pad1 options. rather than multiple Pad1 options.
5.1.1.1.3. PadN 5.1.1.1.3. PadN
The PadN option does not have any alignment requirements. Its format The PadN option does not have any alignment requirements. Its format
is as follows: is as follows:
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Type = 1 | Subopt Length | Subopt Data | Type = 1 | Subopt Length | Subopt Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
Figure 7: Pad N Figure 7: Pad N
The PadN option is used to insert two or more octets of padding in The PadN option is used to insert two or more octets of padding in
the DIO to enable suboptions alignment. For N (N > 1) octets of the RA-DIO message to enable suboptions alignment. For N (N > 1)
padding, the Option Length field contains the value N-2, and the octets of padding, the Option Length field contains the value N-2,
Option Data consists of N-2 zero-valued octets. PadN Option data and the Option Data consists of N-2 zero-valued octets. PadN Option
MUST be ignored by the receiver. data MUST be ignored by the receiver.
5.1.1.1.4. DAG Metric Container 5.1.1.1.4. DAG Metric Container
The DAG Metric Container suboption may be aligned as necessary to The DAG Metric Container suboption may be aligned as necessary to
support its contents. Its format is as follows: support its contents. Its format is as follows:
0 1 0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - - +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Type = 2 | Container Len | DAG Metric Data | Type = 2 | Container Len | DAG Metric Data
skipping to change at page 38, line 36 skipping to change at page 41, line 29
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Lifetime | | Prefix Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Prefix (Variable Length) | | Destination Prefix (Variable Length) |
. . . .
. . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: DAG Destination Prefix Figure 9: DAG Destination Prefix
The Destination Prefix suboption is used when the DAG root needs to The Destination Prefix suboption is used when the DAG root, or
indicate that it offers connectivity to destination prefixes other another node located inwards along the DAG on the path to the DAG
than the default. This may be useful in cases where more than one root, needs to indicate that it offers connectivity to destination
LBR is operating within the LLN and offering connectivity to prefixes other than the default. This may be useful in cases where
different administrative domains, e.g. a home network and a utility more than one LBR is operating within the LLN and offering
network. In such cases, upon observing the Destination Prefixes connectivity to different administrative domains, e.g. a home network
offered by a particular DAG root, a node MAY decide to join multiple and a utility network. In such cases, upon observing the Destination
DAGs in support of a particular application. Prefixes offered by a particular DAG, a node MAY decide to join
multiple DAGs in support of a particular application.
The Length is coded as the length of the suboption in octets, The Length is coded as the length of the suboption in octets,
excluding the Type and Length fields. The Prefix Length is an 8-bit excluding the Type and Length fields.
unsigned integer that indicates the number of leading bits in the
destination prefix. Prf is the Route Preference as in [RFC4191].
The Destination Prefix contains Prefix Length significant bits of the The Prefix Length is an 8-bit unsigned integer that indicates the
destination prefix. The remaining bits of the Destination Prefix, as number of leading bits in the destination prefix. Prf is the Route
required to complete the trailing octet, are set to 0. Preference as in [RFC4191]. The reserved fields MUST be set to zero
on transmission and MUST be ignored on receipt.
The Prefix Lifetime is a 32-bit unsigned integer representing the The Prefix Lifetime is a 32-bit unsigned integer representing the
length of time in seconds (relative to the time the packet is sent) length of time in seconds (relative to the time the packet is sent)
that the Destination Prefix is valid for route determination. A that the Destination Prefix is valid for route determination. A
value of all one bits (0xFFFFFFFF) represents infinity. A value of value of all one bits (0xFFFFFFFF) represents infinity. A value of
all zero bits (0x00000000) indicates a loss of reachability. all zero bits (0x00000000) indicates a loss of reachability.
In the event that a DAG root may need to specify that it offers The Destination Prefix contains Prefix Length significant bits of the
connectivity to more than one destination, the Destination Prefix destination prefix. The remaining bits of the Destination Prefix, as
suboption may be repeated. required to complete the trailing octet, are set to 0.
In the event that a RA-DIO message may need to specify connectivity
to more than one destination, the Destination Prefix suboption may be
repeated.
5.2. Conceptual Data Structures 5.2. Conceptual Data Structures
The RPL implementation must maintain the following conceptual data The RPL implementation MUST maintain the following conceptual data
structures in support of DAG Discovery: structures in support of DAG discovery:
o A set of Candidate Neighbors o A set of candidate neighbors
o For each DAG: o For each DAG:
* A set of Candidate DAG Parents * A set of candidate DAG parents
* A set of DAG Parents (which are a subset of Candidate DAG * A set of DAG parents (which are a subset of candidate DAG
Parents and may be implemented as such) parents and may be implemented as such)
5.2.1. Candidate Neighbors 5.2.1. Candidate Neighbors Data Structure
The set of Candidate Neighbors is to be populated by neighbors who The set of candidate neighbors is to be populated by neighbors who
are discovered by the neighbor discovery mechanism and further are discovered by the neighbor discovery mechanism and further
qualified as statistically stable as per the mechanisms discussed in qualified as statistically stable as per the mechanisms discussed in
[I-D.ietf-roll-routing-metrics]. The Candidate Neighbors, and [I-D.ietf-roll-routing-metrics]. The candidate neighbors, and
related metrics, should demonstrate stability/reliability beyond a related metrics, should demonstrate stability/reliability beyond a
certain threshold, and it is recommended that a local confidence certain threshold, and it is recommended that a local confidence
value be maintained with respect to the neighbor in order to track value be maintained with respect to the neighbor in order to track
this. Implementations may choose to bound the maximum size of the this. Implementations MAY choose to bound the maximum size of the
Candidate Neighbor set, in which case a local confidence value will candidate neighbor set, in which case a local confidence value will
assist in ordering neighbors to determine which ones should remain in assist in ordering neighbors to determine which ones should remain in
the Candidate Neighbor set and which should be evicted. the candidate neighbor set and which should be evicted.
If Neighbor Unreachability Detection (NUD) determines that a If Neighbor Unreachability Detection (NUD) determines that a
Candidate Neighbor is no longer reachable, then it shall be removed candidate neighbor is no longer reachable, then it shall be removed
from the Candidate Neighbor set. In the case that the Candidate from the candidate neighbor set. In the case that the candidate
Neighbor has associated states in the DAG Parent set or active DA neighbor has associated states in the DAG parent set or active DA
entries, then the removal of the Candidate Neighbor shall be entries, then the removal of the candidate neighbor shall be
coordinated with tearing down these states. All provisioned routes coordinated with tearing down these states. All provisioned routes
associated with the Candidate Neighbor should be removed. associated with the candidate neighbor should be removed.
5.2.2. DAGs 5.2.2. Directed Acyclic Graphs (DAGs) Data Structure
A DAG may be uniquely identified by within the LLN by its unique A DAG may be uniquely identified by within the LLN by its unique
DAGID. When a single device is capable to root multiple DAGs in DAGID. When a single device is capable to root multiple DAGs in
support of an application need for multiple optimization objectives support of an application need for multiple optimization objectives
it is expected to produce a different and unique DAGID for each of it is expected to produce a different and unique DAGID for each of
the multiple DAGs. the multiple DAGs.
For each DAG that a node is, or may become, a member of, the For each DAG that a node is, or may become, a member of, the
implementation MUST keep a conceptual record of: implementation MUST keep a DAG table with the following entries:
o DAGID o DAGID
o DAGObjectiveCodePoint o DAGObjectiveCodePoint
o A set of Destination Prefixes offered by the DAG root o A set of Destination Prefixes offered inwards along the DAG
o A set of candidate DAG Parents o A set of candidate DAG parents
o A timer to govern the sending of DIOs for the DAG o A timer to govern the sending of RA-DIO messages for the DAG
o DAGSequenceNumber o DAGSequenceNumber
When a DAG is discovered for which no DAG data structure is When a DAG is discovered for which no DAG data structure is
instantiated, and the node wants to join (i.e. the neighbor is to instantiated, and the node wants to join (i.e. the neighbor is to
become a Candidate DAG Parent in the Held-Up state), then the DAG become a candidate DAG parent in the Held-Up state), then the DAG
data structure is instantiated. data structure is instantiated.
When the Candidate DAG Parent set is depleted (i.e. the last When the candidate DAG parent set is depleted (i.e. the last
Candidate DAG Parent has timed out of the Held-Down state), then the candidate DAG parent has timed out of the Held-Down state), then the
DAG data structure may be deallocated. An implementation should DAG data structure SHOULD be suppressed after the expiration of an
delay before deallocating the DAG data structure in order to observe implementation-specific local timer. An implementation SHOULD delay
that the DAGSequenceNumber has incremented should any new candidate before deallocating the DAG data structure in order to observe that
DAG Parents appear for the DAG. the DAGSequenceNumber has incremented should any new candidate DAG
parents appear for the DAG.
5.2.2.1. Candidate DAG Parents 5.2.2.1. Candidate DAG Parents Structure
When the DAG is self-rooted, the set of candidate DAG Parents is When the DAG is self-rooted, the set of candidate DAG parents is
empty. empty.
In all other cases, for each candidate DAG Parent in the set, the In all other cases, for each candidate DAG parent in the set, the
implementation MUST keep a record of: implementation MUST keep a record of:
o a reference to the neighboring device which is the DAG parent o a reference to the neighboring device which is the DAG parent
o a record of most recent information taken from the DAG Information o a record of most recent information taken from the DAG Information
Object last processed from the candidate DAG Parent Object last processed from the candidate DAG parent
o a state associated with the role of the candidate as a potential o a state associated with the role of the candidate as a potential
DAG Parent {Current, Held-Up, Held-Down, Collision}, further DAG parent {Current, Held-Up, Held-Down, Collision}, further
described in Section 5.8 described in Section 5.7
o A DAG Hop Timer, if instantiated o A DAG Hop Timer, if instantiated
o A Held-Down Timer, if instantiated o A Held-Down Timer, if instantiated
5.2.2.1.1. DAG Parents 5.2.2.1.1. DAG Parents
Note that the subset of candidate DAG Parents in the `Current' state Note that the subset of candidate DAG parents in the `Current' state
comprises the set of DAG Parents, i.e. the nodes actively acting as comprises the set of DAG parents, i.e. the nodes actively acting as
parents in the DAG. parents in the DAG.
DAG Parents may be ordered, according to the OCP. When ordering DAG DAG parents may be ordered, according to the OCP. When ordering DAG
Parents, in consultation with the OCP, the most preferred DAG Parent parents, in consultation with the OCP, 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 or equal to that of the most preferred DAG Parent. than or equal to that of the most preferred DAG parent.
When nodes are added to or removed from the DAG Parent set the most When nodes are added to or removed from the DAG parent set the most
preferred DAG Parent may have changed and should be reevaluated. Any preferred DAG parent may have changed and should be reevaluated. Any
nodes having a rank greater than the most preferred parent after such nodes having a rank greater than self after such a change must be
a change must be placed in the Held-Down state and evicted as per the placed in the Held-Down state and evicted as per the procedures
procedures described in Section 5.8 described in Section 5.7
An implementation may choose to keep these records as an extension of An implementation may choose to keep these records as an extension of
the Default Router List (DRL). the Default Router List (DRL).
5.3. Initialization and Configuration 5.3. DAG Discovery and Maintenance
An implementation must provide a means, e.g. a set of APIs, to allow
the node to initialize/configure the RPL implementation. The RPL
implementation on the node must be provisioned to know:
Is the node serving a role in an application scenario whereby it
should permanently act as a DAG root? (For example, the node may
act as an LBR, provide Internet access, serve as an application
specific data-collection point, or provide application control to
the LLN.) If so,
What is the DAGPreference value for the self-rooted DAG (likely
0)?
What OCP are supported?
Is connectivity to external infrastructure provided (is the DAG
grounded?)
What destination prefixes are offered?
What is the DAGDelay?
Is the Destination Advertisement mechanism in effect?
What are the values for DIOIntervalDoublings, DIOIntervalMin?
Is the node to periodically emit DIOs (e.g. revise the DAG
Sequence Number upwards) in order to provide a heartbeat for
the DAG? If so, with what period?
If the node does not permanently act as a DAG root, should it DAG discovery locates the nearest sink, as determined according to
actively root a (floating, DAGPreference 0xFF) DAG when no other some metrics and constraints, and forms a Directed Acyclic Graph
DAG is available? (For example, a battery powered node may not towards that sink, by identifying a set of DAG parents. During this
wish expend energy to do this, but will instead passively listen process DAG discovery also identifies siblings, which may be used
for other options). later to provide additional path diversity towards the DAG root. DAG
discovery enables nodes to implement different policies for selecting
their DAG parents in the DAG by using implementation specific policy
functions. DAG discovery specifies a set of rules to be followed by
all implementations in order to ensure interoperation. DAG discovery
also standardizes the format that is used to advertise the most
common information that is used in order to select DAG parents.
For each DAG that the node may root, what is the DAGID? One of these information, the DAG rank, is used by DAG discovery to
provide loop avoidance even if nodes implement different policies.
The DAG Rank is computed as specified by the Objective Code Point in
use by the DAG, demonstrating the properties described in
Section 3.3.1. 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.
What are the supported OCP (optimization goals)? The DAG discovery procedures take into account a number of factors,
including:
What, if any, destination prefixes are being sought, associated o RPL rules for loop avoidance based on rank
with supported OCP?
When a node is provisioned with a set of optimization goals, o The OCP function
effectively indicating targeted OCPs for given destinations (possibly
including the default destination), it may conceptually organize
these into a table where each row indicates an optimization goal. As
DAGs are joined in order to satisfy optimization objectives,
references to the DAG supporting the objective may be entered into
each row. In this way a node may track which objectives are
satisfied by which DAGs, as well as which objectives are unsatisfied
by any DAG. This will help to inform a nodes decision to join a new
DAG, or perhaps leave an existing DAG in order to join a better
alternate DAG, in order to meet specific optimization objectives.
5.4. DAG Discovery o The advertised metrics
DAG Discovery locates the nearest sink and forms a Directed Acyclic o Local policy functions (e.g. a bounded number of candidate
Graph towards that sink, by identifying a set of DAG parents. During neighbors).
this process DAG Discovery also identifies siblings, which may be
used later to provide additional path diversity towards the DAG root.
DAG Discovery enables nodes to implement different policies for
selecting their DAG parents in the DAG by using implementation
specific policy functions. DAG Discovery specifies a set of rules to
be followed by all implementations in order to ensure interoperation.
DAG Discovery also standardizes the format that is used to advertise
the most common information that is used in order to select DAG
parents.
One of these information, the DAG rank, is used by DAG Discovery to 5.3.1. DAG Discovery Rules
provide loop avoidance even if nodes implement different policies.
The DAG Rank is computed as specified by the Objective Code Point in
use by the DAG, demonstrating the properties described in
Section 3.4.1. 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.
In order to organize and maintain loopless structure, the DAG In order to organize and maintain loopless structure, the DAG
Discovery implementation in the nodes MUST obey to the following discovery implementation in the nodes MUST obey to the following
rules and definitions: rules and definitions:
1. A node that does not have any DAG parents in a DAG is the root 1. A node that does not have any DAG parents in a DAG is the root
of its own floating DAG. It's rank is 1. A node will end up in of its own floating DAG. It's rank is 1. A node will end up in
that situation when it looses all of its current feasible that situation when it looses all of its current feasible
parents, i.e. the set of DAG parents becomes depleted. In that parents, i.e. the set of DAG parents becomes depleted. In that
case, the node SHOULD remember the DAGID and the sequence case, the node SHOULD remember the DAGID and the sequence
counter in the DIO of the lost parents for a period of time counter of the last RA-DIO message from the lost parents for a
which covers multiple DIO. period of time which covers multiple RA-DIO messages. This is
done so that if the node does encounter another possible
attachment point to the lost DAGID within a period of time, the
node may observe a sequence counter change by comparing the
observed sequence counter to the last observed sequence counter
and thus verify that the new attachment point is a viable and
independent alternative to attach back to the lost DAGID.
2. A LLN Node that is attached to an infrastructure that does not 2. A node that is attached to an infrastructure that does not
support DIO, is the DAG root of its own grounded DAG. It's rank support RA-DIO messages, is the DAG root of its own grounded
is 1. DAG. It's rank is 1. (For example an LBR that is in
communication with a non-LLN router not running RPL).
3. A router sending a RA without DIO is considered a grounded 3. A (non-LLN) router sending a RA messages without DIO is
infrastructure at rank 0. (For example, a router that is in considered a grounded infrastructure at rank 0. (For example, a
communication with an LLN node but not running RPL such as a router that is in communication with an LLN node but not running
backbone router in communication with an LBR) RPL such as a non-LLN public Internet router in communication
with an LBR)
4. The DAG root exposes the DAG in the RA-DIO and nodes propagate 4. The DAG root exposes the DAG in the RA-DIO message and nodes
the DIO outwards along the DAG with the RAs that they forward propagate the RA-DIO message outwards along the DAG with the RAs
over their LLN links. that they forward over their LLN links.
5. A node MAY move at any time, with no delay, within its DAG as 5. A node MAY move at any time, with no delay, within its DAG when
long as such a move does not increase its own DAG rank, as per the move does not cause the node to increase its own DAG rank,
the rank calculation indicated by the OCP. If a node is as per the rank calculation indicated by the OCP.
required to move such that it cannot stay within the DAG without
a rank increase, then it needs to first leave the DAG. In other
words a node that is already part of a DAG MAY move or follow a
DAG parent at any time and with no delay in order to be closer,
or stay as close, to the DAG root of its current DAG as it
already is. But a node MUST NOT move outwards along the DAG
that it is attached, except in the special case when choosing to
follow the last DAG parent in the set of DAG parents. RAs
received from other routers located higher in the same DAG may
be considered as coming from candidate parents. RAs received
from other routers located at the same rank in the same DAG may
be considered as coming from siblings. Nodes MUST ignore RAs
that are received from other routers located deeper within the
same DAG.
6. A node may jump from its current DAG into any different DAG if 6. A node MUST NOT move outwards along a DAG that it is attached
to, causing the DAG rank to increase, except in a special case
where the node MAY choose to follow the last DAG parent in the
set of DAG parents. In the general case, if a node is required
to move such that it cannot stay within the DAG without a rank
increase, then it needs to first leave the DAG. In other words
a node that is already part of a DAG MAY move or follow a DAG
parent at any time and with no delay in order to be closer, or
stay as close, to the DAG root of its current DAG as it already
is, but may not move outwards. RAs received from other routers
located at lesser rank in the same DAG may be considered as
coming from candidate parents. RAs received from other routers
located at the same rank in the same DAG may be considered as
coming from siblings. Nodes MUST ignore RAs that are received
from other routers located at greater rank within the same DAG.
7. A node may jump from its current DAG into any different DAG if
it is preferred for reasons of connectivity, configured it is preferred for reasons of connectivity, configured
preference, free medium time, size, security, bandwidth, DAG preference, free medium time, size, security, bandwidth, DAG
rank, or whatever metrics the LLN cares to use. A node may jump rank, or whatever metrics the LLN cares to use. A node may jump
at any time and to whatever rank it reaches in the new DAG, but at any time and to whatever rank it reaches in the new DAG, but
it may have to wait for a DAG Hop timer to elapse in order to do it may have to wait for a DAG Hop timer to elapse in order to do
so. This allows the new higher parts (closer to the sink) of so. This allows the new higher parts (closer to the sink) of
the DAG to move first, thus allowing stepped DAG the DAG to move first, thus allowing stepped DAG
reconfigurations and limiting relative movements. A node SHOULD reconfigurations and limiting relative movements. A node SHOULD
NOT join a previous DAG (identified by its DAGID) unless the NOT join a previous DAG (identified by its DAGID) unless the
sequence number in the DIO has incremented since the node left sequence number in the RA-DIO message has incremented since the
that DAG. A newer sequence number indicates that the candidate node left that DAG. A newer sequence number indicates that the
parents were not attached behind this node, as they kept getting candidate parents were not attached behind this node, as they
subsequent DIOs with new sequence numbers from the same DAG. In kept getting subsequent RA-DIO messages with new sequence
the event that old sequence numbers (two or more behind the numbers from the same DAG. In the event that old sequence
present value) are encountered they are considered stale and the numbers (two or more behind the present value) are encountered
corresponding parent SHOULD be removed from the set. they are considered stale and the corresponding parent SHOULD be
removed from the set.
7. If a node has selected a new set of DAG parents but has not 8. If a node has selected a new set of DAG parents but has not
moved yet (because it is waiting for DAG Hop timer to elapse), moved yet (because it is waiting for DAG Hop timer to elapse),
the node is unstable and refrains from sending RA-DIOs for that the node is unstable MUST NOT send RA-DIOs for that DAG.
DAG.
8. If a node receives a RA-DIO from one of its DAG parents, and if 9. If a node receives a RA-DIO from one of its DAG parents, and if
the parent contains a different DAGID, indicating that the the parent contains a different DAGID, indicating that the
parent has left the DAG, and if the node can remain in the parent has left the DAG, and if the node can remain in the
current DAG through an alternate DAG parent, then the node current DAG through an alternate DAG parent, then the node
should remove the DAG parent which has joined the new DAG from SHOULD remove the DAG parent which has joined the new DAG from
its DAG parent set and remain in the original DAG. If the node its DAG parent set and remain in the original DAG. If there is
was the last DAG parent then the node SHOULD follow that parent. no alternate parent for the DAG, then the node SHOULD follow
that parent into the new DAG.
9. When a node detects or causes a DAG inconsistency, as described 10. When a node detects or causes a DAG inconsistency, as described
in Section 5.4.3.2, then the node sends an unsolicited RA-DIO in Section 5.3.4.2, then the node SHOULD send an unsolicited RA-
message to its one-hop neighbors. The RA contains an updated DIO message to its one-hop neighbors. The RA-DIO is updated to
DIO to propagate the new DAG information. Such an event will propagate the new DAG information. Such an event MUST also
also cause the trickle timer governing the periodic RAs to be cause the trickle timer governing the periodic sending of RA-DIO
reset. messages to be reset.
10. If a DAG parent increases its rank such that the node rank would 11. If a DAG parent increases its rank such that the node rank would
have to change, and if the node does not wish to follow (e.g. it 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 has alternate options), then the DAG parent SHOULD be evicted
from the DAG parent set. If the DAG parent is the last in the from the DAG parent set. If the DAG parent is the last in the
DAG parent set, then the node may chose to follow it. DAG parent set, then the node SHOULD chose to follow it.
5.4.1. RA-DIO Reception 5.3.2. Reception and Processing of RA-DIO messages
When an DIO is received from a source device SRC, the receiving node When an RA-DIO message is received from a source device named SRC,
must first determine whether or not the DIO should be accepted for the receiving node must first determine whether or not the RA-DIO
further processing, and subsequently present the DIO for further message should be accepted for further processing, and subsequently
processing if eligible. present the RA-DIO message for further processing if eligible.
5.4.1.1. Determination of Eligibility for DIO Processing 5.3.2.1. Determination of Eligibility for DIO Processing
If the DIO is malformed, then the DIO is not eligible for further If the RA-DIO message is malformed, then the RA-DIO message is not
processing. eligible for further processing and is silently discarded. A RPL
implementation MAY log the reception of a malformed RA-DIO
message.
If SRC is not a member of the candidate neighbor set, then the RA- If SRC is not a member of the candidate neighbor set, then the RA-
DIO is not eligible for further processing. (Further evaluation/ DIO is not eligible for further processing. (Further evaluation/
confidence of this neighbor is necessary) confidence of this neighbor is necessary)
If the DIO advertises a DAG that the node is already a member of, If the RA-DIO message advertises a DAG that the node is already a
then: member of, then:
If the rank of SRC as reported in the DIO is less then that of If the rank of SRC as reported in the RA-DIO message is lesser
the node within the DAG, then the DIO MUST be considered for than that of the node within the DAG, then the RA-DIO message
further processing MUST be considered for further processing
If the rank of SRC as reported in the DIO is equal to that of If the rank of SRC as reported in the RA-DIO message is equal
the node within the DAG, then SRC is marked as a sibling and to that of the node within the DAG, then SRC is marked as a
the DIO is not eligible for further processing. sibling and the RA-DIO message is not eligible for further
processing.
If the rank of SRC as reported in the DIO is lesser than that If the rank of SRC as reported in the RA-DIO message is higher
of the node within the DAG, and SRC is not a DAG Parent, then than that of the node within the DAG, and SRC is not a DAG
the DIO is not eligible for further processing parent, then the RA-DIO message MUST NOT be considered for
further processing
If SRC is a DAG Parent for any other DAG that the node is attached If SRC is a DAG parent for any other DAG that the node is attached
to, then the DIO MUST be considered for further processing (the to, then the RA-DIO message MUST be considered for further
DAG Parent may have jumped). processing (the DAG parent may have jumped).
If the DIO advertises a DAG that offers a better (new or If the RA-DIO message advertises a DAG that offers a better (new
alternate) solution to an optimization objective desired by the or alternate) solution to an optimization objective desired by the
node, then the DIO MUST be considered for further processing. node, then the RA-DIO message MUST be considered for further
processing.
5.4.1.2. Overview of DIO Processing 5.3.2.2. Overview of RA-DIO Message Processing
If the DIO is for a new/alternate DAG: If the received RA-DIO message is for a new/alternate DAG:
Instantiate a data structure for the new/alternate DAG if Instantiate a data structure for the new/alternate DAG if
necessary necessary
Place the neighbor in the Candidate DAG Parent set Place the neighbor in the candidate DAG parent set
Has the node sent an RA within the risk window as described in If the node has sent an RA message within the risk window as
Section 5.8.3? If so, perform the collision detection described in Section 5.7.3 then perform the collision detection
described in Section 5.8.3. If a collision occurs, place the described in Section 5.7.3. If a collision occurs, place the
Candidate DAG Parent in the collision state and do not process candidate DAG parent in the collision state and do not process
the DIO any further as described in Section 5.8. the RA-DIO message any further as described in Section 5.7.
If the SRC node is also a DAG Parent for another DAG that the If the SRC node is also a DAG parent for another DAG that the
node is a member of, and if the new/alternate DAG satisfies an node is a member of, and if the new/alternate DAG satisfies an
equivalent optimization objective as the other DAG, then the equivalent optimization objective as the other DAG, then the
DAG Parent is known to have jumped. DAG parent is known to have jumped.
Remove SRC as a DAG Parent from the other DAG (place it in Remove SRC as a DAG parent from the other DAG (place it in
the held-down state) the held-down state)
If the other DAG is now empty of candidate Parents, then If the other DAG is now empty of candidate parents, then
directly follow SRC into the new DAG by adding it as a DAG directly follow SRC into the new DAG by adding it as a DAG
Parent in the Current state parent in the Current state, else ignore the RA-DIO message
(do not follow the parent).
Else ignore the DIO (do not follow the parent).
If the new/alternate DAG offers a better solution to the If the new/alternate DAG offers a better solution to the
optimization objectives, then prepare to jump: copy the DIO optimization objectives, then prepare to jump: copy the DIO
information into the record for the Candidate DAG Parent, place information into the record for the candidate DAG parent, place
the Candidate DAG Parent into the Held-Up state, and start the the candidate DAG parent into the Held-Up state, and start the
DAG Hop timer as per Section 5.8.1. DAG Hop timer as per Section 5.7.1.
If the DIO is for a known/existing DAG: If the RA-DIO message is for a known/existing DAG:
Process the DIO as per the rules in Section 5.4 Process the RA-DIO message as per the rules in Section 5.3
As candidate parents are identified, they may subsequently be As candidate parents are identified, they may subsequently be
promoted to DAG parents by following the rules of DAG Discovery as promoted to DAG parents by following the rules of DAG discovery as
described in Section 5.4. When a node adds another node to its set described in Section 5.3. When a node adds another node to its set
of candidate parents, the node becomes attached to the DAG through of candidate parents, the node becomes attached to the DAG through
the parent node. the 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. All be used to restrict which other nodes may become DAG parents. Some
nodes in the DAG Parent set should 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).
5.4.2. RA-DIO Transmission 5.3.3. RA-DIO Transmission
Each node maintains a timer that governs when to multicast RAs. This Each node maintains a timer that governs when to multicast RA
timer is implemented as a trickle timer operating over a variable messages. This timer is implemented as a trickle timer operating
interval. Trickle timers are further detailed in Section 5.4.3. The over a variable interval. Trickle timers are further detailed in
governing parameters for the timer should be configured consistently Section 5.3.4. The governing parameters for the timer should be
across the DAG, and are provided by the DAG root in the DIO. In configured consistently across the DAG, and are provided by the DAG
addition to periodic RAs, each LLN node will respond to Router root in the RA-DIO message. In addition to periodic RA messages,
Solicitation messages according to [RFC4861]. each LLN node will respond to Router Solicitation (RS) messages
according to [RFC4861].
o When a node is unstable, because any DAG Hop timer is running in o When a node is unstable, because any DAG Hop timer is running in
preparation for a jump, then the node must not transmit preparation for a jump, then the node MUST NOT transmit
unsolicited RA-DIOs (i.e. the node will remain silent when the unsolicited RA-DIOs (i.e. the node will remain silent when the
timer expires). timer expires).
o When a node detects an inconsistency, it may reset the interval of o When a node detects an inconsistency, it SHOULD reset the interval
the trickle timer to a minimum value, causing RAs to be emitted of the trickle timer to a minimum value, causing RA messages to be
more frequently as part of a strategy to quickly correct the emitted more frequently as part of a strategy to quickly correct
inconsistency. Such inconsistencies may be, for example, an the inconsistency. Such inconsistencies may be, for example, an
update to a key parameter (e.g. sequence number) in the DIO or a update to a key parameter (e.g. sequence number) in the RA-DIO
point-to-point loop detected when a node located inwards along the message or a loop detected when a node located inwards along the
DAG forwards traffic intended for the default destination. DAG forwards traffic outwards. Inconsistencies are further
Inconsistencies are further detailed in Section 5.4.3.2. detailed in Section 5.3.4.2.
o When a node enters a mode of consistent operation within a DAG, o When a node enters a mode of consistent operation within a DAG,
i.e. DIOs from its DAG Parents are consistent and no other i.e. RA-DIO messages from its DAG parents are consistent and no
inconsistencies are detected, it may begin to open up the interval other inconsistencies are detected, it may begin to open up the
of the trickle timer towards a maximum value, causing RAs to be interval of the trickle timer towards a maximum value, causing RAs
emitted less frequently, thus reducing network maintenance to be emitted less frequently, thus reducing network maintenance
overhead and saving energy consumption (which is of utmost overhead and saving energy consumption (which is of utmost
importance for battery-operated nodes). importance for battery-operated nodes).
o When a node is initialized, it may be configured to remain silent o When a node is initialized, it MAY be configured to remain silent
and not multicast any RAs until it has encountered and joined a and not multicast any RA messages until it has encountered and
DAG (perhaps initially probing for a nearby DAG with an RS). joined a DAG (perhaps initially probing for a nearby DAG with an
Alternately, it may choose to root its own floating DAG and begin RS message). Alternately, it may choose to root its own floating
multicasting RAs using a default trickle configuration. The DAG and begin multicasting RAs using a default trickle
second case may be advantageous if it is desired for independent configuration. The second case may be advantageous if it is
nodes to begin aggregating into scattered floating DAGs in the desired for independent nodes to begin aggregating into scattered
absence of a grounded node, for example in support of LLN floating DAGs in the absence of a grounded node, for example in
installation and commissioning. support of LLN installation and commissioning.
Note that if multiple DAG roots are participating in the same DAG, Note that if multiple DAG roots are participating in the same DAG,
i.e. offering DIOs with the same DAGID, then they must coordinate i.e. offering RA-DIO messages with the same DAGID, then they must
with each other to ensure that their DIOs are consistent when they coordinate with each other to ensure that their RA-DIO messages are
emit RA-DIOs. In particular the Sequence number must be identical consistent when they emit RA-DIO messages. In particular the
from each DAG root, regardless of which of the multiple DAG roots Sequence number must be identical from each DAG root, regardless of
issues the DIO, and changes to the Sequence number should be issued which of the multiple DAG roots issues the RA-DIO message, and
at the same time. The specific mechanism of this coordination, e.g. changes to the Sequence number should be issued at the same time.
along a backbone between DAG roots, is beyond the scope of this The specific mechanism of this coordination, e.g. along a non-LLN
specification. network between DAG roots, is beyond the scope of this specification.
5.4.3. Trickle Timer for RA Transmission 5.3.4. Trickle Timer for RA Transmission
RPL treats the construction of a DAG as a consistency problem, and RPL treats the construction of a DAG as a consistency problem, and
uses a trickle timer [Levis08] to control the rate of control uses a trickle timer [Levis08] to control the rate of control
broadcasts. The operation of this timer is in support of the broadcasts.
procedures further discussed in Section 5.4
For each DAG that a node is part of, the node must maintain a single For each DAG that a node is part of, the node must maintain a single
trickle timer. The required state contains the following conceptual trickle timer. The required state contains the following conceptual
items: items:
I: The current length of the communication interval I: The current length of the communication interval
T: A timer with a duration set to a random value in the range T: A timer with a duration set to a random value in the range
[I/2, I] [I/2, I]
C: Redundancy Counter C: Redundancy Counter
I_min: The smallest communication interval in milliseconds. This I_min: The smallest communication interval in milliseconds. This
value is learned from the DIO as (2^DIOIntervalMin)ms. The value is learned from the RA-DIO message as
default value is DEFAULT_DIO_INTERVAL_MIN. (2^DIOIntervalMin)ms. 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 as 2^I_doublings. This value is learned from the RA-DIO message
DIOIntervalDoublings. The default value is as DIOIntervalDoublings. The default value is
DEFAULT_DIO_INTERVAL_DOUBLINGS. DEFAULT_DIO_INTERVAL_DOUBLINGS.
5.4.3.1. Resetting the Trickle Timer 5.3.4.1. Resetting the Trickle Timer
The trickle timer for a DAGID is reset by: The trickle timer for a DAGID is reset by:
1. Setting I_min and I_doublings to the values learned from the RA- 1. Setting I_min and I_doublings to the values learned from the RA-
DIO. DIO 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 an LLN learns about a DAG through a RA and makes the decision to When an LLN learns about a DAG through a RA-DIO message and makes the
join it, it initializes the state of the trickle timer by resetting decision to join it, it initializes the state of the trickle timer by
the trickle timer and listening. Each time it hears a consistent RA resetting the trickle timer and listening. Each time it hears a
for this DAG from a DAG Parent, it increments C. consistent RA for this DAG from a DAG parent, it increments C.
When the timer fires at time T, the node compares C to the redundancy When the timer fires at time T, the node compares C to the redundancy
constant, DEFAULT_DIO_REDUNDANCY_CONSTANT. If C is less than that constant, DEFAULT_DIO_REDUNDANCY_CONSTANT. If C is less than that
value, the node generates a new RA and broadcasts it. When the value, the node generates a new RA and broadcasts it. When the
communication interval I expires, the node doubles the interval I so communication interval I expires, the node doubles the interval I so
long as it has previously doubled it fewer then I_doubling times, long as it has previously doubled it fewer than I_doubling times,
resets C, and chooses a new T value. resets C, and chooses a new T value.
5.4.3.2. Determination of Inconsistency 5.3.4.2. 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 DAG, for example:
o The node joins a new DAGID o The node joins a new DAGID
o The node moves within a DAGID o The node moves within a DAGID
o The node receives a modified DIO from a DAG parent o The node receives a modified RA-DIO message from a DAG parent
o A DAG parent forwards a packet intended for the default route, o A DAG parent forwards a packet intended to move inwards,
indicating an inconsistency and possible loop. indicating an inconsistency and possible loop.
o A metric communicated in the DIO is determined to be inconsistent, o A metric communicated in the RA-DIO message is determined to be
as according to a implementation specific path metric selection inconsistent, as according to a implementation specific path
engine. metric selection engine.
o The rank of a DAG parent has changed. o The rank of a DAG parent has changed.
The implementation SHOULD provide an API whereby any procedure that 5.4. DAG Heartbeat
detects an inconsistency may cause the trickle timer to reset.
5.5. DAG Heartbeat
The DAG Root makes the sole determination of when to revise the The DAG root makes the sole determination of when to revise the
DAGSequenceNumber by incrementing it upwards. When the DAGSequenceNumber by incrementing it upwards. When the
DAGSequenceNumber is increased an inconsistency results, causing RA- DAGSequenceNumber is increased an inconsistency results, causing RA-
DIOs to be sent back outwards along the DAG to convey the change. DIO messages to be sent back outwards along the DAG to convey the
The degree to which this mechanism is relied on may be determined by change. The degree to which this mechanism is relied on may be
the implementation- on one hand it may serve as a periodic heartbeat, determined by the implementation- on one hand it may serve as a
refreshing the DAG states, and on the other hand it may result in a periodic heartbeat, refreshing the DAG states, and on the other hand
constant steady-state control cost overhead which is not desirable. it may result in a constant steady-state control cost overhead which
is not desirable.
Some implementations may provide an administrative API at the DAG Some implementations may provide an administrative interface, such as
Root whereby the DAGSequenceNumber may be caused to increment in a command line, at the DAG root whereby the DAGSequenceNumber may be
response to some policy outside of the scope of RPL. caused to increment in response to some policy outside of the scope
of RPL.
Other implementations may make use of a periodic timer to Other implementations may make use of a periodic timer to
automatically increment the DAGSequenceNumber, resulting in a automatically increment the DAGSequenceNumber, resulting in a
periodic DAG Heartbeat at a rate appropriate to the application and periodic DAG Heartbeat at a rate appropriate to the application and
implementation. implementation.
5.6. DAG Selection 5.5. DAG Selection
The DAG selection is implementation and algorithm dependent. Nodes The DAG selection is implementation and algorithm dependent. Nodes
SHOULD prefer to join DAGs advertising OCPs and destinations SHOULD prefer to join DAGs advertising OCPs and destinations
compatible with their implementation specific objectives. In order compatible with their implementation specific objectives. In order
to limit erratic movements, and all metrics being equal, nodes SHOULD to limit erratic movements, and all metrics being equal, nodes SHOULD
keep their previous selection. Also, nodes SHOULD provide a means to keep their previous selection. Also, nodes SHOULD provide a means to
filter out a candidate parent whose availability is detected as filter out a candidate parent whose availability is detected as
fluctuating, at least when more stable choices are available. Nodes fluctuating, at least when more stable choices are available. Nodes
MAY place the failed candidate parent in a Hold Down mode that MAY place the failed candidate parent in a Hold Down mode that
ensures that the candidate parent will not be reused for a given ensures that the candidate parent will not be reused for a given
period of time. period of time.
When connection to a fixed network is not possible or preferable for When connection to a fixed network is not possible or preferable for
security or other reasons, scattered DAGs MAY aggregate as much as security or other reasons, scattered DAGs MAY aggregate as much as
possible into larger DAGs in order to allow connectivity within the possible into larger DAGs in order to allow connectivity within the
LLN. How to balance these DAGs is implementation dependent, and MAY LLN.
use a specific visitor-counter suboption in the DIO.
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 5.6. Administrative rank
When the DAG is formed under a common administration, or when a node When the DAG is formed under a common administration, or when a node
performs a certain role within a community, it might be beneficial to performs a certain role within a community, it might be beneficial to
associate a range of acceptable rank with that node. For instance, a associate a range of acceptable rank with that node. For instance, a
node that has limited battery should be a leaf unless there is no node that has limited battery should be a leaf unless there is no
other choice, and may then augment the rank computation specified by other choice, and may then augment the rank computation specified by
the OCP in order to expose an exaggerated rank. the OCP in order to expose an exaggerated rank.
5.8. Candidate DAG Parent States and Stability 5.7. Candidate DAG Parent States and Stability
Candidate DAG Parents may or may not be eligible to act as DAG Candidate DAG parents may or may not be eligible to act as DAG
Parents depending on runtime conditions. The following states are parents depending on runtime conditions. The following states are
defined: defined:
Current This candidate parent is in the set of DAG parents and Current This candidate parent is in the set of DAG parents and
may be used for forwarding traffic inward along the DAG. may be used for forwarding traffic inward along the DAG.
When a candidate parent is placed into the Current state, When a candidate parent is placed into the Current state,
or taken out of the Current state, it is necessary to re- or taken out of the Current state, it is necessary to re-
evaluate which of the remaining DAG Parents is the most evaluate which of the remaining DAG parents is the most
preferred DAG Parent and its rank. At that time any preferred DAG parent and its rank. At that time any
remaining DAG Parents of greater rank than the most remaining DAG parents of greater rank than this node must
preferred DAG parent must be placed in the Held-Down be placed in the Held-Down state, and the hold-down timer
state, and the hold-down timer started, in order to be started, in order to be evicted as DAG parents. In the
evicted as DAG Parents. same fashion, siblings must also be reevaluated.
Held-Up This parent can not be used until the DAG hop timer Held-Up This parent can not be used until the DAG hop timer
elapses. elapses.
Held-Down This candidate parent can not be used till hold down Held-Down This candidate parent can not be used till hold down
timer elapses. At the end of the hold-down period, the timer elapses. At the end of the hold-down period, the
candidate is removed from the Candidate DAG Parent set, candidate is removed from the candidate DAG parent set,
and may be reinserted if it appears again with a RA. and may be reinserted if it appears again with a RA-DIO
message.
Collision This candidate parent can not be used till its next RA. Collision This candidate parent can not be used till its next RA-
DIO message.
5.8.1. Held-Up 5.7.1. Held-Up
This state is managed by the DAG Hop timer, it serves 2 purposes: This state is managed by the DAG Hop timer, it serves 2 purposes:
Delay the reattachment of a sub-DAG that has been forced to Delay the reattachment of a sub-DAG that has been forced to
detach. This is not as safe as the use of the sequence, but still detach. This is not as safe as the use of the sequence, but still
covers that when a sub-DAG has detached, the Router Advertisement covers that when a sub-DAG has detached, the RA-DIO message that
- DAG Information Option that is initiated by the new DAG root has is initiated by the new DAG root has a chance to spread outward
a chance to spread outward along the sub-DAG so that two different along the sub-DAG, ideally forming a frozen sub-DAG that is aware
DAGs have formed. of the DAG change, such that two different DAGs have formed prior
to an attempted reattachment.
Limit RA-DIO storms when two DAGs collide/merge. The idea is that Limit RA-DIO message storms (control cost / churn) when two DAGs
between the nodes from DAG A that decide to move to DAG B, those collide/merge. The idea is that between the nodes from DAG A that
that see the highest place (closer to the DAG root) in DAG B will decide to move to DAG B, those that see the highest place (closer
move first and advertise their new locations before other nodes to the DAG root) in DAG B will move first and advertise their new
from DAG A actually move. locations before other nodes from DAG A actually move.
A new DAG is discovered upon a router advertisement message with or A new DAG is discovered upon receiving a RA message with or without a
without a RA-DIO. The node joins the DAG by selecting the source of DIO. The node joins the DAG by selecting the source of the RA
the RA message as a DAG parent (and possible default gateway) and message as a DAG parent (and possibly installing the DAG parent as a
propagating the DIO accordingly. default gateway). The node is then a member of the DAG and may begin
to multicast RA-DIO messages containing the DIO for the DAG.
When a new DAG is discovered, the candidate parent that advertises When a new DAG is discovered, the candidate parent that advertises
the new DAG is placed in a held up state for the duration of a DAG the new DAG is placed in a held up state for the duration of a DAG
Hop timer. If the resulting new set of DAG parents is more Hop timer. If the resulting new set of DAG parents is more
preferable than the current one, or if the node is intending to preferable than the current one, or if the node is intending to
maintain a membership in the new DAG in addition to its current DAG, maintain a membership in the new DAG in addition to its current DAG,
the node expects to jump and becomes unstable. the node expects to jump and becomes unstable.
A node that is unstable may discover other candidate parents from the A node that is unstable may discover other candidate parents from the
same new DAG during the instability phase. It needs to start a new same new DAG during the instability phase. It needs to start a new
DAG Hop timer for all these. The first timer that elapses for a DAG Hop timer for all these. The first timer that elapses for a
given new DAG clears them all for that DAG, allowing the node to jump given new DAG clears them all for that DAG, allowing the node to jump
to the highest position available in the new DAG. to the highest position available in the new DAG.
The duration of the DAG Hop timer depends on the DAG Delay of the new The duration of the DAG Hop timer depends on the DAG Delay of the new
DAG and on the rank of candidate parent that triggers it: (candidates DAG and on the rank of candidate parent that triggers it: (candidates
rank + random) * candidate's DAG_delay (where 0 <= random < 1). It rank + random) * candidate's DAG_delay (where 0 <= random < 1). It
is randomized in order to limit collisions and synchronizations. is randomized in order to limit collisions and synchronizations.
5.8.2. Held-Down 5.7.2. Held-Down
When a neighboring node is 'removed' from the Default Router List, it When a neighboring node is 'removed' from the Default Router List, it
is actually held down for a hold down timer period, in order to is actually held down for a hold down timer period, in order to
prevent flapping. This happens when a node disappears (upon prevent flapping. This happens when a node disappears (upon
expiration timer). expiration timer).
When the hold down timer elapses, the node is removed from the When the hold down timer elapses, the node is removed from the
Candidate DAG Parent set. candidate DAG parent set.
5.8.3. Collision 5.7.3. Collision
A race condition occurs if 2 nodes send RA-DIO at the same time and A race condition occurs if 2 nodes send RA-DIO messages at the same
then attempt to join each other. This might happen, for example, time and then attempt to join each other. This might happen, for
between nodes which act as DAG root of their own DAGs. In order to example, between nodes which act as DAG root of their own DAGs. In
detect the situation, LLN Nodes time stamp the sending of RA-DIO. order to detect the situation, LLN Nodes time stamp the sending of
Any RA-DIO received within a short link-layer-dependent period RA-DIO message. Any RA-DIO message received within a short link-
introduces a risk. To resolve the collision, a 32bits extended layer-dependent period introduces a risk. To resolve the collision,
preference is constructed from the DIO by concatenating the a 32bits extended preference is constructed from the RA-DIO message
NodePreference with the BootTimeRandom. by concatenating the NodePreference with the BootTimeRandom.
A node that decides to add a candidate to its DAG parents will do so A node that decides to add a candidate to its DAG parents will do so
between (candidate rank) and (candidate rank + 1) times the candidate between (candidate rank) and (candidate rank + 1) times the candidate
DAG Delay. But since a node is unstable as soon as it receives the DAG Delay. But since a node is unstable as soon as it receives the
RA-DIO from the desired candidate, it will restrain from sending a RA-DIO message from the desired candidate, it will restrain from
RA-DIO between the time it receives the RA and the time it actually sending a RA-DIO message between the time it receives the RA and the
jumps. So the crossing of RA may only happen during the propagation time it actually jumps. So the crossing of RA may only happen during
time between the candidate and the node, plus some internal queuing the propagation time between the candidate and the node, plus some
and processing time within each machine. It is expected that one DAG internal queuing and processing time within each machine. It is
delay normally covers that interval, but ultimately it is up to the expected that one DAG delay normally covers that interval, but
implementation and the configuration of the candidate parent to ultimately it is up to the implementation and the configuration of
define the duration of risk window. the candidate parent to define the duration of risk window.
There is risk of a collision when a node receives an RA, for another There is risk of a collision when a node receives an RA, for another
candidate that is more preferable than the current candidate, within candidate that is more preferable than the current candidate, within
the risk window. In the face of a potential collision, the node with the risk window. In the face of a potential collision, the node with
lowest extended preference processes the RA-DIO normally, while the lowest extended preference processes the RA-DIO message normally,
router with the highest extended preference places the other in while the router with the highest extended preference places the
collision state, does not start the DAG hop timer, and does not other in collision state, does not start the DAG hop timer, and does
become instable. It is expected that next RAs between the two will not become instable. It is expected that next RAs between the two
not cross anyway. will not cross anyway.
5.8.4. Instability For example, consider a case where two nodes are each rooting their
own transient floating DAGs and multicast RA-DIO messages towards
each other in a close enough interval that the RA-DIO messages
`cross'. Then each node may receive the RA-DIO message from the
other node, and in some scenario decide to join each others DAG. RPL
avoids this deadlock scenario via the collision mechanism described
above - after each node sends the RA-DIO message they will enter the
risk window. When the peer RA-DIO message is received in the risk
window, the nodes will calculate the extended preferences as describe
above and the node with the lowest extended preference will proceed
to process the RA-DIO message, while the other node will defer,
avoiding the deadlock scenario.
5.7.4. Instability
A node is instable when it is prepared to shortly replace a set of A node is instable when it is prepared to shortly replace a set of
DAG parents in order to jump to a different DAGID. This happens DAG parents in order to jump to a different DAGID. This happens
typically when the node has selected a more preferred candidate typically when the node has selected a more preferred candidate
parent in a different DAG and has to wait for the DAG hop timer to parent in a different DAG and has to wait for the DAG hop timer to
elapse before adjusting the DAG parent set. Instability may also elapse before adjusting the DAG parent set. Instability may also
occur when the entire current DAG parent set is lost and the next occur when the entire current DAG parent set is lost and the next
best candidates are still held up. Instability is resolved when the best candidates are still held up. Instability is resolved when the
DAG hop timer of all the candidate(s) causing instability elapse. DAG hop timer of all the candidate(s) causing instability elapse.
Such candidates then change state to Current or Held- Down. Such candidates then change state to Current or Held- Down.
Instability is transient (in the order of DAG hop timers). When a Instability is transient (in the order of DAG hop timers). When a
node is unstable, it MUST NOT send RAs with DIO. This avoids loops node is unstable, it MUST NOT send RAs with the DIO message. This
when node A decides to attach to node B and node B decides to attach avoids loops when node A decides to attach to node B and node B
to node A. Unless RAs cross (see Collision section), a node receives decides to attach to node A. Unless RAs cross (see Collision
DIO from stable candidate parents, which do not plan to attach to the section), a node receives RA-DIO messages from stable candidate
node, so the node can safely attach to them. parents, which do not plan to attach to the node, so the node can
safely attach to them.
5.9. Guidelines for Objective Code Points 5.8. Guidelines for Objective Code Points
5.9.1. Objective Function 5.8.1. Objective Function
An objective function (OF) selects a DAG to join, and a number of An Objective Function (OF) allows for the selection of a DAG to join,
peers in that DAG as parents. The OF computes an ordered list of and a number of peers in that DAG as parents. The OF is used to
parents and provides load balancing guidance. The OF is also compute an ordered list of parents and provides load balancing
responsible to compute the rank of the device within the DAG. guidance. The OF is also responsible to compute the rank of the
device within the DAG.
An Objective Function is indicated in the DIO using an objective code The Objective Function is specified in the RA-DIO message using an
point (OCP). The objective code point are administered by IANA that objective code point (OCP) and indicates the objective function that
might delegate some ranges to other organizations. This has been used to compute the DAG (e.g. "minimize the path cost using
specification reserves OCP 0, in support of default operation. the ETX metric and avoid `Blue' links"). The objective code points
are specified in [I-D.ietf-roll-routing-metrics]. This document
specifies the OCP 0, in support of default operation.
Most Objective Functions are expected to follow the same abstract Most Objective Functions are expected to follow the same abstract
behavior: behavior:
o The parent selection is triggered each time an event indicates o The parent selection is triggered each time an event indicates
that a potential next_hop information is updated. This might that a potential next_hop information is updated. This might
happen upon a RA-DIO, a timer elapse, or a trigger indicating that happen upon the reception of a RA-DIO message, a timer elapse, or
the state of a Candidate Neighbor has changed. 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 o An OF scans all the interfaces on the device. Although there may
typically be only one interface in most application scenarios, typically be only one interface in most application scenarios,
there might be multiple of them and an interface might be there might be multiple of them and an interface might be
configured to be usable or not for RPL operation. An interface configured to be usable or not for RPL operation. An interface
can also be configured with a preference or dynamically learned to can also be configured with a preference or dynamically learned to
be better than another by some heuristics that might be link-layer be better than another by some heuristics that might be link-layer
dependent and are out of scope. An interface might not be ready dependent and are out of scope. Finally an interface might or not
for IPv6 operation with a usable link-local address. Finally an match a required criterion for an Objective Function, for instance
interface might or not match a required criterion for an Objective a degree of security. As a result some interfaces might be
Function, for instance a degree of security. As a result some completely excluded from the computation, while others might be
interfaces might be completely excluded from the computation, more or less preferred.
while others might be more or less preferred.
o The OF scans all the Candidate Neighbors on the possible o The OF scans all the candidate neighbors on the possible
interfaces to check whether they can act as an attachment router interfaces to check whether they can act as an attachment router
for a DAG. There might be multiple of them and a Candidate for a DAG. There might be multiple of them and a candidate
Neighbor might need to pass some validation tests before it can be neighbor might need to pass some validation tests before it can be
used. In particular, some link layers require experience on the used. In particular, some link layers require experience on the
activity with a router to enable and raise the router value as a activity with a router to enable the router as a next_hop.
next_hop.
o The OF computes self's rank by adding the step of rank to that o The OF computes self's rank by adding the step of rank to that
candidate to the rank of that candidate. The step of rank is candidate to the rank of that candidate. The step of rank is
estimated as follows: estimated as follows:
* When a router has reached a value that's qualified as normal,
the step of rank for that hop is 4.
* The step of rank might vary from 1 to 16. * The step of rank might vary from 1 to 16.
+ 1 indicates a unusually good link, for instance a link + 1 indicates a unusually good link, for instance a link
between powered devices in a mostly battery operated between powered devices in a mostly battery operated
environment. environment.
+ 4 indicates a `normal'/typical link, as qualified by the
implementation.
+ 16 indicates a link that can hardly be used to forward any + 16 indicates a link that can hardly be used to forward any
packet, for instance a radio link with quality indicator or packet, for instance a radio link with quality indicator or
expected transmission count that flirts with the acceptable expected transmission count that is close to the acceptable
threshold. threshold.
* Candidate Neighbors that would cause self's rank to increase * Candidate neighbors that would cause self's rank to increase
are ignored are ignored
o As it scans all the Candidate Neighbors, the OF keeps the current 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 best parent and compares its capabilities with the current
Candidate Neighbor. The OF defines a number of tests that are candidate neighbor. The OF defines a number of tests that are
critical to reach the Objective. A test between the routers critical to reach the Objective. A test between the routers
determines an order relation. determines an order relation.
* If the routers are roughly equal for that relation then the * If the routers are roughly equal for that relation then the
next test is attempted between the routers, next test is attempted between the routers,
* Else the best of the 2 becomes the current best parent and the * Else the best of the 2 becomes the current best parent and the
scan continues with the next Candidate Neighbor scan continues with the next candidate neighbor
* One of these tests might include comparing the resulting ranks * Some OFs may include a test to compare the ranks that would
but it isn't necessarily so result if the node joined either router
o When the scan is complete, the preferred parent is elected and o When the scan is complete, the preferred parent is elected and
self's rank is computed as the preferred parent rank plus the step self's rank is computed as the preferred parent rank plus the step
in rank with that parent. in rank with that parent.
o Other rounds of scans might be necessary to elect alternate o Other rounds of scans might be necessary to elect alternate
parents and siblings. Self's rank is now determined by the new parents and siblings. In the next rounds:
preferred parent if it has changed. In the next rounds:
* Candidate Neighbors that are not in the same DAG are ignored * Candidate neighbors that are not in the same DAG are ignored
* Candidate Neighbors that would cause self's rank to increase * Candidate neighbors that are of worse rank than self are
are ignored ignored
* Candidate Neighbors of a better rank than self (non-siblings) * Candidate neighbors of a better rank than self (non-siblings)
are preferred are preferred
5.9.2. Objective Code Point 0 (OCP 0) 5.8.2. Objective Code Point 0 (OCP 0)
Here follows the specification for the Objective Function for OCP 0. Here follows the specification for the default Objective Function
This is a very simple references to help design more complex corresponding to OCP codepoint 0. This is a very simple reference to
Objective Functions. In particular, the Objective Function described help design more complex Objective Functions. In particular, the
here does not use physical metrics as described in Objective Function described here does not use physical metrics as
[I-D.ietf-roll-routing-metrics], but are only based on abstract described in [I-D.ietf-roll-routing-metrics], but are only based on
information from the DIO such as rank and administrative preference. abstract information from the RA-DIO message such as rank and
administrative preference.
OCP 0 is as a default fall back behavior when a node joins a DAG but This document specifies a default objective metric, called OF0, and
does not support the OF that's preferred for this DAG. using the OCP 0. OF0 is the default objective function of RPL, and
can be used if allowed by the policy of the processing node when no
objective function is included in the RA-DIO message, or if the OF
indicated in the RA-DIO message is unknown to the node. If not
allowed, then the RA-DIO message is simply ignored and not processed
by the node.
5.9.2.1. OCP 0 Objective Function (OF0) 5.8.2.1. OCP 0 Objective Function (OF0)
OF0 favors the connectivity. That is, the Objective Function is OF0 favors the connectivity. That is, the Objective Function is
designed to find the nearest sink into a 'grounded' topology, and if designed to find the nearest sink into a 'grounded' topology, and if
there's none then join any network per order of administrative there is none then join any network per order of administrative
preference. preference. The metric in use is the rank.
OF0 selects a preferred parent and a backup next_hop if that's OF0 selects a preferred parent and a backup next_hop if one is
available. The backup next_hop might be a parent or a sibling. All available. The backup next_hop might be a parent or a sibling. All
the traffic is routed via the preferred parent. When the link the traffic is routed via the preferred parent. When the link
conditions do not let a packet through to the preferred parent, the conditions do not let a packet through to the preferred parent, the
packet is passed to the backup next_hop. packet is passed to the backup next_hop.
The step of rank is 4 for each hop. The step of rank is 4 for each hop.
5.9.2.2. Selection of the Preferred Parent 5.8.2.2. Selection of the Preferred Parent
As it scans all the Candidate Neighbors, OF0 keeps the parent that is As it scans all the candidate neighbors, OF0 keeps the parent that is
the best for the following criteria (in order): the best for the following criteria (in order):
1. The interface must be usable and the administrative preference 1. The interface must be usable and the administrative preference
(if any) applies first. (if any) applies first.
2. A candidate that would cause the node to augment the rank in the 2. A candidate that would cause the node to augment the rank in the
current DAG is not considered. current DAG is not considered.
3. A router that is validated as usable is better. 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 better DAG preference 4. If none are grounded then a DAG with a more preferred
wins. administrative preference is better.
5. A router that offers connectivity to a grounded DAG is better. 5. A router that offers connectivity to a grounded DAG is better.
6. A lesser resulting rank is better. 6. A lesser resulting rank is better.
7. A DAG for which there is an alternate parent is better. This 7. A DAG for which there is an alternate parent is better. This
check is optional. It is performed by computing the backup check is optional. It is performed by computing the backup
next_hop while assuming that this router won. next_hop while assuming that this router won.
8. The DAG that was in use already is preferred. 8. The DAG that was in use already is preferred.
9. The router with a better router preference wins. 9. The router with a better router preference wins.
10. The preferred parent that was in use already is better. 10. The preferred parent that was in use already is better.
11. A router that is fresher (most recent RA) is better. 11. A router that has announced a RA-DIO message more recently is
preferred.
5.9.2.3. Selection of the Backup next_hop 5.8.2.3. Selection of the Backup next_hop
o The interface must be usable and the administrative preference (if o The interface must be usable and the administrative preference (if
any) applies first. any) applies first.
o A candidate that would cause the node to augment the rank in the o The preferred parent is ignored.
current DAG is not considered.
o The preferred parent is ignored
o Candidate Neighbors that are not in the same DAG are ignored o Candidate neighbors that are not in the same DAG are ignored.
o Candidate Neighbors that would cause self's rank (from that o Candidate neighbors with a higher rank are ignored.
determined by the preferred parent) to increase are ignored
o Candidate Neighbors of a better rank than self (non-siblings) are o Candidate neighbors of a better rank than self (non-siblings) are
preferred preferred.
o A router that is validated as usable is better 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 router with a better router preference wins.
o The backup next_hop that was in use already is better. o The backup next_hop that was in use already is better.
5.10. Establishing Routing State Outward Along the DAG 5.9. Establishing Routing State Outward Along the DAG
The Destination Advertisement mechanism supports the dissemination of The destination advertisement mechanism supports the dissemination of
routing state required to support traffic flows outward along the routing state required to support traffic flows outward along the
DAG, from the DAG root toward nodes. DAG, from the DAG root toward nodes.
Note that some aspects of the Destination Advertisement mechanism are As a result of destination advertisement operation:
still under investigation.
As a result of Destination Advertisement operation:
o DAG Discovery establishes a DAG oriented toward a DAG root using o DAG discovery establishes a DAG oriented toward a DAG root using
extended Neighbor Discovery RS/RA flows, along which inward routes extended Neighbor Discovery RS/RA flows, along which inward routes
toward the DAG root are set up. toward the DAG root are set up.
o Destination Advertisement extends Neighbor Discovery in order to o Destination advertisement extends Neighbor Discovery in order to
establish outward routes along the DAG, along paths containing DA establish outward routes along the DAG. Such paths consist of:
parents. Such paths consist of:
* Hop-By-Hop routing state within islands of `stateful' nodes. * Hop-By-Hop routing state within islands of `stateful' nodes.
* Source Routing `bridges' across nodes who do not retain state. * Source Routing `bridges' across nodes who 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. In this process they may also learn toward destinations by populating their routing tables with the
necessary piecewise source routes to traverse regions of the LLN routes learned from nodes in their sub-DAG. In this process they
that do not maintain routing state. They may perform route may also learn necessary piecewise source routes to traverse
aggregation on known destinations before emitting Destination regions of the LLN that do not maintain routing state. They may
Advertisements. perform route aggregation on known destinations before emitting
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 DAG
roots, are able to learn which destinations are contained in the sub- roots, are able to learn which destinations are contained in the sub-
DAG below the node, and via which next-hop neighbors. The 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 DAG root outwards along the
DAG. The mechanism is further enhance by supporting the construction DAG. The mechanism is further enhance by supporting the construction
of source routes across stateless `gaps' in the DAG, where nodes are of source routes across stateless `gaps' in the DAG, 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 or landmark mechanism allows for the implementation of loose-source routing.
(waypoint) 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 destination prefixes directly available from its one-hop learn destinations directly available from its one-hop neighbors.
neighbors.
The design choice behind this is not to synchronize the parent and A design choice behind advertising routes via destination
children databases along the DAG, but instead to update them advertisements is not to synchronize the parent and children
regularly to cover from the loss of packets. The rationale for that databases along the DAG, but instead to update them regularly to
choice is time variations in connectivity across unreliable links. recover from the loss of packets. The rationale for that choice is
If the topology can be expected to change frequently, synchronization time variations in connectivity across unreliable links. If the
might be an excessive goal in terms of exchanges and protocol topology can be expected to change frequently, synchronization might
complexity. The approach used here results in a simple protocol with be an excessive goal in terms of exchanges and protocol complexity.
no real peering. The Destination Advertisement mechanism hence The approach used here results in a simple protocol with no real
provides for periodic updates of the derivative routing state, as peering. The destination advertisement mechanism hence provides for
cued by occasional RAs and other mechanisms, similarly to other periodic updates of the routing state, as cued by occasional RAs and
protocols such as RIP [RFC2453]. other mechanisms, similarly to other protocols such as RIP [RFC2453].
5.10.1. Destination Advertisement Message Formats 5.9.1. Destination Advertisement Message Formats
5.10.1.1. DAO Option 5.9.1.1. DAO Option
RPL extends Neighbor Discovery [RFC4861] and RFC4191 [RFC4191] to RPL extends Neighbor Discovery [RFC4861] and RFC4191 [RFC4191] to
allow a node to include a Destination Advertisement option, which allow a node to include a destination advertisement option, which
includes prefix information, in the Neighbor Advertisements (NAs). A includes prefix information, in the Neighbor Advertisement (NA)
prefix option is normally present in Router Advertisements (RAs) messages. A prefix option is normally present in RA messages only,
only, but the NA is augmented with this option in order to propagate but the NA is augmented with this option in order to propagate
destination information inwards along the DAG. The option is named destination information inwards along the DAG. The option is named
the Destination Advertisement Option (DAO), and an NA containing this the Destination Advertisement Option (DAO), and an NA message
option may be referred to as a Destination Advertisement. The RPL containing this option may be referred to as a destination
use of Destination Advertisements allows the nodes in the DAG to advertisement, or NA-DAO. The RPL use of destination advertisements
build up routing state for nodes contained in the sub-DAG in support allows the nodes in the DAG to build up routing state for nodes
of traffic flowing outward along the DAG. contained in the sub-DAG in support of traffic flowing outward along
the DAG.
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Prefix Length | RRCount | | Type | Length | Prefix Length | RRCount |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Lifetime | | DAO Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Tag | | Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at page 59, line 29 skipping to change at page 62, line 25
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (Variable Length) | | Prefix (Variable Length) |
. . . .
. . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse Route Stack (Variable Length) | | Reverse Route Stack (Variable Length) |
. . . .
. . . .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: Destination Advertisement Option (DAO) Figure 10: The Destination Advertisement Option (DAO)
Type: 8-bit unsigned identifying the Destination Advertisement Type: 8-bit unsigned identifying the Destination Advertisement
option. The value is to be assigned by the IANA. option. IANA had defined the IPv6 Neighbor Discovery Option
Formats registry. The suggested type value for the Destination
Advertisement Option carried within a NA message is 141, to be
confirmed by IANA.
Length: 8-bit unsigned integer. The length of the option (including Length: 8-bit unsigned integer. The length of the option (including
the Type and Length fields) in units of 8 octets. the Type and Length fields) in units of 8 octets.
Prefix Length: Number of valid leading bits in the IPv6 Prefix. Prefix Length: 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.
skipping to change at page 60, line 12 skipping to change at page 63, line 6
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.
DAO Depth: Set to 0 by the node that owns the prefix and first DAO Depth: Set to 0 by the node that owns the prefix and first
issues the DAO. Incremented by all LLN nodes that propagate issues the NA-DAO message. Incremented by all LLN nodes that
the DAO. propagate the NA-DAO message.
Reserved: 8-bit unused field. It MUST be initialized to zero by the Reserved: 8-bit unused field. The reserved field MUST be set to
sender and MUST be ignored by the receiver. zero on transmission and MUST be ignored on receipt.
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 for that prefix. new NA-DAO message for that prefix.
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 initialized to zero by the sender and ignored by the be set to zero on transmission and MUST be ignored on receipt.
receiver.
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 who adds RRCount (possibly compressed) IPv6 addresses. A node who 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.
5.10.2. Destination Advertisement Operation 5.9.2. Destination Advertisement Operation
5.10.2.1. Overview
Note that some aspects of the Destination Advertisement mechanism are 5.9.2.1. Overview
still under investigation
According to implementation specific policy, a subset or all of the According to implementation specific policy, a subset or all of the
feasible parents in the DAG may be selected to receive prefix feasible parents in the DAG may be selected to receive prefix
information from the Destination Advertisement mechanism. This information from the destination advertisement mechanism. This
subset of DAG parents shall be designated the set of DA parents. subset of DAG parents shall be designated the set of DA parents.
RPL takes advantage of the DAG structure and allows a node capable of As NA-DAO messages for particular destinations move inwards along the
storing sufficient routing state to autonomously discover the DAG, a sequence counter is used to guarantee their freshness. The
destinations below itself through the operation of the Destination sequence counter is incremented by the source of the NA-DAO message
Advertisement mechanism. This allows participating nodes to build up (the node that owns the prefix, or learned the prefix via some other
routing state to support traffic flowing outwards along the DAG. means), each time it issues a NA-DAO message for its prefix. Nodes
Destination Advertisement messages convey the necessary information who receive the NA-DAO message and, if scope allows, will be
to learn the destinations. forwarding a NA-DAO message for the unmodified destination inwards
along the DAG, will leave the sequence number unchanged.
As Destination Advertisements for particular destinations move Intermediate nodes will check the sequence counter before processing
inwards along the DAG, a sequence counter is used to guarantee their a NA-DAO message, and if the DAO is unchanged (the sequence counter
freshness. The sequence counter is incremented by the source of the has not changed), then the NA-DAO message will be discarded without
DAO (the node that owns the prefix), each time it issues a DAO for additional processing. Further, if the NA-DAO message appears to be
its prefix. Nodes who receive the DAO and, if scope allows, will be out of synch (the sequence counter is 2 or more behind the present
forwarding a DAO for the unmodified destination inwards along the value) then the DAO state is considered to be stale and may be
DAG, will leave the sequence number unchanged. Intermediate nodes purged, and the NA-DAO message is discarded. A depth is also added
will check the sequence counter before processing a DAO, and if the for tracking purposes; the depth is incremented at each hop as the
DAO is unchanged (the sequence counter has not changed), then the DAO NA-DAO message is propagated up the DAG. Nodes who are storing
will be discarded without additional processing. Further, if the DAO routing state may use the depth to determine which possible next-hops
appears to be out of synch (the sequence counter is 2 or more behind for the destination are more optimal.
the present value) then the DAO state is considered to be stale and
may be purged, and the DAO is discarded. A depth is also added for
tracking purposes; the depth is incremented at each hop as the DAO is
propagated up the DAG. Nodes who are storing routing state may use
the depth to determine which possible next-hops for the destination
are more optimal.
If Destination Advertisements are activated in the DIO as indicated If destination advertisements are activated in the RA-DIO message as
by the `D' bit, the node sends unicast Destination Advertisements to indicated by the `D' bit, the node sends unicast destination
its DA parents, and only accepts unicast Destination Advertisements advertisements to its DA parents, and only accepts unicast
from any nodes BUT those contained in the DA parent subset. destination advertisements from any nodes but those contained in the
DA parent subset.
Every NA to a DA parent MAY contain one or more DAOs. Receiving a Every NA to a DA parent MAY contain one or more DAOs. Receiving a
DAG Discovery RA-DIO with the `D' Destination Advertisement bit set RA-DIO message with the `D' destination advertisement bit set from a
from a DAG parent stimulates the sending of a delayed Destination DAG parent stimulates the sending of a delayed destination
Advertisement back, with the collection of all known prefixes (that advertisement back, with the collection of all known prefixes (that
is the prefixes learned via Destination Advertisements for nodes is the prefixes learned via destination advertisements for nodes
lower in the DAG, and any connected prefixes). If the Destination lower in the DAG, and any connected prefixes). If the Destination
Advertisement Supported (A) bit is set in the DIO for the DAG, then a Advertisement Supported (A) bit is set in the RA-DIO message for the
Destination Advertisement is also sent to a DAG parent once it has DAG, then a destination advertisement is also sent to a DAG parent
been added to the DA parent set after a movement, or when the list of once it has been added to the DA parent set after a movement, or when
advertised prefixes has changed. Destination Advertisements may also the list of advertised prefixes has changed. Destination
be scheduled for sending when the PathDigest of the DIO has changed, advertisements may also be scheduled for sending when the PathDigest
indicating that some aspect of the inwards paths along the DAG has of the RA-DIO message has changed, indicating that some aspect of the
been modified. inwards paths along the DAG has been modified.
Destination Advertisements may advertise positive (prefix is present) Destination advertisements may advertise positive (prefix is present)
or negative (removed) DAOs. A no-DAO is stimulated by the or negative (removed) NA-DAO messages, termed as no-DAOs. A no-DAO
disappearance of a prefix below. This is discovered by timing out is stimulated by the disappearance of a prefix below. This is
after a request (a RA-DIO) or by receiving a no-DAO. A no-DAO is a discovered by timing out after a request (a RA-DIO message) or by
conveyed as a DAO with a DAO Lifetime of 0. receiving a no-DAO. A no-DAO is a conveyed as a NA-DAO message with
a DAO Lifetime of 0.
A node who is capable of recording the state information conveyed in A node who is capable of recording the state information conveyed in
a unicast DAO will do so upon receiving and processing the DAO, thus a unicast NA-DAO message will do so upon receiving and processing the
building up routing state concerning destinations below it in the NA-DAO message, thus building up routing state concerning
DAG. If a node capable of recording state information receives a DAO destinations below it in the DAG. If a node capable of recording
containing a Reverse Route Stack, then the node knows that the DAO state information receives a NA-DAO message containing a Reverse
has traversed one or more nodes that did not retain any routing state Route Stack, then the node knows that the NA-DAO message has
as it traversed the path from the DAO source to the node. The node traversed one or more nodes that did not retain any routing state as
may then extract the Reverse Route Stack and retain the included it traversed the path from the DAO source to the node. The node may
state in order to specify Source Routing instructions along the then extract the Reverse Route Stack and retain the included state in
return path towards the destination. The node MUST set the RRCount order to specify Source Routing instructions along the return path
back to zero and clear the Reverse Route Stack prior to passing the towards the destination. The node MUST set the RRCount back to zero
DAO information on. and clear the Reverse Route Stack prior to passing the NA-DAO message
information on.
A node who is unable to record the state information conveyed in the A node who is unable to record the state information conveyed in the
DAO will append the next-hop address to the Reverse Route Stack, NA-DAO message will append the next-hop address to the Reverse Route
increment the RRCount, and then pass the Destination Advertisement on Stack, increment the RRCount, and then pass the destination
without recording any additional state. In this way the Reverse advertisement on without recording any additional state. In this way
Route Stack will come to contain a vector of next hops that must be the Reverse Route Stack will contain a vector of next hops that must
traversed along the reverse path that the DAO has traveled. The be traversed along the reverse path that the NA-DAO message has
vector will be ordered such that the node closest to the destination traveled. The vector will be ordered such that the node closest to
will appear first in the list. In such cases the node may choose to the destination will appear first in the list. In such cases, if it
convey the Destination Advertisement to one or more DAG Parents in is useful to the implementation to try and build up redundant paths,
order of preference as guided by an implementation specific policy. the node may choose to convey the destination advertisement to one or
more DAG parents in order of preference as guided by an
implementation specific policy.
In hybrid cases, some nodes along the path a Destination In some cases (called hybrid cases), some nodes along the path a
Advertisement follows inward along the DAG may store state and some destination advertisement follows inward along the DAG may store
may not. The Destination Advertisement mechanism allows for the state and some may not. The destination advertisement mechanism
provisioning of routing state such that when a packet is traversing allows for the provisioning of routing state such that when a packet
outwards along the DAG, some nodes may be able to directly forward to is traversing outwards along the DAG, some nodes may be able to
the next hop, and other nodes may be able to specify a piecewise directly forward to the next hop, and other nodes may be able to
source route in order to bridge spans of stateless nodes within the specify a piecewise source route in order to bridge spans of
path on the way to the desired destination. stateless nodes within the path on the way to the desired
destination.
In the degenerate case, 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 destination advertisements pass by, and the DAG root ends up with NA-
DAOs that contain a completely specified route back to the DAO 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 nor indicate support for Destination DAG root should not request (Destination Advertisement Trigger) nor
Advertisements if it is not able to store the Reverse Route Stack indicate support (Destination Advertisement Supported) for
information in the degenerate case. destination advertisements if it is not able to store the Reverse
Route Stack information in this case.
Information learned through Destination Advertisements can be
redistributed in a routing protocol, MANET or IGP. But the MANET or
the IGP SHOULD NOT be redistributed into Destination Advertisements.
This creates a hierarchy of routing protocols where DA routes stand
somewhere between connected and IGP routes.
The Destination Advertisement mechanism requires stateful nodes to The destination advertisement mechanism requires stateful nodes to
maintain lists of known prefixes. A prefix entry contains the maintain lists of known prefixes. A prefix entry contains the
following abstract information: following abstract information:
o A reference to the ND entry that was created for the advertising o A reference to the ND entry that was created for the advertising
neighbor. neighbor.
o The IPv6 address and interface for the advertising neighbor. o The IPv6 address and interface for the advertising neighbor.
o The logical equivalent of the full Destination Advertisement o The logical equivalent of the full destination advertisement
information (including the prefixes, depth, and Reverse Route information (including the prefixes, depth, and Reverse Route
Stack, if any). Stack, if any).
o A 'reported' Boolean to keep track whether this prefix was o A 'reported' Boolean to keep track whether this prefix was
reported already, and to which of the DA parents. reported already, and to which of the DA parents.
o A counter of retries to count how many RA-DIOs were sent on the o A counter of retries to count how many RA-DIO messages were sent
interface to the advertising neighbor without reachability on the interface to the advertising neighbor without reachability
confirmation for the prefix. confirmation for the prefix.
Note that nodes may receive multiple information from different Note that nodes may receive multiple information from different
neighbors for a specific destination, as different paths through the neighbors for a specific destination, as different paths through the
DAG may be propagating information inwards along the DAG for the same DAG may be propagating information inwards along the DAG for the same
destination. A node who is recording routing state will keep track destination. A node who 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 for a particular prefix to the DA comes time to propagate the NA-DAO message for a particular prefix to
parents, then the DAO information will be selected from among the the DA parents, then the DAO information will be selected from among
advertising neighbors who offer the least depth to the destination. the advertising neighbors who offer the least depth to the
destination.
The Destination Advertisement mechanism stores the prefix entries in The destination advertisement mechanism stores the prefix entries in
one of 3 abstract lists; the Connected, the Reachable and the one of 3 abstract lists; the Connected, the Reachable and the
Unreachable lists. Unreachable lists.
The Connected list corresponds to the prefixes owned and managed by The Connected list corresponds to the prefixes owned and managed by
the local node. the local node.
The Reachable list contains prefixes for which the node keeps The Reachable list contains prefixes for which the node keeps
receiving DAOs, and for those prefixes which have not yet timed out. receiving NA-DAO messages, and for those prefixes which have not yet
timed out.
The Unreachable list keeps track of prefixes which are no longer The Unreachable list keeps track of prefixes which are no longer
valid and in the process of being destroyed, in order to send no-DAOs valid and in the process of being deleted, in order to send NA-DAO
to the DA parents. messages with zero lifetime (also called no-DAO) to the DA parents.
5.10.2.1.1. Destination Advertisement Timers 5.9.2.1.1. Destination Advertisement Timers
The Destination Advertisement mechanism requires 2 timers; the The destination advertisement mechanism requires 2 timers; the
DelayNA timer and the DestroyTimer. DelayNA timer and the RemoveTimer.
o The DelayNA timer is armed upon a stimulation to send a o The DelayNA timer is armed upon a stimulation to send a
Destination Advertisement (such as a DIO from a DA parent). When destination advertisement (such as a RA-DIO message from a DA
the timer is armed, all entries in the Reachable list as well as parent). When the timer is armed, all entries in the Reachable
all entries for Connected list are set to not reported yet for list as well as all entries for Connected list are set to not be
that particular DA parent. reported yet for that particular DA parent.
o The DelayNA timer has a duration that is DEF_NA_LATENCY divided by o The DelayNA timer has a duration that is DEF_NA_LATENCY divided by
a multiple of the DAG rank of the node. The intention is that a multiple of the DAG rank of the node. The intention is that
nodes located deeper in the DAG should have a shorter DelayNA nodes located deeper in the DAG should have a shorter DelayNA
timer, allowing DAOs a chance to be reported from deeper in the timer, allowing NA-DAO messages a chance to be reported from
DAG and potentially aggregated along sub-DAGs before propagating deeper in the DAG and potentially aggregated along sub-DAGs before
further inwards. propagating further inwards.
o The DestroyTimer is armed when at least one entry has exhausted o The RemoveTimer is used to clean up entries for which NA-DAO
its retries, which means that a number of RA-DIO were sent toward messages are no longer being received from the sub-DAG.
the reporting neighbor but that the entry was not confirmed with a
DAO. When the destroy timer elapses, for all exhausted entries,
the associated route is removed, and the entry is scheduled to be
destroyed.
o The Destroy timer has a duration of min (MAX_DESTROY_INTERVAL, * When a RA-DIO message is sent that is requesting destination
advertisements, a flag is set for all DAO entries in the
routing table.
* If the flag has already been set for a DAO entry, the retry
count is incremented.
* If a NA-DAO message is received to confirm the entry, the entry
is refreshed and the flag and count may be cleared.
* If at least one entry has reached a threshold value and the
RemoveTimer is not running, the entry is considered to be
probably gone and the RemoveTimer is started.
* When the RemoveTimer elapse, NA-DAO messages with lifetime 0,
i.e. no-DAOs, are sent to explicitly inform DA parents that the
entries who have reached the threshold are no longer available,
and the related routing states may be propagated and cleaned
up.
o The RemoveTimer has a duration of min (MAX_DESTROY_INTERVAL,
RA_INTERVAL). RA_INTERVAL).
5.10.2.2. Multicast Destination Advertisement messages 5.9.2.2. Multicast Destination Advertisement messages
It is also possible for a node to multicast a DAO to the link-local It is also possible for a node to multicast a NA-DAO message to the
scope all-nodes multicast address FF02::1. This message will be link-local scope all-nodes multicast address FF02::1. This message
received by all node listening in range of the emitting node. The will be received by all node listening in range of the emitting node.
objective is to enable direct P2P communication, between destination The objective is to enable direct P2P communication, between
prefixes directly supported by neighboring nodes, without needing the destinations directly supported by neighboring nodes, without needing
RPL routing structure to relay the packets. the RPL routing structure to relay the packets.
A multicast DAO MUST be used only to advertise information about A multicast NA-DAO message MUST be used only to advertise information
self, i.e. prefixes in the Connected list. This would typically be a about self, i.e. prefixes in the Connected list or addresses owned by
multicast group that this node is listening to or a global address this node. This would typically be a multicast group that this node
owned by this node, though it can be used to advertise any prefix is listening to or a global address owned by this node, though it can
owned by this node as well. A multicast DAO is not used for routing be used to advertise any prefix owned by this node as well. A
and does not presume any DAG relationship between the emitter and the multicast NA-DAO message is not used for routing and does not presume
receiver; it MUST NOT be used to relay information learned (e.g. any DAG relationship between the emitter and the receiver; it MUST
information in the Reachable list) from another node. NOT be used to relay information learned (e.g. information in the
Reachable list) from another node; information obtained from a
multicast NA-DAO MAY be installed in the routing table and MAY be
propagated by a router in unicast NA-DAOs.
A node receiving a multicast DAO addressed to FF02::1 MAY install A node receiving a multicast NA-DAO message addressed to FF02::1 MAY
prefixes contained in the DAO in the routing table for local use. install prefixes contained in the NA-DAO message in the routing table
Such a node MUST NOT perform any other processing on the DAO (i.e. for local use. Such a node MUST NOT perform any other processing on
such a node does not presume it is a DA parent). the NA-DAO message (i.e. such a node does not presume it is a DA
parent).
5.10.2.3. Unicast Destination Advertisement messages from child to 5.9.2.3. Unicast Destination Advertisement messages from child to
parent parent
When sending a Destination Advertisement to a DA parent, a LLN Node When sending a destination advertisement to a DA parent, a node
includes the DAOs about not already reported prefix entries in the includes the DAOs for prefix entries not already reported (since the
Reachable and Connected lists, as well as no-DAOs for all the entries last DA Trigger from an RA-DIO message) in the Reachable and
in the Unreachable list. Depending on its policy and ability to Connected lists, as well as no-DAOs for all the entries in the
retain routing state, the receiving node SHOULD keep a record of the Unreachable list. Depending on its policy and ability to retain
reported DAO. If the DAO offers the best route to the prefix as routing state, the receiving node SHOULD keep a record of the
determined by policy and other prefix records, the node SHOULD reported NA-DAO message. If the NA-DAO message offers the best route
install a route to the prefix in the DAO via the link local address to the prefix as determined by policy and other prefix records, the
of the reporting neighbor and it SHOULD further propagate the node SHOULD install a route to the prefix reported in the NA-DAO
information, either as a DAO or by means of redistribution into a message via the link local address of the reporting neighbor and it
routing protocol. SHOULD further propagate the information in a NA-DAO message.
The RA-DIO from the DAG root is used to synchronize the whole DAG, The RA-DIO message from the DAG root is used to synchronize the whole
including the periodic reporting of Destination Advertisements back DAG, including the periodic reporting of destination advertisements
up the DAG. Its period is expected to vary, depending on the back up the DAG. Its period is expected to vary, depending on the
configuration of the trickle timer that governs the RAs. configuration of the trickle timer that governs the RAs.
When a node receives a RA-DIO over an LLN interface from a DA parent, When a node receives a RA-DIO message over an LLN interface from a DA
the DelayNA is armed to force a full update. parent, the DelayNA is armed to force a full update.
When the node broadcasts a RA-DIO on an LLN interface, for all When the node broadcasts a RA-DIO message on an LLN interface, for
entries on that interface: all entries on that interface:
o If the entry is CONFIRMED, it goes PENDING with the retry count o If the entry is CONFIRMED, it goes PENDING with the retry count
set to 0. set to 0.
o If the entry is PENDING, the retry count is incremented. If it o If the entry is PENDING, the retry count is incremented. If it
reaches a maximum threshold, the entry goes ELAPSED If at least reaches a maximum threshold, the entry goes ELAPSED If at least
one entry is ELAPSED at the end of the process: if the Destroy one entry is ELAPSED at the end of the process: if the Destroy
timer is not running then it is armed with a jitter. timer is not running then it is armed with a jitter.
Since the DelayNA has a duration that decreases with the depth, it is Since the DelayNA timer has a duration that decreases with the depth,
expected to receive all DAOs from all children before the timer it is expected to receive all NA-DAO messages from all children
elapses and the full update is sent to the DA parents. before the timer elapses and the full update is sent to the DA
parents.
Once the Destroy timer is elapsed, the prefix entry is scheduled to Once the RemoveTimer is elapsed, the prefix entry is scheduled to be
be destroyed and moved to the Unreachable list if there are any DA removed and moved to the Unreachable list if there are any DA parents
parents that need to be informed of the change in status for the that need to be informed of the change in status for the prefix,
prefix, otherwise the prefix entry is cleaned up right away. The otherwise the prefix entry is cleaned up right away. The prefix
prefix entry is removed from the Unreachable list when no more DA entry is removed from the Unreachable list when no more DA parents
parents need to be informed. This condition may be satisfied when a need to be informed. This condition may be satisfied when a no-DAO
no-DAO is sent to all current DA parents indicating the loss of the is sent to all current DA parents indicating the loss of the prefix,
prefix, and noting that in some cases parents may have been removed and noting that in some cases parents may have been removed from the
from the set of DA parents. set of DA parents.
5.10.2.4. Other events 5.9.2.4. 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 DAOs are abstract lists are freed. All the routes learned from NA-DAO
destroyed. 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 destroyed. scheduled to be removed.
o Loss of routing adjacency: When the routing adjacency for a o Loss of routing adjacency: When the routing adjacency for a
neighbor is lost, as per the procedures described in Section 5.11, neighbor is lost, as per the procedures described in Section 5.11,
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 DelayNA is armed. set to not 'reported' and DelayNA is armed.
5.10.2.5. Aggregation of prefixes by a node 5.9.2.5. 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 platoon of nodes. In such a case, it is possible to use a group of nodes. In such a case, it is possible to use aggregation
aggregation techniques with Destination Advertisements and improve techniques with destination advertisements and improve scalability.
scalability. For example, consider a platoon formed by firefighters
and their commander. Specifically, the commander may be configured
as the Destination Advertisement aggregator for a group prefix. At
run time, the commander absorbs the individual DAO information
received from the platoon members down its sub-DAG and only reports
the aggregation up the DAG. This works fine when the whole platoon
is attached within the commander's sub-DAG.
Other cases might occur for which additional support is required: Other cases might occur for which additional support is required:
1. The commander is attached within the sub-DAG of one of its 1. The aggregating node is attached within the sub-DAG of the nodes
platoon members. it is aggregating for.
2. A platoon member is somewhere else within the DAG. 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.
3. A platoon member is somewhere else in the LLN. 3. A node that is to be aggregated for is located somewhere else in
the LLN.
In all those cases, a node situated above the commander in the DAG Consider a node M who is performing an aggregation, and a node N who
but not above the platoon member will see the advertisements for the is to be a member of the aggregation group. A node Z situated above
aggregation owned by the commander but not that of the individual the node M in the DAG, but not above node N, will see the
platoon member prefix. So it will route all the packets for the advertisements for the aggregation owned by M but not that of the
platoon member towards the commander, but the commander will have no individual prefix for N. Such a node Z will route all the packets for
route to the individual platoon member and will fail to forward. node N towards node M, but node M will have no route to the node N
and will fail to forward.
Additional protocols may be applied beyond the scope of this Additional protocols may be applied beyond the scope of this
specification to dynamically elect/provision a commander and platoon specification to dynamically elect/provision an aggregating node and
in order to provide route summarization for a sub-DAG. groups of nodes eligible to be aggregated in order to provide route
summarization for a sub-DAG.
5.10.2.6. Default Values 5.9.2.6. Default Values
DEF_NA_LATENCY = To Be Determined DEF_NA_LATENCY = To Be Determined
MAX_DESTROY_INTERVAL = To Be Determined MAX_DESTROY_INTERVAL = To Be Determined
5.10. Multicast Operation
This section describes further the multicast routing operations over
an IPv6 RPL network, and specifically how unicast NA-DAOs can be used
to relay group registrations inwards. Wherever the following text
mentions MLD, one can read MLDv2 or v3.
As is traditional, a listener uses a protocol such as MLD with a
router to register to a multicast group.
Along the path between the router and the root of the DAG, MLD
requests are mapped and transported as NA-DAO messages within the RPL
protocol; each hop coalesces the multiple requests for a same group
as a single NA-DAO message to the parent(s), in a fashion similar to
proxy IGMP, but recursively between child router and parent up to the
root.
A router might select to pass a listener registration NA-DAO message
to its preferred parent only, in which case multicast packets coming
back might be lost for all of its sub-DAG if the transmission fails
over that link. Alternatively the router might select to copy
additional parents as it would do for NA-DAO messages advertising
unicast destinations, in which case there might be duplicates that
the router will need to prune.
As a result, multicast routing states are installed in each router on
the way from the listeners to the root, enabling the root to copy a
multicast packet to all its children routers that had issued a NA-DAO
message including a DAO for that multicast group, as well as all the
attached nodes that registered over MLD.
For unicast traffic, it is expected that the grounded root of an RPL
DAG terminates RPL and MAY redistribute the RPL routes over the
external infrastructure using whatever routing protocol is used
there. For multicast traffic, the root MAY proxy MLD for all the
nodes attached to the RPL routers (this would be needed if the
multicast source is located in the external infrastructure). For
such a source, the packet will be replicated as it flows outwards
along the DAG based on the multicast routing table entries installed
from the NA-DAO message.
For a source inside the DAG, the packet is passed to the preferred
parents, and if that fails then to the alternates in the DAG. The
packet is also copied to all the registered children, except for the
one that passed the packet. Finally, if there is a listener in the
external infrastructure then the DAG root has to further propagate
the packet into the external infrastructure.
As a result, the DAG Root acts as an automatic proxy Rendez-vous
Point for the RPL network, and as source towards the Internet for all
multicast flows started in the RPL LLN. So regardless of whether the
root is actually attached to the Internet, and regardless of whether
the DAG is grounded or floating, the root can serve inner multicast
streams at all times.
5.11. Maintenance of Routing Adjacency 5.11. Maintenance of Routing Adjacency
The selection of successors, along the default paths inward along the The selection of successors, along the default paths inward along the
DAG, or along the paths learned from Destination Advertisements DAG, or along the paths learned from destination advertisements
outward along the DAG, leads to the formation of routing adjacencies outward along the DAG, leads to the formation of routing adjacencies
that require maintenance. that require maintenance.
In IGPs such as OSPF [RFC4915] or IS-IS [RFC5120], the maintenance of In IGPs such as OSPF [RFC4915] or IS-IS [RFC5120], the maintenance of
a routing adjacency involves the use of Keepalive mechanisms (Hellos) a routing adjacency involves the use of Keepalive mechanisms (Hellos)
or other protocols such as BFD ([I-D.ietf-bfd-base]) and MANET or other protocols such as BFD ([I-D.ietf-bfd-base]) and MANET
Neighborhood Discovery Protocol (NHDP [I-D.ietf-manet-nhdp]). Neighborhood Discovery Protocol (NHDP [I-D.ietf-manet-nhdp]).
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
skipping to change at page 67, line 44 skipping to change at page 72, line 8
reception of a Neighbor Advertisement (NA) message with the reception of a Neighbor Advertisement (NA) message with the
"Solicited Flag" set is used to verify the validity of the routing "Solicited Flag" set is used to verify the validity of the routing
adjacency. Such mechanism MAY be used prior to sending a data adjacency. Such mechanism MAY be used prior to sending a data
packet. This allows for detecting whether or not the routing packet. This allows for detecting whether or not the routing
adjacency is still valid, and should it not be the case, select adjacency is still valid, and should it not be the case, select
another feasible successor to forward the packet. another feasible successor to forward the packet.
5.12. Packet Forwarding 5.12. 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, with consideration given to selection of a next-hop successor as follows:
selecting a DAG/OCP to follow as per marking in the IPv6 header, as
follows:
1. If the packet header contains any source routing directives (TBD) 1. It is preferred to select a successor from a DAG who is
then the highest precedence should be given to follow them. supporting an OCP and related optimization that maps to an
objective marked in the IPv6 header of the packet being
forwarded.
2. If there is an entry in the routing table matching the 2. If a local administrative preference favors a route that has been
destination that has been provisioned outside of the context of learned from a different routing protocol than RPL, then use that
RPL, e.g. through an application intervention or a co-hosted successor.
(P2P) routing protocol, then use that successor.
3. If there is an entry in the routing table matching the 3. If there is an entry in the routing table matching the
destination that has been learned from a multicast Destination destination that has been learned from a multicast destination
Advertisement (e.g. the destination is a one-hop neighbor), then advertisement (e.g. the destination is a one-hop neighbor), then
use that successor. use that successor.
4. If there is an entry in the routing table matching the 4. If there is an entry in the routing table matching the
destination that has been learned from a unicast Destination destination that has been learned from a unicast destination
Advertisement (e.g. the destination is located outwards along the advertisement (e.g. the destination is located outwards along the
sub-DAG), then use that successor. sub-DAG), then use that successor.
5. If there is a DAG offering a route to a prefix matching the 5. If there is a DAG offering a route to a prefix matching the
destination, then select one of those DAG Parents as a successor. destination, then select one of those DAG parents as a successor.
6. If there is a DAG offering a default route with a compatible OCP, 6. If there is a DAG offering a default route with a compatible OCP,
then select one of those DAG Parents as a successor. then select one of those DAG parents as a successor.
7. If there is a DAG offering a route to a prefix matching the 7. If there is a DAG offering a route to a prefix matching the
destination, but all DAG Parents have been tried and are destination, but all DAG parents have been tried and are
temporarily unavailable (as determined by the forwarding temporarily unavailable (as determined by the forwarding
procedure), then select a DAG sibling as a successor. procedure), then select a DAG sibling as a successor.
8. Finally, if no DAG siblings are available, the packet is dropped. 8. Finally, if no DAG siblings are available, the packet is dropped.
ICMP Destination Unreachable may be invoked. An inconsistency is ICMP Destination Unreachable may be invoked. An inconsistency is
detected. detected.
TTL MUST be decremented when forwarding. If the packet is being TTL MUST be decremented when forwarding. If the packet is being
forwarded via a sibling, then the TTL may be decremented more forwarded via a sibling, then the TTL MAY be decremented more
aggressively (by more than one) to limit the impact of possible aggressively (by more than one) to limit the impact of possible
loops. loops.
Note that unless overridden by a source routing directive or a route Note that the chosen successor MUST NOT be the neighbor who was the
that has been provisioned outside of RPL, the chosen successor MUST predecessor of the packet (split horizon), except in the case where
NOT be the neighbor who was the predecessor of the packet (split it is intended for the packet to change from an inward to an outward
horizon). flow, such as switching from DIO routes to DAO routes as the
destination is neared.
5.12.1. Loop Taxonomy 5.12.1. Loop Taxonomy
The following is a summary of the sort of loops that may occur within The following is a summary of the sort of loops that may occur within
RPL. This is provided in part as a basis for discussion of loop RPL. This is provided in part as a basis for discussion of loop
detection at forwarding. detection at forwarding.
5.12.1.1. DAG Loops 5.12.1.1. DAG Loops
A DAG loop may occur when a node detaches from the DAG and reattaches A DAG loop may occur when a node detaches from the DAG and reattaches
to a device in its prior sub-DAG that has missed the whole detachment to a device in its prior sub-DAG that has missed the whole detachment
sequence and kept advertising the original DAG. This may happen in sequence and kept advertising the original DAG. This may happen in
particular when RA-DIOs are missed. Use of the DAG sequence number particular when RA-DIO messages are missed. Use of the DAG sequence
can eliminate this type of loop. If the DAG sequence number is not number can eliminate this type of loop. If the DAG sequence number
in use, the protection is limited (it depends on propagation of DIOs is not in use, the protection is limited (it depends on propagation
during DAG hop timer), and temporary loops might occur. RPL will of RA-DIO messages during DAG hop timer), and temporary loops might
move to eliminate such a loop as soon as a DIO is received from a occur. RPL will move to eliminate such a loop as soon as a RA-DIO
parent that appears to be going down, as the child has to detach from message is received from a parent that appears to be going down, as
it immediately. (The alternate choice of staying attached and the child has to detach from it immediately. (The alternate choice
following the parent in its fall would have counted to infinity and of staying attached and following the parent in its fall would have
led to detach as well). counted to infinity and led to detach as well).
Consider Node (24) in the DAG Example depicted in Figure 12, and its Consider node (24) in the DAG Example depicted in Figure 12, and its
sub-DAG Nodes (34), (44), and (45). An example of a DAG loop would sub-DAG nodes (34), (44), and (45). An example of a DAG loop would
be if Node (24) were to detach from the DAG rooted at (LBR), and Node be if node (24) were to detach from the DAG rooted at (LBR), and
(45) were to miss the detachment sequence. Subsequently, if the link nodes (34) and (45) were to miss the detachment sequence.
(24)--(45) were to become viable and Node (24) heard Node (45) Subsequently, if the link (24)--(45) were to become viable and node
advertising the DAG rooted at (LBR), a DAG loop (45->34->24->45) may (24) heard node (45) advertising the DAG rooted at (LBR), a DAG loop
form if Node (24) attaches to Node (45). (45->34->24->45) may form if node (24) attaches to node (45).
5.12.1.2. DAO Loops 5.12.1.2. DAO Loops
A DAO loop may occur when the parent has a route installed by a DAO A DAO loop may occur when the parent has a route installed upon
via a child, but the child has cleaned up the state. This loop receiving and processing a NA-DAO message from a child, but the child
happens when a no-DAO was missed till a heartbeat cleans up all has subsequently cleaned up the state. This loop happens when a no-
states. The DAO loop is not explicitly handled by the current DAO was missed till a heartbeat cleans up all states. The DAO loop
specification. Split horizon, not forwarding a packet back to the is not explicitly handled by the current specification. Split
node it came from, may mitigate the DAO loop in some cases, but does horizon, not forwarding a packet back to the node it came from, may
not eliminate it. mitigate the DAO loop in some cases, but does not eliminate it.
Consider Node (24) in the DAG Example depicted in Figure 12. Suppose Consider node (24) in the DAG Example depicted in Figure 12. Suppose
Node (24) has received a DA from Node (34) advertising a destination node (24) has received a DA from node (34) advertising a destination
at Node (45). Subsequently, if Node (34) tears down the DA state for at node (45). Subsequently, if node (34) tears down the routing
the destination and Node (24) did not hear a no-DAO to clean up the state for the destination and node (24) did not hear a no-DAO message
state, a DAO loop may exist. Node (24) will forward traffic destined to clean up the routing state, a DAO loop may exist. node (24) will
for Node (45) to Node (34), who may then naively return it into a forward traffic destined for node (45) to node (34), who may then
loop (if split horizon is not in place). A more complicated DAO loop naively return it into a loop (if split horizon is not in place). A
may result if Node (34) instead passes the traffic to it's sibling, more complicated DAO loop may result if node (34) instead passes the
Node (33), potentially resulting in a (24->34->33->23->13->24) loop. traffic to it's sibling, node (33), potentially resulting in a
(24->34->33->23->13->24) loop.
5.12.1.3. Sibling Loops 5.12.1.3. Sibling Loops
Sibling loops occur when a group of siblings keep choosing amongst Sibling loops occur when a group of siblings keep choosing amongst
themselves as successors such that a packet does not make forward themselves as successors such that a packet does not make forward
progress. The current draft limits those loops to some degree by progress. The current draft limits those loops to some degree by
split horizon (do not send back to the same sibling) and parent split horizon (do not send back to the same sibling) and parent
preference (always prefer parents vs. siblings). Further approaches preference (always prefer parents vs. siblings).
to mitigate sibling loops may include:
o aggressively dropping the TTL to limit the impact of the loops
o randomizing the next hop to try and exit the loop if there is one
one
o maintaining per packet states
o tunneling or source routing (path vector)
Consider the DAG Example depicted in Figure 12. Suppose that Node Consider the DAG Example depicted in Figure 12. Suppose that Node
(32) and (34) are reliable neighbors, and thus are siblings. Then, (32) and (34) are reliable neighbors, and thus are siblings. Then,
in the case where Nodes (22), (23), and (24) are transiently in the case where Nodes (22), (23), and (24) are transiently
unavailable, and with no other guiding strategy, a sibling loop may unavailable, and with no other guiding strategy, a sibling loop may
exist, e.g. (33->34->32->33) as the siblings keep choosing amongst exist, e.g. (33->34->32->33) as the siblings keep choosing amongst
each other in an uncoordinated manner. each other in an uncoordinated manner.
5.13. Expectations of Link Layer Behavior 6. RPL Variables
This specification does not rely on any particular features of a
specific link layer technologies. It is anticipated that an
implementer should be able to operate RPL over a variety of different
low power wireless or PLC (Power Line Communication) link layer
technologies.
Implementers may find RFC 3819 [RFC3819] a useful reference when
designing a link layer interface between RPL and a particular link
layer technology.
6. Summary of RPL Timers
DIO Timer One instance per DAG that a node is a member of. Expiry DIO Timer One instance per DAG that a node is a member of. Expiry
triggers RA-DIO transmission. Trickle timer with variable triggers RA-DIO message transmission. Trickle timer with
interval in [0, DIOIntervalMin..2^DIOIntervalDoublings]. See variable interval in [0,
Section 5.4.3 DIOIntervalMin..2^DIOIntervalDoublings]. See Section 5.3.4
DAG Hop Timer Up to one instance per candidate DAG Parent in the DAG Hop Timer Up to one instance per candidate DAG parent in the
`Held-Up' state per DAG that a node is going to jump to. `Held-Up' state per DAG that a node is going to jump to.
Expiry triggers candidate DAG Parent to become a DAG Parent in Expiry triggers candidate DAG parent to become a DAG parent in
the `Current' state, as well as cancellation of any other DAG the `Current' state, as well as cancellation of any other DAG
Hop timers associated with other DAG Parents for that DAG. Hop timers associated with other DAG parents for that DAG.
Duration is computed based on the rank of the candidate DAG Duration is computed based on the rank of the candidate DAG
parent and DAG delay, as (candidates rank + random) * parent and DAG delay, as (candidates rank + random) *
candidate's DAG_delay (where 0 <= random < 1). See candidate's DAG_delay (where 0 <= random < 1). See
Section 5.8.1. Section 5.7.1.
Hold-Down Timer Up to one instance per candidate DAG Parent in the Hold-Down Timer Up to one instance per candidate DAG parent in the
`Held-Down' state per DAG. Expiry triggers the eviction of the `Held-Down' state per DAG. Expiry triggers the eviction of the
candidate DAG Parent from the candidate DAG Parent set. The candidate DAG parent from the candidate DAG parent set. The
interval should be chosen as appropriate to prevent flapping. interval should be chosen as appropriate to prevent flapping.
See Section 5.8 See Section 5.7.
DAG Heartbeat Timer Up to one instance per DAG that the node is DAG Heartbeat Timer Up to one instance per DAG that the node is
acting as DAG Root of. May not be supported in all acting as DAG root of. May not be supported in all
implementations. Expiry triggers revision of implementations. Expiry triggers revision of
DAGSequenceNumber, causing a new series of updated RA-DIO to be DAGSequenceNumber, causing a new series of updated RA-DIO
sent. Interval should be chosen appropriate to propagation message to be sent. Interval should be chosen appropriate to
time of DAG and as appropriate to application requirements propagation time of DAG and as appropriate to application
(e.g. response time vs. overhead). See Section 5.5 requirements (e.g. response time vs. overhead). See
Section 5.4
DelayNA Timer Up to one instance per DA Parent (the subset of DAG DelayNA Timer Up to one instance per DA parent (the subset of DAG
Parents chosen to receive Destination Advertisements) per DAG. parents chosen to receive destination advertisements) per DAG.
Expiry triggers sending of NA-DAO to the DA Parent. The Expiry triggers sending of NA-DAO message to the DA parent.
interval is to be proportional to DEF_NA_LATENCY/(node rank), The interval is to be proportional to DEF_NA_LATENCY/(node
such that nodes of greater rank (further outward along the DAG) rank), such that nodes of greater rank (further outward along
expire first, coordinating the sending of DAOs to allow for a the DAG) expire first, coordinating the sending of NA-DAO
chance of aggregation. See Section 5.10.2.1.1 messages to allow for a chance of aggregation. See
Section 5.9.2.1.1
DestroyTimer Up to one instance per DA entry per neighbor (i.e. DestroyTimer Up to one instance per DA entry per neighbor (i.e.
those neighbors who have given DAO to this node as a DAG those neighbors who have given NA-DAO messages to this node as
Parent) Expiry triggers a change in state for the DA entry, a DAG parent) Expiry triggers a change in state for the DA
setting up to do unreachable (No-DAO) advertisements or entry, setting up to do unreachable (No-DAO) advertisements or
immediately deallocating the DA entry if there are no DA immediately deallocating the DA entry if there are no DA
Parents. The interval is min(MAX_DESTROY_INTERVAL, parents. The interval is min(MAX_DESTROY_INTERVAL,
RA_INTERVAL). See Section 5.10.2.1.1 RA_INTERVAL). See Section 5.9.2.1.1
7. Protocol Extensions 7. Manageability Considerations
8. Manageability Considerations The aim of this section is to give consideration to the manageability
of RPL, and how RPL will be operated in LLN beyond the use of a MIB
module. The scope of this section is to consider the following
aspects of manageability: fault management, configuration, accounting
and performance.
9. Security Considerations 7.1. Control of Function and Policy
7.1.1. Initialization Mode
When a node is first powered up, it may either choose to stay silent
and not send any multicast RA-DIO message until it has joined a DAG,
or to immediately root a transient DAG and start sending multicast
RA-DIO messages. A RPL implementation SHOULD allow configuring
whether the node should stay silent or should start advertising RA-
DIO messages.
Furthermore, the implementation SHOULD to allow configuring whether
or not the node should start sending an RS message as an initial
probe for nearby DAGs, or should simply wait until it received RA
messages from other nodes that are part of existing DAGs.
7.1.2. DIO Base option
RPL specifies a number of protocol parameters.
A RPL implementation SHOULD allow configuring the following routing
protocol parameters:
DAGPreference: 8-bit unsigned integer set by the DAG root to its
preference and unchanged at propagation.
NodePreference: The administrative preference of that LLN Node.
DAGDelay: 16-bit unsigned integer set by the DAG root indicating the
delay before changing the DAG configuration,
DIOIntervalDoublings: 8-bit unsigned integer. Configured on the DAG
root and used to configure the trickle timer governing when RA-
DIO messages should be sent within the DAG.
DIOIntervalMin: 8-bit unsigned integer. Configured on the DAG root
and used to configure the trickle timer governing when RA-DIO
messages should be sent within the DAG. The minimum configured
interval for the RA-DIO trickle timer in units of ms is
2^DIOIntervalMin (e.g. a DIOIntervalMin value of 16ms is
expressed as 4).
DAGObjectiveCodePoint The DAG Objective Code Point is used to
indicate the cost metrics, objective functions, and methods of
computation and comparison for DAGRank in use in the DAG. The
DAG OCP is set by the DAG root.
PathDigest: 32-bit unsigned integer CRC, updated by each LLN Node.
This is the result of a CRC-32c computation on a bit string
obtained by appending the received value and the ordered set of
DAG parents at the LLN Node. DAG roots use a 'previous value'
of zeroes to initially set the PathDigest.
DAGID: 128-bit unsigned integer which uniquely identify a DAG. This
value is set by the DAG root. The global IPv6 address of the
DAG root can be used.
Destination Prefixes List of advertised destinations
DAG Root behavior: In some cases, a node may not want to permanently
act as a DAG root if it cannot join a grounded DAG. For
example a battery-operated node may not want to act as a DAG
root for a long period of time. Thus a RPL implementation MAY
support the ability to configure whether or not a node could
act as a DAG root for a configured period of time.
DAG Hop Timer: A RPL implementation MUST provide the ability to
configure the value of the DAG Hop Timer, expressed in ms.
DAG Table Entry Suppression A RPL implementation SHOULD provide the
ability to configure a timer after the expiration of which the
DAG table that contains all the records about a DAG is
suppressed, to be invoked if the DAG parent set becomes empty.
7.1.3. Trickle Timers
A RPL implementation makes use of trickle timer to govern the sending
of RA-DIO message. Such an algorithm is determined a by a set of
configurable parameters that are then advertised by the DAG root
along the DAG in RA-DIO messages.
For each DAG, a RPL implementation MUST allow for the monitoring of
the following parameters:
I: The current length of the communication interval
T: A timer with a duration set to a random value in the range
[I/2, I]
C: Redundancy Counter
I_min: The smallest communication interval in milliseconds. This
value is learned from the RA-DIO message as
(2^DIOIntervalMin)ms. The default value is
DEFAULT_DIO_INTERVAL_MIN.
I_doublings: The number of times I_min should be doubled before
maintaining a constant rate, i.e. I_max = I_min *
2^I_doublings. This value is learned from the RA-DIO message
as DIOIntervalDoublings. The default value is
DEFAULT_DIO_INTERVAL_DOUBLINGS.
A RPL implementation SHOULD provide a command (for example via API,
CLI, or SNMP MIB) whereby any procedure that detects an inconsistency
may cause the trickle timer to reset.
7.1.4. DAG Heartbeat
A RPL implementation may allow by configuration at the DAG root to
refresh the DAG states by updating the DAGSequenceNumber. A RPL
implementation SHOULD allow configuring whether or not periodic or
event triggered mechanism are used by the DAG root to control
DAGSequenceNumber change.
7.1.5. The Destination Advertisement Option
The following set of parameters of the NA-DAO messages SHOULD be
configurable:
o The DelayNA timer
o The Remove timer
7.1.6. Policy Control
DAG discovery enables nodes to implement different policies for
selecting their DAG parents.
A RPL implementation SHOULD allow configuring the set of acceptable
or preferred Objective Functions (OF) referenced by their Objective
Codepoints (OCPs) for a node to join a DAG, and what action should be
taken if none of a node's candidate neighbors advertise one of the
configured allowable Objective Functions.
A node in an LLN may learn routing information from different routing
protocols including RPL. It is in this case desirable to control via
administrative preference which route should be favored. An
implementation SHOULD allow for specifying an administrative
preference for the routing protocol from which the route was learned.
A RPL implementation SHOULD allow for the configuration of the "Route
Tag" field of the NA-DAO messages according to a set of rules defined
by policy.
7.1.7. Data Structures
Some RPL implementation may limit the size of the candidate neighbor
list in order to bound the memory usage, in which case some otherwise
viable candidate neighbors may not be considered and simply dropped
from the candidate neighbor list.
A RPL implementation MAY provide an indicator on the size of the
candidate neighbor list.
7.2. Information and Data Models
The information and data models necessary for the operation of RPL
will be defined in a separate document specifying the RPL SNMP MIB.
7.3. Liveness Detection and Monitoring
The aim of this section is to describe the various RPL mechanisms
specified to monitor the protocol.
As specified in Section 5.2, an implementation must maintain a set of
data structures in support of DAG discovery:
o The candidate neighbors data structure
o For each DAG:
* A set of candidate DAG parents
* A set of DAG parents (which are a subset of candidate DAG
parents and may be implemented as such)
7.3.1. Candidate Neighbor Data Structure
A node in the candidate neighbor list is a node discovered by the
some means and qualified to potentially become of neighbor or a
sibling (with high enough local confidence). A RPL implementation
SHOULD provide a way monitor the candidate neighbors list with some
metric reflecting local confidence (the degree of stability of the
neighbors) measured by some metrics.
A RPL implementation MAY provide a counter reporting the number of
times a candidate neighbor has been ignored, should the number of
candidate neighbors exceeds the maximum authorized value.
7.3.2. Directed Acyclic Graph (DAG) Table
For each DAG, a RPL implementation MUST keep track of the following
DAG table values:
o DAGID
o DAGObjectiveCodePoint
o A set of Destination Prefixes offered inwards along the DAG
o A set of candidate DAG Parents
o timer to govern the sending of RA-DIO messages for the DAG
o DAGSequenceNumber
The set of candidate DAG parents structure is itself a table with the
following entries:
o A reference to the neighboring device which is the DAG parent
o A record of most recent information taken from the DAG Information
Object last processed from the candidate DAG Parent
o a state associated with the role of the candidate as a potential
DAG Parent {Current, Held-Up, Held-Down, Collision}, further
described in Section 5.7
o A DAG Hop Timer, if instantiated
o A Held-Down Timer, if instantiated
o A flag reporting if the Parent is a DA Parent as described in
Section 5.9
7.3.3. Routing Table
To be completed.
7.3.4. Other RPL Monitoring Parameters
A RPL implementation SHOULD provide a counter reporting the number of
a times the node has detected an inconsistency with respect to a DAG
parent, e.g. if the DAGID has changed.
A RPL implementation MAY log the reception of a malformed RA-DIO
message along with the neighbor identification if avialable.
7.3.5. RPL Trickle Timers
A RPL implementation operating on a DAG root MUST allow for the
configuration of the following trickle parameters:
o The DIOIntervalMin expressed in ms
o The DIOIntervalDoublings
A RPL implementation MAY provide a counter reporting the number of
times an inconsistency (and thus the trickle timer has been reset).
7.4. Verifying Correct Operation
This section has to be completed in further revision of this document
to list potential Operations and Management (OAM) tools that could be
used for verifying the correct operation of RPL.
7.5. Requirements on Other Protocols and Functional Components
RPL does not have any impact on the operation of existing protocols.
7.6. Impact on Network Operation
To be completed.
8. Security Considerations
Security Considerations for RPL are to be developed in accordance Security Considerations for RPL are to be developed in accordance
with recommendations laid out in, for example, with recommendations laid out in, for example,
[I-D.tsao-roll-security-framework]. [I-D.tsao-roll-security-framework].
10. IANA Considerations 9. IANA Considerations
10.1. DAG Information Option 9.1. DAG Information Option (DIO) Base Option
IANA is requested to allocate a new Neighbor Discovery Option Type The DAG Information Option is a container option carried within an
from the IPv6 Neighbor Discovery Option Formats Registry in order to IPv6 Router Advertisement message as defined in [RFC4861], which
represent the DAG Information Option as described in Section 5.1 might contain a number of suboptions. The base option regroups the
minimum information set that is mandatory in all cases.
10.2. Objective Code Point IANA had defined the IPv6 Neighbor Discovery Option Formats registry.
The suggested type value for the DAG Information Option (DIO) Base
Option is 140, to be confirmed by IANA.
9.2. New Registry for the Flag Field of the DIO Base Option
IANA is requested to create a registry for the Flag field of the DIO
Base Option.
New bit numbers may be allocated only by an IETF Consensus action.
Each bit should be tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
o Defining RFC
Three flags are currently defined:
+-----+-------------------------------------+---------------+
| Bit | Description | Reference |
+-----+-------------------------------------+---------------+
| 0 | Grounded DAG | This document |
| 1 | Destination Advertisement Trigger | This document |
| 2 | Destination Advertisement Supported | This document |
+-----+-------------------------------------+---------------+
DIO Base Option Flags
9.3. DAG Information Option (DIO) Suboption
IANA is requested to create a registry for the DIO Base Option
Suboptions
+-------+------------------------------+---------------+
| Value | Meaning | Reference |
+-------+------------------------------+---------------+
| 0 | Pad1 - DIO Padding | This document |
| 1 | PadN - DIO suboption padding | This document |
| 2 | DAG Metric Container | This Document |
| 3 | Destination Prefix | This Document |
+-------+------------------------------+---------------+
DAG Information Option (DIO) Base Option Suboptions
9.4. Destination Advertisement Option (DAO) Option
The RPL protocol extends Neighbor Discovery [RFC4861] and [RFC4191]
to allow a node to include a Destination Advertisement Option, which
includes prefix information in the Neighbor Advertisements messages.
The Neighbor Advertisement messages are augmented with the
Destination Advertisement Option (DAO).
IANA had defined the IPv6 Neighbor Discovery Option Formats registry.
The suggested type value for the Destination Advertisement Option
carried within a Neighbor Advertisement message is 141, to be
confirmed by IANA.
9.5. Objective Code Point
This specification requests that an Objective Code Point registry, as This specification requests that an Objective Code Point registry, as
to be specified in [I-D.ietf-roll-routing-metrics], reserve the to be specified in [I-D.ietf-roll-routing-metrics], reserve the
Objective Code Point value 0x0000, for the purposes designated as OCP Objective Code Point value 0x0000, for the purposes designated as OCP
0 in this document. 0 in this document.
10.3. Destination Advertisement Option 10. Acknowledgements
IANA is requested to allocate a new Neighbor Discovery Option Type
from the IPv6 Neighbor Discovery Option Formats Registry in order to
represent the Destination Advertisement Option as described in
Section 5.10.1.1
11. Acknowledgements
The ROLL Design Team would like to acknowledge the review, feedback, The ROLL Design Team would like to acknowledge the review, feedback,
and comments from Dominique Barthel, Yusuf Bashir, Mathilde Durvy, and comments from Dominique Barthel, Yusuf Bashir, Mathilde Durvy,
Manhar Goindi, Mukul Goyal, Richard Kelsey, Quentin Lampin, Philip Manhar Goindi, Mukul Goyal, Quentin Lampin, Philip Levis, Jerry
Levis, Jerry Martocci, Alexandru Petrescu, and Don Sturek. Martocci, Alexandru Petrescu, and Don Sturek.
The ROLL Design Team would like to acknowledge the guidance and input The ROLL Design Team would like to acknowledge the guidance and input
provided by the ROLL Chairs, David Culler and JP Vasseur. provided by the ROLL Chairs, David Culler and JP Vasseur.
The ROLL Design Team would like to acknowledge prior contributions of The ROLL Design Team would like to acknowledge prior contributions of
Richard Kelsey, Robert Assimiti, Mischa Dohler, Julien Abeille, Ryuji Robert Assimiti, Mischa Dohler, Julien Abeille, Ryuji Wakikawa, Teco
Wakikawa, Teco Boot, Patrick Wetterwald, Bryan Mclaughlin, Carlos J. Boot, Patrick Wetterwald, Bryan Mclaughlin, Carlos J. Bernardos,
Bernardos, Thomas Watteyne, Zach Shelby, Dominique Barthel, Caroline Thomas Watteyne, Zach Shelby, Caroline Bontoux, Marco Molteni, Billy
Bontoux, Marco Molteni, Billy Moon, and Arsalan Tavakoli, in addition Moon, and Arsalan Tavakoli, which have provided useful design
to contributions from [I-D.thubert-roll-fundamentals] and considerations to RPL.
[I-D.tavakoli-hydro] which have provided useful design considerations
to RPL.
12. Contributors 11. Contributors
ROLL Design Team in alphabetical order: JP Vasseur
Cisco Systems, Inc
11, Rue Camille Desmoulins
Issy Les Moulineaux, 92782
France
Anders Brandt Email: jpv@cisco.com
Zensys, Inc.
Emdrupvej 26
Copenhagen, DK-2100
Denmark
Email: abr@zen-sys.com Jonathan W. Hui
Arch Rock Corporation
501 2nd St. Ste. 410
San Francisco, CA 94107
USA
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
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
Jonathan W. Hui
Arch Rock Corporation
501 2nd St. Ste. 410
San Francisco, CA 94107
USA
Email: jhui@archrock.com
Kris Pister Kris Pister
Dust Networks Dust Networks
30695 Huntwood Ave. 30695 Huntwood Ave.
Hayward, 94544 Hayward, 94544
USA USA
Email: kpister@dustnetworks.com Email: kpister@dustnetworks.com
Pascal Thubert Anders Brandt
Cisco Systems Zensys, Inc.
Village d'Entreprises Green Side Emdrupvej 26
400, Avenue de Roumanille Copenhagen, DK-2100
Batiment T3 Denmark
Biot - Sophia Antipolis 06410
FRANCE
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
Tim Winter (editor)
wintert@acm.org Email: abr@zen-sys.com
13. References 12. References
13.1. Normative References 12.1. 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.
13.2. Informative References 12.2. Informative References
[I-D.ietf-bfd-base] [I-D.ietf-bfd-base]