One document matched: draft-ietf-roll-rpl-04.txt
Differences from draft-ietf-roll-rpl-03.txt
Networking Working Group T. Winter, Ed.
Internet-Draft
Intended status: Standards Track P. Thubert, Ed.
Expires: April 29, 2010 Cisco Systems
ROLL Design Team
IETF ROLL WG
October 26, 2009
RPL: IPv6 Routing Protocol for Low power and Lossy Networks
draft-ietf-roll-rpl-04
Status of this Memo
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Abstract
Low power and Lossy Networks (LLNs) are a class of network in which
both the routers and their interconnect are constrained: LLN routers
typically operate with constraints on (any subset of) processing
power, memory and energy (battery), and their interconnects are
characterized by (any subset of) high loss rates, low data rates and
instability. LLNs are comprised of anything from a few dozen and up
to thousands of LLN routers, and support point-to- point traffic
(between devices inside the LLN), point-to-multipoint traffic (from a
central control point to a subset of devices inside the LLN) and
multipoint-to- point traffic (from devices inside the LLN towards a
central control point). This document specifies the IPv6 Routing
Protocol for LLNs (RPL), which provides a mechanism whereby
multipoint-to-point traffic from devices inside the LLN towards a
central control point, as well as point-to-multipoint traffic from
the central control point to the devices inside the LLN, is
supported. Support for point-to-point traffic is also available.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1. Design Principles . . . . . . . . . . . . . . . . . . . . 6
1.2. Expectations of Link Layer Type . . . . . . . . . . . . . 7
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 7
3. Protocol Model . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1. Topology Instance and Objectives . . . . . . . . . . . 9
3.1.2. Multipoint-to-Point Traffic Flows and DAGs . . . . . . 11
3.1.3. Point-to-Multipoint Traffic Flows . . . . . . . . . . 11
3.1.4. Point-to-Point Traffic Flows . . . . . . . . . . . . . 12
3.2. Protocol Operation . . . . . . . . . . . . . . . . . . . . 12
3.2.1. DAG Construction . . . . . . . . . . . . . . . . . . . 12
3.2.2. Destination Advertisement . . . . . . . . . . . . . . 15
3.3. Loop Avoidance and Stability . . . . . . . . . . . . . . . 17
3.3.1. Greediness and Rank-based Instabilities . . . . . . . 17
3.3.2. DAG Loops . . . . . . . . . . . . . . . . . . . . . . 18
3.3.3. DAO Loops . . . . . . . . . . . . . . . . . . . . . . 18
3.3.4. Sibling Loops . . . . . . . . . . . . . . . . . . . . 18
4. Routing Metrics and Constraints Used By RPL . . . . . . . . . 18
5. RPL Protocol Specification . . . . . . . . . . . . . . . . . . 19
5.1. RPL Messages . . . . . . . . . . . . . . . . . . . . . . . 19
5.1.1. ICMPv6 RPL Control Message . . . . . . . . . . . . . . 19
5.1.2. DAG Information Solicitation (DIS) . . . . . . . . . . 20
5.1.3. DAG Information Object (DIO) . . . . . . . . . . . . . 20
5.1.4. Destination Advertisement Object (DAO) . . . . . . . . 27
5.2. Conceptual Data Structures . . . . . . . . . . . . . . . . 28
5.2.1. Candidate Neighbors Data Structure . . . . . . . . . . 28
5.2.2. Directed Acyclic Graphs (DAGs) Data Structure . . . . 29
5.3. DAG Rank . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.4. DAG Discovery and Maintenance . . . . . . . . . . . . . . 31
5.4.1. DAG Discovery Rules . . . . . . . . . . . . . . . . . 32
5.4.2. Reception and Processing of DIO messages . . . . . . . 36
5.4.3. DIO Transmission . . . . . . . . . . . . . . . . . . . 38
5.4.4. Trickle Timer for DIO Transmission . . . . . . . . . . 39
5.5. DAG Sequence Number Increment . . . . . . . . . . . . . . 40
5.6. DAG Selection . . . . . . . . . . . . . . . . . . . . . . 41
5.7. Administrative rank . . . . . . . . . . . . . . . . . . . 41
5.8. Collision . . . . . . . . . . . . . . . . . . . . . . . . 42
5.9. Guidelines for Objective Functions . . . . . . . . . . . . 42
5.9.1. Objective Function . . . . . . . . . . . . . . . . . . 42
5.9.2. Objective Function 0 (OF0) . . . . . . . . . . . . . . 44
5.10. Establishing Routing State Outward Along the DAG . . . . . 46
5.10.1. Destination Advertisement Operation . . . . . . . . . 47
5.11. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 54
5.11.1. Host Basic Operation . . . . . . . . . . . . . . . . . 55
5.11.2. Instance Forwarding . . . . . . . . . . . . . . . . . 55
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5.11.3. DAG Inconsistency Loop Detection . . . . . . . . . . . 56
5.11.4. Sibling Loop Avoidance . . . . . . . . . . . . . . . . 56
5.11.5. DAO Inconsistency Loop Detection and Recovery . . . . 57
5.12. Multicast Operation . . . . . . . . . . . . . . . . . . . 57
5.13. Maintenance of Routing Adjacency . . . . . . . . . . . . . 58
5.14. Packet Forwarding . . . . . . . . . . . . . . . . . . . . 59
6. RPL Constants and Variables . . . . . . . . . . . . . . . . . 60
7. Manageability Considerations . . . . . . . . . . . . . . . . . 61
7.1. Control of Function and Policy . . . . . . . . . . . . . . 61
7.1.1. Initialization Mode . . . . . . . . . . . . . . . . . 61
7.1.2. DIO Base option . . . . . . . . . . . . . . . . . . . 61
7.1.3. Trickle Timers . . . . . . . . . . . . . . . . . . . . 62
7.1.4. DAG Sequence Number Increment . . . . . . . . . . . . 63
7.1.5. Destination Advertisement Timers . . . . . . . . . . . 63
7.1.6. Policy Control . . . . . . . . . . . . . . . . . . . . 63
7.1.7. Data Structures . . . . . . . . . . . . . . . . . . . 63
7.2. Information and Data Models . . . . . . . . . . . . . . . 64
7.3. Liveness Detection and Monitoring . . . . . . . . . . . . 64
7.3.1. Candidate Neighbor Data Structure . . . . . . . . . . 64
7.3.2. Directed Acyclic Graph (DAG) Table . . . . . . . . . . 64
7.3.3. Routing Table . . . . . . . . . . . . . . . . . . . . 65
7.3.4. Other RPL Monitoring Parameters . . . . . . . . . . . 65
7.3.5. RPL Trickle Timers . . . . . . . . . . . . . . . . . . 66
7.4. Verifying Correct Operation . . . . . . . . . . . . . . . 66
7.5. Requirements on Other Protocols and Functional
Components . . . . . . . . . . . . . . . . . . . . . . . . 66
7.6. Impact on Network Operation . . . . . . . . . . . . . . . 66
8. Security Considerations . . . . . . . . . . . . . . . . . . . 66
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 66
9.1. RPL Control Message . . . . . . . . . . . . . . . . . . . 66
9.2. New Registry for RPL Control Codes . . . . . . . . . . . . 67
9.3. New Registry for the Control Field of the DIO Base
Option . . . . . . . . . . . . . . . . . . . . . . . . . . 67
9.4. DAG Information Object (DIO) Suboption . . . . . . . . . . 68
9.5. Objective Code Point for the Default Objective
Function OF0 . . . . . . . . . . . . . . . . . . . . . . . 68
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 68
11. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 69
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 70
12.1. Normative References . . . . . . . . . . . . . . . . . . . 70
12.2. Informative References . . . . . . . . . . . . . . . . . . 71
Appendix A. Requirements . . . . . . . . . . . . . . . . . . . . 72
A.1. Protocol Properties Overview . . . . . . . . . . . . . . . 72
A.1.1. IPv6 Architecture . . . . . . . . . . . . . . . . . . 73
A.1.2. Typical LLN Traffic Patterns . . . . . . . . . . . . . 73
A.1.3. Constraint Based Routing . . . . . . . . . . . . . . . 73
A.2. Deferred Requirements . . . . . . . . . . . . . . . . . . 74
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 74
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B.1. Destination Advertisement . . . . . . . . . . . . . . . . 76
B.2. Example: DAG Parent Selection . . . . . . . . . . . . . . 77
B.3. Example: DAG Maintenance . . . . . . . . . . . . . . . . . 78
B.4. Example: Greedy Parent Selection and Instability . . . . . 79
Appendix C. Outstanding Issues . . . . . . . . . . . . . . . . . 81
C.1. Additional Support for P2P Routing . . . . . . . . . . . . 81
C.2. Loop Detection . . . . . . . . . . . . . . . . . . . . . . 81
C.3. Destination Advertisement / DAO Fan-out . . . . . . . . . 81
C.4. Source Routing . . . . . . . . . . . . . . . . . . . . . . 82
C.5. Address / Header Compression . . . . . . . . . . . . . . . 82
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 82
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1. Introduction
Low power and Lossy Networks (LLNs) are made largely of constrained
nodes (with limited processing power, memory, and sometimes energy
when they are battery operated). These routers are interconnected by
lossy links, typically time supporting only low data rates, that are
usually unstable with relatively low packet delivery rates. Another
characteristic of such networks is that the traffic patterns are not
simply unicast, but in many cases point-to-multipoint or multipoint-
to-point. Furthermore such networks may potentially comprise up to
thousands of nodes. These characteristics offer unique challenges to
a routing solution: the IETF ROLL Working Group has defined
application-specific routing requirements for a Low power and Lossy
Network (LLN) routing protocol, specified in
[I-D.ietf-roll-building-routing-reqs],
[I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548]. This
document specifies the IPv6 Routing Protocol for Low power and Lossy
Networks (RPL).
1.1. Design Principles
RPL was designed with the objective to meet the requirements spelled
out in [I-D.ietf-roll-building-routing-reqs],
[I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548]. Because
those requirements are heterogeneous and sometimes incompatible in
nature, the approach is first taken to design a protocol capable of
supporting a core set of functionalities corresponding to the
intersection of the requirements. (Note: it is intended that as this
design evolves optional features may be added to address some
application specific requirements). This is a key protocol design
decision providing a granular approach in order to restrict the core
of the protocol to a minimal set of functionalities, and to allow
each implementation of the protocol to be optimized in terms of,
e.g., minimizing required code space and use of limited computation
resources.
Multiple instances of the protocol can be operated at the same time
in order to serve different and potentially antagonistic constraints.
Instances run independently of one another with no required
interaction. A node might participate to multiple instances and
route independently along the associated topologies. This
specification defines only the protocol operation for the node within
one instance. Consideration is given to default behavior that
enables future extensions for the multiple instances and related
policies.
It must be noted that RPL is not restricted to the aforementioned
applications and is expected to be used in other environments. All
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"MUST" application requirements that cannot be satisfied by RPL will
be specifically listed in the Appendix A, accompanied by a
justification.
The core set of functionalities is to be capable of operating in the
most severely constrained environments, with minimal requirements for
memory, energy, processing, communication, and other consumption of
limited resources from nodes. Trade-offs inherent in the
provisioning of protocol features will be exposed to the implementer
in the form of configurable parameters, such that the implementer can
further tweak and optimize the operation of RPL as appropriate to a
specific application and implementation. Finally, RPL is designed to
consult implementation specific policies to determine, for example,
the evaluation of routing metrics.
A set of companion documents to this specification will provide
further guidance in the form of applicability statements specifying a
set of operating points appropriate to the Building Automation, Home
Automation, Industrial, and Urban application scenarios.
1.2. Expectations of Link Layer Type
This specification does not rely on any particular features of a
specific link layer technologies. It is anticipated that an
implementer should be able to operate RPL over a variety of different
link layers, including but not limited to low power wireless or PLC
(Power Line Communication) technologies.
Implementers may find RFC 3819 [RFC3819] a useful reference when
designing a link layer interface between RPL and a particular link
layer technology.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
This document requires readers to be familiar with the terminology
described in `Terminology in Low power And Lossy Networks'
[I-D.ietf-roll-terminology].
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DAG: Directed Acyclic Graph. A directed graph having the property
that all edges are oriented in such a way that no cycles exist.
In the RPL context, all edges are contained in paths oriented
toward and terminating at one or more root nodes (a DAG root,
or sink- typically a Low power and Lossy Network Border Router
(LBR)). For the purpose of this document, the term DAG is
often used to refer to a DAG Iteration as defined below.
DAG Instance: A DAG Instance is a set of possibly multiple
Destination Oriented DAGs. A network may have more than one
DAG Instance, and a RPL router can participate to multiple DAG
instances. Each DAG Instance operates independently of other
DAG Instances. This document describes operation within a
single DAG instance.
InstanceID: Unique identifier of a DAG Instance.
Destination Oriented DAG: A DAG rooted at a single destination,
which is a node with no outgoing edges. The tuple (InstanceID,
DAGID) uniquely identifies a Destination Oriented DAG. In the
RPL context, a router can can belong to at most one Destination
Oriented DAG per DAG Instance.
DAGID: The identifier of a DAG root. The DAGID must be unique
within the scope of a DAG Instance in the LLN.
DAG Iteration: The DAG that results from the iterative process that
reshapes the Destination Oriented DAG upon a stimulation by the
root.
DAGSequenceNumber: A sequential counter that is incremented by the
root to form a new Iteration of a DAG. A DAG Iteration is
identified uniquely by the (InstanceID, DAGID,
DAGSequenceNumber) tuple.
DAG parent: A parent of a node within a DAG is one of the immediate
successors of the node on a path towards the DAG root.
DAG sibling: A sibling of a node within a DAG is defined in this
specification to be any neighboring node which is located at
the same rank within a DAG. Note that siblings defined in this
manner do not necessarily share a common parent.
DAG root: A DAG root is a node within the DAG that has no outgoing
edges. Because the graph is acyclic, by definition all DAGs
must have at least one DAG root and all paths terminate at a
DAG root.
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Sub-DAG The sub-DAG of a node is the set of other nodes in the DAG
that might use a path towards the DAG root that contains the
node. Nodes in the sub-DAG of a node have a greater rank
(although not all nodes of greater rank are in the sub-DAG).
Grounded: A DAG is grounded if it contains a DAG root offering
connectivity to an external routed infrastructure such as the
public Internet or a private core (non-LLN) IP network.
Floating: A DAG is floating if is not grounded. A floating DAG is
not expected to reach any additional external routed
infrastructure such as the public Internet or a private core
(non-LLN) IP network.
Inward: Inward refers to the direction from leaf nodes towards DAG
roots, following the orientation of the edges within the DAG.
Outward: Outward refers to the direction from DAG roots towards leaf
nodes, going against the orientation of the edges within the
DAG.
OCP: Objective Code Point. The Objective Code Point is used to
indicate which Objective Function is in use in a DAG. The
Objective Code Point is further described in
[I-D.ietf-roll-routing-metrics].
OF: Objective Function. The Objective Function (OF) defines which
routing metrics, optimization objectives, and related functions
are in use in a DAG. The Objective Function is further
described in [I-D.ietf-roll-routing-metrics].
Note that in this document, the terms `node' and `LLN router' are
used interchangeably.
3. Protocol Model
The aim of this section is to describe RPL in the spirit of
[RFC4101]. Protocol details can be found in further sections.
3.1. Overview
3.1.1. Topology Instance and Objectives
A topology instance of RPL exists over the scope of an LLN in support
of a particular application, or service, and is optimized according
to a certain objective, as determined by an Objective Function (OF),
and may be characterized by certain destination prefixes as well. A
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topology instance, or DAG Instance, may be administratively
associated with an InstanceID.
A single topology instance may comprise:
o a single Destination Oriented DAG with a single DAG root
* For example, a DAG optimized to minimize latency rooted at a
single centralized lighting controller in a home automation
application.
o multiple uncoordinated Destination Oriented DAGs with independent
DAG roots (differing DAGIDs)
* For example, multiple data collection points in an urban data
collection application that do not have an always-on backbone
suitable to coordinate to form a single DAG, and further use
the formation of multiple DAGs as a means to dynamically and
autonomously partition the network.
o a single Destination Oriented DAG with multiple DAG roots
coordinating over some backbone
* For example, multiple border routers operating with a reliable
backbone, e.g. in support of a 6LowPAN application, that are
capable to act as logically equivalent sinks to the same DAG.
o a combination of one of the above as suited to some application
scenario
The exact deployment scenario is determined as appropriate to the
application and capabilities of the LLN nodes. What is suitable for
one deployment may not be possible or necessary for another.
Traffic is bound to a specific DAG Instance by a marking in the flow
label of the IPv6 header. Traffic originating in support of a
particular application may be tagged to follow an appropriate
instance, for example to follow paths optimized for low latency or
low energy. The provisioning or automated discovery of a mapping
between an InstanceID and a type or service of application traffic is
beyond the scope of this specification.
Conceptually a node running RPL may capable to maintain a membership
in multiple DAG Instances in support of different application
services and/or optimization objectives. For example, one instance
may optimize for minimizing latency and a separate orthogonal
instance may optimize for minimizing energy. This scenario does
introduce some additional considerations, for example loop avoidance
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and default routing behavior. These considerations are beyond the
scope of this specification and are intended to be elaborated on in a
future revision of this or a companion specification. As such, this
specification will deal exclusively with the scenario where a node
implements RPL in support of a single DAG Instance.
3.1.2. Multipoint-to-Point Traffic Flows and DAGs
Many of the dominant traffic flows in support of the LLN application
scenarios are MP2P flows ([I-D.ietf-roll-building-routing-reqs],
[I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548]). These
flows are rooted at designated nodes that have some application
significance, such as providing connectivity to an external routed
infrastructure. The term "external" is used to refer to the public
Internet or a core private (non-LLN) IP network.
LLN nodes running RPL will construct Directed Acyclic Graphs (DAGs)
rooted at DAG roots, which may be naturally designated according to
their application significance. This structure provides the routing
solution for the dominant MP2P traffic flows. The DAG structure
further provides each node potentially multiple successors for MP2P
flows, which may be used for, e.g., local route repair or load
balancing.
Nodes running RPL are able to further restrict the scope of the
routing problem by using the DAG as a reference topology. By
referencing a rank property that is related to the positions in the
DAG, nodes are able to determine their positions in a DAG relative to
each other. This information is used by RPL in part to construct
rules for movement relative to the DAG that endeavor to avoid loops.
It is important to note that the rank property is derived from
metrics, and not directly from the position in the DAG (Section 5.3).
3.1.3. Point-to-Multipoint Traffic Flows
As DAGs are organized, RPL will use a destination advertisement
mechanism to build up routing tables in support of outward P2MP
traffic flows. This mechanism, using the DAG as a reference,
distributes routing information across intermediate nodes (between
the DAG leaves and the root), guided along the DAG, such that the
routes toward destination prefixes in the outward direction may be
set up. As the DAG undergoes modification during DAG maintenance,
the destination advertisement mechanism can be triggered to update
the outward routing state.
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3.1.4. Point-to-Point Traffic Flows
A baseline support for P2P traffic in RPL is provided by the DAG, as
P2P traffic may flow inward along the DAG until a common parent is
reached that has stored an entry for the destination in its routing
table and is capable of directing the traffic outward along the
correct outward path. RPL also provides support for the trivial case
where a P2P destination may be a `one-hop' neighbor. In the present
document RPL does not specify nor preclude any additional mechanisms
that may be capable to compute and install more optimal routes into
LLN nodes in support of arbitrary P2P traffic according to some
routing metric.
3.2. Protocol Operation
3.2.1. DAG Construction
3.2.1.1. DAG Information Object (DIO)
A DAG Information Object is defined and used by RPL in order to build
and maintain a DAG. This document defines an ICMPv6 Message Type RPL
Control Message, which is capable to carry the DIO. The DIO message
conveys information about the DAG, including:
o A DAGID used to identify the DAG as sourced from the DAG root.
The DAGID must be unique to a single DAG in the scope of the LLN.
o Objective Function identified by an Objective Code Point (OCP) as
described below.
o Rank information used by nodes to determine their positions in the
DAG relative to each other.
o Sequence number originated from the DAG root, used to aid in
identification of dependent sub-DAGs and coordinate topology
changes in a manner so as to avoid loops.
o Indications and configuration for the DAG, e.g. grounded or
floating, administrative preference, ...
o A set of path metrics and constraints, as further described in
[I-D.ietf-roll-routing-metrics].
o List of additional destination prefixes reachable inwards along
the DAG.
The DIO messages are issued whenever a change is detected to the DAG
such that a node is able to determine that a region of the DAG has
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become inconsistent. As the DAG stabilizes the period at which RA
messages occur is configured to taper off, reducing the steady-state
overhead of DAG maintenance. The periodic issue of DIO messages,
along with the triggered DIO messages in response to inconsistency,
is one feature that enables RPL to operate in the presence of
unreliable links.
3.2.1.2. Grounded and Floating DAGs
Certain LLN nodes may offer connectivity to an external routed
infrastructure in support of an application scenario. These nodes
are designated `grounded', and may serve as the DAG roots of a
grounded DAG. DAGs that do not have a grounded DAG root are floating
DAGs. In either case routes may be provisioned toward the DAG root,
although in the floating case there is no expectation to reach an
external infrastructure. Some applications will include permanent
floating DAGs.
3.2.1.3. Administrative Preference
An administrative preference may be associated with each DAG root,
and thereby each DAG, in order that some DAGs in the LLN may be more
preferred over other DAGs. For example, a DAG root that is sinking
traffic in support of a data collection application may be configured
by the application to be very preferred. A transient DAG, e.g. a DAG
that is only existing until a permanent DAG is found, may be
configured to be less preferred. The administrative preference
offers a way to engineer the formation of the DAG in support of the
application.
3.2.1.4. Objective Function (OF)
The Objective Function (OF) conveys and controls the optimization
objectives in use within the DAG. The Objective Function is
indicated by an Objective Code Point (OCP), and is further specified
in [I-D.ietf-roll-routing-metrics]. Each instance of an allocated OF
indicates:
o The set of metrics used within the DAG
o The method used for least cost path determination.
o The method used to compute DAG Rank
o The methods used to prepare metrics for propagation within a DIO
message
By using defined OCPs that are understood by all nodes in a
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particular implementation, and by conveying them in the DIO message,
RPL nodes may work to build optimized LLN using a variety of
application and implementation specific metrics and goals.
A default OF, OF0 (designated by OCP value of 0x0000), is specified
with a well-defined default behavior. OF0 may be used to define RPL
behaviors in the case where a node encounters a DIO message
containing a code point that it does not support, if allowed by
policy.
3.2.1.5. Distributed Algorithm Operation
A high level overview of the distributed algorithm which constructs
the DAG is as follows:
o Some nodes may be initially provisioned to act as DAG roots,
either permanent or transient, with associated preferences.
o Nodes will maintain a data structure containing their candidate
(viable) neighbors, as determined by the implementation. This
data structure will also track DAG information as learned from and
associated with each neighbor.
o Nodes that are members of a DAG, including DAG roots, will
multicast DIO messages as needed (when inconsistency is detected),
to their link-local neighbors. Nodes will also respond to DIS
messages.
o Nodes that receive DIO messages may either discard the DIO based
on several criteria, including rank-based loop avoidance rules, or
process the DIO to maintain a position in an existing DAG or
improve a position as according to an Objective Function (OF) and
current path cost.
o Nodes manage a set of DAG Parents according to the rules specified
by RPL. This set is also augmented to include DAG siblings.
o DIO messages may be emitted more or less frequently as a function
of DAG consistency.
o As less preferred DAGs encounter more preferred DAGs that offer
equivalent or better optimization objectives for the same
InstanceID, the nodes in the less preferred DAGs may leave to join
the more preferred DAGs, finally leaving only the more preferred
DAGs. This is an illustration of the mechanism by which an
application may engineer DAG construction.
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o The nodes provision routing table entries for the destinations
specified by the DIO towards their DAG Parents. Nodes may
provision a DAG Parent as a default gateway.
3.2.2. Destination Advertisement
As RPL constructs DAGs, nodes may provision routes toward
destinations advertised through DIO messages through their selected
parents, and are thus able to send traffic inward along the DAG by
forwarding to their selected parents. However, this mechanism alone
is not sufficient to support P2MP traffic flowing outward along the
DAG from the DAG root toward nodes. A destination advertisement
mechanism is employed by RPL to build up routing state in support of
these outward flows. The destination advertisement mechanism may not
be supported in all implementations, as appropriate to the
application requirements. A DAG root that supports using the
destination advertisement mechanism to build up routing state will
indicate such in the DIO message. A DAG root that supports using the
destination advertisement mechanism must be capable of allocating
enough state to store the routing state received from the LLN.
3.2.2.1. Destination Advertisement Object (DAO)
A Destination Advertisement Object is defined and used by RPL in
order to convey the destination information inward along the DAG
toward the DAG root. This document defines an ICMPv6 Message Type
RPL Control Message, which is capable to carry the DAO. The
information conveyed in the DAO message includes the following:
o A lifetime and sequence counter to determine the freshness of the
destination advertisement.
o Depth information used by nodes to determine how far away the
destination (the source of the destination advertisement) is
o Prefix information to identify the destination, which may be a
prefix, an individual host, or multicast listeners
o Reverse Route information to record the nodes visited (along the
outward path) when the intermediate nodes along the path cannot
support storing state for Hop-By-Hop routing.
3.2.2.2. Destination Advertisement Operation
As the DAG is constructed and maintained, nodes are capable to emit
DAO messages to a subset of their DAG parents.
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3.2.2.2.1. `One-Hop' Neighbors
As a special case, a node may periodically emit a link-local
multicast IPv6 DAO message advertising its locally available
destination prefixes. This mechanism allows for the one-hop
neighbors of a node to learn explicitly of the prefixes on the node,
and in some application specific scenarios this is desirable in
support of provisioning a trivial `one-hop' route. In this case,
nodes that receive the multicast destination advertisement may use it
to provision the one-hop route only, and not engage in any additional
processing (so as not to engage the mechanisms used by a DAG parent).
3.2.2.2.2. Operation in Support of Stateful Nodes
When a (unicast) DAO message reaches a node capable of storing
routing state, the node extracts information from the DAO message and
updates its local database with a record of the DAO message and the
neighbor that it was received from. When the node later propagates
DAO messages, it selects the best (least depth) information for each
destination and conveys this information again in the form of DAO
messages to a subset of its own DAG parents. At this time the node
may perform route aggregation if it is able, thus reducing the
overall number of DAO messages.
3.2.2.2.3. Operation in Support of Stateless Nodes
When a (unicast) DAO message reaches a node incapable of storing
additional state, the node must append the next-hop address (from
which neighbor the DAO message was received) to a Reverse Route Stack
carried within the DAO message. The node then passes the DAO message
on to one or more of its DAG parents without storing any additional
state.
When a node that is capable of storing routing state encounters a
(unicast) DAO message with a Reverse Route Stack that has been
populated, the node knows that the DAO message has traversed a region
of nodes that did not record any routing state. The node is able to
detach and store the Reverse Route State and associate it with the
destination described by the DAO message. Subsequently the node may
use this information to construct a source route in order to bridge
the region of nodes that are unable to support Hop-By-Hop routing to
reach the destination.
3.2.2.2.4. Additional Considerations
Further aggregations of DAO messages prefix reachability information
by destinations are possible in order to support additional
scalability.
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A special case of an DAO message, termed a `no-DAO', may be used to
tear down the routing state that has been established by the
destination advertisement mechanism in case of, e.g., unreachability
or other events that affect the outward routing state.
A further example of the operation of the destination advertisement
mechanism is available in Appendix B.1
3.3. Loop Avoidance and Stability
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.
RPL includes rank-based mechanisms for detecting loops to ensure that
packets make forward progress within the DAG and trigger DAG repair
if necessary.
3.3.1. Greediness and Rank-based Instabilities
Once a node has joined a DAG, RPL disallows certain behaviors,
including greediness, in order to prevent resulting instabilities in
the DAG.
If a node is allowed to be greedy and attempts to move deeper in the
DAG, beyond its most preferred parent, in order to increase the size
of the DAG parent set, then an instability can result. This is
illustrated in Figure 14.
Suppose a node is willing to receive and process a DIO messages from
a node in its own sub-DAG, and in general a node deeper than it. In
such cases a chance exists to create a feedback loop, wherein two or
more nodes continue to try and move in the DAG in order to optimize
against each other. In some cases this will result in an
instability. It is for this reason that RPL mandates that a node
never receive and process DIO messages from deeper nodes. This rule
creates an `event horizon', whereby a node cannot be influenced into
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an instability by the action of nodes that may be in its own sub-DAG.
A further example of the consequences of greedy operation, and
instability related to processing DIO messages from nodes of greater
rank, may be found in Appendix B.4
3.3.2. DAG Loops
A DAG loop may occur when a node detaches from the DAG and reattaches
to a device in its prior sub-DAG. This may happen in particular when
DIO messages are missed. Strict use of the DAG sequence number can
eliminate this type of loop.
3.3.3. DAO Loops
A DAO loop may occur when the parent has a route installed upon
receiving and processing a DAO message from a child, but the child
has subsequently cleaned up the state. This loop happens when a no-
DAO was missed till a heartbeat cleans up all states. RPL includes
loop detection mechanisms that may mitigate the impact of DAO loops
and trigger their repair.
In the case where stateless DAO operation is used, i.e. source
routing specifies the outwards routes, then DAO Loops should not
occur on the stateless portions of the path.
3.3.4. Sibling Loops
Sibling loops could occur if a group of siblings kept choosing
amongst themselves as successors such that a packet does not make
forward progress. This specification limits the number of times that
sibling forwarding may be used at a given rank to prevent sibling
loops.
4. Routing Metrics and Constraints Used By RPL
Routing metrics are used by routing protocols to compute the shortest
paths. Interior Gateway Protocols (IGPs) such as IS-IS ([RFC5120])
and OSPF ([RFC4915]) use static link metrics. Such link metrics can
simply reflect the bandwidth or can also be computed according to a
polynomial function of several metrics defining different link
characteristics; in all cases they are static metrics. Some routing
protocols support more than one metric: in the vast majority of the
cases, one metric is used per (sub)topology. Less often, a second
metric may be used as a tie-breaker in the presence of Equal Cost
Multiple Paths (ECMP). The optimization of multiple metrics is known
as an NP complete problem and is sometimes supported by some
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centralized path computation engine.
In contrast, LLNs do require the support of both static and dynamic
metrics. Furthermore, both link and node metrics are required. In
the case of RPL, it is virtually impossible to define one metric, or
even a composite, that will satisfy all use cases.
In addition, RPL supports constrained-based routing where constraints
may be applied to link and nodes. If a link or a node does not
satisfy a required constraint, it is `pruned' from the candidate list
thus leading to a constrained shortest path.
The set of supported link/node constraints and metrics is specified
in [I-D.ietf-roll-routing-metrics].
The role of the Objective Function is to advertise routing metrics
and constraints in addition to the objectives used to compute the
(constrained) shortest path.
Example 1: Shortest path: path offering the shortest end-to-end delay
Example 2: Constrained shortest path: the path that does traverse any
battery-operated node and that optimizes the path
reliability
5. RPL Protocol Specification
5.1. RPL Messages
5.1.1. ICMPv6 RPL Control Message
This document defines the RPL Control Message, a new ICMPv6 message.
The RPL Control Message has the following general format, in
accordance with [RFC4443]:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Message Body +
| |
Figure 1: RPL Control Message
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The RPL Control message is an ICMPv6 information message with a
requested Type of 155.
The Code will be used to identify RPL Control Messages as follows:
o 0x01: DAG Information Solicitation (Section 5.1.2)
o 0x02: DAG Information Object (Section 5.1.3)
o 0x04: Destination Advertisement Object (Section 5.1.4)
5.1.2. DAG Information Solicitation (DIS)
The DAG Information Solicitation (DIS) message may be used to solicit
a DAG Information Object from a RPL node. Its use is analogous to
that of a Router Solicitation; a node may use DIS to probe its
neighborhood for nearby DAGs. The DAG Information Solicitation
carries no additional message body.
5.1.3. DAG Information Object (DIO)
The DAG Information Object carries a number of metrics and other
information that allows a node to discover a DAG, select its DAG
parents, and identify its siblings while employing loop avoidance
strategies.
5.1.3.1. DIO Base Option
The DIO Base Option is a container option, which is always present,
and might contain a number of suboptions. The base option regroups
the minimum information set that is mandatory in all cases.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|G|D|A|0|0| Prf | Sequence | InstanceID | DAGRank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| DAGID |
+ +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: DIO Base Option
Control Field: The DAG Control Field is currently allocated as
follows:
Grounded (G): The Grounded (G) flag is set when the DAG root
is offering connectivity to an external routed
infrastructure such as the Internet.
Destination Advertisement Trigger (D): The Destination
Advertisement Trigger (D) flag is set when the DAG root
or another node in the successor chain decides to trigger
the sending of destination advertisements in order to
update routing state for the outward direction along the
DAG, as further detailed in Section 5.10. Note that the
use and semantics of this flag are still under
investigation.
Destination Advertisement Supported (A): The Destination
Supported (A) bit is set when the DAG root is capable to
support the collection of destination advertisement
related routing state and enables the operation of the
destination advertisement mechanism within the DAG.
DAGPreference (Prf): 3-bit unsigned integer set by the DAG
root to its preference and unchanged at propagation.
DAGPreference ranges from 0x00 (least preferred) to 0x07
(most preferred). The default is 0 (least preferred).
The DAG preference provides an administrative mechanism
to engineer the self-organization of the LLN, for example
indicating the most preferred LBR. If a node has the
option to join a more preferred DAG while still meeting
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other optimization objectives, then the node will
generally seek to join the more preferred DAG as
determined by the OF.
Unassigned bits of the Control Field are considered as
reserved. They MUST be set to zero on transmission and MUST be
ignored on receipt.
Sequence Number: 8-bit unsigned integer set by the DAG root,
incremented according to a policy provisioned at the DAG root,
and propagated with no change outwards along the DAG. Each
increment SHOULD have a value of 1 and may cause a wrap back to
zero.
InstanceID: 8-bit field indicating the topology instance associated
with the DAG, as provisioned at the DAG root.
DAGRank: 8-bit unsigned integer indicating the DAG rank of the node
sending the DIO message. The DAGRank of the DAG root is
ROOT_RANK. DAGRank is further described in Section 5.4.
DAGID: 128-bit unsigned integer which uniquely identify a DAG. This
value is set by the DAG root. The global IPv6 address of the
DAG root can be used, however. the DAGID MUST be unique per DAG
within the scope of the LLN. In the case where a DAG root is
rooting multiple DAGs the DAGID MUST be unique for each DAG
rooted at a specific DAG root.
The following values MUST NOT change during the propagation of DIO
messages outwards along the DAG:
Grounded (G)
Destination Advertisement Supported (A)
DAGPreference (Prf)
Sequence
InstanceID
DAGID
All other fields of the DIO message may be updated at each hop of the
propagation.
5.1.3.1.1. DIO Suboptions
In addition to the minimum options presented in the base option,
several suboptions are defined for the DIO message:
5.1.3.1.1.1. Format
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0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Subopt. Type | Subopt Length | Subopt Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
Figure 3: DIO Suboption Generic Format
Suboption Type: 8-bit identifier of the type of suboption. When
processing a DIO message containing a suboption for which the
Suboption Type value is not recognized by the receiver, the
receiver MUST silently ignore the unrecognized option, continue
to process the following suboption, correctly handling any
remaining options in the message.
Suboption Length: 16-bit unsigned integer, representing the length
in octets of the suboption, not including the suboption Type
and Length fields.
Suboption Data: A variable length field that contains data specific
to the option.
The following subsections specify the DIO message suboptions which
are currently defined for use in the DAG Information Object.
Implementations MUST silently ignore any DIO message suboptions
options that they do not understand.
DIO message suboptions may have alignment requirements. Following
the convention in IPv6, these options are aligned in a packet such
that multi-octet values within the Option Data field of each option
fall on natural boundaries (i.e., fields of width n octets are placed
at an integer multiple of n octets from the start of the header, for
n = 1, 2, 4, or 8).
5.1.3.1.1.2. Pad1
The Pad1 suboption does not have any alignment requirements. Its
format is as follows:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type = 0 |
+-+-+-+-+-+-+-+-+
Figure 4: Pad 1
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NOTE! the format of the Pad1 option is a special case - it has
neither Option Length nor Option Data fields.
The Pad1 option is used to insert one or two octets of padding in the
DIO message to enable suboptions alignment. If more than two octets
of padding is required, the PadN option, described next, should be
used rather than multiple Pad1 options.
5.1.3.1.1.3. PadN
The PadN option does not have any alignment requirements. Its format
is as follows:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Type = 1 | Subopt Length | Subopt Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
Figure 5: Pad N
The PadN option is used to insert three or more octets of padding in
the DIO message to enable suboptions alignment. For N (N > 2) octets
of padding, the Option Length field contains the value N-3, and the
Option Data consists of N-3 zero-valued octets. PadN Option data
MUST be ignored by the receiver.
5.1.3.1.1.4. DAG Metric Container
The DAG Metric Container suboption may be aligned as necessary to
support its contents. Its format is as follows:
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Type = 2 | Container Length | DAG Metric Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
Figure 6: DAG Metric Container
The DAG Metric Container is used to report aggregated path metrics
along the DAG. The DAG Metric Container may contain a number of
discrete node, link, and aggregate path metrics as chosen by the
implementer. The Container Length field contains the length in
octets of the DAG Metric Data. The order, content, and coding of the
DAG Metric Container data is as specified in
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[I-D.ietf-roll-routing-metrics].
The processing and propagation of the DAG Metric Container is
governed by implementation specific policy functions.
5.1.3.1.1.5. Destination Prefix
The Destination Prefix suboption does not have any alignment
requirements. Its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 | Length |Resvd|Prf|Resvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | |
+-+-+-+-+-+-+-+-+ |
| Destination Prefix (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: DAG Destination Prefix
The Destination Prefix suboption is used when the DAG root, or
another node located inwards along the DAG on the path to the DAG
root, needs to indicate that it offers connectivity to destination
prefixes other than the default. This may be useful in cases where
more than one LBR is operating within the LLN and offering
connectivity to different administrative domains, e.g. a home network
and a utility network. In such cases, upon observing the Destination
Prefixes offered by a particular DAG, a node MAY decide to join
multiple DAGs in support of a particular application.
The Length is coded as the length of the suboption in octets,
excluding the Type and Length fields.
Prf is the Route Preference as in [RFC4191]. The reserved fields
MUST be set to zero on transmission and MUST be ignored on receipt.
The Prefix Lifetime is a 32-bit unsigned integer representing the
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.
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The Prefix Length is an 16-bit unsigned integer that indicates the
number of leading bits in the destination prefix.
The Destination Prefix contains Prefix Length significant bits of the
destination prefix. The remaining bits of the Destination Prefix, as
required to complete the trailing octet, are set to 0.
In the event that a DIO message may need to specify connectivity to
more than one destination, the Destination Prefix suboption may be
repeated.
5.1.3.1.1.6. DAG Timer Configuration
The DAG Timer Configuration suboption does not have any alignment
requirements. Its format is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 4 | Length | DIOIntDoubl. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DIOIntMin. |
+-+-+-+-+-+-+-+-+
Figure 8: DAG Timer Configuration
The DAG Timer Configuration suboption is used to distribute
configuration information for DAG Timer Operation through the DAG.
The information communicated in this suboption is generally static
and unchanging within the DAG, therefore it is not necessary to
include in every DIO. This suboption MAY be included periodically by
the DAG Root, and SHOULD be included in response to a unicast
request, e.g. a DAG Information Solicitation (DIS) message.
The Length is coded as 2.
DIOIntervalDoublings is an 8-bit unsigned integer. Configured on the
DAG root and used to configure the trickle timer governing when DIO
message should be sent within the DAG. DIOIntervalDoublings is the
number of times that the DIOIntervalMin is allowed to be doubled
during the trickle timer operation.
DIOIntervalMin is an 8-bit unsigned integer. Configured on the DAG
root and used to configure the trickle timer governing when DIO
message should be sent within the DAG. The minimum configured
interval for the DIO trickle timer in units of ms is
2^DIOIntervalMin. For example, a DIOIntervalMin value of 16ms is
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expressed as 4.
5.1.4. Destination Advertisement Object (DAO)
The Destination Advertisement Object (DAO) is used to propagate
destination information inwards along the DAG. The RPL use of the
DAO allows the nodes in the DAG to build up routing state for nodes
contained in the sub-DAG in support of traffic flowing outward along
the DAG.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Sequence | InstanceID | DAO Rank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | RRCount | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Prefix (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse Route Stack (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: The Destination Advertisement Object (DAO)
DAO Sequence: Incremented by the node that owns the prefix for each
new DAO message for that prefix.
InstanceID: 8-bit field indicating the topology instance associated
with the DAG, as learned from the DIO.
DAO Rank: Set by the node that owns the prefix and first issues the
DAO message to its rank.
DAO Lifetime: 32-bit unsigned integer. The length of time in
seconds (relative to the time the packet is sent) that the
prefix is valid for route determination. A value of all one
bits (0xFFFFFFFF) represents infinity. A value of all zero
bits (0x00000000) indicates a loss of reachability.
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Route Tag: 32-bit unsigned integer. The Route Tag may be used to
give a priority to prefixes that should be stored. This may be
useful in cases where intermediate nodes are capable of storing
a limited amount of routing state. The further specification
of this field and its use is under investigation.
Prefix Length: Number of valid leading bits in the IPv6 Prefix.
RRCount: 8-bit unsigned integer. This counter is used to count the
number of entries in the Reverse Route Stack. A value of `0'
indicates that no Reverse Route Stack is present.
Prefix: Variable-length field containing an IPv6 address or a prefix
of an IPv6 address. The Prefix Length field contains the
number of valid leading bits in the prefix. The bits in the
prefix after the prefix length (if any) are reserved and MUST
be set to zero on transmission and MUST be ignored on receipt.
Reverse Route Stack: Variable-length field containing a sequence of
RRCount (possibly compressed) IPv6 addresses. A node that adds
on to the Reverse Route Stack will append to the list and
increment the RRCount.
5.2. Conceptual Data Structures
The RPL implementation MUST maintain the following conceptual data
structures in support of DAG discovery:
o A set of candidate neighbors
o For each DAG:
* A set of DAG parents and siblings
5.2.1. Candidate Neighbors Data Structure
The set of candidate neighbors is to be populated by neighbors that
are discovered by the neighbor discovery mechanism and further
qualified as statistically stable as per the mechanisms discussed in
[I-D.ietf-roll-routing-metrics]. 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.
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If Neighbor Unreachability Detection (NUD) determines that a
candidate neighbor is no longer reachable, then it shall be removed
from the candidate neighbor set. In the case that the candidate
neighbor has associated states in the DAG parent set or active DA
entries, then the removal of the candidate neighbor shall be
coordinated with tearing down these states. All provisioned routes
associated with the candidate neighbor should be removed.
5.2.2. Directed Acyclic Graphs (DAGs) Data Structure
At a given point of time, a DAG Iteration is uniquely identified by
the tuple (DagID, InstanceID, DAGSequenceNumber) where a change in
the sequence denotes the iteration of a given DAG over time. When a
single device is capable to root multiple DAGs in support of an
application need for multiple optimization objectives it MUST produce
a different and unique (DagID, InstanceID) pair for each of the
multiple DAGs.
For each DAG that a node is, or may become, a member of, the
implementation MUST keep a DAG table with the following entries:
o InstanceID
o DAGID
o DAGSequenceNumber
o DAG Metric Container, including DAGObjectiveCodePoint
o A set of Destination Prefixes offered inwards along the DAG
o A set of DAG parents and siblings
o A timer to govern the sending of DIO messages for the DAG
When a DAG is discovered for which no DAG data structure is
instantiated, and the node wants to join, then the DAG data structure
is instantiated.
When the DAG parent set is depleted (i.e. the last DAG is removed),
then the DAG data structure SHOULD be suppressed after the expiration
of an implementation-specific local timer. An implementation SHOULD
delay before deallocating the DAG data structure in order to observe
that the DAGSequenceNumber has incremented should any new DAG parents
appear for the DAG.
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5.2.2.1. DAG Parents/Siblings Structure
When the DAG is self-rooted, the set of DAG parents/siblings is
empty.
In all other cases, for each node in the set, the implementation MUST
keep a record of:
o a reference to the neighboring device which is the DAG parent or
sibling
o a record of most recent information taken from the DAG Information
Object last processed from the DAG parent
DAG parents may be ordered, according to the OF. When ordering DAG
parents, in consultation with the OF, the most preferred DAG parent
may be identified. All current DAG parents must have a rank less
than self. All current DAG siblings must have a rank equal to self.
When nodes are added to or removed from the DAG set the most
preferred DAG parent may have changed. The role of all the nodes in
the list should be reevaluated. In particular, any nodes having a
rank greater than self after such a change must be evicted from the
set.
An implementation may choose to keep these records as an extension of
the Default Router List (DRL).
5.3. DAG Rank
Based on the selection of DAG Parents, the metrics conveyed by the
most preferred DAG parent, the nodes own metrics and configuration,
and a related function defined by the OF, a node will be able to
compute a value for its rank as a consequence of selecting a most
preferred DAG parent.
The rank value feeds back into the DAG parent selection according to
a loop-avoidance strategy. Once a DAG parent has been added, and a
rank value for the node within the DAG has been computed, the nodes
further options with regard to DAG parent selection and movement
within the DAG are restricted in favor of loop avoidance.
It is important to note that the DAG Rank is not itself a metric,
although its value is derived from and influenced by the use of
metrics to select DAG parents and take up a position in the DAG. The
only aim of the rank is to inform loop avoidance and detection.
The computation of the DAG Rank MUST be done in such a way so as to
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maintain the following properties for any nodes M and N that are
neighbors in the LLN:
DAGRank(M) is less than DAGRank(N): In this case, M is probably
located in a more preferred position than N in the DAG with
respect to the metrics and optimizations defined by the
objective code point. In any fashion, Node M may safely be a
DAG parent for Node N without risk of creating a loop.
Further, for a node N, all parents in the DAG parent set must
be of rank less than self's DAGRank(N). In other words, the
rank presented by a node N MUST be greater (deeper) than that
presented by any of its parents.
DAGRank(M) equals DAGRank(N): In this case M and N are located
positions of relatively the same optimality within the DAG.
In some cases, Node M may be used as a successor by Node N,
but with related chance of creating a loop that must be
detected and broken by some other means.
DAGRank(M) is greater than DAGRank(N): In this case, then node M is
located in a less preferred position than N in the DAG with
respect to the metrics and optimizations defined by the
objective code point. Further, Node (M) may in fact be in
Node (N)'s sub-DAG. There is a higher risk to Node (N)
selecting Node (M) as a DAG parent, as such a selection may
create a loop.
As an example, the DAG Rank could be computed in such a way so as to
closely track ETX when the objective function is to minimize ETX, or
latency when the objective function is to minimize latency, or in a
more complicated way as appropriate to the objective code point being
used within the DAG.
5.4. DAG Discovery and Maintenance
DAG discovery locates the nearest sink (aka root), as determined
according to some metrics and constraints, and forms a Directed
Acyclic Graph towards that sink, by identifying a set of DAG parents.
During this process DAG discovery also identifies siblings, which may
be used later to provide additional path diversity towards the DAG
root. DAG discovery enables nodes to implement different policies
for selecting their DAG parents in the DAG by using implementation
specific policy functions. DAG discovery specifies a set of rules to
be followed by all implementations in order to ensure interoperation.
DAG discovery also standardizes the format that is used to advertise
the most common information that is used in order to select DAG
parents.
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One of these information, the DAG rank, is used by DAG discovery to
provide loop avoidance even if nodes implement different policies.
The DAG Rank is computed as specified by the OF in use by the DAG,
demonstrating the properties described in Section 5.3. The rank
should be computed in such a way so as to provide a comparable basis
with other nodes which may not use the same metric at all.
The DAG discovery procedures take into account a number of factors,
including:
o RPL rules for loop avoidance based on DAGs and ranks
o The Objective Function
o The advertised metrics
o Local policy functions (e.g. a bounded number of candidate
neighbors).
5.4.1. DAG Discovery Rules
In order to organize and maintain loopless structure, the DAG
discovery implementation in the nodes MUST obey to the following
rules and definitions:
5.4.1.1. DAGs
1. DAG discovery instantiates LLN topologies that are each optimized
for specific constraints and goals. A topology assumes the shape
of a DAG, and a DAG Instance is uniquely identified by its
instanceID.
2. For reasons of scalability and operations of the protocol, a DAG
Instance is partitioned into a set of DAGs rooted at a
destination, aka Destination Oriented DAGs. A destination is
uniquely identified by a DAGID so a DAG rooted at a destination
is uniquely identified by the pair (InstanceID, DAGID).
3. A Destination Oriented DAG is periodically reconstructed from the
root, by incrementing a DAGSequenceNumber. An Iteration of a
Destination Oriented DAG is thus uniquely identified by the tuple
(InstanceID, DAGID, DAGSequenceNumber). Through this document,
the graph formed by this iterative process is referred to as the
DAG Iteration, or in short, the DAG.
4. The rank is defined within the scope of a DAG Iteration as an
abstract coordinate to compare the relative position of nodes and
ensure forward progress of the traffic.
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5. A node MUST belong at most to one DAG Iteration per InstanceID
and MUST select all its parents and siblings within that same DAG
Iteration.
5.4.1.2. DAG Sequence Number
1. The DAGSequenceNumber is incremented by the root and flooded
through DIOs.
2. The root floods a new DAGSequenceNumber periodically, at a rate
that depends on the deployment. This rate can be set to 0 if
other methods such as loop detection are considered sufficient to
solve the routing issues in that deployment.
3. The root MAY also flood a new DAGSequenceNumber on-demand. The
details of the mechanism to signal the root to do so are to be
specified in a future revision of this document.
4. A parent that advertises the new DAGSequenceNumber can not
possibly belong to the sub-DAG of a node that still advertises an
older DAGSequenceNumber. The node MAY thus attach to that parent
regardless of the relative rank, and this situation is equivalent
to jumping onto a different Destination Oriented DAG.
5. Thus, as a new DAGSequenceNumber spreads, a new DAG Iteration
forms that supersedes the previous one. During a
DAGSequenceNumber transition, a node MAY decide to forward
packets via 'future parents' that belong to the same Destination
Oriented DAG (same InstanceID and DagID), but a more recent
(incremented) DAGSequenceNumber.
5.4.1.3. DAG Root
1. A node that does not have any DAG parent MAY become the root of
its own floating DAG. It's rank is ROOT_RANK.
2. A (non-LLN) router is considered connected to a grounded
infrastructure at rank BASE_RANK. A LLN node that is attached to
such an infrastructure router is the DAG root of its own grounded
DAG. It's rank is ROOT_RANK.
3. In a deployment that uses a backbone link to federate a number of
LLN roots, it is possible to run RPL over the backbone and use
one router as a backbone root. The backbone root exposes a rank
of BASE_RANK over the backbone. All the LLN roots that are
parented to that backbone root, including the backbone root if it
also serves as LLN root, expose a rank of ROOT_RANK over the LLN
and act as multiple roots for a same DAG, coordinated by the
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backbone root.
4. The DAG root exposes the DAG in the DIO message and LLN nodes
propagate the DIO message outwards along the DAG.
5.4.1.4. Moving Inside a DAG
1. A node moves when it changes its parent selection within the same
DAG Iteration. When a node moves (within its DAG) in a fashion
that cause its rank to decrease, the node MUST abandon all
parents and siblings with a rank larger than self, and MAY adopt
as siblings nodes with the same rank.
2. A node MAY move at any time, with no delay, within its DAG when
the move does not cause the node to increase its own DAG rank, as
per the rank calculation indicated by the OF.
3. A node MUST NOT move outwards along a DAG that it is attached to,
causing the DAG rank to increase. If a node cannot stay within
the DAG without a rank increase, then it MUST poison its routes
as described in Section 5.4.1.6.
4. When DIO messages are received from other routers located at
lesser rank in the same DAG, those routers are eligible for
consideration as DAG parents. DIO messages received from other
routers located at the same rank in the same DAG may be
considered as coming from siblings. DIO messages that are
received from other routers located at greater rank within the
same DAG might cause greedy behaviors and loops; such a DIO is
ignored unless:
1. The DIO comes from an existing parent or sibling; in which
case that parent must be removed.
2. The DIO comes from a node that has better OF ratings than any
parent known at this point; in that case, this potential
parent MAY be remembered in order to jump at a better
position when the next sequence is flooded.
5.4.1.5. Jumping Onto Another DAG
1. A node jumps when it performs a new parent selection whereby its
DAG Iteration changes within the same DAG Instance. When a node
jumps onto a new DAG Iteration, it MUST abandon all parents and
siblings from its previous position.
2. A node MAY jump from its current DAG onto any other DAG that
provides service for the same InstanceID if it is preferred by
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the OF, for example for reasons such as connectivity, configured
preference, free medium time, size, security, bandwidth, DAG
rank, or whatever metrics the LLN uses. This is allowed
regardless of the rank that the node reaches in the new DAG.
3. A node that jumps should attempt to transmit all the packets
received as part of the previous DAG along the previous DAG. In
other words, it should switch the parent set only after the
outstanding packet queue of packets received prior to announcing
the jump is exhausted.
4. Jumping back onto a previous DAG is equivalent to moving inside
that DAG and obeys the same rules. To satisfy this, a node
detaching from a DAG SHOULD remember its DAG as identified by the
tuple (InstanceID, DagID, DAGSequenceNumber) as well as its rank
within that DAG for long as that DAG exists.
5.4.1.6. Poisoning a Broken Path
1. A node SHOULD poison its inwards routes when it looses all of its
current feasible parents, i.e. the set of DAG parents becomes
depleted, and it can not jump onto an alternate DAG.
2. In order to poison its inwards routes, a node MAY stay at its
position within its DAG (that is maintain its InstanceID, DagID,
DAGSequenceNumber and Rank) but it SHOULD immediately advertise a
rank of INFINITE_RANK in a DIO so as to force all its children to
remove it from their parent list and try an alternate path. The
node SHOULD then wait for a new DAG Iteration (DAGSequenceNumber
increment) before resuming its operation in the same Destination
Oriented DAG.
3. Alternatively, a node MAY detach from its DAG. A node that
detaches becomes root of its own floating DAG and MUST
immediately advertise its new situation in a DIO.
4. Either way, the route poisoning will recursively be flooded
throughout the impacted sub-DAG as children lose their last
parent in the original DAG.
5. The loss of a DIO message may interrupt the flooding. This can
be compensated by cheer repetition through the trickle algorithm.
If that also fails, packet loops will be prevented by the
detection mechanism described in Section 5.11.
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5.4.1.7. Following a Parent
1. If a node that receives a DIO from one of its DAG parents
indicating that the parent has left the DAG, it may either follow
that parent or stay in its current DAG through an alternate DAG
parent if that is possible.
2. If a DAG parent increases its rank such that the node rank would
have to change, and if the node does not wish to follow (e.g. it
has alternate options), then the DAG parent SHOULD be evicted
from the DAG parent set. If the DAG parent is the last in the
DAG parent set, then the node SHOULD chose to follow it.
5.4.1.8. DAG Inconsistency
1. When a node detects or causes a DAG inconsistency, as described
in Section 5.4.4.2, then the node SHOULD send an unsolicited DIO
message to its one-hop neighbors. The DIO is updated to
propagate the new DAG information. Such an event MUST also cause
the trickle timer governing the periodic sending of DIO messages
to be reset.
5.4.2. Reception and Processing of DIO messages
When an DIO message is received from a source device named SRC, the
receiving node must first determine whether or not the DIO message
should be accepted for further processing, and subsequently present
the DIO message for further processing if eligible.
1. If the DIO message is malformed, then the DIO message is not
eligible for further processing and is silently discarded. A RPL
implementation MAY log the reception of a malformed DIO message.
2. If SRC is not a member of the candidate neighbor set, then the
DIO is not eligible for further processing. (Further evaluation/
confidence of this neighbor is necessary)
3. If the DIO message advertises a DAG that the node is already a
member of, then:
* If the rank of SRC as reported in the DIO message is lesser
than that of the node within the DAG, then the DIO message
MUST be considered for further processing.
* If the rank of SRC as reported in the DIO message is equal to
that of the node within the DAG, then SRC is marked as a
sibling and the DIO message is not eligible for further
processing.
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* If the rank of SRC as reported in the DIO message is higher
than that of the node within the DAG, and SRC is not a DAG
parent, then the DIO message MUST NOT be considered for
further processing
4. Even if not processed further, information from a DIO might be
remembered for instance if SRC is preferable to the current
parents per the OF selection process.
5. If SRC is a DAG parent for any other DAG that the node is
attached to, then the DIO message MUST be considered for further
processing (the DAG parent may have jumped).
6. If the DIO message advertises a DAG that offers a better (new or
alternate) solution to an optimization objective desired by the
node, then the DIO message MUST be considered for further
processing.
5.4.2.1. Overview of DIO Message Processing
If the received DIO message is for a new/alternate DAG:
If the node has sent an DIO message within the risk window as
described in Section 5.8 then a collision has occurred; do not
process the DIO message any further.
If the SRC node is also a DAG parent for another DAG that the
node is a member of, and if the new/alternate DAG is the same
InstanceID as the other DAG, then the DAG parent is known to
have jumped.
Remove SRC as a DAG parent from the other DAG
If the other DAG is now empty of candidate parents, then
prepare to directly follow SRC into the new DAG by adding it
as a DAG parent for the new DAG, else ignore the DIO message
(do not follow the parent).
Instantiate a data structure for the new/alternate DAG if
necessary
If the new/alternate DAG offers a better solution to the
optimization objectives, then jump: copy the DIO information
place the neighbor into the DAG parent set.
If the DIO message is for a known/existing DAG:
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Process the DIO message as per the rules in Section 5.4
As DIO messages are received from candidate neighbors, the neighbors
may be promoted to DAG parents by following the rules of DAG
discovery as described in Section 5.4. When a node places a neighbor
into the DAG Parent set, the node becomes attached to the DAG through
the new parent node.
In the DAG discovery implementation, the most preferred parent should
be used to restrict which other nodes may become DAG parents. Some
nodes in the DAG parent set may be of a rank less than or equal to
the most preferred DAG parent. (This case may occur, for example, if
an energy constrained device is at a lesser rank but should be
avoided as per an optimization objective, resulting in a more
preferred parent at a greater rank).
5.4.3. DIO Transmission
Each node maintains a timer that governs when to multicast DIO
messages. This timer is implemented as a trickle timer operating
over a variable interval. Trickle timers are further detailed in
Section 5.4.4. The governing parameters for the timer should be
configured consistently across the DAG, and are provided by the DAG
root in the DIO message. In addition to periodic DIO messages, each
node may respond to a DIS message with a DIO message.
o When a node detects an inconsistency, it SHOULD reset the interval
of the trickle timer to a minimum value, causing DIO messages to
be emitted more frequently as part of a strategy to quickly
correct the inconsistency. Such inconsistencies may be, for
example, an update to a key parameter (e.g. sequence number) in
the DIO message or a loop detected when a node located inwards
along the DAG forwards traffic outwards. Inconsistencies are
further detailed in Section 5.4.4.2.
o When a node enters a mode of consistent operation within a DAG,
i.e. DIO messages from its DAG parents are consistent and no
other inconsistencies are detected, it may begin to open up the
interval of the trickle timer towards a maximum value, causing DIO
messages to be emitted less frequently, thus reducing network
maintenance overhead and saving energy consumption.
o When a node is initialized, it MAY be configured to remain silent
and not multicast any DIO messages until it has encountered and
joined a DAG (perhaps initially probing for a nearby DAG with an
DIS message). Alternately, it may choose to root its own floating
DAG and begin multicasting DIO messages using a default trickle
configuration. The second case may be advantageous if it is
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desired for independent nodes to begin aggregating into scattered
floating DAGs in the absence of a grounded node, for example in
support of LLN installation and commissioning.
Note that if multiple DAG roots are participating in the same DAG,
i.e. offering DIO messages with the same DAGID, then they must
coordinate with each other to ensure that their DIO messages are
consistent when they emit DIO messages. In particular the Sequence
number must be identical from each DAG root, regardless of which of
the multiple DAG roots issues the DIO message, and changes to the
Sequence number should be issued at the same time. The specific
mechanism of this coordination, e.g. along a non-LLN network between
DAG roots, is beyond the scope of this specification.
5.4.4. Trickle Timer for DIO Transmission
RPL treats the construction of a DAG as a consistency problem, and
uses a trickle timer [Levis08] to control the rate of control
broadcasts.
For each DAG that a node is part of, the node must maintain a single
trickle timer. The required state contains the following conceptual
items:
I: The current length of the communication interval
T: A timer with a duration set to a random value in the range
[I/2, I]
C: Redundancy Counter
I_min: The smallest communication interval in milliseconds. This
value is learned from the DIO message as (2^DIOIntervalMin)ms.
The default value is DEFAULT_DIO_INTERVAL_MIN.
I_doublings: The number of times I_min should be doubled before
maintaining a constant rate, i.e. I_max = I_min *
2^I_doublings. This value is learned from the DIO message as
DIOIntervalDoublings. The default value is
DEFAULT_DIO_INTERVAL_DOUBLINGS.
5.4.4.1. Resetting the Trickle Timer
The trickle timer for a DAGID is reset by:
1. Setting I_min and I_doublings to the values learned from the DIO
message.
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2. Setting C to zero.
3. Setting I to I_min.
4. Setting T to a random value as described above.
5. Restarting the trickle timer to expire after a duration T
When node learns about a DAG through a DIO message and makes the
decision to join it, it initializes the state of the trickle timer by
resetting the trickle timer and listening. Each time it hears a
consistent DIO message for this DAG from a DAG parent, it MAY
increment C.
When the timer fires at time T, the node compares C to the redundancy
constant, DEFAULT_DIO_REDUNDANCY_CONSTANT. If C is less than that
value, the node generates a new DIO message and multicasts it. When
the communication interval I expires, the node doubles the interval I
so long as it has previously doubled it fewer than I_doubling times,
resets C, and chooses a new T value.
5.4.4.2. Determination of Inconsistency
The trickle timer is reset whenever an inconsistency is detected
within the DAG, for example:
o The node joins a new DAGID
o The node moves within a DAGID
o The node receives a modified DIO message from a DAG parent
o A DAG parent forwards a packet intended to move inwards,
indicating an inconsistency and possible loop.
o A metric communicated in the DIO message is determined to be
inconsistent, as according to a implementation specific path
metric selection engine.
o The rank of a DAG parent has changed.
5.5. DAG Sequence Number Increment
The DAG root makes the sole determination of when to revise the
DAGSequenceNumber by incrementing it upwards. When the
DAGSequenceNumber is increased an inconsistency results, causing DIO
messages to be sent back outwards along the DAG to convey the change.
The degree to which this mechanism is relied on may be determined by
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the implementation- on one hand it may serve as a periodic heartbeat,
refreshing the DAG states, and on the other hand it may result in a
constant steady-state control cost overhead which is not desirable.
Some implementations may provide an administrative interface, such as
a command line, at the DAG root whereby the DAGSequenceNumber may be
caused to increment in response to some policy outside of the scope
of RPL.
Other implementations may make use of a periodic timer to
automatically increment the DAGSequenceNumber, resulting in a
periodic DAG iteration at a rate appropriate to the application and
implementation. Other automated mechanisms to determine
DAGSequenceNumber increments are also possible as appropriate to a
deployment.
5.6. DAG Selection
The DAG selection is implementation and algorithm dependent. Nodes
SHOULD prefer to join DAGs for InstanceIDs advertising OCPs and
destinations compatible with their implementation specific
objectives. In order to limit erratic movements, and all metrics
being equal, nodes SHOULD keep their previous selection. Also, nodes
SHOULD provide a means to filter out a candidate parent whose
availability is detected as fluctuating, at least when more stable
choices are available.
When connection to a fixed network is not possible or preferable for
security or other reasons, scattered DAGs MAY aggregate as much as
possible into larger DAGs in order to allow connectivity within the
LLN.
A node SHOULD verify that bidirectional connectivity and adequate
link quality is available with a candidate neighbor before it
considers that candidate as a DAG parent.
5.7. Administrative rank
When the DAG is formed under a common administration, or when a node
performs a certain role within a community, it might be beneficial to
associate a range of acceptable rank with that node. For instance, a
node that has limited battery should be a leaf unless there is no
other choice, and may then augment the rank computation specified by
the OF in order to expose an exaggerated rank.
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5.8. Collision
A race condition occurs if 2 nodes send DIO messages at the same time
and then attempt to join each other. This might happen, for example,
between nodes which act as DAG root of their own DAGs. In order to
detect the situation, LLN Nodes time stamp the sending of DIO
message. Any DIO message received within a short link-layer-
dependent period introduces a risk. It is up to the implementation
to define the duration of the risk window.
There is risk of a collision when a node receives and processes a DIO
within the risk window. For example, it may occur that two nodes are
associated with different DAGs and near-simultaneously send DIO
messages, which are received and processed by both, and possibly
result in both nodes simultaneously deciding to attach to each other.
As a remedy, in the face of a potential collision, as determined by
receiving a DIO within the risk window, the DIO message is not
processed. It is expected that subsequent DIOs would not cross.
5.9. Guidelines for Objective Functions
5.9.1. Objective Function
An Objective Function (OF) allows for the selection of a DAG to join,
and a number of peers in that DAG as parents. The OF is used to
compute an ordered list of parents. The OF is also responsible to
compute the rank of the device within the DAG.
The Objective Function is specified in the DIO message within a DAG
Metric Container using an Objective Code Point (OCP), as specified in
[I-D.ietf-roll-routing-metrics], and indicates the method that must
be used to compute the DAG (e.g. "minimize the path cost using the
ETX metric and avoid `Blue' links"). The Objective Code Points are
specified in [I-D.ietf-roll-routing-metrics]. This document
specifies an Objective Function, OF0, in support of default
operation. In the case where the DIO does not include an OCP
specification in the DAG Metric Container, OF0 MAY be presumed.
Most Objective Functions are expected to follow the same abstract
behavior:
o The parent selection is triggered each time an event indicates
that a potential next hop information is updated. This might
happen upon the reception of a DIO message, a timer elapse, or a
trigger indicating that the state of a candidate neighbor has
changed.
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o An OF scans all the interfaces on the device. Although there may
typically be only one interface in most application scenarios,
there might be multiple of them and an interface might be
configured to be usable or not for RPL operation. An interface
can also be configured with a preference or dynamically learned to
be better than another by some heuristics that might be link-layer
dependent and are out of scope. Finally an interface might or not
match a required criterion for an Objective Function, for instance
a degree of security. As a result some interfaces might be
completely excluded from the computation, while others might be
more or less preferred.
o An OF scans all the candidate neighbors on the possible interfaces
to check whether they can act as a router for a DAG. There might
be multiple of them and a candidate neighbor might need to pass
some validation tests before it can be used. In particular, some
link layers require experience on the activity with a router to
enable the router as a next hop.
o An OF computes self's rank by adding the step of rank to that
candidate to the rank of that candidate. The step of rank is
computed by estimating the link as follows:
* The step of rank might vary from 1 to 16.
+ 1 indicates a unusually good link, for instance a link
between powered devices in a mostly battery operated
environment.
+ 4 indicates a `normal'/typical link, as qualified by the
implementation.
+ 16 indicates a link that can hardly be used to forward any
packet, for instance a radio link with quality indicator or
expected transmission count that is close to the acceptable
threshold.
* Candidate neighbors that would cause self's rank to increase
are ignored
o Candidate neighbors that advertise an OF incompatible with the set
of OF specified by the policy functions are ignored.
o As it scans all the candidate neighbors, the OF keeps the current
best parent and compares its capabilities with the current
candidate neighbor. The OF defines a number of tests that are
critical to reach the objective. A test between the routers
determines an order relation.
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* If the routers are roughly equal for that relation then the
next test is attempted between the routers,
* Else the best of the 2 becomes the current best parent and the
scan continues with the next candidate neighbor
* Some OFs may include a test to compare the ranks that would
result if the node joined either router
o When the scan is complete, the preferred parent is elected and
self's rank is computed as the preferred parent rank plus the step
in rank with that parent.
o Other rounds of scans might be necessary to elect alternate
parents and siblings. In the next rounds:
* Candidate neighbors that are not in the same DAG are ignored
* Candidate neighbors that are of greater rank than self are
ignored
* Candidate neighbors of an equal rank to self (siblings) are
ignored
* Candidate neighbors of a lesser rank than self (non-siblings)
are preferred
5.9.2. Objective Function 0 (OF0)
This document specifies a default objective function, called OF0,
indicated by an OCP value of 0x0000. OF0 is the default objective
function of RPL, and can be used if allowed by the policy of the
processing node when the OF indicated in the DIO message is unknown
to the node. If not allowed, then the DIO message is simply ignored
and not processed by the node. OF0 is notable in that it does not
use physical metrics as described in [I-D.ietf-roll-routing-metrics],
but is only based on abstract information from the DIO message such
as rank and administrative preference.
OF0 favors connectivity. That is, the Objective Function is designed
to find the nearest sink into a 'grounded' topology, and if there is
none then join any network per order of administrative preference.
The metric in use is the rank.
OF0 selects a preferred parent and a backup next hop if one is
available. The backup next hop might be a parent or a sibling. All
the traffic is routed via the preferred parent. When the link
conditions do not let a packet through to the preferred parent, the
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packet is passed to the backup next hop.
The step of rank is 4 for each hop.
5.9.2.1. Selection of the Preferred Parent
As it scans all the candidate neighbors, OF0 keeps the parent that is
the best for the following criteria (in order):
1. The interface must be usable and any administrative preference
associated with the interface applies first.
2. A candidate that would cause the node to augment the rank in the
current DAG is not considered.
3. A router that has been validated as usable, e.g. with a local
confidence that has exceeded some pre-configured threshold, is
better.
4. If none are grounded then a DAG with a more preferred
administrative preference (DAGPreference) is better.
5. A router that offers connectivity to a grounded DAG is better.
6. A lesser resulting rank is better.
7. A DAG for which there is an alternate parent is better. This
check is optional. It is performed by computing the backup next
hop while assuming that this router won.
8. The DAG that was in use already is preferred.
9. The preferred parent that was in use already is better.
10. A router that has announced a DIO message more recently is
preferred.
5.9.2.2. Selection of the Backup Next Hop
o The interface must be usable and the administrative preference (if
any) applies first.
o The preferred parent is ignored.
o Candidate neighbors that are not in the same DAG are ignored.
o Candidate neighbors with a higher rank are ignored.
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o Candidate neighbors of a better rank than self (non-siblings) are
preferred.
o A router that has been validated as usable, e.g. with a local
confidence that has exceeded some pre-configured threshold, is
better.
o The router with a better router preference wins.
o The backup next hop that was in use already is better.
5.10. Establishing Routing State Outward Along the DAG
The destination advertisement mechanism supports the dissemination of
routing state required to support traffic flows outward along the
DAG, from the DAG root toward nodes.
As a result of destination advertisement operation:
o DAG discovery establishes a DAG oriented toward a DAG root along
which inward routes toward the DAG root are set up.
o Destination advertisement establishes outward routes along the
DAG. Such paths consist of:
* Hop-By-Hop routing state within islands of `stateful' nodes.
* Source Routing `bridges' across nodes that do not retain state.
Destinations disseminated with the destination advertisement
mechanism may be prefixes, individual hosts, or multicast listeners.
The mechanism supports nodes of varying capabilities as follows:
o When nodes are capable of storing routing state, they may inspect
destination advertisements and learn hop-by-hop routing state
toward destinations by populating their routing tables with the
routes learned from nodes in their sub-DAG. In this process they
may also learn necessary piecewise source routes to traverse
regions of the LLN that do not maintain routing state. They may
perform route aggregation on known destinations before emitting
Destination Advertisements.
o When nodes are incapable of storing routing state, they may
forward destination advertisements, recording the reverse route as
the go in order to support the construction of piecewise source
routes.
Nodes that are capable of storing routing state, and finally the DAG
roots, are able to learn which destinations are contained in the sub-
DAG below the node, and via which next-hop neighbors. The
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dissemination and installation of this routing state into nodes
allows for Hop-By-Hop routing from the DAG root outwards along the
DAG. The mechanism is further enhance by supporting the construction
of source routes across stateless `gaps' in the DAG, where nodes are
incapable of storing additional routing state. An adaptation of this
mechanism allows for the implementation of loose-source routing.
A special case, the reception of a destination advertisement
addressed to a link-local multicast address, allows for a node to
learn destinations directly available from its one-hop neighbors.
A design choice behind advertising routes via destination
advertisements is not to synchronize the parent and children
databases along the DAG, but instead to update them regularly to
recover from the loss of packets. The rationale for that choice is
time variations in connectivity across unreliable links. If the
topology can be expected to change frequently, synchronization might
be an excessive goal in terms of exchanges and protocol complexity.
The approach used here results in a simple protocol with no real
peering. The destination advertisement mechanism hence provides for
periodic updates of the routing state, as cued by occasional RAs and
other mechanisms, similarly to other protocols such as RIP [RFC2453].
5.10.1. Destination Advertisement Operation
5.10.1.1. Overview
According to implementation specific policy, a subset or all of the
feasible parents in the DAG may be selected to receive prefix
information from the destination advertisement mechanism. This
subset of DAG parents shall be designated the set of DA parents.
As DAO messages for particular destinations move inwards along the
DAG, a sequence counter is used to guarantee their freshness. The
sequence counter is incremented by the source of the DAO message (the
node that owns the prefix, or learned the prefix via some other
means), each time it issues a DAO message for its prefix. Nodes that
receive the DAO message and, if scope allows, will be forwarding a
DAO message for the unmodified destination inwards along the DAG,
will leave the sequence number unchanged. Intermediate nodes will
check the sequence counter before processing a DAO message, and if
the DAO is unchanged (the sequence counter has not changed), then the
DAO message will be discarded without additional processing.
Further, if the DAO message appears to be out of synch (the sequence
counter is 2 or more behind the present value) then the DAO state is
considered to be stale and may be purged, and the DAO message is
discarded. A depth is also added for tracking purposes; the depth is
incremented at each hop as the DAO message is propagated up the DAG.
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Nodes that are storing routing state may use the depth to determine
which possible next-hops for the destination are more optimal.
If destination advertisements are activated in the DIO message as
indicated by the `D' bit, the node sends unicast destination
advertisements to one of its DA parents, that is selected as most
favored for incoming outwards traffic. The node only accepts unicast
destination advertisements from any nodes but those contained in the
DA parent subset.
Receiving a DIO message with the `D' destination advertisement bit
set from a DAG parent stimulates the sending of a delayed destination
advertisement back, with the collection of all known prefixes (that
is the prefixes learned via destination advertisements for nodes
lower in the DAG, and any connected prefixes). If the Destination
Advertisement Supported (A) bit is set in the DIO message for the
DAG, then a destination advertisement is also sent to a DAG parent
once it has been added to the DA parent set after a movement, or when
the list of advertised prefixes has changed.
A node that modifies its DAG Parent set may set the `D' bit in
subsequent DIO propagation in order to trigger destination
advertisements to be updated to its DAG Parents and other inward
nodes on the DAG. Additional recommendations and guidelines
regarding the use of this mechanism are still under consideration and
will be elaborated in a future revision of this specification.
Destination advertisements may advertise positive (prefix is present)
or negative (removed) DAO messages, termed as no-DAOs. A no-DAO is
stimulated by the disappearance of a prefix below. This is
discovered by timing out after a request (a DIO message) or by
receiving a no-DAO. A no-DAO is a conveyed as a DAO message with a
DAO Lifetime of ZERO_LIFETIME.
A node that is capable of recording the state information conveyed in
a unicast DAO message will do so upon receiving and processing the
DAO message, thus building up routing state concerning destinations
below it in the DAG. If a node capable of recording state
information receives a DAO message containing a Reverse Route Stack,
then the node knows that the DAO message has traversed one or more
nodes that did not retain any routing state as it traversed the path
from the DAO source to the node. The node may then extract the
Reverse Route Stack and retain the included state in order to specify
Source Routing instructions along the return path towards the
destination. The node MUST set the RRCount back to zero and clear
the Reverse Route Stack prior to passing the DAO message information
on.
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A node that is unable to record the state information conveyed in the
DAO message will append the next-hop address to the Reverse Route
Stack, increment the RRCount, and then pass the destination
advertisement on without recording any additional state. In this way
the Reverse Route Stack will contain a vector of next hops that must
be traversed along the reverse path that the DAO message has
traveled. The vector will be ordered such that the node closest to
the destination will appear first in the list. In such cases, if it
is useful to the implementation to try and build up redundant paths,
the node may choose to convey the destination advertisement to one or
more DAG parents in order of preference as guided by an
implementation specific policy.
In some cases (called hybrid cases), some nodes along the path a
destination advertisement follows inward along the DAG may store
state and some may not. The destination advertisement mechanism
allows for the provisioning of routing state such that when a packet
is traversing outwards along the DAG, some nodes may be able to
directly forward to the next hop, and other nodes may be able to
specify a piecewise source route in order to bridge spans of
stateless nodes within the path on the way to the desired
destination.
In the case where no node is able to store any routing state as
destination advertisements pass by, and the DAG root ends up with DAO
messages that contain a completely specified route back to the
originating node in the form of the inverted Reverse Route Stack. A
DAG root should not request (Destination Advertisement Trigger) nor
indicate support (Destination Advertisement Supported) for
destination advertisements if it is not able to store the Reverse
Route Stack information in this case.
The destination advertisement mechanism requires stateful nodes to
maintain lists of known prefixes. A prefix entry contains the
following abstract information:
o A reference to the ND entry that was created for the advertising
neighbor.
o The IPv6 address and interface for the advertising neighbor.
o The logical equivalent of the full destination advertisement
information (including the prefixes, depth, and Reverse Route
Stack, if any).
o A 'reported' Boolean to keep track whether this prefix was
reported already, and to which of the DA parents.
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o A counter of retries to count how many DIO messages were sent on
the interface to the advertising neighbor without reachability
confirmation for the prefix.
Note that nodes may receive multiple information from different
neighbors for a specific destination, as different paths through the
DAG may be propagating information inwards along the DAG for the same
destination. A node that is recording routing state will keep track
of the information from each neighbor independently, and when it
comes time to propagate the DAO message for a particular prefix to
the DA parents, then the DAO information will be selected from among
the advertising neighbors who offer the least depth to the
destination.
The destination advertisement mechanism stores the prefix entries in
one of 3 abstract lists; the Connected, the Reachable and the
Unreachable lists.
The Connected list corresponds to the prefixes owned and managed by
the local node.
The Reachable list contains prefixes for which the node keeps
receiving DAO messages, and for those prefixes which have not yet
timed out.
The Unreachable list keeps track of prefixes which are no longer
valid and in the process of being deleted, in order to send DAO
messages with zero lifetime (also called no-DAO) to the DA parents.
5.10.1.1.1. Destination Advertisement Timers
The destination advertisement mechanism requires 2 timers; the
DelayDAO timer and the RemoveTimer.
o The DelayDAO timer is armed upon a stimulation to send a
destination advertisement (such as a DIO message from a DA
parent). When the timer is armed, all entries in the Reachable
list as well as all entries for Connected list are set to not be
reported yet for that particular DA parent.
o The DelayDAO timer has a duration that is DEF_DAO_LATENCY divided
by a multiple of the DAG rank of the node. The intention is that
nodes located deeper in the DAG should have a shorter DelayDAO
timer, allowing DAO messages a chance to be reported from deeper
in the DAG and potentially aggregated along sub-DAGs before
propagating further inwards.
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o The RemoveTimer is used to clean up entries for which DAO messages
are no longer being received from the sub-DAG.
* When a DIO message is sent that is requesting destination
advertisements, a flag is set for all DAO entries in the
routing table.
* If the flag has already been set for a DAO entry, the retry
count is incremented.
* If a DAO message is received to confirm the entry, the entry is
refreshed and the flag and count may be cleared.
* If at least one entry has reached a threshold value and the
RemoveTimer is not running, the entry is considered to be
probably gone and the RemoveTimer is started.
* When the RemoveTimer elapse, DAO messages with lifetime 0, i.e.
no-DAOs, are sent to explicitly inform DA parents that the
entries which have reached the threshold are no longer
available, and the related routing states may be propagated and
cleaned up.
o The RemoveTimer has a duration of min (MAX_DESTROY_INTERVAL,
TBD(DIO Trickle Timer Interval)).
5.10.1.2. Multicast Destination Advertisement Messages
It is also possible for a node to multicast a DAO message to the
link-local scope all-nodes multicast address FF02::1. This message
will be received by all node listening in range of the emitting node.
The objective is to enable direct P2P communication, between
destinations directly supported by neighboring nodes, without needing
the RPL routing structure to relay the packets.
A multicast DAO message MUST be used only to advertise information
about self, i.e. prefixes in the Connected list or addresses owned by
this node. This would typically be a multicast group that this node
is listening to or a global address owned by this node, though it can
be used to advertise any prefix owned by this node as well. A
multicast DAO message is not used for routing and does not presume
any DAG relationship between the emitter and the receiver; it MUST
NOT be used to relay information learned (e.g. information in the
Reachable list) from another node; information obtained from a
multicast DAO MAY be installed in the routing table and MAY be
propagated by a router in unicast DAOs.
A node receiving a multicast DAO message addressed to FF02::1 MAY
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install prefixes contained in the DAO message in the routing table
for local use. Such a node MUST NOT perform any other processing on
the DAO message (i.e. such a node does not presume it is a DA
parent).
5.10.1.3. Unicast Destination Advertisement Messages from Child to
Parent
When sending a destination advertisement to a DA parent, a node
includes the DAOs for prefix entries not already reported (since the
last DA Trigger from an DIO message) in the Reachable and Connected
lists, as well as no-DAOs for all the entries in the Unreachable
list. Depending on its policy and ability to retain routing state,
the receiving node SHOULD keep a record of the reported DAO message.
If the DAO message offers the best route to the prefix as determined
by policy and other prefix records, the node SHOULD install a route
to the prefix reported in the DAO message via the link local address
of the reporting neighbor and it SHOULD further propagate the
information in a DAO message.
The DIO message from the DAG root is used to synchronize the whole
DAG, including the periodic reporting of destination advertisements
back up the DAG. Its period is expected to vary, depending on the
configuration of the trickle timer that governs the RAs.
When a node receives a DIO message over an LLN interface from a DA
parent, the DelayDAO is armed to force a full update.
When the node broadcasts a DIO message on an LLN interface, for all
entries on that interface:
o If the entry is CONFIRMED, it goes PENDING with the retry count
set to 0.
o If the entry is PENDING, the retry count is incremented. If it
reaches a maximum threshold, the entry goes ELAPSED If at least
one entry is ELAPSED at the end of the process: if the RemoveTimer
is not running then it is armed with a jitter.
Since the DelayDAO timer has a duration that decreases with the
depth, it is expected to receive all DAO messages from all children
before the timer elapses and the full update is sent to the DA
parents.
Once the RemoveTimer is elapsed, the prefix entry is scheduled to be
removed and moved to the Unreachable list if there are any DA parents
that need to be informed of the change in status for the prefix,
otherwise the prefix entry is cleaned up right away. The prefix
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entry is removed from the Unreachable list when no more DA parents
need to be informed. This condition may be satisfied when a no-DAO
is sent to all current DA parents indicating the loss of the prefix,
and noting that in some cases parents may have been removed from the
set of DA parents.
5.10.1.4. Other Events
Finally, the destination advertisement mechanism responds to a series
of events, such as:
o Destination advertisement operation stopped: All entries in the
abstract lists are freed. All the routes learned from DAO
messages are removed.
o Interface going down: for all entries in the Reachable list on
that interface, the associated route is removed, and the entry is
scheduled to be removed.
o Loss of routing adjacency: When the routing adjacency for a
neighbor is lost, as per the procedures described in Section 5.13,
and if the associated entries are in the Reachable list, the
associated routes are removed, and the entries are scheduled to be
destroyed.
o Changes to DA parent set: all entries in the Reachable list are
set to not 'reported' and DelayDAO is armed.
5.10.1.5. Aggregation of Prefixes by a Node
There may be number of cases where a aggregation may be shared within
a group of nodes. In such a case, it is possible to use aggregation
techniques with destination advertisements and improve scalability.
Other cases might occur for which additional support is required:
1. The aggregating node is attached within the sub-DAG of the nodes
it is aggregating for.
2. A node that is to be aggregated for is located somewhere else
within the DAG, not in the sub-DAG of the aggregating node.
3. A node that is to be aggregated for is located somewhere else in
the LLN.
Consider a node M that is performing an aggregation, and a node N
that is to be a member of the aggregation group. A node Z situated
above the node M in the DAG, but not above node N, will see the
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advertisements for the aggregation owned by M but not that of the
individual prefix for N. Such a node Z will route all the packets for
node N towards node M, but node M will have no route to the node N
and will fail to forward.
Additional protocols may be applied beyond the scope of this
specification to dynamically elect/provision an aggregating node and
groups of nodes eligible to be aggregated in order to provide route
summarization for a sub-DAG.
5.11. Loop Detection
RPL loop avoidance mechanisms are kept simple and designed to
minimize churn and states. Loops may form for a number of reasons,
from control packet loss to sibling forwarding. RPL includes a
reactive loop detection technique that protects from meltdown and
triggers repair of broken paths.
RPL loop detection uses information that is placed into the packet in
the flow label. It assumes that the flow label may be overloaded for
this purpose. The flow label is constructed as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|O|S|R|D| SenderRank | InstanceID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: RPL Flow Label
Outwards 'O' bit: 1-bit flag indicating whether the packet is
expected to progress inwards or outwards. A router sets the
'O' bit when the packet is expect to progress outwards (using
DAO routes), and resets it when forwarding towards the root of
the DAG. A host MUST set the bit to 0.
Sibling 'S' bit: 1-bit flag indicating whether the packet has been
forwarded via a sibling at the present rank, and denotes a risk
of a sibling loop. A host sets the bit to 0.
Rank-Error 'R' bit: 1-bit flag indicating whether a rank error was
detected. A rank error is detected when there is a mismatch in
the relative ranks and the direction as indicated in the 'O'
bit. A host MUST set the bit to 0.
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DAO-Error 'D' bit: 1-bit flag indicating whether a DAO error was
detected. An undetected DAO error would have resulted in an
inward to outward transition that is not expected with this
spec. A host MUST set the bit to 0.
SenderRank: 8-bit field indicating the rank of the sender. A host
MUST set the rank to INFINITE_RANK. A router MUST place its
own rank in the flow label when forwarding.
InstanceID: 8-bit field indicating the DAG instance along which the
packet is sent.
5.11.1. Host Basic Operation
It is expected that a host that does not participate to RPL in any
fashion is configured to set the flow label to all zeroes in its
outgoing packets. The host MAY send a packet to any router
regardless of the DAG and RPL operations at large.
A host that participates to RPL SHOULD zero out all the flags, and it
MUST set the sender rank to INFINITE_RANK. If the host can map a
flow to a given InstanceID then it MUST set the flow label
accordingly. Forwarding rules are the same for this host and a
router, and are described in the next section.
5.11.2. Instance Forwarding
Instance IDs is used to avoid loops between DAGs from different
origins. DAGs that constructed for antagonistic constraints might
contain paths that, if mixed together, would yield loops. Those
loops are avoided by forwarding a packet along the DAG that is
associated to a given instance.
The InstanceID is placed by the source in the flow label. It is not
meaningful if the packet has the flow label set to all zeroes.
Otherwise it MUST match the DAG instance onto which the packet is
placed by any node, be it a host or router.
When a router receives a packet that is flagged with a given instance
ID and the node can forward the packet along the DAG associated to
that instance, then the router MUST do so and leave the instance ID
flag unchanged.
If any node can not forward a packet along the DAG associated to the
instance ID in the flow label, then the node MAY either change the
InstanceID to match a DAG that it is using for this packet or discard
the packet. That decision is based on a policy.
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The default policy is as follows: if the node can forward along the
DAG associated to the instance RPL_DEFAULT_INSTANCE then it should do
so. Otherwise it should drop the packet.
5.11.3. DAG Inconsistency Loop Detection
The DAG is inconsistent is the direction of a packet does not match
the rank relationship. A receiver detects an inconsistency if it
receives a packet with either:
the 'O' bit set (to outwards) from a node of a higher rank.
the 'O' bit reset (for inwards) from a node of a lesser rank.
the 'S' bit set (to sibling) from a node of a different rank.
The propagation of a new sequence creates local inconsistencies. In
particular, it is possible for a router to forward a packet to a
future parent (same instance, same DAGID, higher sequence) without a
loop, regardless of the rank of that parent. In that case, the
sending router MUST present itself as a host on the future DAG and
use a rank of INFINITE_RANK as it forwards the packets via a future
parent to avoid a false positive.
One inconsistency along the path is not considered as a critical
error and the packet may continue. But a second detection along the
path of a same packet should not occur and the packet is dropped.
This process is controlled by the Rank-Error bit in the Flow Label.
When an inconsistency, is detected on a packet, if the Rank-Error bit
was not set then the Rank-Error bit is set. If it was set the packet
is discarded and the trickle timer is reset.
5.11.4. Sibling Loop Avoidance
When a packet is forwarded along siblings, it cannot be checked for
forward progress and may loop between siblings. Experimental
evidence has shown that one sibling hop can be very useful but is
generally sufficient to avoid loops. Based on that evidence, this
specification enforces the simple rule that a packet may not make 2
sibling hops in a row.
When a host issues a packet or when a router forwards a packet to a
non sibling, the Sibling bit in the packet must be reset. When a
router forwards to a sibling: if the Sibling bit was not set then the
Sibling bit is set. If the Sibling bit was set then the packet is
discarded. This does not denote a graph inconsistency but indicates
that a new graph should probably be formed with a new sequence.
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5.11.5. DAO Inconsistency Loop Detection and Recovery
A DAO inconsistency happens when router that has an outwards DAO
route via a child that is a remnant from an obsolete state that is
not matched in the child. With DAO inconsistency loop recovery, a
packet can be used to recursively explore and cleanup the obsolete
DAO states along a sub-DAG.
In a general manner, a packet that goes outwards should never go
inwards again. So rather than routing inwards a packet with the
Outwards bit set, the router MUST discard the packet. If DAO
inconsistency loop recovery is applied, then the router SHOULD send
the packet to the parent that passed it with the DAO-Error bit set.
Upon a packet with a DAO bit set, the parent MUST remove the routing
states that caused forwarding to that child, clear DAO-Error bit and
send the packet again. The packet will make its way either to an
alternate child or inwards to a parent. If that parent still has an
inconsistent DAO state via self, the process will recurse and that
state will be cleaned up as well.
5.12. Multicast Operation
This section describes further the multicast routing operations over
an IPv6 RPL network, and specifically how unicast DAOs can be used to
relay group registrations inwards. Wherever the following text
mentions MLD, one can read MLDv2 or v3.
As is traditional, a listener uses a protocol such as MLD with a
router to register to a multicast group.
Along the path between the router and the root of the DAG, MLD
requests are mapped and transported as DAO messages within the RPL
protocol; each hop coalesces the multiple requests for a same group
as a single DAO message to the parent(s), in a fashion similar to
proxy IGMP, but recursively between child router and parent up to the
root.
A router might select to pass a listener registration DAO message to
its preferred parent only, in which case multicast packets coming
back might be lost for all of its sub-DAG if the transmission fails
over that link. Alternatively the router might select to copy
additional parents as it would do for DAO messages advertising
unicast destinations, in which case there might be duplicates that
the router will need to prune.
As a result, multicast routing states are installed in each router on
the way from the listeners to the root, enabling the root to copy a
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multicast packet to all its children routers that had issued a DAO
message including a DAO for that multicast group, as well as all the
attached nodes that registered over MLD.
For unicast traffic, it is expected that the grounded root of an RPL
DAG terminates RPL and MAY redistribute the RPL routes over the
external infrastructure using whatever routing protocol is used
there. For multicast traffic, the root MAY proxy MLD for all the
nodes attached to the RPL routers (this would be needed if the
multicast source is located in the external infrastructure). For
such a source, the packet will be replicated as it flows outwards
along the DAG based on the multicast routing table entries installed
from the DAO message.
For a source inside the DAG, the packet is passed to the preferred
parents, and if that fails then to the alternates in the DAG. The
packet is also copied to all the registered children, except for the
one that passed the packet. Finally, if there is a listener in the
external infrastructure then the DAG root has to further propagate
the packet into the external infrastructure.
As a result, the DAG Root acts as an automatic proxy Rendezvous Point
for the RPL network, and as source towards the Internet for all
multicast flows started in the RPL LLN. So regardless of whether the
root is actually attached to the Internet, and regardless of whether
the DAG is grounded or floating, the root can serve inner multicast
streams at all times.
5.13. Maintenance of Routing Adjacency
The selection of successors, along the default paths inward along the
DAG, or along the paths learned from destination advertisements
outward along the DAG, leads to the formation of routing adjacencies
that require maintenance.
In IGPs such as OSPF [RFC4915] or IS-IS [RFC5120], the maintenance of
a routing adjacency involves the use of Keepalive mechanisms (Hellos)
or other protocols such as BFD ([I-D.ietf-bfd-base]) and MANET
Neighborhood Discovery Protocol (NHDP [I-D.ietf-manet-nhdp]).
Unfortunately, such an approach is not desirable in constrained
environments such as LLN and would lead to excessive control traffic
in light of the data traffic with a negative impact on both link
loads and nodes resources. Overhead to maintain the routing
adjacency should be minimized. Furthermore, it is not always
possible to rely on the link or transport layer to provide
information of the associated link state. The network layer needs to
fall back on its own mechanism.
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Thus RPL makes use of a different approach consisting of probing the
neighbor using a Neighbor Solicitation message (see [RFC4861]). The
reception of a Neighbor Advertisement (NA) message with the
"Solicited Flag" set is used to verify the validity of the routing
adjacency. Such mechanism MAY be used prior to sending a data
packet. This allows for detecting whether or not the routing
adjacency is still valid, and should it not be the case, select
another feasible successor to forward the packet.
5.14. Packet Forwarding
When forwarding a packet to a destination, precedence is given to
selection of a next-hop successor as follows:
1. In the scope of this specification, it is preferred to select a
successor from a DAG that matches the InstanceID marked in the
IPv6 header of the packet being forwarded.
2. If a local administrative preference favors a route that has been
learned from a different routing protocol than RPL, 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 parent offering a default route then select
that DAG parent 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
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forwarded via a sibling, then the TTL MAY be decremented more
aggressively (by more than one) to limit the impact of possible
loops.
Note that the chosen successor MUST NOT be the neighbor that was the
predecessor of the packet (split horizon), except in the case where
it is intended for the packet to change from an inward to an outward
flow, such as switching from DIO routes to DAO routes as the
destination is neared.
6. RPL Constants and Variables
ZERO_LIFETIME This is the special value of a lifetime that indicates
immediate death and removal. ZERO_LIFETIME has a value of 0.
BASE_RANK This is the rank for a virtual root that might be used to
coordinate multiple roots. BASE_RANK has a value of 0.
ROOT_RANK This is the rank for a DAG root. ROOT_RANK has a value of
1.
INFINITE_RANK This is the constant maximum for the rank.
INFINITE_RANK has a value of 0xFF.
RPL_DEFAULT_INSTANCE This is the instance ID that is used by this
protocol by a node without a policy to know any better.
RPL_DEFAULT_INSTANCE has a value of 0.
DEFAULT_DIO_INTERVAL_MIN To be determined
DEFAULT_DIO_INTERVAL_DOUBLINGS To be determined
DEF_DAO_LATENCY To be determined
MAX_DESTROY_INTERVAL To be determined
DIO Timer One instance per DAG that a node is a member of. Expiry
triggers DIO message transmission. Trickle timer with variable
interval in [0, DIOIntervalMin..2^DIOIntervalDoublings]. See
Section 5.4.4
DAG Sequence Number Increment Timer Up to one instance per DAG that
the node is acting as DAG root of. May not be supported in all
implementations. Expiry triggers revision of
DAGSequenceNumber, causing a new series of updated DIO message
to be sent. Interval should be chosen appropriate to
propagation time of DAG and as appropriate to application
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requirements (e.g. response time vs. overhead). See
Section 5.5
DelayDAO Timer Up to one instance per DA parent (the subset of DAG
parents chosen to receive destination advertisements) per DAG.
Expiry triggers sending of DAO message to the DA parent. The
interval is to be proportional to DEF_DAO_LATENCY/(node rank),
such that nodes of greater rank (further outward along the DAG)
expire first, coordinating the sending of DAO messages to allow
for a chance of aggregation. See Section 5.10.1.1.1
RemoveTimer Up to one instance per DA entry per neighbor (i.e. those
neighbors that have given DAO messages to this node as a DAG
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, TBD(DIO
Trickle Timer Interval)). See Section 5.10.1.1.1
7. Manageability Considerations
The aim of this section is to give consideration to the manageability
of RPL, and how RPL will be operated in LLN beyond the use of a MIB
module. The scope of this section is to consider the following
aspects of manageability: fault management, configuration, accounting
and performance.
7.1. Control of Function and Policy
7.1.1. Initialization Mode
When a node is first powered up, it may either choose to stay silent
and not send any multicast DIO message until it has joined a DAG, or
to immediately root a transient DAG and start sending multicast DIO
messages. A RPL implementation SHOULD allow configuring whether the
node should stay silent or should start advertising DIO messages.
Furthermore, the implementation SHOULD to allow configuring whether
or not the node should start sending an DIS message as an initial
probe for nearby DAGs, or should simply wait until it received RA
messages from other nodes that are part of existing DAGs.
7.1.2. DIO Base option
RPL specifies a number of protocol parameters.
A RPL implementation SHOULD allow configuring the following routing
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protocol parameters, which are further described in Section 5.1.3.1:
DAGPreference
InstanceID
DAGObjectiveCodePoint
DAGID
Destination Prefixes
DIOIntervalDoublings
DIOIntervalMin
DAG Root behavior: In some cases, a node may not want to permanently
act as a DAG root if it cannot join a grounded DAG. For
example a battery-operated node may not want to act as a DAG
root for a long period of time. Thus a RPL implementation MAY
support the ability to configure whether or not a node could
act as a DAG root for a configured period of time.
DAG Table Entry Suppression A RPL implementation SHOULD provide the
ability to configure a timer after the expiration of which the
DAG table that contains all the records about a DAG is
suppressed, to be invoked if the DAG parent set becomes empty.
7.1.3. Trickle Timers
A RPL implementation makes use of trickle timer to govern the sending
of DIO message. Such an algorithm is determined a by a set of
configurable parameters that are then advertised by the DAG root
along the DAG in DIO messages.
For each DAG, a RPL implementation MUST allow for the monitoring of
the following parameters, further described in Section 5.4.4:
I
T
C
I_min
I_doublings:
A RPL implementation SHOULD provide a command (for example via API,
CLI, or SNMP MIB) whereby any procedure that detects an inconsistency
may cause the trickle timer to reset.
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7.1.4. DAG Sequence Number Increment
A RPL implementation may allow by configuration at the DAG root to
refresh the DAG states by updating the DAGSequenceNumber. A RPL
implementation SHOULD allow configuring whether or not periodic or
event triggered mechanism are used by the DAG root to control
DAGSequenceNumber change.
7.1.5. Destination Advertisement Timers
The following set of parameters of the DAO messages SHOULD be
configurable:
o The DelayDAO timer
o The Remove timer
7.1.6. Policy Control
DAG discovery enables nodes to implement different policies for
selecting their DAG parents.
A RPL implementation SHOULD allow configuring the set of acceptable
or preferred Objective Functions (OF) referenced by their Objective
Codepoints (OCPs) for a node to join a DAG, and what action should be
taken if none of a node's candidate neighbors advertise one of the
configured allowable Objective Functions.
A node in an LLN may learn routing information from different routing
protocols including RPL. It is in this case desirable to control via
administrative preference which route should be favored. An
implementation SHOULD allow for specifying an administrative
preference for the routing protocol from which the route was learned.
A RPL implementation SHOULD allow for the configuration of the "Route
Tag" field of the DAO messages according to a set of rules defined by
policy.
7.1.7. Data Structures
Some RPL implementation may limit the size of the candidate neighbor
list in order to bound the memory usage, in which case some otherwise
viable candidate neighbors may not be considered and simply dropped
from the candidate neighbor list.
A RPL implementation MAY provide an indicator on the size of the
candidate neighbor list.
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7.2. Information and Data Models
The information and data models necessary for the operation of RPL
will be defined in a separate document specifying the RPL SNMP MIB.
7.3. Liveness Detection and Monitoring
The aim of this section is to describe the various RPL mechanisms
specified to monitor the protocol.
As specified in Section 5.2, an implementation must maintain a set of
data structures in support of DAG discovery:
o The candidate neighbors data structure
o For each DAG:
* A set of DAG parents
7.3.1. Candidate Neighbor Data Structure
A node in the candidate neighbor list is a node discovered by the
some means and qualified to potentially become of neighbor or a
sibling (with high enough local confidence). A RPL implementation
SHOULD provide a way monitor the candidate neighbors list with some
metric reflecting local confidence (the degree of stability of the
neighbors) measured by some metrics.
A RPL implementation MAY provide a counter reporting the number of
times a candidate neighbor has been ignored, should the number of
candidate neighbors exceeds the maximum authorized value.
7.3.2. Directed Acyclic Graph (DAG) Table
For each DAG, a RPL implementation MUST keep track of the following
DAG table values:
o DAGID
o DAGObjectiveCodePoint
o A set of Destination Prefixes offered inwards along the DAG
o A set of DAG Parents
o timer to govern the sending of DIO messages for the DAG
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o DAGSequenceNumber
The set of DAG parents structure is itself a table with the following
entries:
o A reference to the neighboring device which is the DAG parent
o A record of most recent information taken from the DAG Information
Object last processed from the DAG Parent
o A flag reporting if the Parent is a DA Parent as described in
Section 5.10
7.3.3. Routing Table
For each route provisioned by RPL operation, a RPL implementation
MUST keep track of the following:
o Destination Prefix
o Destination Prefix Length
o Lifetime Timer
o Next Hop
o Next Hop Interface
o Flag indicating that the route was provisioned from one of:
* Unicast DAO message
* DIO message
* Multicast DAO message
7.3.4. Other RPL Monitoring Parameters
A RPL implementation SHOULD provide a counter reporting the number of
a times the node has detected an inconsistency with respect to a DAG
parent, e.g. if the DAGID has changed.
A RPL implementation MAY log the reception of a malformed DIO message
along with the neighbor identification if avialable.
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7.3.5. RPL Trickle Timers
A RPL implementation operating on a DAG root MUST allow for the
configuration of the following trickle parameters:
o The DIOIntervalMin expressed in ms
o The DIOIntervalDoublings
A RPL implementation MAY provide a counter reporting the number of
times an inconsistency (and thus the trickle timer has been reset).
7.4. Verifying Correct Operation
This section has to be completed in further revision of this document
to list potential Operations and Management (OAM) tools that could be
used for verifying the correct operation of RPL.
7.5. Requirements on Other Protocols and Functional Components
RPL does not have any impact on the operation of existing protocols.
7.6. Impact on Network Operation
To be completed.
8. Security Considerations
Security Considerations for RPL are to be developed in accordance
with recommendations laid out in, for example,
[I-D.tsao-roll-security-framework].
9. IANA Considerations
9.1. RPL Control Message
The RPL Control Message is an ICMP information message type that is
to be used carry DAG Information Objects, DAG Information
Solicitations, and Destination Advertisement Objects in support of
RPL operation.
IANA has defined a ICMPv6 Type Number Registry. The suggested type
value for the RPL Control Message is 155, to be confirmed by IANA.
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9.2. New Registry for RPL Control Codes
IANA is requested to create a registry, RPL Control Codes, for the
Code field of the ICMPv6 RPL Control Message.
New codes may be allocated only by an IETF Consensus action. Each
code should be tracked with the following qualities:
o Code
o Description
o Defining RFC
Three codes are currently defined:
+------+----------------------------------+---------------+
| Code | Description | Reference |
+------+----------------------------------+---------------+
| 0x01 | DAG Information Solicitation | This document |
| 0x02 | DAG Information Object | This document |
| 0x04 | Destination Advertisement Object | This document |
+------+----------------------------------+---------------+
RPL Control Codes
9.3. New Registry for the Control Field of the DIO Base Option
IANA is requested to create a registry for the Control field of the
DIO Base Option.
New bit numbers may be allocated only by an IETF Consensus action.
Each bit should be tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
o Defining RFC
Four groups are currently defined:
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+-------+-------------------------------------+---------------+
| Bit | Description | Reference |
+-------+-------------------------------------+---------------+
| 0 | Grounded DAG | This document |
| 1 | Destination Advertisement Trigger | This document |
| 2 | Destination Advertisement Supported | This document |
| 5,6,7 | DAG Preference | This document |
+-------+-------------------------------------+---------------+
DIO Base Option Flags
9.4. DAG Information Object (DIO) Suboption
IANA is requested to create a registry for the DIO Base Option
Suboptions
+-------+------------------------------+---------------+
| Value | Meaning | Reference |
+-------+------------------------------+---------------+
| 0 | Pad1 - DIO Padding | This document |
| 1 | PadN - DIO suboption padding | This document |
| 2 | DAG Metric Container | This Document |
| 3 | Destination Prefix | This Document |
| 4 | DAG Timer Configuration | This Document |
+-------+------------------------------+---------------+
DAG Information Option (DIO) Base Option Suboptions
9.5. Objective Code Point for the Default Objective Function OF0
This specification specifies the Default Objective Function (called
OF0) for which the OCP field of the OF object, as defined in
[I-D.ietf-roll-routing-metrics], is equal to 0x0000
+-------+---------+---------------+
| Value | Meaning | Reference |
+-------+---------+---------------+
| 0 | OF0 | This document |
+-------+---------+---------------+
OCP Allocation
10. Acknowledgements
The authors would like to acknowledge the review, feedback, and
comments from Emmanuel Baccelli, Dominique Barthel, Yusuf Bashir,
Mathilde Durvy, Manhar Goindi, Mukul Goyal, Anders Jagd, Quentin
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Lampin, Jerry Martocci, Alexandru Petrescu, and Don Sturek.
The authors would like to acknowledge the guidance and input provided
by the ROLL Chairs, David Culler and JP Vasseur.
The authors would like to acknowledge prior contributions of Robert
Assimiti, Mischa Dohler, Julien Abeille, Ryuji Wakikawa, Teco Boot,
Patrick Wetterwald, Bryan Mclaughlin, Carlos J. Bernardos, Thomas
Watteyne, Zach Shelby, Caroline Bontoux, Marco Molteni, Billy Moon,
and Arsalan Tavakoli, which have provided useful design
considerations to RPL.
11. Contributors
RPL is the result of the contribution of the following members of the
ROLL Design Team, including the editors, and additional contributors
as listed below:
JP Vasseur
Cisco Systems, Inc
11, Rue Camille Desmoulins
Issy Les Moulineaux, 92782
France
Email: jpv@cisco.com
Jonathan W. Hui
Arch Rock Corporation
501 2nd St. Ste. 410
San Francisco, CA 94107
USA
Email: jhui@archrock.com
Thomas Heide Clausen
LIX, Ecole Polytechnique, France
Phone: +33 6 6058 9349
EMail: T.Clausen@computer.org
URI: http://www.ThomasClausen.org/
Richard Kelsey
Ember Corporation
Boston, MA
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USA
Phone: +1 617 951 1225
Email: kelsey@ember.com
Philip Levis
Stanford University
358 Gates Hall, Stanford University
Stanford, CA 94305-9030
USA
Email: pal@cs.stanford.edu
Stephen Dawson-Haggerty
UC Berkeley
Soda Hall, UC Berkeley
Berkeley, CA 94720
USA
Email: stevedh@cs.berkeley.edu
Kris Pister
Dust Networks
30695 Huntwood Ave.
Hayward, 94544
USA
Email: kpister@dustnetworks.com
Anders Brandt
Zensys, Inc.
Emdrupvej 26
Copenhagen, DK-2100
Denmark
Email: abr@zen-sys.com
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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12.2. Informative References
[I-D.ietf-bfd-base]
Katz, D. and D. Ward, "Bidirectional Forwarding
Detection", draft-ietf-bfd-base-09 (work in progress),
February 2009.
[I-D.ietf-manet-nhdp]
Clausen, T., Dearlove, C., and J. Dean, "MANET
Neighborhood Discovery Protocol (NHDP)",
draft-ietf-manet-nhdp-10 (work in progress), July 2009.
[I-D.ietf-roll-building-routing-reqs]
Martocci, J., Riou, N., Mil, P., and W. Vermeylen,
"Building Automation Routing Requirements in Low Power and
Lossy Networks", draft-ietf-roll-building-routing-reqs-07
(work in progress), September 2009.
[I-D.ietf-roll-home-routing-reqs]
Brandt, A., Buron, J., and G. Porcu, "Home Automation
Routing Requirements in Low Power and Lossy Networks",
draft-ietf-roll-home-routing-reqs-08 (work in progress),
September 2009.
[I-D.ietf-roll-routing-metrics]
Vasseur, J. and D. Networks, "Routing Metrics used for
Path Calculation in Low Power and Lossy Networks",
draft-ietf-roll-routing-metrics-01 (work in progress),
October 2009.
[I-D.ietf-roll-terminology]
Vasseur, J., "Terminology in Low power And Lossy
Networks", draft-ietf-roll-terminology-02 (work in
progress), October 2009.
[I-D.tsao-roll-security-framework]
Tsao, T., Alexander, R., Dohler, M., Daza, V., and A.
Lozano, "A Security Framework for Routing over Low Power
and Lossy Networks", draft-tsao-roll-security-framework-01
(work in progress), September 2009.
[Levis08] Levis, P., Brewer, E., Culler, D., Gay, D., Madden, S.,
Patel, N., Polastre, J., Shenker, S., Szewczyk, R., and A.
Woo, "The Emergence of a Networking Primitive in Wireless
Sensor Networks", Communications of the ACM, v.51 n.7,
July 2008,
<http://portal.acm.org/citation.cfm?id=1364804>.
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[RFC2453] Malkin, G., "RIP Version 2", STD 56, RFC 2453,
November 1998.
[RFC3819] Karn, P., Bormann, C., Fairhurst, G., Grossman, D.,
Ludwig, R., Mahdavi, J., Montenegro, G., Touch, J., and L.
Wood, "Advice for Internet Subnetwork Designers", BCP 89,
RFC 3819, July 2004.
[RFC4101] Rescorla, E. and IAB, "Writing Protocol Models", RFC 4101,
June 2005.
[RFC4191] Draves, R. and D. Thaler, "Default Router Preferences and
More-Specific Routes", RFC 4191, November 2005.
[RFC4443] Conta, A., Deering, S., and M. Gupta, "Internet Control
Message Protocol (ICMPv6) for the Internet Protocol
Version 6 (IPv6) Specification", RFC 4443, March 2006.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
September 2007.
[RFC4915] Psenak, P., Mirtorabi, S., Roy, A., Nguyen, L., and P.
Pillay-Esnault, "Multi-Topology (MT) Routing in OSPF",
RFC 4915, June 2007.
[RFC5120] Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
Topology (MT) Routing in Intermediate System to
Intermediate Systems (IS-ISs)", RFC 5120, February 2008.
[RFC5548] Dohler, M., Watteyne, T., Winter, T., and D. Barthel,
"Routing Requirements for Urban Low-Power and Lossy
Networks", RFC 5548, May 2009.
[RFC5673] Pister, K., Thubert, P., Dwars, S., and T. Phinney,
"Industrial Routing Requirements in Low-Power and Lossy
Networks", RFC 5673, October 2009.
Appendix A. Requirements
A.1. Protocol Properties Overview
RPL demonstrates the following properties, consistent with the
requirements specified by the application-specific requirements
documents.
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A.1.1. IPv6 Architecture
RPL is strictly compliant with layered IPv6 architecture.
Further, RPL is designed with consideration to the practical support
and implementation of IPv6 architecture on devices which may operate
under severe resource constraints, including but not limited to
memory, processing power, energy, and communication. The RPL design
does not presume high quality reliable links, and operates over lossy
links (usually low bandwidth with low packet delivery success rate).
A.1.2. Typical LLN Traffic Patterns
Multipoint-to-Point (MP2P) and Point-to-multipoint (P2MP) traffic
flows from nodes within the LLN from and to egress points are very
common in LLNs. Low power and lossy network Border Router (LBR)
nodes may typically be at the root of such flows, although such flows
are not exclusively rooted at LBRs as determined on an application-
specific basis. In particular, several applications such as building
or home automation do require P2P (Point-to-Point) communication.
As required by the aforementioned routing requirements documents, RPL
supports the installation of multiple paths. The use of multiple
paths include sending duplicated traffic along diverse paths, as well
as to support advanced features such as Class of Service (CoS) based
routing, or simple load balancing among a set of paths (which could
be useful for the LLN to spread traffic load and avoid fast energy
depletion on some, e.g. battery powered, nodes). Conceptually,
multiple instances of RPL can be used to send traffic along different
topology instances, the construction of which is governed by
different Objective Functions (OF). Details of RPL operation in
support of multiple instances are beyond the scope of the present
specification.
A.1.3. Constraint Based Routing
The RPL design supports constraint based routing, based on a set of
routing metrics and constraints. The routing metrics and constraints
for links and nodes with capabilities supported by RPL are specified
in a companion document to this specification,
[I-D.ietf-roll-routing-metrics]. RPL signals the metrics,
constraints, and related Objective Functions (OFs) in use in a
particular implementation by means of an Objective Code Point (OCP).
Both the routing metrics, constraints, and the OF help determine the
construction of the Directed Acyclic Graphs (DAG) using a distributed
path computation algorithm.
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A.2. Deferred Requirements
NOTE: RPL is still a work in progress. At this time there remain
several unsatisfied application requirements, but these are to be
addressed as RPL is further specified.
Appendix B. Examples
Consider the example LLN physical topology in Figure 11. In this
example the links depicted are all usable L2 links. Suppose that all
links are equally usable, and that the implementation specific policy
function is simply to minimize hops. This LLN physical topology then
yields the DAG depicted in Figure 12, where the links depicted are
the edges toward DAG parents. This topology includes one DAG, rooted
by an LBR node (LBR) at rank 1. The LBR node will issue DIO
messages, as governed by a trickle timer. Nodes (11), (12), (13),
have selected (LBR) as their only parent, attached to the DAG at rank
2, and periodically multicast DIOs. Node (22) has selected (11) and
(12) in its DAG parent set, and advertises itself at rank 3. Node
(22) thus has a set of DAG parents {(11), (12)} and siblings {((21),
(23)}.
(LBR)
/ | \
.---` | `----.
/ | \
(11)------(12)------(13)
| \ | \ | \
| `----. | `----. | `----.
| \| \| \
(21)------(22)------(23) (24)
| /| /| |
| .----` | .----` | |
| / | / | |
(31)------(32)------(33)------(34)
| /| \ | \ | \
| .----` | `----. | `----. | `----.
| / | \| \| \
.--------(41) (42) (43)------(44)------(45)
/ / /| \ | \
.----` .----` .----` | `----. | `----.
/ / / | \| \
(51)------(52)------(53)------(54)------(55)------(56)
Note that the links depicted represent the usable L2 connectivity
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available in the LLN. For example, Node (31) can communicate
directly with its neighbors, Nodes (21), (22), (32), and (41). Node
(31) cannot communicate directly with any other nodes, e.g. (33),
(23), (42). In this example these links offer bidirectional
communication, and `bad' links are not depicted.
Figure 11: Example LLN Topology
(LBR)
/ | \
.---` | `----.
/ | \
(11) (12) (13)
| \ | \ | \
| `----. | `----. | `----.
| \| \| \
(21) (22) (23) (24)
| /| /| |
| .----` | .----` | |
| / | / | |
(31) (32) (33) (34)
| /| \ | \ | \
| .----` | `----. | `----. | `----.
| / | \| \| \
.--------(41) (42) (43) (44) (45)
/ / /| \ | \
.----` .----` .----` | `----. | `----.
/ / / | \| \
(51) (52) (53) (54) (55) (56)
Note that the links depicted represent directed links in the DAG
overlaid on top of the physical topology depicted in Figure 11. As
such, the depicted edges represent the relationship between nodes and
their DAG parents, wherein all depicted edges are directed and
oriented `up' on the page toward the DAG root (LBR). The DAG may
provide default routes within the LLN, and serves as the foundation
on which RPL builds further routing structure, e.g. through the
destination advertisement mechanism.
Figure 12: Example DAG
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B.1. Destination Advertisement
Consider the example DAG depicted in Figure 12. Suppose that Nodes
(22) and (32) are unable to record routing state. Suppose that Node
(42) is able to perform prefix aggregation on behalf of Nodes (53),
(54), and (55).
o Node (53) would send a DAO message to Node (42), indicating the
availability of destination (53).
o Node (54) and Node (55) would similarly send DAO messages to Node
(42) indicating their own destinations.
o Node (42) would collect and store the routing state for
destinations (53), (54), and (55).
o In this example, Node (42) may then be capable of representing
destinations (42), (53), (54), and (55) in the aggregation (42').
o Node (42) sends a DAO message advertising destination (42') to
Node 32.
o Node (32) does not want to maintain any routing state, so it adds
onto to the Reverse Route Stack in the DAO message and passes it
on to Node (22) as (42'):[(42)]. It may send a separate DAO
message to indicate destination (32).
o Node (22) does not want to maintain any routing state, so it adds
on to the Reverse Route Stack in the DAO message and passes it on
to Node (12) as (42'):[(42), (32)]. It also relays the DAO
message containing destination (32) to Node 12 as (32):[(32)], and
finally may send a DAO message for itself indicating destination
(22).
o Node (12) is capable to maintain routing state again, and receives
the DAO messages from Node (22). Node (12) then learns:
* Destination (22) is available via Node (22)
* Destination (32) is available via Node (22) and the piecewise
source route to (32)
* Destination (42') is available via Node (22) and the piecewise
source route to (32), (42').
o Node (12) sends DAO messages to (LBR), allowing (LBR) to learn
routes to the destinations (12), (22), (32), and (42'). (42),
(53), (54), and (55) are available via the aggregation (42'). It
is not necessary for Node (12) to propagate the piecewise source
routes to (LBR).
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B.2. Example: DAG Parent Selection
For example, suppose that a node (N) is not attached to any DAG, and
that it is in range of nodes (A), (B), (C), (D), and (E). Let all
nodes be configured to use an OCP which defines a policy such that
ETX is to be minimized and paths with the attribute `Blue' should be
avoided. Let the rank computation indicated by the OCP simply
reflect the ETX aggregated along the path. Let the links between
node (N) and its neighbors (A-E) all have an ETX of 1 (which is
learned by node (N) through some implementation specific method).
Let node (N) be configured to send RPL DIS messages to probe for
nearby DAGs.
o Node (N) transmits a RPL DIS message.
o Node (B) responds. Node (N) investigates the DIO message, and
learns that 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.
o Similarly, Node (N) hears from Node (A) at rank 9, Node (C) at
rank 5, and Node (E) at rank 4.
o Node (D) responds. Node (D) has a DIO message that indicates that
it is a member of DAGID 1 at rank 2, but it carries the attribute
`Blue'. Node (N)'s policy function rejects Node (D), and no
further consideration is given.
o This process continues until Node (N), based on implementation
specific policy, builds up enough confidence to trigger a decision
to join DAGID 1. Let Node (N) determine its most preferred parent
to be Node (E).
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
that the ETX is 1 for the link between (N) and (E), Node (N) is
now at rank 5 in DAGID 1.
o Node (N) adds Node (B) (rank 4) to its set of DAG parents for
DAGID 1.
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
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
also choose to forward traffic to its sibling node (C), without
making inward progress but with the intention that node (C) or a
following successor can make inward progress. Should Node (C) not
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have a viable parent, it should never send the packet back to Node
(N) (to avoid a 2-node loop).
B.3. Example: DAG Maintenance
: : :
: : :
(A) (A) (A)
|\ | |
| `-----. | |
| \ | |
(B) (C) (B) (C) (B)
| | \
| | `-----.
| | \
(D) (D) (C)
|
|
|
(D)
-1- -2- -3-
Figure 13: DAG Maintenance
Consider the example depicted in Figure 13-1. In this example, Node
(A) is attached to a DAG at some rank d. Node (A) is a DAG parent of
Nodes (B) and (C). Node (C) is a DAG parent of Node (D). There is
also an undirected sibling link between Nodes (B) and (C).
In this example, Node (C) may safely forward to Node (A) without
creating a loop. Node (C) may not safely forward to Node (D),
contained within it's own sub-DAG, without creating a loop. Node (C)
may forward to Node (B) in some cases, e.g. the link (C)->(A) is
temporarily unavailable, but with some chance of creating a loop
(e.g. if multiple nodes in a set of siblings start forwarding
`sideways' in a cycle) and requiring the intervention of additional
mechanisms to detect and break the loop.
Consider the case where Node (C) hears a DIO message from a Node (Z)
at a lesser rank and superior position in the DAG than node (A).
Node (C) may safely undergo the process to evict node (A) from its
DAG parent set and attach directly to Node (Z) without creating a
loop, because its rank will decrease.
Now consider the case where the link (C)->(A) becomes nonviable, and
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node (C) must move to a deeper rank within the DAG:
o Node (C) must first detach from the DAG by removing Node (A) from
its DAG parent set, leaving an empty DAG parent set. Node (C) may
become the root of its own floating, less preferred, DAG.
o Node (D), hearing a modified DIO message from Node (C), follows
Node (C) into the floating DAG. This is depicted in Figure 13-2.
In general, any node with no other options in the sub-DAG of Node
(C) will follow Node (C) into the floating DAG, maintaining the
structure of the sub-DAG.
o Node (C) hears a DIO message with an incremented DAGSequenceNumber
from Node (B) and determines it is able to rejoin the grounded DAG
by reattaching at a deeper rank to Node (B). Node (C) adds Node
(B) to its DAG parent set. Node (C) has now safely moved deeper
within the grounded DAG without creating any loops.
o Node (D), and any other sub-DAG of Node (C), will hear the
modified DIO message sourced from Node (C) and follow Node (C) in
a coordinated manner to reattach to the grounded DAG. The final
DAG is depicted in Figure 13-3
B.4. Example: Greedy Parent Selection and Instability
(A) (A) (A)
|\ |\ |\
| `-----. | `-----. | `-----.
| \ | \ | \
(B) (C) (B) \ | (C)
\ | | /
`-----. | | .-----`
\| |/
(C) (B)
-1- -2- -3-
Figure 14: Greedy DAG Parent Selection
Consider the example depicted in Figure 14. A DAG is depicted in 3
different configurations. A usable link between (B) and (C) exists
in all 3 configurations. In Figure 14-1, Node (A) is a DAG parent
for Nodes (B) and (C), and (B)--(C) is a sibling link. In
Figure 14-2, Node (A) is a DAG parent for Nodes (B) and (C), and Node
(B) is also a DAG parent for Node (C). In Figure 14-3, Node (A) is a
DAG parent for Nodes (B) and (C), and Node (C) is also a DAG parent
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for Node (B).
If a RPL node is too greedy, in that it attempts to optimize for an
additional number of parents beyond its preferred parent, then an
instability can result. Consider the DAG illustrated in Figure 14-1.
In this example, Nodes (B) and (C) may most prefer Node (A) as a DAG
parent, but are operating under the greedy condition that will try to
optimize for 2 parents.
When the preferred parent selection causes a node to have only one
parent and no siblings, the node may decide to insert itself at a
slightly higher rank in order to have at least one sibling and thus
an alternate forwarding solution. This does not deprive other nodes
of a forwarding solution and this is considered acceptable
greediness.
o Let Figure 14-1 be the initial condition.
o Suppose Node (C) first is able to leave the DAG and rejoin at a
lower rank, taking both Nodes (A) and (B) as DAG parents as
depicted in Figure 14-2. Now Node (C) is deeper than both Nodes
(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
process a DIO message from Node (C) (against the rules of RPL),
and then Node (B) leaves the DAG and rejoins at a lower rank,
taking both Nodes (A) and (C) as DAG parents. Now Node (B) is
deeper than both Nodes (A) and (C) and is satisfied with 2 DAG
parents.
o Then Node (C), because it is also greedy, will leave and rejoin
deeper, to again get 2 parents and have a lower rank then both of
them.
o Next Node (B) will again leave and rejoin deeper, to again get 2
parents
o And again Node (C) leaves and rejoins deeper...
o The process will repeat, and the DAG will oscillate between
Figure 14-2 and Figure 14-3 until the nodes count to infinity and
restart the cycle again.
o This cycle can be averted through mechanisms in RPL:
* Nodes (B) and (C) stay at a rank sufficient to attach to their
most preferred parent (A) and don't go for any deeper (worse)
alternate parents (Nodes are not greedy)
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* Nodes (B) and (C) do not process DIO messages from nodes deeper
than themselves (because such nodes are possibly in their own
sub-DAGs)
Appendix C. Outstanding Issues
This section enumerates some outstanding issues that are to be
addressed in future revisions of the RPL specification.
C.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.
C.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.
C.3. Destination Advertisement / DAO Fan-out
When DAO messages are relayed to more than one DAG parent, in some
cases a situation may be created where a large number of DAO messages
conveying information about the same destination flow inward along
the DAG. It is desirable to bound/limit the multiplication/fan-out
of DAO messages in this manner. Some aspects of the Destination
Advertisement mechanism remain under investigation, such as behavior
in the face of links that may not be symmetric.
In general, the utility of providing redundancy along outwards routes
by sending DAO messages to more than one parent is under
investigation.
The use of suitable triggers, such as the `D' bit, to trigger DA
operation within an affected sub-DAG, is under investigation.
Further, the ability to limit scope of the affected depth within the
sub-DAG is under investigation (e.g. if a stateful node can proxy for
all nodes `behind' it, then there may be no need to propagate the
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triggered `D' bit further).
C.4. Source Routing
In support of nodes that 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.
C.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
Tim Winter (editor)
Email: wintert@acm.org
Pascal Thubert (editor)
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis 06410
FRANCE
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
ROLL Design Team
IETF ROLL WG
Email: rpl-authors@external.cisco.com
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