draft-ietf-roll-rpl-00.txt   draft-ietf-roll-rpl-01.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 ROLL Design Team
Expires: February 4, 2010 IETF ROLL WG Expires: March 18, 2010 IETF ROLL WG
August 3, 2009 September 14, 2009
RPL: Routing Protocol for Low Power and Lossy Networks RPL: Routing Protocol for Low Power and Lossy Networks
draft-ietf-roll-rpl-00 draft-ietf-roll-rpl-01
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
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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 . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Design Principles . . . . . . . . . . . . . . . . . . . . 4 1.1. Design Principles . . . . . . . . . . . . . . . . . . . . 4
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Protocol Model . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Protocol Model . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Problem . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Problem . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Protocol Properties Overview . . . . . . . . . . . . . . . 7 3.2. Protocol Properties Overview . . . . . . . . . . . . . . . 7
3.2.1. IPv6 Architecture . . . . . . . . . . . . . . . . . . 7 3.2.1. IPv6 Architecture . . . . . . . . . . . . . . . . . . 7
3.2.2. Path Properties for LLN Traffic Flows . . . . . . . . 7 3.2.2. Path Properties for LLN Traffic Flows . . . . . . . . 7
3.2.3. Constraint Based Routing . . . . . . . . . . . . . . . 7 3.2.3. Constraint Based Routing . . . . . . . . . . . . . . . 8
3.2.4. Autonomous Operation . . . . . . . . . . . . . . . . . 8 3.2.4. Autonomous Operation . . . . . . . . . . . . . . . . . 8
3.3. Protocol Operation . . . . . . . . . . . . . . . . . . . . 8 3.3. Protocol Operation . . . . . . . . . . . . . . . . . . . . 8
3.3.1. DAG Construction . . . . . . . . . . . . . . . . . . . 9 3.3.1. DAG Construction . . . . . . . . . . . . . . . . . . . 9
3.3.2. Source Routing . . . . . . . . . . . . . . . . . . . . 17 3.3.2. Source Routing . . . . . . . . . . . . . . . . . . . . 19
3.3.3. Destination Advertisement . . . . . . . . . . . . . . 17 3.3.3. Destination Advertisement . . . . . . . . . . . . . . 19
3.4. Other Considerations . . . . . . . . . . . . . . . . . . . 19 3.4. Other Considerations . . . . . . . . . . . . . . . . . . . 21
3.4.1. DAG Depth and Loop Avoidance . . . . . . . . . . . . . 19 3.4.1. DAG Rank and Loop Avoidance . . . . . . . . . . . . . 21
3.4.2. DAG Parent Selection, Stability, and Greediness . . . 21 3.4.2. DAG Parent Selection, Stability, and Greediness . . . 25
3.4.3. Merging DAGs . . . . . . . . . . . . . . . . . . . . . 23 3.4.3. Merging DAGs . . . . . . . . . . . . . . . . . . . . . 27
3.4.4. Local and Temporary Routing Decision . . . . . . . . . 25 3.4.4. Local and Temporary Routing Decision . . . . . . . . . 29
3.4.5. Scalability . . . . . . . . . . . . . . . . . . . . . 26 3.4.5. Scalability . . . . . . . . . . . . . . . . . . . . . 30
3.4.6. Maintenance of Routing Adjacency . . . . . . . . . . . 26 3.4.6. Maintenance of Routing Adjacency . . . . . . . . . . . 30
4. Constraint Based Routing in LLNs . . . . . . . . . . . . . . . 27 4. Constraint Based Routing in LLNs . . . . . . . . . . . . . . . 30
4.1. Routing Metrics . . . . . . . . . . . . . . . . . . . . . 27 4.1. Routing Metrics . . . . . . . . . . . . . . . . . . . . . 30
4.2. Routing Constraints . . . . . . . . . . . . . . . . . . . 28 4.2. Routing Constraints . . . . . . . . . . . . . . . . . . . 32
4.3. Constraint Based Routing . . . . . . . . . . . . . . . . . 28 4.3. Constraint Based Routing . . . . . . . . . . . . . . . . . 32
5. Specification of Core Protocol . . . . . . . . . . . . . . . . 29 5. Specification of Core Protocol . . . . . . . . . . . . . . . . 32
5.1. DAG Information Option . . . . . . . . . . . . . . . . . . 29 5.1. DAG Information Option . . . . . . . . . . . . . . . . . . 33
5.1.1. DIO base option . . . . . . . . . . . . . . . . . . . 29 5.1.1. DIO base option . . . . . . . . . . . . . . . . . . . 33
5.2. Neighbor Discovery . . . . . . . . . . . . . . . . . . . . 35 5.2. Conceptual Data Structures . . . . . . . . . . . . . . . . 39
5.2.1. RA-DIO Reception . . . . . . . . . . . . . . . . . . . 35 5.2.1. Candidate Neighbors . . . . . . . . . . . . . . . . . 39
5.2.2. RA-DIO Transmission . . . . . . . . . . . . . . . . . 37 5.2.2. DAGs . . . . . . . . . . . . . . . . . . . . . . . . . 40
5.2.3. Trickle Timer for RA Transmission . . . . . . . . . . 38 5.3. Initialization and Configuration . . . . . . . . . . . . . 41
5.3. DAG Discovery . . . . . . . . . . . . . . . . . . . . . . 39 5.4. DAG Discovery . . . . . . . . . . . . . . . . . . . . . . 42
5.3.1. DAG Selection . . . . . . . . . . . . . . . . . . . . 41 5.4.1. RA-DIO Reception . . . . . . . . . . . . . . . . . . . 45
5.3.2. Administrative depth . . . . . . . . . . . . . . . . . 42 5.4.2. RA-DIO Transmission . . . . . . . . . . . . . . . . . 47
5.3.3. DRL entries states and stability . . . . . . . . . . . 42 5.4.3. Trickle Timer for RA Transmission . . . . . . . . . . 48
5.4. Establishing Routing State Outward Along the DAG . . . . . 45 5.5. DAG Heartbeat . . . . . . . . . . . . . . . . . . . . . . 49
5.4.1. Destination Advertisement Message Formats . . . . . . 46 5.6. DAG Selection . . . . . . . . . . . . . . . . . . . . . . 50
5.4.2. Destination Advertisement Operation . . . . . . . . . 48 5.7. Administrative rank . . . . . . . . . . . . . . . . . . . 50
5.5. Maintenance of Routing Adjacency . . . . . . . . . . . . . 54 5.8. Candidate DAG Parent States and Stability . . . . . . . . 51
5.6. Expectations of Link Layer Behavior . . . . . . . . . . . 55 5.8.1. Held-Up . . . . . . . . . . . . . . . . . . . . . . . 51
6. Protocol Extensions . . . . . . . . . . . . . . . . . . . . . 55 5.8.2. Held-Down . . . . . . . . . . . . . . . . . . . . . . 52
7. Manageability Considerations . . . . . . . . . . . . . . . . . 55 5.8.3. Collision . . . . . . . . . . . . . . . . . . . . . . 52
8. Security Considerations . . . . . . . . . . . . . . . . . . . 55 5.8.4. Instability . . . . . . . . . . . . . . . . . . . . . 53
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 55 5.9. Guidelines for Objective Code Points . . . . . . . . . . . 53
9.1. DAG Information Option . . . . . . . . . . . . . . . . . . 55 5.9.1. Objective Function . . . . . . . . . . . . . . . . . . 53
9.2. Destination Advertisement Option . . . . . . . . . . . . . 55 5.9.2. Objective Code Point 0 (OCP 0) . . . . . . . . . . . . 55
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 55 5.10. Establishing Routing State Outward Along the DAG . . . . . 57
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 56 5.10.1. Destination Advertisement Message Formats . . . . . . 58
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 57 5.10.2. Destination Advertisement Operation . . . . . . . . . 60
12.1. Normative References . . . . . . . . . . . . . . . . . . . 57 5.11. Maintenance of Routing Adjacency . . . . . . . . . . . . . 67
12.2. Informative References . . . . . . . . . . . . . . . . . . 57 5.12. Packet Forwarding . . . . . . . . . . . . . . . . . . . . 67
Appendix A. Deferred Requirements . . . . . . . . . . . . . . . . 59 5.12.1. Loop Taxonomy . . . . . . . . . . . . . . . . . . . . 68
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 60 5.13. Expectations of Link Layer Behavior . . . . . . . . . . . 70
B.1. Moving Down a DAG . . . . . . . . . . . . . . . . . . . . 61 6. Summary of RPL Timers . . . . . . . . . . . . . . . . . . . . 70
B.2. Link Removed . . . . . . . . . . . . . . . . . . . . . . . 62 7. Protocol Extensions . . . . . . . . . . . . . . . . . . . . . 71
B.3. Link Added . . . . . . . . . . . . . . . . . . . . . . . . 62 8. Manageability Considerations . . . . . . . . . . . . . . . . . 71
B.4. Node Removed . . . . . . . . . . . . . . . . . . . . . . . 63 9. Security Considerations . . . . . . . . . . . . . . . . . . . 71
B.5. New LBR Added . . . . . . . . . . . . . . . . . . . . . . 63 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 72
B.6. Destination Advertisement . . . . . . . . . . . . . . . . 64 10.1. DAG Information Option . . . . . . . . . . . . . . . . . . 72
Appendix C. Additional Examples . . . . . . . . . . . . . . . . . 65 10.2. Objective Code Point . . . . . . . . . . . . . . . . . . . 72
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 69 10.3. Destination Advertisement Option . . . . . . . . . . . . . 72
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 72
12. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 72
13. References . . . . . . . . . . . . . . . . . . . . . . . . . . 74
13.1. Normative References . . . . . . . . . . . . . . . . . . . 74
13.2. Informative References . . . . . . . . . . . . . . . . . . 74
Appendix A. Deferred Requirements . . . . . . . . . . . . . . . . 76
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 76
B.1. Moving Down a DAG . . . . . . . . . . . . . . . . . . . . 78
B.2. Link Removed . . . . . . . . . . . . . . . . . . . . . . . 79
B.3. Link Added . . . . . . . . . . . . . . . . . . . . . . . . 79
B.4. Node Removed . . . . . . . . . . . . . . . . . . . . . . . 80
B.5. New LBR Added . . . . . . . . . . . . . . . . . . . . . . 80
B.6. Destination Advertisement . . . . . . . . . . . . . . . . 81
Appendix C. Additional Examples . . . . . . . . . . . . . . . . . 82
Appendix D. Outstanding Issues . . . . . . . . . . . . . . . . . 86
D.1. Additional Support for P2P Routing . . . . . . . . . . . . 86
D.2. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 86
D.3. DAO Fan-out . . . . . . . . . . . . . . . . . . . . . . . 86
D.4. Source Routing . . . . . . . . . . . . . . . . . . . . . . 86
D.5. Address / Header Compression . . . . . . . . . . . . . . . 86
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 87
1. Introduction 1. Introduction
The defining characteristics of Low Power and Lossy Networks (LLNs) The defining characteristics of Low Power and Lossy Networks (LLNs)
offer unique challenges to a routing solution. The IETF ROLL Working offer unique challenges to a routing solution. The IETF ROLL Working
Group has defined application-specific routing requirements for a Low Group has defined application-specific routing requirements for a Low
Power and Lossy Network (LLN) routing protocol Power and Lossy Network (LLN) routing protocol
[I-D.ietf-roll-building-routing-reqs] [I-D.ietf-roll-building-routing-reqs]
[I-D.ietf-roll-home-routing-reqs] [I-D.ietf-roll-indus-routing-reqs] [I-D.ietf-roll-home-routing-reqs] [I-D.ietf-roll-indus-routing-reqs]
[RFC5548]. RPL is a new routing protocol designed to meet these [RFC5548]. RPL is a new routing protocol designed to meet these
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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 to be any DAG Sibling: A sibling of a node within a DAG is defined in this
neighboring node which is located at the same depth, or rank, specification to be any neighboring node which is located at
within a DAG. Note that siblings defined in this manner do not the same rank (depth) within a DAG. Note that siblings defined
necessarily share a common parent. For each DAGID that a node in this manner do not necessarily share a common parent. For
is a member of, the node will maintain a set of DAG Siblings. each DAG that a node is a member of, the node will maintain a
If a node is a member of multiple DAGs then it must set of DAG Siblings. If a node is a member of multiple DAGs
conceptually maintain a set of DAG Siblings for each DAGID. then it must conceptually maintain a set of DAG Siblings for
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 graph. All paths in
the DAG terminate at a DAG root, and all DAG edges contained in the DAG terminate at a DAG root, and all DAG edges contained in
the paths terminating at a DAG root are oriented toward the DAG the paths terminating at a DAG root are oriented toward the DAG
root. There must be at least one DAG Root per DAGID, and in root. There must be at least one DAG Root per DAG, and in some
some cases there may be more than one. In many use cases, cases there may be more than one. In many use cases, source-
source-sink represents a dominant traffic flow, where the sink sink represents a dominant traffic flow, where the sink is a
is a DAG root. Maintaining default routing towards DAG roots DAG root. Maintaining default routing towards DAG roots is
is therefore a prominent functionality for RPL. 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 a
default route. default route to an external routed infrastructure such as the
Internet.
Floating: A DAG is floating if it contains a DAG root that does not Floating: A DAG is floating if is not Grounded. A floating DAG may
offer a default route. install a default route, although it is not expected to reach
any additional external routed infrastructure such as the
Internet.
Inward: In the context of RPL, inward refers to the direction from Inward: In the context of RPL, inward refers to the direction from
leaf nodes towards DAG roots, following the orientation of the leaf nodes towards DAG 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: In the context of RPL, outward refers to the direction from
DAG roots towards leaf nodes, going against the orientation of DAG roots towards leaf nodes, going against the orientation of
the edges within the DAG. the edges within the 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]). [RFC4875]). A common RPL use case involves P2MP flows from or
through a DAG Root outward towards other nodes contained in the
DAG.
MP2P: Multipoint-to-Point; used to describe a particular traffic 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)
indicates which routing metrics, optimization objectives, and indicates which routing metrics, optimization objectives, and
related functions are in use in a DAG. It is recommended that related functions are in use in a DAG. Instances of the
a companion document define instances of the Objective Code Objective Code Point are further described in
Point and request the creation of a registry to manage them. [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]. An architectural protocol overview (the big picture of
the protocol) is provided in this section. Protocol details can be the protocol) is provided in this section. Protocol details can be
found in further sections. found in further sections.
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operate over lossy links (usually low bandwidth with low packet operate over lossy links (usually low bandwidth with low packet
delivery success rate). delivery success rate).
3.2.2. Path Properties for LLN Traffic Flows 3.2.2. Path Properties for LLN Traffic Flows
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. specific basis. In particular, several applications such as building
or home automation do require P2P (Point-to-Point) communication.
As required by the aforementioned routing requirements documents, RPL 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 nodes).
3.2.3. Constraint Based Routing 3.2.3. 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 supported by RPL are specified
in a companion document to this specification, in a companion document to this specification,
[I-D.ietf-roll-routing-metrics]. RPL signals the metrics and related [I-D.ietf-roll-routing-metrics]. RPL signals the metrics and related
objective functions in use in a particular implementation by means of objective functions in use in a particular implementation by means of
an Objective Code Point (OCP). an Objective Code Point (OCP). Both the routing metrics and the OCP
help determine the construction of the Directed Acyclic Graphs (DAG)
using a distributed path computation algorithm.
RPL supports the computation and installation of different paths in 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 3.2.4. Autonomous Operation
Nodes running RPL are able to independently and autonomously discover Nodes running RPL are able to independently and autonomously discover
a network topology and compute and install routes, without requiring a network topology and compute and install routes, without requiring
further administrative interaction. further administrative interaction.
3.3. Protocol Operation 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 provide default routes. rooted at designated nodes that generally have some application
The DAG is sufficient to support inward MP2P traffic flows, flowing significance, such as providing a default route to an external routed
inward along the LLN towards a sink (DAG Root), which is one of the infrastructure. The DAG is sufficient to support inward MP2P traffic
dominant traffic flows described in the requirements documents flows, flowing inward along the LLN towards a sink (DAG Root), which
([I-D.ietf-roll-building-routing-reqs], 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 26 skipping to change at page 9, line 38
This section further describes the high level operation of RPL. This section further describes the high level operation of RPL.
3.3.1. DAG Construction 3.3.1. DAG Construction
3.3.1.1. Overview of a Typical Case 3.3.1.1. Overview of a Typical Case
RPL constructs one or more base routing topologies, in the form of RPL constructs one or more base routing topologies, in the form of
DAGs, over gradients defined by optimizing cost metrics along paths DAGs, over gradients defined by optimizing cost metrics along paths
rooted at designated nodes. rooted at designated nodes.
DAGs may be grounded, in which case the DAG Root is offering a DAGs may be grounded, in which case the DAG Root (e.g. an LBR) is
default route. A typical goal for a node participating in DAG offering a default route to an external routed infrastructure such as
Construction will be to find and join a grounded DAG. the Internet. A typical goal for a node participating in DAG
Construction may be to find and join a grounded DAG. Any DAG which
is not grounded is floating, and default routes may still be
provisioned toward the DAG root 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 offering a default route and thus be provisioned to act be capable of, e.g. serving as an LBR or acting as a data collection
as DAG roots. These nodes will begin the process of constructing a point, and thus be provisioned to act as the most preferred DAG
grounded DAG by occasionally emitting Router Advertisements roots. These nodes will begin the process of constructing a DAG by
containing the necessary information for neighboring nodes to occasionally emitting Router Advertisements containing the necessary
evaluate the DAG Root as a potential DAG parent. This information information for neighboring nodes to evaluate the DAG Root as a
will include a DAGID and an Objective Code Point (OCP). The OCP potential DAG parent. This information will include a DAGID, a
provides information as to which metrics and optimization goals are DAGPreference, and an Objective Code Point (OCP). The DAGID is an
being employed across the DAG. Note that a single DAG Root may identifier unique to the DAG. The DAGPreference offers a way to
conceptually root different DAGs with different OCPs as required to engineer the formation of the DAG in support of the application, by
support different sets of routing constraints. Note that if multiple providing a mechanism by which the DAG may look attractive for other
DAG roots are rooting the same DAG, i.e. presenting the same DAGID, nodes to join. The OCP provides information as to which metrics and
then they must have some means of coordinating with each other when optimization goals are being employed across the DAG. Note that a
emitting Router Advertisements. This is described further below. single DAG Root may conceptually root different DAGs with different
OCPs as required to support different sets of routing constraints.
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 Router Advertisements, advertising a specific DAGID,
and OCP, will take into consideration several criteria when will take into consideration several criteria when processing the
processing the extracted DAG information. A node may seek a DAG extracted DAG information. A node may seek a DAG advertising a
advertising a specific OCP, reflecting the implementation specific specific OCP, reflecting the implementation specific routing
routing constraints understood by the node. In particular, a node constraints understood by the node. In particular, a node will be
will be seeking to find a least cost path satisfying some objective seeking to find a least cost path satisfying some objective function
function as indicated by the OCP according to some routing metrics as indicated by the OCP according to some routing metrics defined in
defined in [I-D.ietf-roll-routing-metrics]. For example, the least [I-D.ietf-roll-routing-metrics]. For example, the least cost path
cost path may be determined in part by minimizing energy along a may be determined in part by minimizing energy along a path, or
path, or latency, or avoiding the use of battery powered nodes. 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
the minimum DAGPreference when selecting a DAG, all else being equal.
Based on the evaluation of such criteria, a node may determine if the Based on the evaluation of such criteria, a node may determine if the
node who emitted the Router Advertisement should be considered as a node who emitted the Router Advertisement should be considered as a
potential DAG parent. If so, then the node may add the advertising potential DAG parent. If so, then the node may add the advertising
node to its set of DAG parents for the advertised DAGID, and can be node to its set of candidate DAG parents for the advertised DAGID,
considered to have joined the DAG designated by DAGID. and after waiting for a designated delay, the node may follow the
procedures to activate the advertising node as a DAG 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 beginning
to multicast the DAG information in Router Advertisements. In this to multicast the DAG information in Router Advertisements (to
way, nodes are able to join the DAG at ever-increasing depths outward neighbors with a link-local scope). In this way, nodes are able to
from the DAG root. As nodes continue to receive DAG multicasts they join the DAG at ever-increasing rank outward from the DAG root. As
may continue to expand their set of DAG parents, while employing loop nodes continue to receive DAG multicasts they may continue to expand
avoidance strategies as describe below, in order to build path their set of DAG parents, while employing loop avoidance strategies
diversity inwards toward the DAG root. as describe below, in order to build path diversity inwards toward
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 depth 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 In addition to identifying DAG parents, a node also may hear the
Router Advertisements of other neighboring nodes at the same depth Router Advertisements of other neighboring nodes at the same rank
within the DAG. In this way a node can discover DAG Siblings. within the DAG. In this way a node can discover DAG Siblings.
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. To this list the node may also
append a similarly ordered set of DAG siblings. By forwarding append a similarly ordered set of DAG siblings. By forwarding
traffic intended for the default destination towards the DAG parents, traffic intended for the default destination towards the DAG parents,
the node is able to support the main Multipoint-to-point (MP2P) the node is able to support the main Multipoint-to-point (MP2P)
traffic flows required by a typical LLN application. By using the traffic flows required by a typical LLN application. By using the
ordered set of DAG parents and DAG siblings the node is able to take 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
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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. In a further
example, a forwarding implementation may choose to decrease the hop example, a forwarding implementation may choose to decrease the hop
limit more quickly when forwarding along sibling paths than along limit more quickly when forwarding along sibling paths than along
parent paths. A forwarding engine may behave in a manner similar to parent paths. A forwarding engine may behave in a manner similar to
these examples, however the specific implementation of a forwarding these examples, however the specific implementation of a forwarding
engine and related path diversity strategies is beyond the scope of engine and related path diversity strategies is beyond the scope of
this specification. 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 Note that the further interaction of the routing solution and the
forwarding engine, in particular how they utilize and react to forwarding engine, in particular how they utilize and react to
changes in metrics, and how the forwarding engine may use the changes in metrics, and how the forwarding engine may use the
constrained set of successors provided by the routing engine based on constrained set of successors provided by the routing engine based on
L2 triggers and metrics, is under investigation. 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 default destination flows inward along the traffic intended for the destinations available to or through the DAG
DAG towards the root, and nodes forwarding traffic are able to root, e.g. the default destination or other advertised prefixes,
leverage the path diversity of the DAG as necessary. flows inward along the DAG towards the root, and nodes forwarding
traffic are able to leverage the path diversity of the DAG as
necessary.
The DAG is then used by RPL as a reference topology, constraining the The DAG is then used by RPL as a reference topology, constraining the
LLN routing problem, on which to build additional routing mechanisms. LLN routing problem, on which to build additional routing mechanisms.
3.3.1.2. Further Operation 3.3.1.2. Further Operation
The sub-DAG of a node is the set of other nodes of greater depth 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. Depth 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 tend to have a greater depth than sub-DAG of a specific node should have a greater rank than the node.
the node. Paths through siblings are not contained in this set. This is an important property that is leveraged for loop avoidance-
if a node has lesser rank then it is NOT in the sub-DAG. (An
arbitrary node with greater rank may or may not be contained in the
sub-DAG). Paths through siblings are not contained in this set.
As a further illustration, consider the DAG examples in Appendix B. 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, in which case the node rooting the DAG is A DAG may also be floating. Floating DAGs may be encountered, for
not offering a default route. 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. 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
transient and carries a marking to make it less attractive. Some
specific application scenarios may employ permanent floating DAGS,
e.g. DAGs without connectivity to an external routed infrastructure,
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
make it more attractive. DAGPreference, a configurable property that
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 LBR. This specification does not necessarily) to or through a grounded DAG rooted at an LBR. In some
preclude a node from joining multiple DAGs. In one such case, a cases, as suitable to the application scenario, a DAG may still
particular application may require the node to maintain membership in provision the default route toward DAG Parents and not be connected
multiple DAGs in order to satisfy competing constraints, for example to a backbone network or the Internet.
to support different types of traffic, similar to the concept of MTR
(Multi-topology routing) as supported by other routing protocols such This specification does not preclude a node from joining multiple
as IS-IS [RFC5120] or OSPF [RFC4915], although the RPL mechanisms DAGs. In one such case, a particular application may require the
will significantly differ from the ones specified for these node to maintain membership in multiple DAGs in order to satisfy
protocols. (Note that not all constrained traffic cases may require competing constraints, for example to support different types of
multiple DAGs). In support of such cases the RPL implementation must traffic, similar to the concept of MTR (Multi-topology routing) as
independently maintain requisite information and state for each DAG supported by other routing protocols such as IS-IS [RFC5120] or OSPF
in parallel. In cases where a competing constraints must be [RFC4915], although the RPL mechanisms will significantly differ from
satisfied toward the same DAG root, the OCP should differ by the ones specified for these protocols. (Note that not all
definition and may serve to coordinate the maintenance of the constrained traffic cases may require multiple DAGs). In support of
multiple DAGs. such cases the RPL implementation must independently maintain
requisite information and state for each DAG in parallel. In cases
where a competing constraints must be satisfied toward the same DAG
root, the OCP should differ by definition and may serve to coordinate
the maintenance of the multiple DAGs. Further, additional
recommendations for the operation of loop avoidance/loop detection
mechanisms in the presence of multiple DAGs are under investigation.
An administered preference (DAGPreference) shall be associated with
each DAG. In cases where a RPL node has a choice of joining more
than one DAG to satisfy a particular constraint, and all else being
equal, the node will seek to join the DAG with the lowest preference
value. In practice this mechanism may be assist in engineering the
construction of a DAG as appropriate to an application. For example,
nodes that are to become DAG roots in support of a particular
application role, e.g. as a data sink or a controller, may be
provisioned with a low DAG preference, e.g. 0x00. Nodes who are
serving as the DAG root of a transient DAG, e.g. for DAG repair, may
take on a high DAG preference, e.g. 0xFF. Nodes will then be able to
yield their transient DAGs to join the DAGs with lower DAGPreference.
3.3.1.3. Router Advertisement - DAG Information Option (RA-DIO) 3.3.1.3. Router Advertisement - DAG Information Option (RA-DIO)
The IPv6 Router Advertisement mechanism (as specified in [RFC4861]) The IPv6 Router Advertisement mechanism (as specified in [RFC4861])
is used by RPL in order to build and maintain a DAG. is used by RPL in order to build and maintain a DAG.
The IPv6 Router Advertisement message is augmented with a DAG The IPv6 Router Advertisement message is augmented with a DAG
Information Option (DIO) in order to facilitate the formation and Information Option (DIO) 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 DIO includes
the following: 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. Root. May be tested for equality. The DAGID MUST be unique to a
single DAG in the scope of the LLN. If the DAG Root is rooting
multiple DAGs, each must be provisioned with their own IPv6
address and thus derive unique DAGIDs.
o Objective Code Point (OCP) as described below. o Objective Code Point (OCP) as described below.
o Depth information used by nodes to determine their relationships o Rank information used by nodes to determine their relationships in
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. Used to support loop avoidance strategies and in support
of ordering alternate successors when engaged in path maintenance. 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 destinations 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 Router Advertisements are issued whenever a change is detected to
the DAG such that a node is able to determine that a region of the the DAG such that a node is able to determine that a region of the
DAG has become inconsistent. As the DAG stabilizes the period at DAG has become inconsistent. As the DAG stabilizes the period at
which Router Advertisements occur is configured to taper off, which Router Advertisements occur is configured to taper off,
reducing the steady-state overhead of DAG maintenance. The periodic reducing the steady-state overhead of DAG maintenance. The periodic
issue of Router Advertisements, along with the triggered Router issue of Router Advertisements, along with the triggered Router
Advertisements in response to inconsistency, is one feature that Advertisements in response to inconsistency, is one feature that
enables RPL to operate in the presence of unreliable links. enables RPL to operate in the presence of unreliable 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) 3.3.1.4. 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 envisaged the optimization functions used within the DAG. The OCP is further
to serve as an reference into a TBA Registry. Each instance of an specified in [I-D.ietf-roll-routing-metrics]. Each instance of an
allocated OCP MUST indicate: 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 Depth
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 DIO
For example, and objective code point might indicate that the DAG is For example, an objective code point might indicate that the DAG is
using ETX, that the optimization goal is to minimize ETX, that DAG using ETX as a metric, that the optimization goal is to minimize ETX,
Depth is equivalent to ETX, and that DIO propagation entails adding that DAG Rank is equivalent to ETX, and that DIO propagation entails
the advertised ETX of the most preferred parent to the ETX of the adding the advertised ETX of the most preferred parent to the ETX of
link to the most preferred parent. 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 DIO, RPL
nodes may work to build optimized LLN using a variety of application nodes may work to build optimized LLN using a variety of application
and implementation specific metrics and goals. and implementation specific metrics and goals.
NOTE: A NULL OCP MUST be specified with a well-defined default A default OCP, OCP 0, is specified with a well-defined default
behavior. The NULL code point will subsequently be used to define behavior. OCP 0 is used to define RPL behaviors in the case where a
RPL behaviors in the case where a node encounters a DIO containing a node encounters a DIO containing a code point that it does not
code point that it does not support. support.
3.3.1.5. Selection of Feasible DAG Parents 3.3.1.5. 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 in parallel may be constructed and maintained in the Thus two DAGs, with two unique DAGIDs, may be constructed and
LLN, DAG-1 would be optimized according to OCP-1, whereas DAG-2 would maintained in the LLN: DAG-1 would be optimized according to OCP-1,
be optimized according to OCP-2. A node may then maintain two whereas DAG-2 would be optimized according to OCP-2. A node may then
parallel sets of DAG parents. Note that in such a case traffic may maintain two parallel sets of DAG parents and related data
directed along the appropriate constrained DAG based on traffic structures. Note that in such a case traffic may directed along the
marking within the IPv6 header. appropriate constrained DAG based on traffic marking within the IPv6
header.
As a node hears RAs from its neighbors it may process their DIOs. At As a node hears RAs from its neighbors it may process their DIOs. At
this time the node may be able to take into consideration, for this time the node may be able to take into consideration, for
example, the following: 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?
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example, the following: 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
for an otherwise currently satisfied 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. neighboring node into the set of DAG parents according to
implementation specific algorithms that are outside the scope of this
document.
When the node inserts the first DAG parent into the empty DAG parent When the node inserts the first DAG parent into the empty DAG parent
set, it is able to join the DAG. After the DAG parent set is set, it is able to join the DAG. After the DAG parent set is
updated, the node will consult a depth computation function indicated updated, the node will consult a rank computation function indicated
by the OCP for the DAG in order to determine its depth value, which by the OCP for the DAG in order to determine its rank value, which it
it will subsequently advertise when it emits its own DIOs. A general will subsequently advertise when it emits its own DIOs. A general
property of the depth value presented by the node is that it should property of the rank value presented by the node is that it should be
be greater than that presented by any of its DAG parents. It is greater than that presented by any of its DAG parents. A node must
recommended that a node maintain its DAG Parent set such that its maintain its DAG Parent set such that its most preferred parent from
most preferred parent from the OCP goals also has the greatest depth the OCP goals also has the greatest rank value in the DAG parent set.
value in the DAG parent set. All reliable neighboring nodes of a All reliable neighboring nodes of a lesser rank than the node may
lesser depth then the node may then be considered as potential DAG then be considered as potential DAG parents (Note that as a
parents. All neighboring nodes of equal depth may come to be consequence of satisfying a particular OCP goal, the most preferred
considered as siblings within the DAG (even though they may not have parent may not necessarily be the potential parent of least rank, for
parents in common, they may still provide path diversity towards the example a potential parent of lesser rank may also be an energy
DAG root). constrained device that is to generally be avoided and thus not the
most preferred). No nodes of greater rank than the most preferred
parent 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 depth, and related properties, are further The computation of rank, and related properties, are further
described in Section 3.4.1. described in Section 3.4.1.
3.3.1.5.1. Example 3.3.1.5.1. 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 depth 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 Router Solicitations to probe for
nearby DAGs. 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 DIO, and learns that
Node (B) is a member of DAGID 1 at depth 4, and not `Blue'. Node Node (B) is a member of DAGID 1 at rank 4, and not `Blue'. Node
(N) takes note of this, but is not yet confident. (N) takes note of this, but is not yet confident.
o Similarly, Node (N) hears from Node (A) at depth 9, Node (C) at o Similarly, Node (N) hears from Node (A) at rank 9, Node (C) at
depth 5, and Node (E) at depth 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 DIO that indicates that it is a
member of DAGID 1 at depth 2, but it carries the attribute `Blue'. member of DAGID 1 at rank 2, but it carries the attribute `Blue'.
Node (N)'s policy function rejects Node (D), and no further Node (N)'s policy function rejects Node (D), and no further
consideration is given. 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) (depth 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 depth 5 in DAGID. 1. now at rank 5 in DAGID 1.
o Node (N) adds Node (B) (depth 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 depth 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. following successor can make inward progress. Should Node (C) not
have a viable parent, it should never send the packet back to Node
(N) (to avoid a 2-node loop).
3.3.1.6. DAG Maintenance 3.3.1.6. 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 depth. 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 in the context of a DAG is attaching to a new DAGID, in such a
way that an old DAGID is replaced by the new DAGID. In particular, way that an old DAGID is replaced by the new DAGID. In particular,
when an old DAGID is left, all associated parents are no longer when an old DAGID is left, all associated parents are no longer
feasible, and a new DAGID is joined. 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 3.4.1.1.
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, discarding a deeper node and possibly causing its preferred parent, possibly causing its own advertised rank to
own advertised depth to decrease. This case is `moving inwards along decrease, and discarding any former parents now of a deeper rank.
the DAG' and does not require any additional coordination for loop This case is `moving inwards along the DAG' and does not require any
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 will become the root of its node will have detached from the DAG, and may, if so configured,
own floating DAG (thus establishing the frozen sub-DAG), and then may become the root of its own transient floating DAG with a high
reattach to the original DAG at a lower point if it is able. DAGPreference (0xFF) (thus beginning the process of establishing the
frozen sub-DAG), and then may reattach to the original DAG at a lower
point if it is able (after hearing RA-DIOs 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, it may do so safely without regard to loop avoidance.
However, it may not return immediately to the current DAG as such However, it may not return immediately to the current DAG as such
movement may trigger the creation of loops. movement may result in the creation of loops.
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 3.4.3.
Section 5 contains more detail on the processes and rules used for Section 5 contains more detail on the processes and rules used for
skipping to change at page 17, line 33 skipping to change at page 19, line 19
maintenance. maintenance.
3.3.2. Source Routing 3.3.2. Source Routing
A Source Routing mechanism for RPL is currently under investigation. A Source Routing mechanism for RPL is currently under investigation.
3.3.3. Destination Advertisement 3.3.3. Destination Advertisement
As RPL constructs DAGs, nodes are able to learn a set of default As RPL constructs DAGs, nodes are able to learn a set of default
routes in order to send traffic to the sink. However, this mechanism routes in order to send traffic to the sink. However, this mechanism
alone does is not sufficient to support P2MP traffic flowing outward alone is not sufficient to support P2MP traffic flowing outward along
along the DAG from the DAG root toward nodes. A Destination the DAG from the DAG root toward nodes. A Destination Advertisement
Advertisement mechanism is employed by RPL to build up routing state mechanism is employed by RPL to build up routing state in support of
in support of these outward flows. these outward flows. The Destination Advertisement mechanism may not
be supported in all implementations, as appropriate to the
application requirements. A DAG Root that supports using the
Destination Advertisement mechanism to build up routing state will
indicate such in the DIO. A DAG Root that supports using the
Destination Advertisement mechanism MUST be capable of allocating
enough state to store the routing state received from the LLN.
3.3.3.1. Destination Advertisement Option (DAO) 3.3.3.1. Destination Advertisement Option (DAO)
A Destination Advertisement Option (DAO) is used to convey the A Destination Advertisement Option (DAO) is used to convey the
Destination information inward along the DAG toward the DAG root. Destination information inward along the DAG toward the DAG root.
The information conveyed in the DAO includes the following: The information conveyed in the DAO 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.
skipping to change at page 18, line 4 skipping to change at page 19, line 45
The information conveyed in the DAO includes the following: The information conveyed in the DAO 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 3.3.3.2. Destination Advertisement Operation
As the DAG is constructed and maintained, nodes will emit messages As the DAG is constructed and maintained, nodes are capable to emit
containing Destination Advertisement Options to a subset of their DAG messages containing Destination Advertisement Options to a subset of
Parents. The selection of this subset is according to an their DAG Parents. The selection of this subset is according to an
implementation specific policy. implementation specific policy.
Note that further details of the message and its triggers are still As a special case, a node may periodically emit a link-local
under investigation, including whether or not the DAO should be a multicast message containing a Destination Advertisement Options
IPv6 Hop-By-Hop option or a Neighbor Discovery option. advertising its locally available destination prefixes. This
mechanism allows for the one-hop neighbors of a node to learn
explicitly of the prefixes on the node, and in some application
specific scenarios this is desirable in support of provisioning a
trivial `one-hop' route. In this case, nodes who receive the
multicast Destination Advertisement may use it to provision the one-
hop route only, and not engage in any additional processing (so as
not to engage the mechanisms used by a DAG Parent).
When a DAO reaches a node capable of storing routing state, the node When a (unicast) DAO reaches a node capable of storing routing state,
extracts information from the DAO and updates its local database with the node extracts information from the DAO and updates its local
a record of the DAO and who it was received from. When the node database with a record of the DAO and who it was received from. When
later propagates DAOs, it selects the best (least depth) information the node later propagates DAOs, it selects the best (least depth)
for each destination and conveys this information again in the form information for each destination and conveys this information again
of DAOs to a subset of its own DAG parents. At this time the node in the form of DAOs to a subset of its own DAG parents. At this time
may perform route aggregation if it is able, thus reducing the the node may perform route aggregation if it is able, thus reducing
overall number of DAOs. the overall number of DAOs.
When a DAO reaches a node incapable of storing additional state, the When a (unicast) DAO reaches a node incapable of storing additional
node MUST append its own address to a Reverse Route Stack carried state, the node MUST append the next-hop address (from which neighbor
within the DAO. The node then passes the DAO on to one or more of the DAO was received) to a Reverse Route Stack carried within the
its DAG parents without storing any additional state. DAO. The node then passes the DAO 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 DAO When a node that is capable of storing routing state encounters a
with a Reverse Route Stack that has been populated, the node knows (unicast) DAO with a Reverse Route Stack that has been populated, the
that the DAO has traversed a region of nodes that did not record any node knows that the DAO has traversed a region of nodes that did not
routing state. The node is able to detach and store the Reverse record any routing state. The node is able to detach and store the
Route State and associate it with the destination described by the Reverse Route State and associate it with the destination described
DAO. Subsequently the node may use this information to construct a by the DAO. Subsequently the node may use this information to
source route in order to bridge the region of nodes that are unable construct a source route in order to bridge the region of nodes that
to support Hop-By-Hop routing to reach the destination. 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 LLN along the DAG, and as according to the available resources in the
nodes. network.
Further aggregations of DAOs by destinations are possible in order to Further aggregations of DAOs by destinations are possible in order to
support additional scalability. 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 3.4. Other Considerations
3.4.1. DAG Depth and Loop Avoidance 3.4.1. 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 DIOs along the DAG in conjunction
with the related objective functions as specified by the OCP. 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 depth 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 Depth 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. The metrics to select DAG parents and take up a position in the DAG. In
computation of the DAG Depth MUST be done in such a way so as to other words, routing metrics and OCP (not rank directly) are used to
determine the DAG structure and consequently the path cost. The only
aim of the rank is to inform loop avoidance as explained hereafter.
The computation of the DAG Rank MUST be done in such a way so as to
maintain the following properties for any nodes M and N who are 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, DAGDepth(N) > For a node N, and its most preferred parent M, DAGRank(N) >
DAGDepth(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 depth less than or equal to DAGDepth(M). (This must be of a rank less than or equal to DAGRank(M). In other
mechanism serves to avoid loops in the case where an alternate words, the rank presented by a node N MUST be greater (deeper)
parent is used- if no alternate parent is deeper than the than that presented by any of its parents. (This mechanism serves
preferred parent then loops are avoided. The risk of loops occurs to avoid loops in the case where an alternate parent is used- if
when an alternate parent goes deeper, and traffic then makes no alternate parent is deeper than the preferred parent then loops
backwards progress and comes back to the node again). are avoided. The risk of loops occurs if there is a chance for an
alternate parent to forward traffic to a deeper successor, which
may be in the sub-DAG, and traffic then makes backwards progress
and comes back to the node again).
If DAGDepth(M) < DAGDepth(N), then M is located in a more optimum If DAGRank(M) < DAGRank(N), then M is located in a more optimum
position than N in the DAG with respect to the metrics and 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. Node M may
safely be a DAG Parent for Node N without risk of creating a loop. safely be a DAG Parent for Node N without risk of creating a loop.
For example, a Node M of rank 3 is located in a more optimum
position than a Node N of rank 5. A packet directed inwards and
forwarded from Node N to Node M will always make forward progress
with respect to the DAG organization on that link; there is no
risk of Node M at rank 3 forwarding the packet back into Node N's
sub-DAG at rank of 5 or greater (which would be a sufficient
condition for a loop to occur).
If DAGDepth(M) == DAGDepth(N), then M and N are located positions If DAGRank(M) == DAGRank(N), then M and N are located positions of
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. other means. If Node M is at rank 3 and node N is at rank 3, then
they are siblings; by definition Node M and N cannot be in each
others sub-DAG. They may then forward to each other failing
serviceable parents, making `sideways' progress (but not reverse
progress). If another sibling or more gets involved there may
then be some chance for 3 or more way loops, which is the risk of
sibling forwarding.
If DAGDepth(M) > DAGDepth(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. selection may create a loop. For example, if Node M is of rank 3
and Node N is of rank 5, then by definition Node N is in a less
optimum position than Node N. Further, Node N at rank 5 may in
fact be in Node M's own sub-DAG, and forwarding a packet directed
inwards towards the DAG root from M to N will result in backwards
progress and possibly a loop.
For example, the DAG Depth 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
more complicated way as appropriate to the objective code point being more complicated way as appropriate to the objective code point being
used within the DAG. used within the DAG.
The DAG depth is subsequently used to restrict the options a node has The DAG rank is subsequently used to restrict the options a node has
for movement within the DAG and to coordinate movements in order to for movement within the DAG and to coordinate movements in order to
avoid the creation of loops. avoid the creation of loops.
A node may safely move `up' in the DAG, causing its DAG depth to A node may safely move `up' in the DAG, causing its DAG rank to
decrease and moving closer to the DAG root without risking the decrease and moving closer to the DAG root without risking the
formation of a loop. formation of a loop.
A node may not consider to move `down' the DAG, causing its DAG depth A node may not consider to move `down' the DAG, causing its DAG rank
to increase and moving further from the DAG root. Such a move will to increase and moving further from the DAG root. Such a move will
entail moving to a less optimum position in the DAG in all cases, as entail moving to a less optimum position in the DAG in all cases, as
defined by the objective code point. In the case where a node looses defined by the objective code point. In the case where a node looses
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. then rejoin at a deeper point. This allows for the node to
coordinate moving down, freezing its own sub-DAG and poisoning stale
routes to the DAG, and minimizing the chances of re-attaching to its
own sub-DAG thinking that it has found the original DAG again. If a
node where allowed to re-attach into its own sub-DAG a loop would
most certainly occur, and may not be broken until a count-to-infinity
process elapses. The procedure of detaching before moving down
eliminates the need to count-to-infinity.
Any neighboring nodes of lesser or equal depth are eligible to be Any neighboring nodes of lesser or equal rank to the current most
considered as DAG parents. preferred DAG parent are eligible to be considered as alternate DAG
parents.
The goal of a guaranteed consistent and loop free global routing
solution for an LLN may not be practically achieved given the real
behavior and volatility of the underlying metrics. The trade offs to
achieve a stable approximation of global convergence may be too
restrictive with respect to the need of the LLN to react quickly in
response to the lossy environment. Globally the LLN may be able to
achieve a weak convergence, in particular as link changes are able to
be handled locally and result in minimal changes to global topology.
RPL does not aim to guarantee loop free path selection, or strong
global convergence. In order to reduce control overhead, in
particular the expense of mechanisms such as count-to-infinity, RPL
does try to avoid the creation of loops when undergoing topology
changes. Further mechanisms to mitigate the impact of loops, such as
loop detection when forwarding, are under investigation.
3.4.1.1. Example 3.4.1.1. Example
: : : : : :
: : : : : :
(A) (A) (A) (A) (A) (A)
|\ | | |\ | |
| `-----. | | | `-----. | |
| \ | | | \ | |
(B) (C) (B) (C) (B) (B) (C) (B) (C) (B)
| | \ | | \
| | `-----. | | `-----.
| | \ | | \
(D) (D) (C) (D) (D) (C)
| |
| |
| |
(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 depth d. Node (A) is a DAG Parent (A) is attached to a DAG at some rank d. Node (A) is a DAG Parent of
of Nodes (B) and (C). Node (C) is a DAG Parent of Node (D). There Nodes (B) and (C). Node (C) is a DAG Parent of Node (D). There is
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 and temporarily unavailable, but with some chance of creating a loop
requiring the intervention of additional mechanisms to detect and (e.g. if multiple nodes in a set of siblings start forwarding
break the loop. `sideways' in a cycle) and requiring the intervention of additional
mechanisms to detect and break the loop.
Consider the case where Node (C) hears a DIO from a Node (Z) at a Consider the case where Node (C) hears a DIO from a Node (Z) at a
lesser depth and superior position in the DAG than node (A). Node lesser rank and superior position in the DAG than node (A). Node (C)
(C) may safely undergo the process to evict node (A) from its DAG may safely undergo the process to evict node (A) from its DAG Parent
Parent set and attach directly to Node (Z) without creating a loop, set and attach directly to Node (Z) without creating a loop, because
because its depth will decrease. its rank will decrease.
Consider the case where the link (C)->(A) becomes nonviable, and node Consider the case where the link (C)->(A) becomes nonviable, and node
(C) must move to a deeper depth within the DAG: (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 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 from Node (C), follows Node
(C) into the floating DAG. This is depicted in Figure 1-2. In (C) into the floating DAG. This is depicted in Figure 1-2. In
general, any node with no other options in the sub-DAG of Node (C) general, any node with no other options in the sub-DAG of Node (C)
will follow Node (C) into the floating DAG, maintaining the 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 from Node (B) and determines it is able to
rejoin the grounded DAG by reattaching at a deeper depth to Node rejoin the grounded DAG by reattaching at a deeper rank to Node
(B). Node (C) starts a DAG Hop timer to coordinate this move. (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 sourced from Node (C) and
follow Node (C) in a coordinated manner to reattach to the 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 3.4.2. DAG Parent Selection, Stability, and Greediness
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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-DIOs from a
node in its own sub-DAG, and in general a node deeper than it. In node in its own sub-DAG, and in general a node deeper than it. In
such cases a chance exists to create a feedback loop, wherein two or such cases a chance exists to create a feedback loop, wherein two or
more nodes continue to try and move in the DAG in order to optimize more nodes continue to try and move in the DAG in order to optimize
against each other. In some cases this will result in an against each other. In some cases this will result in an
instability. It is for this reason that RPL recommends 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 MUST NOT receive and process RA-DIOs from deeper nodes. This rule
creates an `event horizon', whereby a node cannot be influenced into creates an `event horizon', whereby a node cannot be influenced into
an instability by the action of nodes that may be in its own sub-DAG. an instability by the action of nodes that may be in its own sub-DAG.
3.4.2.1. Example 3.4.2.1. 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
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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.
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 depth, 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 DIO from Node (C) (against the rules of RPL), and then
Node (B) leaves the DAG and rejoins at a lower depth, taking both Node (B) leaves the DAG and rejoins at a lower rank, taking both
Nodes (A) and (C) as DAG Parents. Now Node (B) is deeper than Nodes (A) and (C) as DAG Parents. Now Node (B) is deeper than
both Nodes (A) and (C) and is satisfied with 2 DAG parents. 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) will leave and rejoin deeper, to again get 2 parents
o Then Node (B) will leave and rejoin deeper, to again get 2 parents o Then Node (B) will leave and rejoin deeper, to again get 2 parents
o ... o ...
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 depth sufficient to attach to * Nodes (B) and (C) stick at a rank sufficient to attach to their
their most preferred parent (A) and don't go for any deeper most preferred parent (A) and don't go for any deeper (worse)
(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) don't process DIOs from nodes deeper than
themselves (possibly in their own sub-DAGs) themselves (possibly in their own sub-DAGs)
3.4.3. Merging DAGs 3.4.3. 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 loops
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| | | |
(B) (E) (B) (E)
| | | |
| | | |
| | | |
(C) (F) (C) (F)
Figure 3: Merging DAGs Figure 3: Merging DAGs
Consider the example depicted in Figure 3. Nodes (A), (B), and (C) Consider the example depicted in Figure 3. Nodes (A), (B), and (C)
are part of some larger grounded DAG, where Node (A) is at a depth of are part of some larger grounded DAG, where Node (A) is at a rank of
d, Node (B) at d+1, and Node (C) at d+2. The DAG comprised of Nodes d, Node (B) at d+1, and Node (C) at d+2. The DAG comprised of Nodes
(D), (E), and (F) is a floating DAG, with Node (D) as the DAG root. (D), (E), and (F) is a floating, less preferred, DAG, with Node (D)
This floating DAG may have been formed, for example, in the absence as the DAG root. This floating DAG may have been formed, for
of a grounded DAG or when Node (D) had to detach from a grounded DAG example, in the absence of a grounded DAG or when Node (D) had to
and (E) and (F) followed. All nodes are using compatible objective detach from a grounded DAG and (E) and (F) followed. All nodes are
code points. using compatible objective code points.
Nodes (D), (E), and (F) would rather join the grounded DAG if they Nodes (D), (E), and (F) would rather join the more preferred grounded
are able than to remain in the floating DAG. DAG if they are able than to remain in the less preferred floating
DAG.
Next, let links (C)--(D) and (A)--(E) become viable. The following 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 from Node (C) on link
(C)--(D). Node (D) will consider Node (C) a candidate neighbor, (C)--(D). Node (D) will consider Node (C) a candidate neighbor,
will note that Node (C) is in a grounded DAG at depth d+2, and will note that Node (C) is in a grounded DAG at rank d+2, and will
will begin the process to join the grounded DAG at depth d+3. begin the process to join the grounded DAG at rank d+3. Node (D)
Node (D) will start a DAG Hop timer, logically associated with the will start a DAG Hop timer, logically associated with the grounded
grounded DAG at Node (C), to coordinate the jump. The DAG Hop DAG at Node (C), to coordinate the jump. The DAG Hop timer will
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 from Node
(A) on link (A)--(E). Node (E) will consider Node (A) a candidate (A) on link (A)--(E). Node (E) will consider Node (A) a candidate
neighbor, will note that Node (A) is in a grounded DAG at depth d, neighbor, will note that Node (A) is in a grounded DAG at rank d,
and will begin the process to join the grounded DAG at depth d+1. and will begin the process to join the grounded DAG at rank d+1.
Node (E) will start a DAG Hop timer, logically associated with the Node (E) will start a DAG Hop timer, logically associated with the
grounded DAG at Node (A), to coordinate the jump. The DAG Hop grounded DAG at Node (A), to coordinate the jump. The DAG Hop
timer will have a duration proportional to d. 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, is removes Node
(D), thus emptying the DAG parent set for the floating DAG and (D), thus emptying the DAG parent set for the floating DAG and
leaving the floating DAG. Node (E) then jumps to the grounded DAG leaving the floating DAG. Node (E) then jumps to the grounded DAG
by entering Node (A) into the set of DAG Parents for the grounded by entering Node (A) into the set of DAG Parents for the grounded
DAG. Node (E) is now in the grounded DAG at depth d+1. Node (E), DAG. Node (E) is now in the grounded DAG at rank d+1. Node (E),
by jumping into the grounded DAG, has created an inconsistency and by jumping into the grounded DAG, has created an inconsistency and
will begin to emit RA-DIOs more frequently. will begin to emit RA-DIOs 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 from Node (E). Node
(F) will observe that Node (E) has changed its DAGID and will (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 depth 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 from Node (E). Since Node (E) is
now in a different DAG, Node (D) may process the RA-DIO from Node now in a different DAG, Node (D) may process the RA-DIO from Node
(E). Node (D) will observe that, via node (E), it could attach to (E). Node (D) will observe that, via node (E), it could attach to
the grounded DAG at depth d+2. Node (D) will start another DAG the grounded DAG at rank d+2. Node (D) will start another DAG Hop
Hop timer, logically associated with the grounded DAG at Node (E), timer, logically associated with the grounded DAG at Node (E),
with a duration proportional to d+1. Node (D) now is running two with a duration proportional to d+1. Node (D) now is running two
DAG hop timers, one which was started with duration proportional DAG hop timers, one which was started with duration proportional
to d+1 and one proportional to d+2. 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 depth 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 grounded DAG o In this way RPL has coordinated a merge between the more preferred
and the floating DAG, such that the nodes within the two DAGs come grounded DAG and the less preferred floating DAG, such that the
together in a generally ordered manner, avoiding the formation of nodes within the two DAGs come together in a generally ordered
loops in the process. manner, avoiding the formation of loops in the process.
3.4.4. Local and Temporary Routing Decision 3.4.4. 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 DIO. For example,
the routing may reflect a set of successors (next-hop), along with the routing may reflect a set of successors (next-hop), along with
various aggregated metrics used to load balance the traffic according various aggregated metrics used to load balance the traffic according
to some local policy. Such decisions are local and implementation to some local policy. Such decisions are local and implementation
specific. specific.
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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 5.3, after reporting Discovery mechanism further detailed in Section 5.4, 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
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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
optimizations that nodes along the way expect. The to-be-defined optimizations that nodes along the way expect. The default operation
NULL OCP and related behaviors will further clarify this point. specified in OCP 0 clarifies this point.
4.2. Routing Constraints 4.2. Routing Constraints
A constraint is a link or a node characteristic that must be A constraint is a link or a node characteristic that must be
satisfied by the computed path (using boolean values or lower/upper satisfied by the computed path (using boolean values or lower/upper
bounds) and is by definition neither additive nor multiplicative. bounds) and is by definition neither additive nor multiplicative.
Examples of links constraints are "available bandwidth", Examples of links constraints are "available bandwidth",
"administrative values (e.g. link coloring)", "protected versus non- "administrative values (e.g. link coloring)", "protected versus non-
protected links", "link quality" whereas a node constraint can be the protected links", "link quality" whereas a node constraint can be the
level of battery power, CPU processing power, etc. level of battery power, CPU processing power, etc.
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5.1.1. DIO base option 5.1.1. DIO base option
The DAG Information Option is a container option, which might contain The DAG Information Option is a container option, which might contain
a number of suboptions. The base option regroups the minimum a number of suboptions. The base option regroups the minimum
information set that is mandatory in all cases. 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| Reserved | Sequence | | Type | Length |G|D|A| Rsvd | Sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAGPreference | BootTimeRandom | | DAGPreference | BootTimeRandom |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NodePref. | DAGDepth | DAGDelay | | NodePref. | DAGRank | DAGDelay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DIOIntDoubl. | DIOIntMin. | DAGObjectiveCodePoint | | DIOIntDoubl. | DIOIntMin. | DAGObjectiveCodePoint |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PathDigest | | PathDigest |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | | |
+ + + +
| DAGID | | DAGID |
+ + + +
| | | |
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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 value is
to be assigned by the IANA. to be assigned 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 Grounded (G): The Grounded (G) flag is set when the DAG root is
offering a default route. offering connectivity to an external routed infrastructure such
as the Internet.
Destination Advertisement (D): The Destination Advertisement (D) Destination Advertisement Trigger (D): The Destination Advertisement
flag is set when the DAG root or another node in the successor Trigger (D) flag is set when the DAG root or another node in
chain decides to trigger the sending of Destination the successor chain decides to trigger the sending of
Advertisements in order to update routing state for the outward Destination Advertisements in order to update routing state for
direction along the DAG, as further detailed in Section 5.4. the outward direction along the DAG, as further detailed in
Note that the use and semantics of this flag are still under Section 5.10. Note that the use and semantics of this flag are
investigation. still under investigation.
Reserved: 6-bit unsigned integer set to 0 by the DAG root and left Destination Advertisement Supported (A) : The Destination Supported
(A) bit is set when the DAG root is capable to support the
collection of Destination Advertisement related routing state
and enables the Destination Advertisement mechanism within the
DAG.
Reserved: 5-bit unsigned integer set to 0 by the DAG root and left
unchanged by nodes propagating the DIO. unchanged by nodes propagating the DIO.
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 with each new DIO it sends on a link, and
propagated with no change outwards along the DAG. propagated with no change outwards along the DAG.
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. Default is 0 (lowest
preference). The DAG preference provides an administrative preference). The DAG preference provides an administrative
mechanism to engineer the self-organization of the LLN, for mechanism to engineer the self-organization of the LLN, for
example indicating the most preferred LBR. example indicating the most preferred LBR. If a node has the
option to join a DAG of lower preference while still meeting
other optimization objectives, then the node will seek the
minimum available preference.
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.
DAGDepth: 8-bit unsigned integer. The DAG depth of the DAG root is DAGRank: 8-bit unsigned integer. The DAG rank of the DAG root is 0.
0. The DAG Depth of a node attached to the DAG should be The DAG Rank of a node attached to the DAG should be greater
greater than depth of its deepest DAG parent, as computed by an than rank of its deepest DAG parent, as computed by an
implementation specific routine. All nodes in the DAG implementation specific routine. All nodes in the DAG
advertise their DAG depth in the DAG Information Options that advertise their DAG rank in the DAG Information Options that
they append to the RA messages over their LLN interfaces as they append to the RA messages over their LLN interfaces as
part of the propagation process. 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.
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2^(DIOIntervalDoublings). 2^(DIOIntervalDoublings).
DIOIntervalMin: 8-bit unsigned integer. Used to configure the DIOIntervalMin: 8-bit unsigned integer. Used to configure the
trickle timer governing when RA-DIO should be sent within the trickle timer governing when RA-DIO should be sent within the
DAG. The minimum configured interval for the RA-DIO trickle DAG. The minimum configured interval for the RA-DIO trickle
timer in units of ms is 2^DIOIntervalMin. For example, a timer in units of ms is 2^DIOIntervalMin. For example, a
DIOIntervalMin value of 16ms is expressed as 4. 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 DAGDepth 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. (Note: this specification DAG OCP is set by the DAG Root. (Objective Code Points are to
recommends that another document, e.g. be further defined in [I-D.ietf-roll-routing-metrics].
[I-D.ietf-roll-routing-metrics], define Objective Code Points
and recommend a registry to manage them)
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.
The following values MUST NOT change during the propagation of the The following values MUST NOT change during the propagation of the
DIO outwards along the DAG: Type, Length, G, DAGPreference, DAGDelay DIO outwards along the DAG: Type, Length, G, DAGPreference, DAGDelay
and DAGID. All other fields of the DIO are updated at each hop of and DAGID. All other fields of the DIO are updated at each hop of
the propagation. the propagation.
5.1.1.1. DIO suboptions 5.1.1.1. 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, a
number of suboptions are defined for the DIO: number of suboptions are defined for the DIO:
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...
skipping to change at page 35, line 10 skipping to change at page 38, line 27
5.1.1.1.5. Destination Prefix 5.1.1.1.5. Destination Prefix
The Destination Prefix suboption has an alignment requirement of The Destination Prefix suboption has an alignment requirement of
4n+1. Its format is as follows: 4n+1. Its format is as follows:
0 1 2 3 0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 | Length | Prefix Length |Resvd|Prf|Resvd| | Type = 3 | Length | Prefix Length |Resvd|Prf|Resvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 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 needs to
indicate that it offers connectivity to destination prefixes other indicate that it offers connectivity to destination prefixes other
than the default. This may be useful in cases where more than one than the default. This may be useful in cases where more than one
LBR is operating within the LLN and offering connectivity to LBR is operating within the LLN and offering connectivity to
different administrative domains, e.g. a home network and a utility different administrative domains, e.g. a home network and a utility
network. (Note that a grounded DIO offers the default route without network. In such cases, upon observing the Destination Prefixes
any other qualification needed). In such cases, upon observing the offered by a particular DAG root, a node MAY decide to join multiple
Destination Prefixes offered by a particular DAG root, a node MAY DAGs in support of a particular application.
decide to join multiple DAGs in support of a particular application.
Note that Destination Prefixes specified in this manner inherit the
Router Lifetime of their parent RA.
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. The Prefix Length is an 8-bit
unsigned integer that indicates the number of leading bits in the unsigned integer that indicates the number of leading bits in the
destination prefix. Prf is the Route Preference as in [RFC4191]. destination prefix. Prf is the Route Preference as in [RFC4191].
The Destination Prefix contains Prefix Length significant bits of the The Destination Prefix contains Prefix Length significant bits of the
destination prefix. The remaining bits of the Destination Prefix, as destination prefix. The remaining bits of the Destination Prefix, as
required to complete the trailing octet, are set to 0. required to complete the trailing octet, are set to 0.
The Prefix Lifetime is a 32-bit unsigned integer representing the
length of time in seconds (relative to the time the packet is sent)
that the Destination Prefix is valid for route determination. A
value of all one bits (0xFFFFFFFF) represents infinity. A value of
all zero bits (0x00000000) indicates a loss of reachability.
In the event that a DAG root may need to specify that it offers In the event that a DAG root may need to specify that it offers
connectivity to more than one destination, the Destination Prefix connectivity to more than one destination, the Destination Prefix
suboption may be repeated. suboption may be repeated.
5.2. Neighbor Discovery 5.2. Conceptual Data Structures
5.2.1. RA-DIO Reception The RPL implementation must maintain the following conceptual data
structures in support of DAG Discovery:
An node will come to discover its link layer neighbors by a o A set of Candidate Neighbors
combination of link layer mechanisms and by hearing the multicast RA
messages from the neighbors. Through these mechanisms a node is able
to construct a set of known neighbors.
When receiving and processing the RA-DIO messages from known o For each DAG:
neighbors, in addition to link layer states and characteristics, the
node will come to determine that a neighbor is of particular
interest. As the LLN node periodically observes the neighbor and
determines its behavior to be reliable beyond a certain threshold,
the node may select the neighbor to be part of the candidate neighbor
set and begin to maintain a local confidence value with respect to
the neighbor.
As RA-DIOs are received from candidate neighbors, the DIO information * A set of Candidate DAG Parents
will be consulted to determine, for example:
1. Does the candidate neighbor offer a position in a different DAG, * A set of DAG Parents (which are a subset of Candidate DAG
or a better position in the current DAG? Is the OCP of the Parents and may be implemented as such)
candidate neighbor compatible with the goals of this node? Do
the related path metrics pass the criteria of a implementation
specific policy function such that the candidate neighbor is
considered feasible? If so then consider the candidate neighbor
as a candidate parent. The decision to move up the DAG is a
policy decision and a node may choose not to move up the DAG if
the path metric is not significantly better than the current one.
2. Does the candidate neighbor exist at the same depth in the 5.2.1. Candidate Neighbors
current DAG as this node? Do the related path metrics pass the
criteria of a implementation specific policy function such that
the candidate neighbor is feasible? If so then consider the
candidate neighbor as a DAG sibling.
3. Otherwise, ignore the candidate neighbor. Ignored neighbors may The set of Candidate Neighbors is to be populated by neighbors who
periodically be re-evaluated to see if their situation has are discovered by the neighbor discovery mechanism and further
improved. qualified as statistically stable as per the mechanisms discussed in
[I-D.ietf-roll-routing-metrics]. The Candidate Neighbors, and
related metrics, should demonstrate stability/reliability beyond a
certain threshold, and it is recommended that a local confidence
value be maintained with respect to the neighbor in order to track
this. Implementations may choose to bound the maximum size of the
Candidate Neighbor set, in which case a local confidence value will
assist in ordering neighbors to determine which ones should remain in
the Candidate Neighbor set and which should be evicted.
The implementation SHOULD provide the ability to bound the size of If Neighbor Unreachability Detection (NUD) determines that a
the candidate neighbor set, and a scheme SHOULD be applied to add Candidate Neighbor is no longer reachable, then it shall be removed
and/or evict neighbors from the candidate neighbor set as necessary from the Candidate Neighbor set. In the case that the Candidate
so as not to exceed the bounds. Neighbor has associated states in the DAG Parent set or active DA
entries, then the removal of the Candidate Neighbor shall be
coordinated with tearing down these states. All provisioned routes
associated with the Candidate Neighbor should be removed.
5.2.2. DAGs
A DAG may be uniquely identified by within the LLN by its unique
DAGID. When a single device is capable to root multiple DAGs in
support of an application need for multiple optimization objectives
it is expected to produce a different and unique DAGID for each of
the multiple DAGs.
For each DAG that a node is, or may become, a member of, the
implementation MUST keep a conceptual record of:
o DAGID
o DAGObjectiveCodePoint
o A set of Destination Prefixes offered by the DAG root
o A set of candidate DAG Parents
o A timer to govern the sending of DIOs for the DAG
o DAGSequenceNumber
When a DAG is discovered for which no DAG data structure is
instantiated, and the node wants to join (i.e. the neighbor is to
become a Candidate DAG Parent in the Held-Up state), then the DAG
data structure is instantiated.
When the Candidate DAG Parent set is depleted (i.e. the last
Candidate DAG Parent has timed out of the Held-Down state), then the
DAG data structure may be deallocated. An implementation should
delay before deallocating the DAG data structure in order to observe
that the DAGSequenceNumber has incremented should any new candidate
DAG Parents appear for the DAG.
5.2.2.1. Candidate DAG Parents
When the DAG is self-rooted, the set of candidate DAG Parents is
empty.
In all other cases, for each candidate DAG Parent in the set, the
implementation MUST keep a record of:
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 5.8
o A DAG Hop Timer, if instantiated
o A Held-Down Timer, if instantiated
5.2.2.1.1. DAG Parents
Note that the subset of candidate DAG Parents in the `Current' state
comprises the set of DAG Parents, i.e. the nodes actively acting as
parents in the DAG.
DAG Parents may be ordered, according to the OCP. When ordering DAG
Parents, in consultation with the OCP, the most preferred DAG Parent
may be identified. All current DAG Parents must have a rank less
than or equal to that of the most preferred DAG Parent.
When nodes are added to or removed from the DAG Parent set the most
preferred DAG Parent may have changed and should be reevaluated. Any
nodes having a rank greater than the most preferred parent after such
a change must be placed in the Held-Down state and evicted as per the
procedures described in Section 5.8
An implementation may choose to keep these records as an extension of
the Default Router List (DRL).
5.3. Initialization and Configuration
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
actively root a (floating, DAGPreference 0xFF) DAG when no other
DAG is available? (For example, a battery powered node may not
wish expend energy to do this, but will instead passively listen
for other options).
For each DAG that the node may root, what is the DAGID?
What are the supported OCP (optimization goals)?
What, if any, destination prefixes are being sought, associated
with supported OCP?
When a node is provisioned with a set of optimization goals,
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
DAG Discovery locates the nearest sink and forms a Directed Acyclic
Graph towards that sink, by identifying a set of DAG parents. During
this process DAG Discovery also identifies siblings, which may be
used later to provide additional path diversity towards the DAG root.
DAG Discovery enables nodes to implement different policies for
selecting their DAG parents in the DAG by using implementation
specific policy functions. DAG Discovery specifies a set of rules to
be followed by all implementations in order to ensure interoperation.
DAG Discovery also standardizes the format that is used to advertise
the most common information that is used in order to select DAG
parents.
One of these information, the DAG rank, is used by DAG Discovery to
provide loop avoidance even if nodes implement different policies.
The DAG Rank is computed as specified by the Objective Code Point in
use by the DAG, demonstrating the properties described in
Section 3.4.1. 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
Discovery implementation in the nodes MUST obey to the following
rules and definitions:
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
that situation when it looses all of its current feasible
parents, i.e. the set of DAG parents becomes depleted. In that
case, the node SHOULD remember the DAGID and the sequence
counter in the DIO of the lost parents for a period of time
which covers multiple DIO.
2. A LLN Node that is attached to an infrastructure that does not
support DIO, is the DAG root of its own grounded DAG. It's rank
is 1.
3. A router sending a RA without DIO is considered a grounded
infrastructure at rank 0. (For example, a router that is in
communication with an LLN node but not running RPL such as a
backbone router in communication with an LBR)
4. The DAG root exposes the DAG in the RA-DIO and nodes propagate
the DIO outwards along the DAG with the RAs that they forward
over their LLN links.
5. A node MAY move at any time, with no delay, within its DAG as
long as such a move does not increase its own DAG rank, as per
the rank calculation indicated by the OCP. 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 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
it is preferred for reasons of connectivity, configured
preference, free medium time, size, security, bandwidth, DAG
rank, or whatever metrics the LLN cares to use. A node may jump
at any time and to whatever rank it reaches in the new DAG, but
it may have to wait for a DAG Hop timer to elapse in order to do
so. This allows the new higher parts (closer to the sink) of
the DAG to move first, thus allowing stepped DAG
reconfigurations and limiting relative movements. A node SHOULD
NOT join a previous DAG (identified by its DAGID) unless the
sequence number in the DIO has incremented since the node left
that DAG. A newer sequence number indicates that the candidate
parents were not attached behind this node, as they kept getting
subsequent DIOs with new sequence numbers from the same DAG. In
the event that old sequence numbers (two or more behind the
present value) are encountered they are considered stale and the
corresponding parent SHOULD be removed from the set.
7. 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),
the node is unstable and refrains from sending RA-DIOs for that
DAG.
8. If a node receives a RA-DIO from one of its DAG parents, and if
the parent contains a different DAGID, indicating that the
parent has left the DAG, and if the node can remain in the
current DAG through an alternate DAG parent, then the node
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
was the last DAG parent then the node SHOULD follow that parent.
9. 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
message to its one-hop neighbors. The RA contains an updated
DIO to propagate the new DAG information. Such an event will
also cause the trickle timer governing the periodic RAs to be
reset.
10. If a DAG parent increases its rank such that the node rank would
have to change, and if the node does not wish to follow (e.g. it
has alternate options), then the DAG parent should be evicted
from the DAG parent set. If the DAG parent is the last in the
DAG parent set, then the node may chose to follow it.
5.4.1. RA-DIO Reception
When an DIO is received from a source device SRC, the receiving node
must first determine whether or not the DIO should be accepted for
further processing, and subsequently present the DIO for further
processing if eligible.
5.4.1.1. Determination of Eligibility for DIO Processing
If the DIO is malformed, then the DIO is not eligible for further
processing.
If SRC is not a member of the candidate neighbor set, then the RA-
DIO is not eligible for further processing. (Further evaluation/
confidence of this neighbor is necessary)
If the DIO advertises a DAG that the node is already a member of,
then:
If the rank of SRC as reported in the DIO is less then that of
the node within the DAG, then the DIO MUST be considered for
further processing
If the rank of SRC as reported in the DIO is equal to that of
the node within the DAG, then SRC is marked as a sibling and
the DIO is not eligible for further processing.
If the rank of SRC as reported in the DIO is lesser than that
of the node within the DAG, and SRC is not a DAG Parent, then
the DIO is not eligible for further processing
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
DAG Parent may have jumped).
If the DIO advertises a DAG that offers a better (new or
alternate) solution to an optimization objective desired by the
node, then the DIO MUST be considered for further processing.
5.4.1.2. Overview of DIO Processing
If the DIO is for a new/alternate DAG:
Instantiate a data structure for the new/alternate DAG if
necessary
Place the neighbor in the Candidate DAG Parent set
Has the node sent an RA within the risk window as described in
Section 5.8.3? If so, perform the collision detection
described in Section 5.8.3. If a collision occurs, place the
Candidate DAG Parent in the collision state and do not process
the DIO any further as described in Section 5.8.
If the SRC node is also a DAG Parent for another DAG that the
node is a member of, and if the new/alternate DAG satisfies an
equivalent optimization objective as the other DAG, then the
DAG Parent is known to have jumped.
Remove SRC as a DAG Parent from the other DAG (place it in
the held-down state)
If the other DAG is now empty of candidate Parents, then
directly follow SRC into the new DAG by adding it as a DAG
Parent in the Current state
Else ignore the DIO (do not follow the parent).
If the new/alternate DAG offers a better solution to the
optimization objectives, then prepare to jump: copy the DIO
information into the record for the Candidate DAG Parent, place
the Candidate DAG Parent into the Held-Up state, and start the
DAG Hop timer as per Section 5.8.1.
If the DIO is for a known/existing DAG:
Process the DIO as per the rules in Section 5.4
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 below. When a node adds another node to its set of described in Section 5.4. When a node adds another node to its set
candidate parents, the node becomes attached to the DAG through the of candidate parents, the node becomes attached to the DAG through
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. All
nodes in the DAG Parent set should be of a depth less than or equal nodes in the DAG Parent set should be of a rank less than or equal to
to the most preferred DAG parent. the most preferred DAG parent. (This case may occur, for example, if
an energy constrained device is at a lesser rank but should be
avoided as per an optimization objective, resulting in a more
preferred parent at a greater rank).
5.2.2. RA-DIO Transmission 5.4.2. 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 RAs. This
timer is implemented as a trickle timer operating over a variable timer is implemented as a trickle timer operating over a variable
interval. Trickle timers are further detailed in Section 5.2.3. The interval. Trickle timers are further detailed in Section 5.4.3. The
governing parameters for the timer should be configured consistently governing parameters for the timer should be configured consistently
across the DAG, and are provided by the DAG root in the DIO. In across the DAG, and are provided by the DAG root in the DIO. In
addition to periodic RAs, each LLN node will respond to Router addition to periodic RAs, each LLN node will respond to Router
Solicitation messages according to [RFC4861]. Solicitation messages according to [RFC4861].
o When a node is unstable, because any DAG Hop timer is running in
preparation for a jump, then the node must not transmit
unsolicited RA-DIOs (i.e. the node will remain silent when the
timer expires).
o When a node detects an inconsistency, it may reset the interval of o When a node detects an inconsistency, it may reset the interval of
the trickle timer to a minimum value, causing RAs to be emitted the trickle timer to a minimum value, causing RAs to be emitted
more frequently as part of a strategy to quickly correct the more frequently as part of a strategy to quickly correct the
inconsistency. Such inconsistencies may be, for example, an 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 DIO or a
point-to-point loop detected when a node located inwards along the point-to-point loop detected when a node located inwards along the
DAG forwards traffic intended for the default destination. DAG forwards traffic intended for the default destination.
Inconsistencies are further detailed in Section 5.2.3.2. Inconsistencies are further detailed in Section 5.4.3.2.
o When a node enters a mode of consistent operation within a DAG, it o When a node enters a mode of consistent operation within a DAG,
may begin to open up the interval of the trickle timer towards a i.e. DIOs from its DAG Parents are consistent and no other
maximum value, causing RAs to be emitted less frequently, thus inconsistencies are detected, it may begin to open up the interval
reducing network maintenance overhead and saving energy of the trickle timer towards a maximum value, causing RAs to be
consumption (which is of utmost importance for battery-operated emitted less frequently, thus reducing network maintenance
nodes). overhead and saving energy consumption (which is of utmost
importance for battery-operated nodes).
o When a node is initialized, it may choose to remain silent and not o When a node is initialized, it may be configured to remain silent
multicast any RAs until it has encountered and joined a DAG and not multicast any RAs until it has encountered and joined a
(perhaps initially probing for a nearby DAG with an RS). DAG (perhaps initially probing for a nearby DAG with an RS).
Alternately, it may choose to root its own floating DAG and begin Alternately, it may choose to root its own floating DAG and begin
multicasting RAs using a default trickle configuration. The multicasting RAs using a default trickle configuration. The
second case may be advantageous if it is desired for independent second case may be advantageous if it is desired for independent
nodes to begin aggregating into scattered floating DAGs in the nodes to begin aggregating into scattered floating DAGs in the
absence of a grounded node, for example in support of LLN absence of a grounded node, for example in support of LLN
installation and commissioning. 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 DIOs with the same DAGID, then they must coordinate
with each other to ensure that their DIOs are consistent when they with each other to ensure that their DIOs are consistent when they
emit RA-DIOs. In particular the Sequence number must be identical emit RA-DIOs. In particular the Sequence number must be identical
from each DAG root, regardless of which of the multiple DAG roots from each DAG root, regardless of which of the multiple DAG roots
issues the DIO, and changes to the Sequence number should be issued issues the DIO, and changes to the Sequence number should be issued
at the same time. The specific mechanism of this coordination is at the same time. The specific mechanism of this coordination, e.g.
beyond the scope of this specification. along a backbone between DAG roots, is beyond the scope of this
specification.
5.2.3. Trickle Timer for RA Transmission 5.4.3. 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. The operation of this timer is in support of the
procedures further discussed in Section 5.3 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]
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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 DIO as (2^DIOIntervalMin)ms. The
default value is DEFAULT_DIO_INTERVAL_MIN. 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 DIO as
DIOIntervalDoublings. The default value is DIOIntervalDoublings. The default value is
DEFAULT_DIO_INTERVAL_DOUBLINGS. DEFAULT_DIO_INTERVAL_DOUBLINGS.
5.2.3.1. Resetting the Trickle Timer 5.4.3.1. 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.
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 and makes the decision to
join it, it initializes the state of the trickle timer by resetting join it, it initializes the state of the trickle timer by resetting
the trickle timer and listening. Each time it hears an RA for this the trickle timer and listening. Each time it hears a consistent RA
DAG, it increments C. 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 then I_doubling times,
resets C, and chooses a new T value. resets C, and chooses a new T value.
5.2.3.2. Determination of Inconsistency 5.4.3.2. 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 DIO from a DAG parent
o A DAG parent forwards a packet intended for the default route, o A DAG parent forwards a packet intended for the default route,
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 DIO is determined to be inconsistent,
as according to a implementation specific path metric selection as according to a implementation specific path metric selection
engine. engine.
o The depth of a DAG parent has changed. o The rank of a DAG parent has changed.
5.3. DAG Discovery
DAG Discovery is a form of distance vector protocol for use in LLNs.
DAG Discovery locates the nearest sink and forms a Directed Acyclic
Graph towards that sink, by identifying a set of DAG parents. During
this process DAG Discovery also identifies siblings, which may be
used later to provide additional path diversity towards the DAG root.
DAG Discovery enables nodes to implement different policies for
selecting their DAG parents in the DAG by using implementation
specific policy functions. DAG Discovery specifies a set of rules to
be followed by all implementations in order to ensure interoperation.
DAG Discovery also standardizes the format that is used to advertise
the most common information that is used in order to select DAG
parents.
One of these information, the DAG depth, is used by DAG Discovery to
provide loop avoidance even if nodes implement different policies.
The DAG Depth is computed as specified by the Objective Code Point in
use by the DAG, demonstrating the properties described in
Section 3.4.1. The depth should be computed in such a way so as to
provide a comparable basis with other nodes which may not use the
same metric at all. (The to-be-defined NULL OCP and related
behaviors will clarify this point).
In order to organize and maintain loopless structure, the DAG
Discovery implementation in the nodes MUST obey to the following
rules and definitions:
1. A node that does not have any DAG parents in a DAG is the root
of its own floating DAG. It's depth is 1. A node will end up
in that situation when it looses all of its current feasible
parents, i.e. the set of DAG parents becomes depleted. In that
case, the node SHOULD remember the DAGID and the sequence
counter in the DIO of the lost parents for a period of time
which covers multiple DIO.
2. A LLN Node that is attached to an infrastructure that does not
support DIO, is the DAG root of its own grounded DAG. It's
depth is 1.
3. A router sending a RA without DIO is considered a grounded
infrastructure at depth 0. (For example, a router that is in
communication with an LLN node but not running RPL such as a
backbone router in communication with an LBR)
4. The DAG root exposes the DAG in the Router Advertisement DAG
Information Option and nodes propagate the DIO outwards along
the DAG with the RAs that they forward over their LLN links.
5. A node MAY move at any time, with no delay, within its DAG as
long as such a move does not increase its own DAG depth, as per
the depth calculation indicated by the OCP. If a node is
required to move such that it cannot stay within the DAG without
a depth increase, then it needs to first leave the DAG. In
other words a 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 depth 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 The implementation SHOULD provide an API whereby any procedure that
it is preferred for reasons of connectivity, configured detects an inconsistency may cause the trickle timer to reset.
preference, free medium time, size, security, bandwidth, DAG
depth, or whatever metrics the LLN cares to use. A node may
jump at any time and to whatever depth it reaches in the new
DAG, but it may have to wait for a DAG Hop timer to elapse in
order to do so. This allows the new higher parts (closer to the
sink) of the DAG to move first, thus allowing stepped DAG
reconfigurations and limiting relative movements. A node SHOULD
NOT join a previous DAG (identified by its DAGID) unless the
sequence number in the DIO has incremented since the node left
that DAG. A newer sequence number indicates that the candidate
parents were not attached behind this node, as they kept getting
subsequent DIOs with new sequence numbers from the same DAG. In
the event that old sequence numbers (two or more behind the
present value) are encountered they are considered stale and the
corresponding parent SHOULD be removed from the set.
7. If a node has selected a new set of DAG parents but has not 5.5. DAG Heartbeat
moved yet (because it is waiting for DAG Hop timer to elapse),
the node is unstable and refrains from sending Router
Advertisement - DAG Information Options.
8. If a node receives a Router Advertisement - DAG Information The DAG Root makes the sole determination of when to revise the
Option from one of its DAG parents, and if the parent contains a DAGSequenceNumber by incrementing it upwards. When the
different DAGID, indicating that the parent has left the DAG, DAGSequenceNumber is increased an inconsistency results, causing RA-
and if the node can remain in the current DAG through an DIOs to be sent back outwards along the DAG to convey the change.
alternate DAG parent, then the node should remove the DAG parent The degree to which this mechanism is relied on may be determined by
which has joined the new DAG from its DAG parent set and remain the implementation- on one hand it may serve as a periodic heartbeat,
in the original DAG. If the node was the last DAG parent then refreshing the DAG states, and on the other hand it may result in a
the node SHOULD follow that parent. constant steady-state control cost overhead which is not desirable.
9. When a node detects or causes a DAG inconsistency, as described Some implementations may provide an administrative API at the DAG
in Section 5.2.3.2, then the node sends an unsolicited Router Root whereby the DAGSequenceNumber may be caused to increment in
Advertisement message to its one-hop neighbors. The RA contains response to some policy outside of the scope of RPL.
a DIO that propagates the new DAG information. Such an event
will also cause the trickle timer governing the periodic RAs to
be reset.
10. If a DAG parent increases its depth such that the node depth Other implementations may make use of a periodic timer to
would have to change, and if the node does not wish to follow automatically increment the DAGSequenceNumber, resulting in a
(e.g. it has alternate options), then the DAG parent should be periodic DAG Heartbeat at a rate appropriate to the application and
evicted from the DAG parent set. If the DAG parent is the last implementation.
in the DAG parent set, then the node may chose to follow it.
5.3.1. DAG Selection 5.6. DAG Selection
The DAG selection is implementation and algorithm dependent. Nodes The DAG selection is implementation and algorithm dependent. Nodes
SHOULD prefer to join DAGs advertising OCPs compatible with their SHOULD prefer to join DAGs advertising OCPs and destinations
implementation specific objectives. In order to limit erratic compatible with their implementation specific objectives. In order
movements, and all metrics being equal, nodes SHOULD keep their to limit erratic movements, and all metrics being equal, nodes SHOULD
previous selection. Also, nodes SHOULD provide a means to filter out keep their previous selection. Also, nodes SHOULD provide a means to
a candidate parent whose availability is detected as fluctuating, at filter out a candidate parent whose availability is detected as
least when more stable choices are available. Nodes MAY place the fluctuating, at least when more stable choices are available. Nodes
failed candidate parent in a Hold Down mode that ensures that the MAY place the failed candidate parent in a Hold Down mode that
candidate parent will not be reused for a given period of time. ensures that the candidate parent will not be reused for a given
period of time.
The known DAGs are associated with the candidate parents that
advertise them and kept in a list by extending the Default Router
List (DRL). DRL entries are extended to store the information
received from the last DIO. The DRL MAY need to be modified in order
to keep track of membership to multiple DAGs simultaneously. The DRL
entries are managed by states and timers described in the next
section.
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. How to balance these DAGs is implementation dependent, and MAY
use a specific visitor-counter suboption in the DIO. 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.
5.3.2. Administrative depth 5.7. Administrative rank
When the DAG is formed under a common administration, or when a node When the DAG is formed under a common administration, or when a node
performs a certain role within a community, it might be beneficial to performs a certain role within a community, it might be beneficial to
associate a range of acceptable depth with that node. For instance, associate a range of acceptable rank with that node. For instance, a
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 depth computation specified by other choice, and may then augment the rank computation specified by
the OCP in order to expose an exaggerated depth. the OCP in order to expose an exaggerated rank.
5.3.3. DRL entries states and stability 5.8. Candidate DAG Parent States and Stability
Candidate parents in the DRL may or may not be usable for forwarding Candidate DAG Parents may or may not be eligible to act as DAG
traffic inward along the DAG toward the root depending on runtime Parents depending on runtime conditions. The following states are
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,
or taken out of the Current state, it is necessary to re-
evaluate which of the remaining DAG Parents is the most
preferred DAG Parent and its rank. At that time any
remaining DAG Parents of greater rank than the most
preferred DAG parent must be placed in the Held-Down
state, and the hold-down timer started, in order to be
evicted as DAG Parents.
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 DRL, and may be reinserted candidate is removed from the Candidate DAG Parent set,
if it appears again with a RA. and may be reinserted if it appears again with a RA.
Collision This candidate parent can not be used till its next RA. Collision This candidate parent can not be used till its next RA.
5.3.3.1. Held-Up 5.8.1. 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 Router Advertisement
- DAG Information Option that is initiated by the new DAG root has - DAG Information Option that is initiated by the new DAG root has
a chance to spread outward along the sub-DAG so that two different a chance to spread outward along the sub-DAG so that two different
DAGs have formed. DAGs have formed.
Limit Router Advertisement - DAG Information Option storms when Limit RA-DIO storms when two DAGs collide/merge. The idea is that
two DAGs collide/merge. The idea is that between the nodes from between the nodes from DAG A that decide to move to DAG B, those
DAG A that decide to move to DAG B, those that see the highest that see the highest place (closer to the DAG root) in DAG B will
place (closer to the DAG root) in DAG B will move first and move first and advertise their new locations before other nodes
advertise their new locations before other nodes from DAG A from DAG A actually move.
actually move.
A new DAG is discovered upon a router advertisement message with or A new DAG is discovered upon a router advertisement message with or
without a Router Advertisement - DAG Information Option. The node without a RA-DIO. The node joins the DAG by selecting the source of
joins the DAG by selecting the source of the RA message as a DAG the RA message as a DAG parent (and possible default gateway) and
parent (and possible default gateway) and propagating the DIO propagating the DIO accordingly.
accordingly.
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 depth of candidate parent that triggers it: DAG and on the rank of candidate parent that triggers it: (candidates
(candidates depth + random) * candidate's DAG_delay (where 0 <= rank + random) * candidate's DAG_delay (where 0 <= random < 1). It
random < 1). It is randomized in order to limit collisions and is randomized in order to limit collisions and synchronizations.
synchronizations.
5.3.3.2. Held-Down 5.8.2. 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).
An node that is held down is not considered for the purpose of When the hold down timer elapses, the node is removed from the
forwarding traffic inward along the DAG toward the root. When the Candidate DAG Parent set.
hold down timer elapses, the node is removed from the DRL.
5.3.3.3. Collision 5.8.3. 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 at the same time and
then attempt to join each other. This might happen, for example, then attempt to join each other. This might happen, for example,
between nodes which act as DAG root of their own DAGs. In order to between nodes which act as DAG root of their own DAGs. In order to
detect the situation, LLN Nodes time stamp the sending of RA-DIO. detect the situation, LLN Nodes time stamp the sending of RA-DIO.
Any RA-DIO received within a short link-layer-dependent period Any RA-DIO received within a short link-layer-dependent period
introduces a risk. To resolve the collision, a 32bits extended introduces a risk. To resolve the collision, a 32bits extended
preference is constructed from the DIO by concatenating the preference is constructed from the DIO by concatenating the
NodePreference with the BootTimeRandom. 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 depth) and (candidate depth + 1) times the between (candidate rank) and (candidate rank + 1) times the candidate
candidate DAG Delay. But since a node is unstable as soon as it DAG Delay. But since a node is unstable as soon as it receives the
receives the RA-DIO from the desired candidate, it will restrain from RA-DIO from the desired candidate, it will restrain from sending a
sending a RA-DIO between the time it receives the RA and the time it RA-DIO between the time it receives the RA and the time it actually
actually jumps. So the crossing of RA may only happen during the jumps. So the crossing of RA may only happen during the propagation
propagation time between the candidate and the node, plus some time between the candidate and the node, plus some internal queuing
internal queuing and processing time within each machine. It is and processing time within each machine. It is expected that one DAG
expected that one DAG delay normally covers that interval, but delay normally covers that interval, but ultimately it is up to the
ultimately it is up to the implementation and the configuration of implementation and the configuration of the candidate parent to
the candidate parent to define the duration of risk window. 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 normally, while the
router with the highest extended preference places the other in router with the highest extended preference places the other in
collision state, does not start the DAG hop timer, and does not collision state, does not start the DAG hop timer, and does not
become instable. It is expected that next RAs between the two will become instable. It is expected that next RAs between the two will
not cross anyway. not cross anyway.
5.3.3.4. Instability 5.8.4. 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 DIO. This avoids loops
when node A decides to attach to node B and node B decides to attach when node A decides to attach to node B and node B decides to attach
to node A. Unless RAs cross (see Collision section), a node receives to node A. Unless RAs cross (see Collision section), a node receives
DIO from stable candidate parents, which do not plan to attach to the DIO from stable candidate parents, which do not plan to attach to the
node, so the node can safely attach to them. node, so the node can safely attach to them.
5.4. Establishing Routing State Outward Along the DAG 5.9. Guidelines for Objective Code Points
5.9.1. Objective Function
An objective function (OF) selects a DAG to join, and a number of
peers in that DAG as parents. The OF computes an ordered list of
parents and provides load balancing guidance. The OF is also
responsible to compute the rank of the device within the DAG.
An Objective Function is indicated in the DIO using an objective code
point (OCP). The objective code point are administered by IANA that
might delegate some ranges to other organizations. This
specification reserves OCP 0, in support of default operation.
Most Objective Functions are expected to follow the same abstract
behavior:
o The parent selection is triggered each time an event indicates
that a potential next_hop information is updated. This might
happen upon a RA-DIO, a timer elapse, or a trigger indicating that
the state of a Candidate Neighbor has changed.
o An OF scans all the interfaces on the device. Although there may
typically be only one interface in most application scenarios,
there might be multiple of them and an interface might be
configured to be usable or not for RPL operation. An interface
can also be configured with a preference or dynamically learned to
be better than another by some heuristics that might be link-layer
dependent and are out of scope. An interface might not be ready
for IPv6 operation with a usable link-local address. Finally an
interface might or not match a required criterion for an Objective
Function, for instance a degree of security. As a result some
interfaces might be completely excluded from the computation,
while others might be more or less preferred.
o The OF scans all the Candidate Neighbors on the possible
interfaces to check whether they can act as an attachment router
for a DAG. There might be multiple of them and a Candidate
Neighbor might need to pass some validation tests before it can be
used. In particular, some link layers require experience on the
activity with a router to enable and raise the router value as a
next_hop.
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
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.
+ 1 indicates a unusually good link, for instance a link
between powered devices in a mostly battery operated
environment.
+ 16 indicates a link that can hardly be used to forward any
packet, for instance a radio link with quality indicator or
expected transmission count that flirts with the acceptable
threshold.
* Candidate Neighbors that would cause self's rank to increase
are ignored
o As it scans all the Candidate Neighbors, the OF keeps the current
best parent and compares its capabilities with the current
Candidate Neighbor. The OF defines a number of tests that are
critical to reach the Objective. A test between the routers
determines an order relation.
* If the routers are roughly equal for that relation then the
next test is attempted between the routers,
* Else the best of the 2 becomes the current best parent and the
scan continues with the next Candidate Neighbor
* One of these tests might include comparing the resulting ranks
but it isn't necessarily so
o When the scan is complete, the preferred parent is elected and
self's rank is computed as the preferred parent rank plus the step
in rank with that parent.
o Other rounds of scans might be necessary to elect alternate
parents and siblings. Self's rank is now determined by the new
preferred parent if it has changed. In the next rounds:
* Candidate Neighbors that are not in the same DAG are ignored
* Candidate Neighbors that would cause self's rank to increase
are ignored
* Candidate Neighbors of a better rank than self (non-siblings)
are preferred
5.9.2. Objective Code Point 0 (OCP 0)
Here follows the specification for the Objective Function for OCP 0.
This is a very simple references to help design more complex
Objective Functions. In particular, the Objective Function described
here does not use physical metrics as described in
[I-D.ietf-roll-routing-metrics], but are only based on abstract
information from the DIO such as rank and administrative preference.
OCP 0 is as a default fall back behavior when a node joins a DAG but
does not support the OF that's preferred for this DAG.
5.9.2.1. OCP 0 Objective Function (OF0)
OF0 favors the connectivity. That is, the Objective Function is
designed to find the nearest sink into a 'grounded' topology, and if
there's none then join any network per order of administrative
preference.
OF0 selects a preferred parent and a backup next_hop if that's
available. The backup next_hop might be a parent or a sibling. All
the traffic is routed via the preferred parent. When the link
conditions do not let a packet through to the preferred parent, the
packet is passed to the backup next_hop.
The step of rank is 4 for each hop.
5.9.2.2. Selection of the Preferred Parent
As it scans all the Candidate Neighbors, OF0 keeps the parent that is
the best for the following criteria (in order):
1. The interface must be usable and the administrative preference
(if any) applies first.
2. A candidate that would cause the node to augment the rank in the
current DAG is not considered.
3. A router that is validated as usable is better.
4. If none are grounded then a DAG with a better DAG preference
wins.
5. A router that offers connectivity to a grounded DAG is better.
6. A lesser resulting rank is better.
7. A DAG for which there is an alternate parent is better. This
check is optional. It is performed by computing the backup
next_hop while assuming that this router won.
8. The DAG that was in use already is preferred.
9. The router with a better router preference wins.
10. The preferred parent that was in use already is better.
11. A router that is fresher (most recent RA) is better.
5.9.2.3. Selection of the Backup next_hop
o The interface must be usable and the administrative preference (if
any) applies first.
o A candidate that would cause the node to augment the rank in the
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 would cause self's rank (from that
determined by the preferred parent) to increase are ignored
o Candidate Neighbors of a better rank than self (non-siblings) are
preferred
o A router that is validated as usable is better
o The router with a better router preference wins
o The backup next_hop that was in use already is better.
5.10. Establishing Routing State Outward Along the DAG
The Destination Advertisement mechanism supports the dissemination of The Destination Advertisement mechanism supports the dissemination of
routing state required to support traffic flows outward along the routing state required to support traffic flows 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 Note that some aspects of the Destination Advertisement mechanism are
still under investigation. still under investigation.
As a result of Destination Advertisement operation: As a result of Destination Advertisement operation:
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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 or landmark
(waypoint) routing. (waypoint) routing.
A special case, the reception of a Destination Advertisement
addressed to a link-local multicast address, allows for a node to
learn destination prefixes directly available from its one-hop
neighbors.
The design choice behind this is not to synchronize the parent and The design choice behind this is not to synchronize the parent and
children databases along the DAG, but instead to update them children databases along the DAG, but instead to update them
regularly to cover from the loss of packets. The rationale for that regularly to cover from the loss of packets. The rationale for that
choice is time variations in connectivity across unreliable links. choice is time variations in connectivity across unreliable links.
If the topology can be expected to change frequently, synchronization If the topology can be expected to change frequently, synchronization
might be an excessive goal in terms of exchanges and protocol might be an excessive goal in terms of exchanges and protocol
complexity. The approach used here results in a simple protocol with complexity. The approach used here results in a simple protocol with
no real peering. The Destination Advertisement mechanism hence no real peering. The Destination Advertisement mechanism hence
provides for periodic updates of the derivative routing state, as provides for periodic updates of the derivative routing state, as
cued by occasional RAs and other mechanisms. cued by occasional RAs and other mechanisms, similarly to other
protocols such as RIP [RFC2453].
5.4.1. Destination Advertisement Message Formats 5.10.1. Destination Advertisement Message Formats
5.4.1.1. DAO Option 5.10.1.1. 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 Advertisements (NAs). A
prefix option is normally present in Router Advertisements (RAs) prefix option is normally present in Router Advertisements (RAs)
only, but the NA is augmented with this option in order to propagate only, 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 containing this
option may be referred to as a Destination Advertisement. The RPL option may be referred to as a Destination Advertisement. The RPL
use of Destination Advertisements allows the nodes in the DAG to use of Destination Advertisements allows the nodes in the DAG to
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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 initialized to zero by the sender and ignored by the
receiver. 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.
5.4.2. Destination Advertisement Operation 5.10.2. Destination Advertisement Operation
5.4.2.1. Overview 5.10.2.1. Overview
Note that some aspects of the Destination Advertisement mechanism are Note that some aspects of the Destination Advertisement mechanism are
still under investigation 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 RPL takes advantage of the DAG structure and allows a node capable of
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If Destination Advertisements are activated in the DIO as indicated If Destination Advertisements are activated in the DIO as indicated
by the `D' bit, the node sends unicast Destination Advertisements to by the `D' bit, the node sends unicast Destination Advertisements to
its DA parents, and only accepts unicast Destination Advertisements its DA parents, and only accepts unicast Destination Advertisements
from any nodes BUT those contained in the DA parent subset. 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 DAG Discovery RA-DIO with the `D' Destination Advertisement bit set
from a DAG parent stimulates the sending of a delayed Destination from a DAG parent stimulates the sending of a delayed Destination
Advertisement back, with the collection of all known prefixes (that Advertisement back, with the collection of all known prefixes (that
is the prefixes learned via Destination Advertisements for nodes is the prefixes learned via Destination Advertisements for nodes
lower in the DAG, and any connected prefixes). A Destination lower in the DAG, and any connected prefixes). If the Destination
Advertisement is also sent to a DAG parent once it has been added to Advertisement Supported (A) bit is set in the DIO for the DAG, then a
the DA parent set after a movement, or when the list of advertised Destination Advertisement is also sent to a DAG parent once it has
prefixes has changed. Destination Advertisements may also be been added to the DA parent set after a movement, or when the list of
scheduled for sending when the PathDigest of the DIO has changed, advertised prefixes has changed. Destination Advertisements may also
be scheduled for sending when the PathDigest of the DIO has changed,
indicating that some aspect of the inwards paths along the DAG has indicating that some aspect of the inwards paths along the DAG has
been modified. 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) DAOs. A no-DAO is stimulated by the
disappearance of a prefix below. This is discovered by timing out disappearance of a prefix below. This is discovered by timing out
after a request (a RA-DIO) or by receiving a no-DAO. A no-DAO is a after a request (a RA-DIO) or by receiving a no-DAO. A no-DAO is a
conveyed as a DAO with a DAO Lifetime of 0. conveyed as a DAO 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 DAO will do so upon receiving and processing the DAO, thus building a unicast DAO will do so upon receiving and processing the DAO, thus
up routing state concerning destinations below it in the DAG. If a building up routing state concerning destinations below it in the
node capable of recording state information receives a DAO containing DAG. If a node capable of recording state information receives a DAO
a Reverse Route Stack, then the node knows that the DAO has traversed containing a Reverse Route Stack, then the node knows that the DAO
one or more nodes that did not retain any routing state as it has traversed one or more nodes that did not retain any routing state
traversed the path from the DAO source to the node. The node may as it traversed the path from the DAO source to the node. The node
then extract the Reverse Route Stack and retain the included state in may then extract the Reverse Route Stack and retain the included
order to specify Source Routing instructions along the return path state in order to specify Source Routing instructions along the
towards the destination. The node MUST set the RRCount back to zero return path towards the destination. The node MUST set the RRCount
and clear the Reverse Route Stack prior to passing the DAO back to zero and clear the Reverse Route Stack prior to passing the
information on. DAO 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, DAO will append the next-hop address to the Reverse Route Stack,
increment the RRCount, and then pass the Destination Advertisement on increment the RRCount, and then pass the Destination Advertisement on
without recording any additional state. In this way the Reverse without recording any additional state. In this way the Reverse
Route Stack will come to contain a vector of next hops that must be Route Stack will come to contain a vector of next hops that must be
traversed along the reverse path that the DAO has traveled. The traversed along the reverse path that the DAO has traveled. The
vector will be ordered such that the node closest to the destination vector will be ordered such that the node closest to the destination
will appear first in the list. In such cases the node may choose to will appear first in the list. In such cases the node may choose to
convey the Destination Advertisement to one or more DAG Parents in convey the Destination Advertisement to one or more DAG Parents in
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In hybrid cases, some nodes along the path a Destination In hybrid cases, some nodes along the path a Destination
Advertisement follows inward along the DAG may store state and some Advertisement follows inward along the DAG may store state and some
may not. The Destination Advertisement mechanism allows for the may not. The Destination Advertisement mechanism allows for the
provisioning of routing state such that when a packet is traversing provisioning of routing state such that when a packet is traversing
outwards along the DAG, some nodes may be able to directly forward to outwards along the DAG, some nodes may be able to directly forward to
the next hop, and other nodes may be able to specify a piecewise the next hop, and other nodes may be able to specify a piecewise
source route in order to bridge spans of stateless nodes within the source route in order to bridge spans of stateless nodes within the
path on the way to the desired destination. path on the way to the desired destination.
In the degenerate case, no node is able to store any routing state as In the degenerate case, no node is able to store any routing state as
Destination Advertisements pass by, and the DAG sink ends up with Destination Advertisements pass by, and the DAG Root ends up with
DAOs that contain a completely specified route back to the DAOs that contain a completely specified route back to the
originating node in the form of the inverted Reverse Route Stack. originating node in the form of the inverted Reverse Route Stack. A
DAG Root should not request nor indicate support for Destination
Advertisements if it is not able to store the Reverse Route Stack
information in the degenerate case.
Information learned through Destination Advertisements can be Information learned through Destination Advertisements can be
redistributed in a routing protocol, MANET or IGP. But the MANET or redistributed in a routing protocol, MANET or IGP. But the MANET or
the IGP SHOULD NOT be redistributed into Destination Advertisements. the IGP SHOULD NOT be redistributed into Destination Advertisements.
This creates a hierarchy of routing protocols where DA routes stand This creates a hierarchy of routing protocols where DA routes stand
somewhere between connected and IGP routes. 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:
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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 DAOs, 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 destroyed, in order to send no-DAOs
to the DA parents. to the DA parents.
5.10.2.1.1. 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 DestroyTimer.
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 DIO from a DA parent). When
the timer is armed, all entries in the Reachable list as well as the timer is armed, all entries in the Reachable list as well as
all entries for Connected list are set to not reported yet for all entries for Connected list are set to not reported yet for
that particular DA parent. 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 depth. The intention is that nodes located a multiple of the DAG rank of the node. The intention is that
deeper in the DAG should have a shorter DelayNA timer, allowing nodes located deeper in the DAG should have a shorter DelayNA
DAOs a chance to be reported from deeper in the DAG and timer, allowing DAOs a chance to be reported from deeper in the
potentially aggregated by sub-DAGs before propagating further DAG and potentially aggregated along sub-DAGs before propagating
inwards. further inwards.
o The DestroyTimer is armed when at least one entry has exhausted o The DestroyTimer is armed when at least one entry has exhausted
its retries, which means that a number of RA-DIO were sent toward its retries, which means that a number of RA-DIO were sent toward
the reporting neighbor but that the entry was not confirmed with a the reporting neighbor but that the entry was not confirmed with a
DAO. When the destroy timer elapses, for all exhausted entries, DAO. When the destroy timer elapses, for all exhausted entries,
the associated route is removed, and the entry is scheduled to be the associated route is removed, and the entry is scheduled to be
destroyed. destroyed.
o The Destroy timer has a duration of min (MAX_DESTROY_INTERVAL, o The Destroy timer has a duration of min (MAX_DESTROY_INTERVAL,
RA_INTERVAL). RA_INTERVAL).
5.4.2.2. Unicast Destination Advertisement messages from child to 5.10.2.2. Multicast Destination Advertisement messages
It is also possible for a node to multicast a DAO to the link-local
scope all-nodes multicast address FF02::1. This message will be
received by all node listening in range of the emitting node. The
objective is to enable direct P2P communication, between destination
prefixes directly supported by neighboring nodes, without needing the
RPL routing structure to relay the packets.
A multicast DAO MUST be used only to advertise information about
self, i.e. prefixes in the Connected list. This would typically be a
multicast group that this node is listening to or a global address
owned by this node, though it can be used to advertise any prefix
owned by this node as well. A multicast DAO is not used for routing
and does not presume any DAG relationship between the emitter and the
receiver; it MUST NOT be used to relay information learned (e.g.
information in the Reachable list) from another node.
A node receiving a multicast DAO addressed to FF02::1 MAY install
prefixes contained in the DAO in the routing table for local use.
Such a node MUST NOT perform any other processing on the DAO (i.e.
such a node does not presume it is a DA parent).
5.10.2.3. 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 LLN Node
includes the DAOs about not already reported prefix entries in the includes the DAOs about not already reported prefix entries in the
Reachable and Connected lists, as well as no-DAOs for all the entries Reachable and Connected lists, as well as no-DAOs for all the entries
in the Unreachable list. Depending on its policy and ability to in the Unreachable list. Depending on its policy and ability to
retain routing state, the receiving node SHOULD keep a record of the retain routing state, the receiving node SHOULD keep a record of the
reported DAO. If the DAO offers the best route to the prefix as reported DAO. If the DAO offers the best route to the prefix as
determined by policy and other prefix records, the node SHOULD determined by policy and other prefix records, the node SHOULD
install a route to the prefix in the DAO via the link local address install a route to the prefix in the DAO via the link local address
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Once the Destroy timer is elapsed, the prefix entry is scheduled to Once the Destroy timer is elapsed, the prefix entry is scheduled to
be destroyed and moved to the Unreachable list if there are any DA be destroyed and moved to the Unreachable list if there are any DA
parents that need to be informed of the change in status for the parents that need to be informed of the change in status for the
prefix, otherwise the prefix entry is cleaned up right away. The prefix, otherwise the prefix entry is cleaned up right away. The
prefix entry is removed from the Unreachable list when no more DA prefix entry is removed from the Unreachable list when no more DA
parents need to be informed. This condition may be satisfied when a parents need to be informed. This condition may be satisfied when a
no-DAO is sent to all current DA parents indicating the loss of the no-DAO is sent to all current DA parents indicating the loss of the
prefix, and noting that in some cases parents may have been removed prefix, and noting that in some cases parents may have been removed
from the set of DA parents. from the set of DA parents.
5.4.2.3. Other events 5.10.2.4. 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 DAOs are
destroyed. destroyed.
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 destroyed.
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 5.5, 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.
5.4.2.4. Aggregation of prefixes by a node 5.10.2.5. 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 platoon of nodes. In such a case, it is possible to use
aggregation techniques with Destination Advertisements and improve aggregation techniques with Destination Advertisements and improve
scalability. For example, consider a platoon formed by firefighters scalability. For example, consider a platoon formed by firefighters
and their commander. Specifically, the commander may be configured and their commander. Specifically, the commander may be configured
as the Destination Advertisement aggregator for a group prefix. At as the Destination Advertisement aggregator for a group prefix. At
run time, the commander absorbs the individual DAO information run time, the commander absorbs the individual DAO information
received from the platoon members down its sub-DAG and only reports received from the platoon members down its sub-DAG and only reports
the aggregation up the DAG. This works fine when the whole platoon the aggregation up the DAG. This works fine when the whole platoon
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but not above the platoon member will see the advertisements for the but not above the platoon member will see the advertisements for the
aggregation owned by the commander but not that of the individual aggregation owned by the commander but not that of the individual
platoon member prefix. So it will route all the packets for the platoon member prefix. So it will route all the packets for the
platoon member towards the commander, but the commander will have no platoon member towards the commander, but the commander will have no
route to the individual platoon member and will fail to forward. route to the individual platoon member 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 a commander and platoon
in order to provide route summarization for a sub-DAG. in order to provide route summarization for a sub-DAG.
5.4.2.5. Default Values 5.10.2.6. 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
5.5. 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]).
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Thus RPL makes use of a different approach consisting of probing the Thus RPL makes use of a different approach consisting of probing the
neighbor using a Neighbor Solicitation message (see [RFC4861]). The neighbor using a Neighbor Solicitation message (see [RFC4861]). The
reception of a Neighbor Advertisement (NA) message with the reception of a Neighbor Advertisement (NA) message with the
"Solicited Flag" set is used to verify the validity of the routing "Solicited Flag" set is used to verify the validity of the routing
adjacency. Such mechanism MAY be used prior to sending a data adjacency. Such mechanism MAY be used prior to sending a data
packet. This allows for detecting whether or not the routing packet. This allows for detecting whether or not the routing
adjacency is still valid, and should it not be the case, select adjacency is still valid, and should it not be the case, select
another feasible successor to forward the packet. another feasible successor to forward the packet.
5.6. Expectations of Link Layer Behavior 5.12. Packet Forwarding
When forwarding a packet to a destination, precedence is given to
selection of a next-hop successor, with consideration given to
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)
then the highest precedence should be given to follow them.
2. If there is an entry in the routing table matching the
destination that has been provisioned outside of the context of
RPL, e.g. through an application intervention or a co-hosted
(P2P) routing protocol, then use that successor.
3. If there is an entry in the routing table matching the
destination that has been learned from a multicast Destination
Advertisement (e.g. the destination is a one-hop neighbor), then
use that successor.
4. If there is an entry in the routing table matching the
destination that has been learned from a unicast Destination
Advertisement (e.g. the destination is located outwards along the
sub-DAG), then use that successor.
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.
6. If there is a DAG offering a default route with a compatible OCP,
then select one of those DAG Parents as a successor.
7. If there is a DAG offering a route to a prefix matching the
destination, but all DAG Parents have been tried and are
temporarily unavailable (as determined by the forwarding
procedure), then select a DAG sibling as a successor.
8. Finally, if no DAG siblings are available, the packet is dropped.
ICMP Destination Unreachable may be invoked. An inconsistency is
detected.
TTL MUST be decremented when forwarding. If the packet is being
forwarded via a sibling, then the TTL may be decremented more
aggressively (by more than one) to limit the impact of possible
loops.
Note that unless overridden by a source routing directive or a route
that has been provisioned outside of RPL, the chosen successor MUST
NOT be the neighbor who was the predecessor of the packet (split
horizon).
5.12.1. Loop Taxonomy
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
detection at forwarding.
5.12.1.1. DAG Loops
A DAG loop may occur when a node detaches from the DAG and reattaches
to a device in its prior sub-DAG that has missed the whole detachment
sequence and kept advertising the original DAG. This may happen in
particular when RA-DIOs are missed. Use of the DAG sequence number
can eliminate this type of loop. If the DAG sequence number is not
in use, the protection is limited (it depends on propagation of DIOs
during DAG hop timer), and temporary loops might occur. RPL will
move to eliminate such a loop as soon as a DIO is received from a
parent that appears to be going down, as the child has to detach from
it immediately. (The alternate choice of staying attached and
following the parent in its fall would have counted to infinity and
led to detach as well).
Consider Node (24) in the DAG Example depicted in Figure 12, and its
sub-DAG Nodes (34), (44), and (45). An example of a DAG loop would
be if Node (24) were to detach from the DAG rooted at (LBR), and Node
(45) were to miss the detachment sequence. Subsequently, if the link
(24)--(45) were to become viable and Node (24) heard Node (45)
advertising the DAG rooted at (LBR), a DAG loop (45->34->24->45) may
form if Node (24) attaches to Node (45).
5.12.1.2. DAO Loops
A DAO loop may occur when the parent has a route installed by a DAO
via a child, but the child has cleaned up the state. This loop
happens when a no-DAO was missed till a heartbeat cleans up all
states. The DAO loop is not explicitly handled by the current
specification. Split horizon, not forwarding a packet back to the
node it came from, may mitigate the DAO loop in some cases, but does
not eliminate it.
Consider Node (24) in the DAG Example depicted in Figure 12. Suppose
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
the destination and Node (24) did not hear a no-DAO to clean up the
state, a DAO loop may exist. Node (24) will forward traffic destined
for Node (45) to Node (34), who may then naively return it into a
loop (if split horizon is not in place). A more complicated DAO loop
may result if Node (34) instead passes the traffic to it's sibling,
Node (33), potentially resulting in a (24->34->33->23->13->24) loop.
5.12.1.3. Sibling Loops
Sibling loops occur when a group of siblings keep choosing amongst
themselves as successors such that a packet does not make forward
progress. The current draft limits those loops to some degree by
split horizon (do not send back to the same sibling) and parent
preference (always prefer parents vs. siblings). Further approaches
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
(32) and (34) are reliable neighbors, and thus are siblings. Then,
in the case where Nodes (22), (23), and (24) are transiently
unavailable, and with no other guiding strategy, a sibling loop may
exist, e.g. (33->34->32->33) as the siblings keep choosing amongst
each other in an uncoordinated manner.
5.13. Expectations of Link Layer Behavior
This specification does not rely on any particular features of a This specification does not rely on any particular features of a
specific link layer technologies. It is anticipated that an specific link layer technologies. It is anticipated that an
implementer should be able to operate RPL over a variety of different implementer should be able to operate RPL over a variety of different
low power wireless or PLC (Power Line Communication) link layer low power wireless or PLC (Power Line Communication) link layer
technologies. technologies.
Implementers may find RFC 3819 [RFC3819] a useful reference when Implementers may find RFC 3819 [RFC3819] a useful reference when
designing a link layer interface between RPL and a particular link designing a link layer interface between RPL and a particular link
layer technology. layer technology.
6. Protocol Extensions 6. Summary of RPL Timers
7. Manageability Considerations DIO Timer One instance per DAG that a node is a member of. Expiry
triggers RA-DIO transmission. Trickle timer with variable
interval in [0, DIOIntervalMin..2^DIOIntervalDoublings]. See
Section 5.4.3
8. Security Considerations DAG Hop Timer Up to one instance per candidate DAG Parent in the
`Held-Up' state per DAG that a node is going to jump to.
Expiry triggers candidate DAG Parent to become a DAG Parent in
the `Current' state, as well as cancellation of any other DAG
Hop timers associated with other DAG Parents for that DAG.
Duration is computed based on the rank of the candidate DAG
parent and DAG delay, as (candidates rank + random) *
candidate's DAG_delay (where 0 <= random < 1). See
Section 5.8.1.
9. IANA Considerations Hold-Down Timer Up to one instance per candidate DAG Parent in the
`Held-Down' state per DAG. Expiry triggers the eviction of the
candidate DAG Parent from the candidate DAG Parent set. The
interval should be chosen as appropriate to prevent flapping.
See Section 5.8
9.1. DAG Information Option DAG Heartbeat Timer Up to one instance per DAG that the node is
acting as DAG Root of. May not be supported in all
implementations. Expiry triggers revision of
DAGSequenceNumber, causing a new series of updated RA-DIO to be
sent. Interval should be chosen appropriate to propagation
time of DAG and as appropriate to application requirements
(e.g. response time vs. overhead). See Section 5.5
DelayNA Timer Up to one instance per DA Parent (the subset of DAG
Parents chosen to receive Destination Advertisements) per DAG.
Expiry triggers sending of NA-DAO to the DA Parent. The
interval is to be proportional to DEF_NA_LATENCY/(node rank),
such that nodes of greater rank (further outward along the DAG)
expire first, coordinating the sending of DAOs to allow for a
chance of aggregation. See Section 5.10.2.1.1
DestroyTimer Up to one instance per DA entry per neighbor (i.e.
those neighbors who have given DAO to this node as a DAG
Parent) Expiry triggers a change in state for the DA entry,
setting up to do unreachable (No-DAO) advertisements or
immediately deallocating the DA entry if there are no DA
Parents. The interval is min(MAX_DESTROY_INTERVAL,
RA_INTERVAL). See Section 5.10.2.1.1
7. Protocol Extensions
8. Manageability Considerations
9. Security Considerations
Security Considerations for RPL are to be developed in accordance
with recommendations laid out in, for example,
[I-D.tsao-roll-security-framework].
10. IANA Considerations
10.1. DAG Information Option
IANA is requested to allocate a new Neighbor Discovery Option Type IANA is requested to allocate a new Neighbor Discovery Option Type
from the IPv6 Neighbor Discovery Option Formats Registry in order to from the IPv6 Neighbor Discovery Option Formats Registry in order to
represent the DAG Information Option as described in Section 5.1 represent the DAG Information Option as described in Section 5.1
9.2. Destination Advertisement Option 10.2. Objective Code Point
This specification requests that an Objective Code Point registry, as
to be specified in [I-D.ietf-roll-routing-metrics], reserve the
Objective Code Point value 0x0000, for the purposes designated as OCP
0 in this document.
10.3. Destination Advertisement Option
IANA is requested to allocate a new Neighbor Discovery Option Type IANA is requested to allocate a new Neighbor Discovery Option Type
from the IPv6 Neighbor Discovery Option Formats Registry in order to from the IPv6 Neighbor Discovery Option Formats Registry in order to
represent the Destination Advertisement Option as described in represent the Destination Advertisement Option as described in
Section 5.4.1.1 Section 5.10.1.1
10. Acknowledgements 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, Richard Kelsey, Quentin Lampin, Philip
Levis, Jerry Martocci, Alexandru Petrescu, and Don Sturek. Levis, Jerry 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 Richard Kelsey, Robert Assimiti, Mischa Dohler, Julien Abeille, Ryuji
Wakikawa, Teco Boot, Patrick Wetterwald, Bryan Mclaughlin, Carlos J. Wakikawa, Teco Boot, Patrick Wetterwald, Bryan Mclaughlin, Carlos J.
Bernardos, Thomas Watteyne, Zach Shelby, Dominique Barthel, Caroline Bernardos, Thomas Watteyne, Zach Shelby, Dominique Barthel, Caroline
Bontoux, Marco Molteni, Billy Moon, and Arsalan Tavakoli, which have Bontoux, Marco Molteni, Billy Moon, and Arsalan Tavakoli, in addition
provided useful design considerations to RPL. to contributions from [I-D.thubert-roll-fundamentals] and
[I-D.tavakoli-hydro] which have provided useful design considerations
to RPL.
11. Contributors 12. Contributors
ROLL Design Team in alphabetical order: ROLL Design Team in alphabetical order:
Anders Brandt Anders Brandt
Zensys, Inc. Zensys, Inc.
Emdrupvej 26 Emdrupvej 26
Copenhagen, DK-2100 Copenhagen, DK-2100
Denmark Denmark
Email: abr@zen-sys.com Email: abr@zen-sys.com
skipping to change at page 57, line 26 skipping to change at page 74, line 14
Biot - Sophia Antipolis 06410 Biot - Sophia Antipolis 06410
FRANCE FRANCE
Phone: +33 497 23 26 34 Phone: +33 497 23 26 34
Email: pthubert@cisco.com Email: pthubert@cisco.com
Tim Winter (editor) Tim Winter (editor)
wintert@acm.org wintert@acm.org
12. References 13. References
12.1. Normative References 13.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
12.2. Informative References 13.2. Informative References
[I-D.ietf-bfd-base] [I-D.ietf-bfd-base]
Katz, D. and D. Ward, "Bidirectional Forwarding Katz, D. and D. Ward, "Bidirectional Forwarding
Detection", draft-ietf-bfd-base-09 (work in progress), Detection", draft-ietf-bfd-base-09 (work in progress),
February 2009. February 2009.
[I-D.ietf-manet-nhdp] [I-D.ietf-manet-nhdp]
Clausen, T., Dearlove, C., and J. Dean, "MANET Clausen, T., Dearlove, C., and J. Dean, "MANET
Neighborhood Discovery Protocol (NHDP)", Neighborhood Discovery Protocol (NHDP)",
draft-ietf-manet-nhdp-10 (work in progress), July 2009. draft-ietf-manet-nhdp-10 (work in progress), July 2009.
[I-D.ietf-roll-building-routing-reqs] [I-D.ietf-roll-building-routing-reqs]
Martocci, J., Riou, N., Mil, P., and W. Vermeylen, Martocci, J., Riou, N., Mil, P., and W. Vermeylen,
"Building Automation Routing Requirements in Low Power and "Building Automation Routing Requirements in Low Power and
Lossy Networks", draft-ietf-roll-building-routing-reqs-05 Lossy Networks", draft-ietf-roll-building-routing-reqs-06
(work in progress), February 2009. (work in progress), August 2009.
[I-D.ietf-roll-home-routing-reqs] [I-D.ietf-roll-home-routing-reqs]
Porcu, G., "Home Automation Routing Requirements in Low Porcu, G., "Home Automation Routing Requirements in Low
Power and Lossy Networks", Power and Lossy Networks",
draft-ietf-roll-home-routing-reqs-06 (work in progress), draft-ietf-roll-home-routing-reqs-06 (work in progress),
November 2008. November 2008.
[I-D.ietf-roll-indus-routing-reqs] [I-D.ietf-roll-indus-routing-reqs]
Networks, D., Thubert, P., Dwars, S., and T. Phinney, Networks, D., Thubert, P., Dwars, S., and T. Phinney,
"Industrial Routing Requirements in Low Power and Lossy "Industrial Routing Requirements in Low Power and Lossy
skipping to change at page 59, line 6 skipping to change at page 75, line 41
and Lossy Networks", draft-tsao-roll-security-framework-00 and Lossy Networks", draft-tsao-roll-security-framework-00
(work in progress), February 2009. (work in progress), February 2009.
[Levis08] Levis, P., Brewer, E., Culler, D., Gay, D., Madden, S., [Levis08] Levis, P., Brewer, E., Culler, D., Gay, D., Madden, S.,
Patel, N., Polastre, J., Shenker, S., Szewczyk, R., and A. Patel, N., Polastre, J., Shenker, S., Szewczyk, R., and A.
Woo, "The Emergence of a Networking Primitive in Wireless Woo, "The Emergence of a Networking Primitive in Wireless
Sensor Networks", Communications of the ACM, v.51 n.7, Sensor Networks", Communications of the ACM, v.51 n.7,
July 2008, July 2008,
<http://portal.acm.org/citation.cfm?id=1364804>. <http://portal.acm.org/citation.cfm?id=1364804>.
[RFC2453] Malkin, G., "RIP Version 2", STD 56, RFC 2453,
November 1998.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., [RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L. Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89, Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004. RFC 3819, July 2004.
[RFC4101] Rescorla, E. and IAB, "Writing Protocol Models", RFC 4101, [RFC4101] Rescorla, E. and IAB, "Writing Protocol Models", RFC 4101,
June 2005. June 2005.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and [RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005. More-Specific Routes", RFC 4191, November 2005.
skipping to change at page 69, line 6 skipping to change at page 86, line 6
| /| | \ | \ | /| | \ | \
| .----` | | `----. | `----. | .----` | | `----. | `----.
| / | | \| \ | / | | \| \
.--------(41) (42) (43) (44) (45) .--------(41) (42) (43) (44) (45)
/ / /| | \ / / /| | \
.----` .----` .----` | | `----. .----` .----` .----` | | `----.
/ / / | | \ / / / | | \
(51) (52) (53) (54) (55) (56) (51) (52) (53) (54) (55) (56)
Figure 18: DAG Construction Step 5 Figure 18: DAG Construction Step 5
Appendix D. Outstanding Issues
This section enumerates some outstanding issues that are to be
addressed in future revisions of the RPL specification.
D.1. Additional Support for P2P Routing
In some situations the baseline mechanism to support arbitrary P2P
traffic, by flowing inward along the DAG until a common parent is
reached and then flowing outward, may not be suitable for all
application scenarios. A related scenario may occur when the outward
paths setup along the DAG by the destination advertisement mechanism
are not be the most desirable outward paths for the specific
application scenario (in part because the DAG links may not be
symmetric). It may be desired to support within RPL the discovery
and installation of more direct routes `across' the DAG. Such
mechanisms need to be investigated.
D.2. Loop Detection
It is under investigation to complement the loop avoidance strategies
provided by RPL with a loop detection mechanism that may be employed
when traffic is forwarded.
D.3. DAO Fan-out
When DAOs are relayed to more than one DAG Parent, in some cases a
situation may be created where a large number of DAOs conveying
information about the same destination flow inward along the DAG. It
is desirable to bound/limit the multiplication/fan-out of DAOs in
this manner.
D.4. Source Routing
In support of nodes who maintain minimal routing state, and to make
use of the collection of piecewise source routes from the Destination
Advertisement mechanism, there needs to be some investigation of a
mechanism to specify, attach, and follow source routes for packets
traversing the LLN.
D.5. Address / Header Compression
In order to minimize overhead within the LLN it is desirable to
perform some sort of address and/or header compression, perhaps via
labels, addresses aggregation, or some other means. This is still
under investigation.
Authors' Addresses Authors' Addresses
Tim Winter (editor) Tim Winter (editor)
Email: wintert@acm.org Email: wintert@acm.org
ROLL Design Team ROLL Design Team
IETF ROLL WG IETF ROLL WG
Email: dtroll@external.cisco.com Email: dtroll@external.cisco.com
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