Networking Working Group T. Winter, Ed.
Internet-Draft
Intended status: Standards Track P. Thubert, Ed.
Expires: September 9, 2010 Cisco Systems
ROLL Design Team
IETF ROLL WG
March 8, 2010
RPL: IPv6 Routing Protocol for Low power and Lossy Networks
draft-ietf-roll-rpl-07
Abstract
Low power and Lossy Networks (LLNs) are a class of network in which
both the routers and their interconnect are constrained: LLN routers
typically operate with constraints on (any subset of) processing
power, memory and energy (battery), and their interconnects are
characterized by (any subset of) high loss rates, low data rates and
instability. LLNs are comprised of anything from a few dozen and up
to thousands of LLN routers, and support point-to-point traffic
(between devices inside the LLN), point-to-multipoint traffic (from a
central control point to a subset of devices inside the LLN) and
multipoint-to-point traffic (from devices inside the LLN towards a
central control point). This document specifies the IPv6 Routing
Protocol for LLNs (RPL), which provides a mechanism whereby
multipoint-to-point traffic from devices inside the LLN towards a
central control point, as well as point-to-multipoint traffic from
the central control point to the devices inside the LLN, is
supported. Support for point-to-point traffic is also available.
Status of this Memo
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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 Overview . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Topology . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1.1. Topology Identifiers . . . . . . . . . . . . . . . . . 9
3.1.2. DODAG Information . . . . . . . . . . . . . . . . . . 10
3.2. Instances, DODAGs, and DODAG Iterations . . . . . . . . . 11
3.3. Traffic Flows . . . . . . . . . . . . . . . . . . . . . . 13
3.3.1. Multipoint-to-Point Traffic . . . . . . . . . . . . . 13
3.3.2. Point-to-Multipoint Traffic . . . . . . . . . . . . . 13
3.3.3. Point-to-Point Traffic . . . . . . . . . . . . . . . . 13
3.4. Upward Routes and DODAG Construction . . . . . . . . . . . 13
3.4.1. DODAG Information Object (DIO) . . . . . . . . . . . . 14
3.4.2. DAG Repair . . . . . . . . . . . . . . . . . . . . . . 14
3.4.3. Grounded and Floating DODAGs . . . . . . . . . . . . . 15
3.4.4. Administrative Preference . . . . . . . . . . . . . . 15
3.4.5. Objective Function (OF) . . . . . . . . . . . . . . . 15
3.4.6. Distributed Algorithm Operation . . . . . . . . . . . 15
3.5. Downward Routes and Destination Advertisement . . . . . . 16
3.5.1. Destination Advertisement Object (DAO) . . . . . . . . 16
3.6. Routing Metrics and Constraints Used By RPL . . . . . . . 17
3.6.1. Loop Avoidance . . . . . . . . . . . . . . . . . . . . 18
3.6.2. Rank Properties . . . . . . . . . . . . . . . . . . . 19
4. ICMPv6 RPL Control Message . . . . . . . . . . . . . . . . . . 21
5. Upward Routes . . . . . . . . . . . . . . . . . . . . . . . . 22
5.1. DODAG Information Object (DIO) . . . . . . . . . . . . . . 22
5.1.1. DIO Base Format . . . . . . . . . . . . . . . . . . . 22
5.1.2. DIO Base Rules . . . . . . . . . . . . . . . . . . . . 24
5.1.3. DIO Suboptions . . . . . . . . . . . . . . . . . . . . 25
5.2. DODAG Information Solicitation (DIS) . . . . . . . . . . . 30
5.3. Upward Route Discovery and Maintenance . . . . . . . . . . 30
5.3.1. RPL Instance . . . . . . . . . . . . . . . . . . . . . 30
5.3.2. Neighbors and Parents within a DODAG Iteration . . . . 30
5.3.3. Neighbors and Parents across DODAG Iterations . . . . 31
5.3.4. DIO Message Communication . . . . . . . . . . . . . . 36
5.3.5. DIO Transmission . . . . . . . . . . . . . . . . . . . 36
5.3.6. DODAG Selection . . . . . . . . . . . . . . . . . . . 39
5.4. Operation as a Leaf Node . . . . . . . . . . . . . . . . . 39
5.5. Administrative Rank . . . . . . . . . . . . . . . . . . . 40
5.6. Collision . . . . . . . . . . . . . . . . . . . . . . . . 40
6. Downward Routes . . . . . . . . . . . . . . . . . . . . . . . 40
6.1. Destination Advertisement Object (DAO) . . . . . . . . . . 41
6.1.1. DAO Suboptions . . . . . . . . . . . . . . . . . . . . 42
6.2. Downward Route Discovery and Maintenance . . . . . . . . . 43
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6.2.1. Overview . . . . . . . . . . . . . . . . . . . . . . . 43
6.2.2. Mode of Operation . . . . . . . . . . . . . . . . . . 44
6.2.3. Destination Advertisement Parents . . . . . . . . . . 44
6.2.4. Operation of DAO Storing Nodes . . . . . . . . . . . . 45
6.2.5. Operation of DAO Non-storing Nodes . . . . . . . . . . 48
6.2.6. Scheduling to Send DAO (or no-DAO) . . . . . . . . . . 49
6.2.7. Triggering DAO Message from the Sub-DODAG . . . . . . 49
6.2.8. Sending DAO Messages to DAO Parents . . . . . . . . . 51
6.2.9. Multicast Destination Advertisement Messages . . . . . 52
7. Packet Forwarding and Loop Avoidance/Detection . . . . . . . . 52
7.1. Suggestions for Packet Forwarding . . . . . . . . . . . . 53
7.2. Loop Avoidance and Detection . . . . . . . . . . . . . . . 54
7.2.1. Source Node Operation . . . . . . . . . . . . . . . . 55
7.2.2. Router Operation . . . . . . . . . . . . . . . . . . . 55
8. Multicast Operation . . . . . . . . . . . . . . . . . . . . . 57
9. Maintenance of Routing Adjacency . . . . . . . . . . . . . . . 58
10. Guidelines for Objective Functions . . . . . . . . . . . . . . 59
11. RPL Constants and Variables . . . . . . . . . . . . . . . . . 61
12. Manageability Considerations . . . . . . . . . . . . . . . . . 62
12.1. Control of Function and Policy . . . . . . . . . . . . . . 62
12.1.1. Initialization Mode . . . . . . . . . . . . . . . . . 62
12.1.2. DIO Base option . . . . . . . . . . . . . . . . . . . 63
12.1.3. Trickle Timers . . . . . . . . . . . . . . . . . . . . 63
12.1.4. DAG Sequence Number Increment . . . . . . . . . . . . 64
12.1.5. Destination Advertisement Timers . . . . . . . . . . . 64
12.1.6. Policy Control . . . . . . . . . . . . . . . . . . . . 64
12.1.7. Data Structures . . . . . . . . . . . . . . . . . . . 65
12.2. Information and Data Models . . . . . . . . . . . . . . . 65
12.3. Liveness Detection and Monitoring . . . . . . . . . . . . 65
12.3.1. Candidate Neighbor Data Structure . . . . . . . . . . 65
12.3.2. Directed Acyclic Graph (DAG) Table . . . . . . . . . . 65
12.3.3. Routing Table . . . . . . . . . . . . . . . . . . . . 66
12.3.4. Other RPL Monitoring Parameters . . . . . . . . . . . 67
12.3.5. RPL Trickle Timers . . . . . . . . . . . . . . . . . . 67
12.4. Verifying Correct Operation . . . . . . . . . . . . . . . 67
12.5. Requirements on Other Protocols and Functional
Components . . . . . . . . . . . . . . . . . . . . . . . . 67
12.6. Impact on Network Operation . . . . . . . . . . . . . . . 67
13. Security Considerations . . . . . . . . . . . . . . . . . . . 67
14. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 67
14.1. RPL Control Message . . . . . . . . . . . . . . . . . . . 68
14.2. New Registry for RPL Control Codes . . . . . . . . . . . . 68
14.3. New Registry for the Control Field of the DIO Base . . . . 68
14.4. DODAG Information Object (DIO) Suboption . . . . . . . . . 69
15. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 69
16. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 70
17. References . . . . . . . . . . . . . . . . . . . . . . . . . . 71
17.1. Normative References . . . . . . . . . . . . . . . . . . . 71
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17.2. Informative References . . . . . . . . . . . . . . . . . . 72
Appendix A. Requirements . . . . . . . . . . . . . . . . . . . . 74
A.1. Protocol Properties Overview . . . . . . . . . . . . . . . 74
A.1.1. IPv6 Architecture . . . . . . . . . . . . . . . . . . 74
A.1.2. Typical LLN Traffic Patterns . . . . . . . . . . . . . 74
A.1.3. Constraint Based Routing . . . . . . . . . . . . . . . 74
A.2. Deferred Requirements . . . . . . . . . . . . . . . . . . 75
Appendix B. Examples . . . . . . . . . . . . . . . . . . . . . . 75
B.1. DAO Operation When Only the Root Node Stores DAO
Information . . . . . . . . . . . . . . . . . . . . . . . 75
B.2. DAO Operation When All Nodes Fully Store DAO
Information . . . . . . . . . . . . . . . . . . . . . . . 77
B.3. DAO Operation When Nodes Have Mixed Capabilities . . . . . 79
Appendix C. Outstanding Issues . . . . . . . . . . . . . . . . . 81
C.1. Additional Support for P2P Routing . . . . . . . . . . . . 81
C.2. Destination Advertisement / DAO Fan-out . . . . . . . . . 81
C.3. Source Routing . . . . . . . . . . . . . . . . . . . . . . 81
C.4. Address / Header Compression . . . . . . . . . . . . . . . 81
C.5. Managing Multiple Instances . . . . . . . . . . . . . . . 82
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 82
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1. Introduction
Low power and Lossy Networks (LLNs) consist of 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 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. As the RPL 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 differently. All "MUST" application
requirements that cannot be satisfied by RPL will be specifically
listed in the Appendix A, accompanied by a justification.
A network may run multiple instances of RPL concurrently. Each such
instance may serve different and potentially antagonistic constraints
or performance criteria. This document defines how a single instance
operates.
RPL is a generic protocol that is to be deployed by instantiating the
generic operation described in this document with a specific
objective function (OF) (which ties together metrics, constraints,
and an optimization objective) to realize a desired objective in a
given environment.
A set of companion documents to this specification will provide
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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
RPL does not rely on any particular features of a specific link layer
technology. RPL is designed to be able to operate 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].
Additionally, this document uses terminology from
[I-D.ietf-roll-terminology], and introduces the following
terminology:
DAG: Directed Acyclic Graph. A directed graph having the property
that all edges are oriented in such a way that no cycles exist.
All edges are contained in paths oriented toward and
terminating at one or more root nodes.
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.
Destination Oriented DAG (DODAG): A DAG rooted at a single
destination, i.e. at a single DAG root (the DODAG root) with no
outgoing edges.
DODAG root: A DODAG root is the DAG root of a DODAG.
Rank: The rank of a node in a DAG identifies the nodes position with
respect to a DODAG root. The farther away a node is from a
DODAG root, the higher is the rank of that node. The rank of a
node may be a simple topological distance, or may more commonly
be calculated as a function of other properties as described
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later.
DODAG parent: A parent of a node within a DODAG is one of the
immediate successors of the node on a path towards the DODAG
root. The DODAG parent of a node will have a lower rank than
the node itself. (See Section 3.6.2.1).
DODAG sibling: A sibling of a node within a DODAG is defined in this
specification to be any neighboring node which is located at
the same rank within a DODAG. Note that siblings defined in
this manner do not necessarily share a common DODAG parent.
(See Section 3.6.2.1).
Sub-DODAG The sub-DODAG of a node is the set of other nodes in the
DODAG that might use a path towards the DODAG root that
contains that node. Nodes in the sub-DODAG of a node have a
greater rank than that node itself (although not all nodes of
greater rank are necessarily in the sub-DODAG of that node).
(See Section 3.6.2.1).
DODAGID: The identifier of a DODAG root. The DODAGID must be unique
within the scope of a RPL Instance in the LLN.
DODAG Iteration: A specific sequence number iteration ("version") of
a DODAG with a given DODAGID.
RPL Instance: A set of possibly multiple DODAGs. A network may have
more than one RPL Instance, and a RPL node can participate in
multiple RPL Instances. Each RPL Instance operates
independently of other RPL Instances. This document describes
operation within a single RPL Instance. In RPL, a node can
belong to at most one DODAG per RPL Instance. The tuple
(RPLInstanceID, DODAGID) uniquely identifies a DODAG.
RPLInstanceID: Unique identifier of a RPL Instance.
DODAGSequenceNumber: A sequential counter that is incremented by the
root to form a new Iteration of a DODAG. A DODAG Iteration is
identified uniquely by the (RPLInstanceID, DODAGID,
DODAGSequenceNumber) tuple.
Up: Up refers to the direction from leaf nodes towards DODAG roots,
following the orientation of the edges within the DODAG.
Down: Down refers to the direction from DODAG roots towards leaf
nodes, going against the orientation of the edges within the
DODAG.
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Objective Code Point (OCP): An identifier, used to indicate which
Objective Function is in use for forming a DODAG. The
Objective Code Point is further described in
[I-D.ietf-roll-routing-metrics].
Objective Function (OF): Defines which routing metrics, optimization
objectives, and related functions are in use in a DODAG. The
Objective Function is further described in
[I-D.ietf-roll-routing-metrics].
Goal: The Goal is a host or set of hosts that satisfy a particular
application objective / OF. Whether or not a DODAG can provide
connectivity to a goal is a property of the DODAG. For
example, a goal might be a host serving as a data collection
point, or a gateway providing connectivity to an external
infrastructure.
Grounded: A DODAG is said to be grounded, when the root can reach
the Goal of the objective function.
Floating: A DODAG is floating if is not Grounded. A floating DODAG
is not expected to reach the Goal defined for the OF.
As they form networks, LLN devices often mix the roles of 'host' and
'router' when compared to traditional IP networks. In this document,
'host' refers to an LLN device that can generate but does not forward
RPL traffic, 'router' refers to an LLN device that can forward as
well as generate RPL traffic, and 'node' refers to any RPL device,
either a host or a router.
3. Protocol Overview
The aim of this section is to describe RPL in the spirit of
[RFC4101]. Protocol details can be found in further sections.
3.1. Topology
This section describes how the basic RPL topologies, and the rules by
which these are constructed, i.e. the rules governing DODAG
formation.
3.1.1. Topology Identifiers
RPL uses four identifiers to track and control the topology:
o The first is a RPLInstanceID. A RPLInstanceID identifies a set of
one or more DODAGs. All DODAGs in the same RPL Instance use the
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same OF. A network may have multiple RPLInstanceIDs, each of
which defines an independent set of DODAGs, which may be optimized
for different OFs and/or applications. The set of DODAGs
identified by a RPLInstanceID is called a RPL Instance.
o The second is a DODAGID. The scope of a DODAGID is a RPL
Instance. The combination of RPLInstanceID and DODAGID uniquely
identifies a single DODAG in the network. A RPL Instance may have
multiple DODAGs, each of which has an unique DODAGID.
o The third is a DODAGSequenceNumber. The scope of a
DODAGSequenceNumber is a DODAG. A DODAG is sometimes
reconstructed from the DODAG root, by incrementing the
DODAGSequenceNumber. The combination of RPLInstanceID, DODAGID,
and DODAGSequenceNumber uniquely identifies a DODAG Iteration.
o The fourth is rank. The scope of rank is a DODAG Iteration. Rank
establishes a partial order over a DODAG Iteration, defining
individual node positions with respect to the DODAG root.
3.1.2. DODAG Information
For each DODAG that a node is, or may become, a member of, the
implementation should conceptually keep track of the following
information. The data structures described in this section are
intended to illustrate a possible implementation to aid in the
description of the protocol, but are not intended to be normative.
o RPLInstanceID
o DODAGID
o DODAGSequenceNumber
o DAG Metric Container, including DAGObjectiveCodePoint
o A set of Destination Prefixes offered by the DODAG root and
available via paths upwards along the DODAG
o A set of DODAG parents
o A set of DODAG siblings
o A timer to govern the sending of RPL control messages
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3.2. Instances, DODAGs, and DODAG Iterations
Each RPL Instance constructs a routing topology optimized for a
certain Objective Function (OF). A RPL Instance may provide routes
to certain destination prefixes, reachable via the DODAG roots. A
single RPL Instance contains one or more Destination Oriented DAG
(DODAG) roots. These roots may operate independently, or may
coordinate over a non-LLN backchannel.
Each root has a unique identifier, the DODAGID.
A RPL Instance may comprise:
o a single DODAG with a single root
* For example, a DODAG optimized to minimize latency rooted at a
single centralized lighting controller in a home automation
application.
o multiple uncoordinated DODAGs with independent roots (differing
DODAGIDs)
* 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 DODAG, and further use
the formation of multiple DODAGs as a means to dynamically and
autonomously partition the network.
o a single DODAG with a single virtual root coordinating LLN sinks
(with the same DODAGID) over some non-LLN 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 DODAG.
o a combination of the above as suited to some application scenario.
Traffic is bound to a specific RPL 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 RPL
instance which enables certain (path) properties, for example to
follow paths optimized for low latency or low energy. The
provisioning or automated discovery of a mapping between a
RPLInstanceID and a type or service of application traffic is beyond
the scope of this specification.
An example of a RPL Instance comprising a number of DODAGs is
depicted in Figure 1. A DODAG Iteration (two "versions" of the same
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DODAG) is depicted in Figure 2.
+----------------------------------------------------------------+
| |
| +--------------+ |
| | | |
| | (R1) | (R2) (Rn) |
| | / \ | /| \ / | \ |
| | / \ | / | \ / | \ |
| | (A) (B) | (C) | (D) ... (F) (G) (H) |
| | /|\ |\ | / | |\ | | | |
| | : : : : : | : (E) : : : : : |
| | | / \ |
| +--------------+ : : |
| DODAG |
| |
+----------------------------------------------------------------+
RPL Instance
Figure 1: RPL Instance
+----------------+ +----------------+
| | | |
| (R1) | | (R1) |
| / \ | | / |
| / \ | | / |
| (A) (B) | \ | (A) |
| /|\ |\ | ------\ | /|\ |
| : : (C) : : | \ | : : (C) |
| | / | \ |
| | ------/ | \ |
| | / | (B) |
| | | |\ |
| | | : : |
| | | |
+----------------+ +----------------+
Sequence N Sequence N+1
Figure 2: DODAG Iteration
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3.3. Traffic Flows
3.3.1. Multipoint-to-Point Traffic
Multipoint-to-Point (MP2P) is a dominant traffic flow in many LLN
applications ([I-D.ietf-roll-building-routing-reqs],
[I-D.ietf-roll-home-routing-reqs], [RFC5673], [RFC5548]). The
destinations of MP2P flows are designated nodes that have some
application significance, such as providing connectivity to the
larger Internet or core private IP network. RPL supports MP2P
traffic by allowing MP2P destinations to be reached via DODAG roots.
3.3.2. Point-to-Multipoint Traffic
Point-to-multipoint (P2MP) is a traffic pattern required by several
LLN applications ([I-D.ietf-roll-building-routing-reqs],
[I-D.ietf-roll-home-routing-reqs], [RFC5673], [RFC5548]). RPL
supports P2MP traffic by using a destination advertisement mechanism
that provisions routes toward destination prefixes and away from
roots. Destination advertisements can update routing tables as the
underlying DODAG topology changes.
3.3.3. Point-to-Point Traffic
RPL DODAGs provide a basic structure for point-to-point (P2P)
traffic. For a RPL network to support P2P traffic, a root must be
able to route packets to a destination. Nodes within the network may
also have routing tables to destinations. A packet flows towards a
root until it reaches an ancestor that has a known route to the
destination.
RPL also supports the case where a P2P destination is a 'one-hop'
neighbor.
RPL neither specifies nor precludes additional mechanisms for
computing and installing more optimal routes to support arbitrary P2P
traffic.
3.4. Upward Routes and DODAG Construction
RPL provisions routes up towards DODAG roots, forming a DODAG
optimized according to the Objective Function (OF) in use. RPL nodes
construct and maintain these DODAGs through exchange of DODAG
Information Object (DIO) messages. Undirected links between siblings
are also identified during this process, which can be used to provide
additional diversity.
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3.4.1. DODAG Information Object (DIO)
A DIO identifies the RPL Instance, the DODAGID, the values used to
compute the RPL Instance's objective function, and the present DODAG
Sequence Number. It can also include additional routing and
configuration information. The DIO includes a measure derived from
the position of the node within the DODAG, the rank, which is used
for nodes to determine their positions relative to each other and to
inform loop avoidance/detection procedures. RPL exchanges DIO
messages to establish and maintain routes.
RPL adapts the rate at which nodes send DIO messages. When a DODAG
is detected to be inconsistent or needs repair, RPL sends DIO
messages more frequently. As the DODAG stabilizes, the DIO message
rate tapers off, reducing the maintenance cost of a steady and well-
working DODAG.
This document defines an ICMPv6 Message Type "RPL Control Message",
which is capable of carrying a DIO.
3.4.2. DAG Repair
RPL supports global repair over the DODAG. A DODAG Root may
increment the DODAG Sequence Number, thereby initiating a new DODAG
iteration. This institutes a global repair operation, revising the
DODAG and allowing nodes to choose an arbitrary new position within
the new DODAG iteration.
RPL supports mechanisms which may be used for local repair within the
DODAG iteration. The DIO message specifies the necessary parameters
as configured from the DODAG root. Local repair options include the
allowing a node, upon detecting a loss of connectivity to a DODAG it
is a member of, to:
o Poison its sub-DODAG by advertising an effective rank of INFINITY
to its sub-DODAG, OR detach and form a floating DODAG in order to
preserve inner connectivity within its sub-DODAG.
o Move down within the DODAG iteration (i.e. increase its rank) in a
limited manner, no further than a bound configured by the DODAG
root via the DIO so as not to count all the way to infinity. Such
a move may be undertaken after waiting an appropriate poisoning
interval, and should allow the node to restore connectivity to the
DODAG Iteration, if at all possible.
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3.4.3. Grounded and Floating DODAGs
DODAGs can be grounded or floating. A grounded DODAG offers
connectivity to to a goal. A floating DODAG offers no such
connectivity, and provides routes only to nodes within the DODAG.
Floating DODAGs may be used, for example, to preserve inner
connectivity during repair.
3.4.4. Administrative Preference
An implementation/deployment may specify that some DODAG roots should
be used over others through an administrative preference.
Administrative preference offers a way to control traffic and
engineer DODAG formation in order to better support application
requirements or needs.
3.4.5. Objective Function (OF)
The Objective Function (OF) implements the optimization objectives of
route selection within the RPL Instance. The OF is identified by an
Objective Code Point (OCP) within the DIO, and its specification also
indicates the metrics and constraints in use. The OF also specifies
the procedure used to compute rank within a DODAG iteration. Further
details may be found in [I-D.ietf-roll-routing-metrics],
[I-D.ietf-roll-of0], and related companion specifications.
By using defined OFs that are understood by all nodes in a particular
deployment, and by referencing these in the DIO message, RPL nodes
may work to build optimized LLN routes using a variety of application
and implementation specific metrics and goals.
In the case where a node is unable to encounter a suitable RPL
Instance using a known Objective Function, it may be configured to
join a RPL Instance using an unknown Objective Function - but in that
case only acting as a leaf node.
3.4.6. Distributed Algorithm Operation
A high level overview of the distributed algorithm which constructs
the DODAG is as follows:
o Some nodes are configured to be DODAG roots, with associated DODAG
configuration.
o Nodes advertise their presence, affiliation with a DODAG, routing
cost, and related metrics by sending link-local multicast DIO
messages.
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o Nodes may adjust the rate at which DIO messages are sent in
response to stability or detection of routing inconsistencies.
o Nodes listen for DIOs and use their information to join a new
DODAG, or to maintain an existing DODAG, as according to the
specified Objective Function and rank-based loop avoidance rules.
o Nodes provision routing table entries, for the destinations
specified by the DIO, via their DODAG parents in the DODAG
iteration. Nodes may provision a DODAG parent as a default
gateway.
o Nodes may identify DODAG siblings within the DODAG iteration to
increase path diversity.
o Using DIOs, and possibly information in data packets, RPL nodes
detect possible routing loops. When a RPL node detects a possible
routing loop, it may adapt its DIO transmission rate to apply a
local repair to the topology.
3.5. Downward Routes and Destination Advertisement
RPL constructs and maintains DODAGs with DIO messages to establish
upward routes: it uses Destination Advertisement Object (DAO)
messages to establish downward routes along the DODAG as well as
other routes. DAO messages are an optional feature for applications
that require P2MP or P2P traffic. DIO messages advertise whether
destination advertisements are enabled within a given DODAG.
3.5.1. Destination Advertisement Object (DAO)
A Destination Advertisement Object (DAO) conveys destination
information upwards along the DODAG so that a DODAG root (and other
intermediate nodes) can provision downward routes. A DAO message
includes prefix information to identify destinations, a capability to
record routes in support of source routing, and information to
determine the freshness of a particular advertisement.
Nodes that are capable of maintaining routing state may aggregate
routes from DAO messages that they receive before transmitting a DAO
message. Nodes that are not capable of maintaining routing state may
attach a next-hop address to the Reverse Route Stack contained within
the DAO message. The Reverse Route Stack is subsequently used to
generate piecewise source routes over regions of the LLN that are
incapable of storing downward routing state.
A special case of the DAO message, termed a no-DAO, is used to clear
downward routing state that has been provisioned through DAO
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operation.
This document defines an ICMPv6 Message Type "RPL Control Message",
which is capable of carrying a DAO.
3.5.1.1. 'One-Hop' Neighbors
In addition to sending DAOs toward DODAG roots, RPL nodes may
occasionally emit a link-local multicast DAO message advertising
available destination prefixes. This mechanism allow provisioning a
trivial 'one-hop' route to local neighbors.
3.6. Routing Metrics and Constraints Used By RPL
Routing metrics are used by routing protocols to compute shortest
paths. Interior Gateway Protocols (IGPs) such as IS-IS ([RFC5120])
and OSPF ([RFC4915]) use static link metrics. Such link metrics can
simply reflect the bandwidth or can also be computed according to a
polynomial function of several metrics defining different link
characteristics; in all cases they are static metrics. Some routing
protocols support more than one metric: in the vast majority of the
cases, one metric is used per (sub)topology. Less often, a second
metric may be used as a tie-breaker in the presence of Equal Cost
Multiple Paths (ECMP). The optimization of multiple metrics is known
as an NP complete problem and is sometimes supported by some
centralized path computation engine.
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 metric, that will satisfy all use cases.
In addition, RPL supports constrained-based routing where constraints
may be applied to both 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 specify which routing
metrics and constraints are in use, and how these are used, in
addition to the objectives used to compute the (constrained) shortest
path.
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Example 1: Shortest path: path offering the shortest end-to-end delay
Example 2: Constrained shortest path: the path that does not traverse
any battery-operated node and that optimizes the path
reliability
3.6.1. Loop Avoidance
RPL guarantees neither loop free path selection nor strong global
convergence. In order to reduce control overhead, however, such as
the cost of the count-to-infinity problem, RPL avoids creating loops
when undergoing topology changes. Furthermore, RPL includes rank-
based mechanisms for detecting loops when they do occur. RPL uses
this loop detection to ensure that packets make forward progress
within the DODAG iteration and trigger repairs when necessary.
3.6.1.1. Greediness and Rank-based Instabilities
Once a node has joined a DODAG iteration, RPL disallows certain
behaviors, including greediness, in order to prevent resulting
instabilities in the DODAG iteration.
If a node is allowed to be greedy and attempts to move deeper in the
DODAG iteration, beyond its most preferred parent, in order to
increase the size of the parent set, then an instability can result.
Suppose a node is willing to receive and process a DIO messages from
a node in its own sub-DODAG, and in general a node deeper than
itself. In this case, a possibility exists that a feedback loop is
created, wherein two or more nodes continue to try and move in the
DODAG iteration while attempting to optimize against each other. In
some cases, this will result in instability. It is for this reason
that RPL limits the cases where a node may process DIO messages from
deeper nodes to some forms of local repair. This approach creates an
'event horizon', whereby a node cannot be influenced beyond some
limit into an instability by the action of nodes that may be in its
own sub-DODAG.
3.6.1.2. DODAG Loops
A DODAG loop may occur when a node detaches from the DODAG and
reattaches to a device in its prior sub-DODAG. This may happen in
particular when DIO messages are missed. Strict use of the DAG
sequence number can eliminate this type of loop, but this type of
loop may possibly be encountered when using some local repair
mechanisms.
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3.6.1.3. 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 related DAO state. This loop happens
when a no-DAO was missed and persists until all state has been
cleaned up. 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 down routes, then DAO Loops should not occur on
the stateless portions of the path.
3.6.1.4. 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, in order to prevent
sibling loops.
3.6.2. Rank Properties
The rank of a node is a scalar representation of the location of that
node within a DODAG iteration. The rank is used to avoid and detect
loops, and as such must demonstrate certain properties. The exact
calculation of the rank is left to the Objective Function, and may
depend on parents, link metrics, and the node configuration and
policies.
The rank is not a cost metric, although its value can be derived from
and influenced by metrics. The rank has properties of its own that
are not necessarily those of all metrics:
Type: Rank is an abstract scalar. Some metrics are boolean (e.g.
grounded), others are statistical and better expressed as a
tuple like an expected value and a variance. Some OCPs use
not one but a set of metrics bound by a piece of logic.
Function: Rank is the expression of a relative position within a
DODAG iteration with regard to neighbors and is not
necessarily a good indication or a proper expression of a
distance or a cost to the root.
Stability: The stability of the rank determines the stability of the
routing topology. Some dampening or filtering might be
applied to keep the topology stable, and thus the rank does
not necessarily change as fast as some physical metrics
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would. A new DODAG iteration would be a good opportunity to
reconcile the discrepancies that might form over time between
metrics and ranks within a DODAG iteration.
Granularity: Rank is coarse grained. A fine granularity would
prevent the selection of siblings.
Properties: Rank is strictly monotonic, and can be used to validate
a progression from or towards the root. A metric, like
bandwidth or jitter, does not necessarily exhibit this
property.
Abstract: Rank does not have a physical unit, but rather a range of
increment per hop, where the assignment of each increment is
to be determined by the implementation.
The rank value feeds into DODAG parent selection, according to the
RPL loop-avoidance strategy. Once a parent has been added, and a
rank value for the node within the DODAG has been advertised, the
nodes further options with regard to DODAG parent selection and
movement within the DODAG are restricted in favor of loop avoidance.
3.6.2.1. Rank Comparison (DAGRank())
Rank may be thought of as a fixed point number, where the position of
the decimal point between the integer part and the fractional part is
determined by MinHopRankIncrease. MinHopRankIncrease is the minimum
increase in rank between a node and any of its DODAG parents. When
an objective function computes rank, the objective function operates
on the entire (i.e. 16-bit) rank quantity. When rank is compared,
e.g. for determination of parent/sibling relationships or loop
detection, the integer portion of the rank is to be used. The
integer portion of the Rank is computed by the DAGRank() macro as
follows:
DAGRank(rank) = floor(rank/MinHopRankIncrease)
MinHopRankIncrease is provisioned at the DODAG Root and propagated in
the DIO message. For efficient implementation the MinHopRankIncrease
SHOULD be a power of 2. An implementation may configure a value
MinHopRankIncrease as appropriate to balance between the loop
avoidance logic of RPL (i.e. selection of eligible parents and
siblings) and the metrics in use.
By convention in this document, using the macro DAGRank(node) may be
interpreted as DAGRank(node.rank), where node.rank is the rank value
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as maintained by the node.
A node A has a rank less than the rank of a node B if DAGRank(A) is
less than DAGRank(B).
A node A has a rank equal to the rank of a node B if DAGRank(A) is
equal to DAGRank(B).
A node A has a rank greater than the rank of a node B if DAGRank(A)
is greater than DAGRank(B).
3.6.2.2. Rank Relationships
The computation of the rank MUST be done in such a way so as to
maintain the following properties for any nodes M and N that are
neighbors in the LLN:
DAGRank(M) is less than DAGRank(N): In this case, the position of M
is closer to the DODAG root than the position of N. Node M
may safely be a DODAG parent for Node N without risk of
creating a loop. Further, for a node N, all parents in the
DODAG parent set must be of rank less than DAGRank(N). In
other words, the rank presented by a node N MUST be greater
than that presented by any of its parents.
DAGRank(M) equals DAGRank(N): In this case the positions of M and N
within the DODAG and with respect to the DODAG root are
similar (identical). In some cases, Node M may be used as a
successor by Node N, which however entails the chance of
creating a loop (which must be detected and resolved by some
other means).
DAGRank(M) is greater than DAGRank(N): In this case, the position of
M is farther from the DODAG root than the position of N.
Further, Node M may in fact be in the sub-DODAG of Node N. If
node N selects node M as DODAG parent there is a risk to
create a loop.
As an example, the 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 function being
used within the DODAG.
4. ICMPv6 RPL Control Message
This document defines the RPL Control Message, a new ICMPv6 message.
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In accordance with [RFC4443], the RPL Control Message has the
following format:
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 3: RPL Control Message
The RPL Control message is an ICMPv6 information message with a
requested Type of 155.
The Code field identifies the type of RPL Control Message. This
document defines three codes for the following RPL Control Message
types:
o 0x01: DODAG Information Solicitation (Section 5.2)
o 0x02: DODAG Information Object (Section 5.1)
o 0x04: Destination Advertisement Object (Section 6.1)
5. Upward Routes
This section describes how RPL discovers and maintains upward routes.
It describes DODAG Information Objects (DIOs), the messages used to
discover and maintain these routes. It specifies how RPL generates
and responds to DIOs. It also describes DODAG Information
Solicitation (DIS) messages, which are used to trigger DIO
transmissions.
5.1. DODAG Information Object (DIO)
The DODAG Information Object carries information that allows a node
to discover a RPL Instance, learn its configuration parameters,
select a DODAG parent set, and maintain the upward routing topology.
5.1.1. DIO Base Format
DIO Base is an always-present container option in a DIO message.
Every DIO MUST include a DIO Base.
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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|A|T|S|0| Prf | Sequence | Rank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID | DTSN | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+ +
| DODAGID |
+ +
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | sub-option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: DIO Base
Control Field: The DAG Control Field has three flags and one field:
Grounded (G): The Grounded (G) flag indicates whether the
upward routes this node advertises provide connectivity
to the set of addresses which are application-defined
goals. If the flag is set, the DODAG is grounded and
provides such connectivity. If the flag is cleared, the
DODAG is floating and may not provide such connectivity.
Destination Advertisement Supported (A): The Destination
Advertisement Supported (A) flag indicates whether the
root of this DODAG can collect and use downward route
state. If the flag is set, nodes in the network are
enabled to exchange destination advertisements messages
to build downward routes (Section 6). If the flag is
cleared, destination advertisement messages are disabled
and the DODAG maintains only upward routes.
Destination Advertisement Trigger (T): The Destination
Advertisement Trigger (T) flag indicates a complete
refresh of downward routes. If the flag is set, then a
refresh of downward route state is to take place over the
entire DODAG. If the flag is cleared, the downward route
maintenance is in its normal mode of operation. The
further details of this process are described in
Section 6.
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Destination Advertisements Stored (S): The Destination
Advertisements Stored (S) flag is used to indicate that a
non-root ancestor is storing routing table entries
learned from DAO messaging. If the flag is set, then a
non-root ancestor is known to be storing routing table
entries learned from DAO messages. If the flag is
cleared, only the root node may be storing routing table
entries learned from DAO messaging. This flag is further
described in Section 6.
DODAGPreference (Prf): A 3-bit unsigned integer that defines
how preferable the root of this DODAG is compared to
other DODAG roots within the instance. DAGPreference
ranges from 0x00 (least preferred) to 0x07 (most
preferred). The default is 0 (least preferred).
Section 5.3 describes how DAGPreference affects DIO
processing.
Unassigned bits of the Control Field are reserved. They MUST
be set to zero on transmission and MUST be ignored on
reception.
Sequence Number: 8-bit unsigned integer set by the DODAG root.
Section 5.3 describes the rules for sequence numbers and how
they affect DIO processing.
Rank: 16-bit unsigned integer indicating the DODAG rank of the node
sending the DIO message. Section 5.3 describes how Rank is set
and how it affects DIO processing.
RPLInstanceID: 8-bit field set by the DODAG root that indicates
which RPL Instance the DODAG is part of.
Destination Advertisement Trigger Sequence Number (DTSN): 8-bit
unsigned integer set by the node issuing the DIO message. The
Destination Advertisement Trigger Sequence Number (DTSN) flag
is used as part of the procedure to maintain downward routes.
The details of this process are described in Section 6.
DODAGID: 128-bit unsigned integer set by a DODAG root which uniquely
identifies a DODAG. Possibly derived from the IPv6 address of
the DODAG root.
5.1.2. DIO Base Rules
1. If the 'A' flag of a DIO Base is cleared, the 'T' flag MUST also
be cleared.
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2. For the following DIO Base fields, a node that is not a DODAG
root MUST advertise the same values as its preferred DODAG parent
(defined in Section 5.3.2). Therefore, if a DODAG root does not
change these values, every node in a route to that DODAG root
eventually advertises the same values for these fields. These
fields are:
1. Grounded (G)
2. Destination Advertisement Supported (A)
3. Destination Advertisement Trigger (T)
4. DAGPreference (Prf)
5. Sequence
6. RPLInstanceID
7. DODAGID
3. A node MAY update the following fields at each hop:
1. Destination Advertisements Stored (S)
2. DAGRank
3. DTSN
4. The DODAGID field each root sets MUST be unique within the RPL
Instance.
5.1.3. DIO Suboptions
This section describes the format of DIO suboptions and the five
suboptions this document defines: Pad 1, Pad N, DAG Metric Container,
DAG Destination Prefix, and DAG Configuration.
5.1.3.1. DIO Suboption Format
The Pad N, DAG Metric Container, DAG Destination Prefix, and DAG
Configuration suboptions all follow this format:
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 | Suboption Length | Suboption Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
Figure 5: DIO Suboption Generic Format
Suboption Type: 8-bit identifier of the type of suboption.
Suboption Length: 16-bit unsigned integer, representing the length
in octets of the suboption, not including the suboption Type
and Length fields.
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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 DODAG Information Object.
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 and continue to process
the following suboption, correctly handling any remaining options in
the message.
DIO message suboptions may have alignment requirements. Following
the convention in IPv6, options with alignment requirements 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).
5.1.3.2. Pad1
The Pad1 suboption format is as follows:
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type = 0 |
+-+-+-+-+-+-+-+-+
Figure 6: Pad 1
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.
5.1.3.3. PadN
The PadN suboption format is as follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Type = 1 | Suboption Length | Suboption Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
Figure 7: Pad N
The PadN suboption 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 Suboption 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.
5.1.3.4. Metric Container
The Metric Container suboption 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 | Suboption Length | Metric Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
Figure 8: Metric Container
The Metric Container is used to report metrics along the DODAG. The
Metric Container may contain a number of discrete node, link, and
aggregate path metrics as chosen by the implementer. The Suboption
Length field contains the length in octets of the Metric Data. The
order, content, and coding of the Metric Container data is as
specified in [I-D.ietf-roll-routing-metrics].
The processing and propagation of the Metric Container is governed by
implementation specific policy functions.
5.1.3.5. Destination Prefix
The Destination Prefix suboption format is as follows:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 | Suboption Length |Resvd|Prf|Resvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | |
+-+-+-+-+-+-+-+-+ |
| Destination Prefix (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9: DAG Destination Prefix
The Destination Prefix suboption is used to indicate that
connectivity to the specified destination prefix is available from
the DODAG root, or from another node located upwards along the DODAG
on the path to the DODAG root. 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 DODAG, a node MAY decide to join
multiple DODAGs in support of a particular application.
The Suboption 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. The
lifetime is initially set by the node that owns the prefix and
denotes the valid lifetime for that prefix (similar to
AdvValidLifetime [RFC4861]). The value might be reduced by the
originator and/or en-route nodes that will not provide connectivity
for the whole valid lifetime. A value of all one bits (0xFFFFFFFF)
represents infinity. A value of all zero bits (0x00000000) indicates
a loss of reachability.
The Prefix Length is an 8-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
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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.
5.1.3.6. DODAG Configuration
The DODAG Configuration suboption 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. | DIORedun. | MaxRankInc | MinHopRankInc |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: DODAG Configuration
The DODAG Configuration suboption is used to distribute configuration
information for DODAG Operation through the DODAG. The information
communicated in this suboption is generally static and unchanging
within the DODAG, therefore it is not necessary to include in every
DIO. This suboption MAY be included occasionally by the DODAG Root,
and MUST be included in response to a unicast request, e.g. a unicast
DODAG Information Solicitation (DIS) message.
The Length is coded as 5.
DIOIntervalDoublings is an 8-bit unsigned integer, configured on the
DODAG root and used to configure the trickle timer (see
Section 5.3.5.1 for details on trickle timers) governing when DIO
message should be sent within the DODAG. 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 DODAG
root and used to configure the trickle timer governing when DIO
message should be sent within the DODAG. The minimum configured
interval for the DIO trickle timer in units of ms is
2^DIOIntervalMin. For example, a DIOIntervalMin value of 16ms is
expressed as 4.
DIORedundancyConstant is an 8-bit unsigned integer used to configure
suppression of DIO transmissions. DIORedundancyConstant is the
minimum number of relevant incoming DIOs required to suppress a DIO
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transmission. If the value is 0xFF then the suppression mechanism is
disabled.
MaxRankInc, 8-bit unsigned integer, is the DAGMaxRankIncrease. This
is the allowable increase in rank in support of local repair. If
DAGMaxRankIncrease is 0 then this mechanism is disabled.
MinHopRankInc, 8-bit unsigned integer, is the MinHopRankIncrease as
described in Section 3.6.2.1.
5.2. DODAG Information Solicitation (DIS)
The DODAG Information Solicitation (DIS) message may be used to
solicit a DODAG 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 DODAGs. The DODAG Information
Solicitation carries no additional message body. Section 5.3.5
describes how nodes respond to a DIS.
5.3. Upward Route Discovery and Maintenance
Upward route discovery allows a node to join a DODAG by discovering
neighbors that are members of the DODAG and identifying a set of
parents. The exact policies for selecting neighbors and parents is
implementation-dependent. This section specifies the set of rules
those policies must follow for interoperability.
5.3.1. RPL Instance
A RPLInstanceID MUST be unique across an LLN.
A node MAY belong to multiple RPL Instances.
Within a given LLN, there may be multiple, logically independent RPL
instances. This document describes how a single instance behaves.
5.3.2. Neighbors and Parents within a DODAG Iteration
RPL's upward route discovery algorithms and processing are in terms
of three logical sets of link-local nodes. First, the candidate
neighbor set is a subset of the nodes that can be reached via link-
local multicast. The selection of this set is implementation-
dependent and OF-dependent. Second, the parent set is a restricted
subset of the candidate neighbor set. Finally, the preferred parent,
a set of size one, is an element of the parent set that is the
preferred next hop in upward routes.
More precisely:
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1. The DODAG parent set MUST be a subset of the candidate neighbor
set.
2. A DODAG root MUST have a DODAG parent set of size zero.
3. A node that is not a DODAG root MAY maintain a DODAG parent set
of size greater than or equal to one.
4. A node's preferred DODAG parent MUST be a member of its DODAG
parent set.
5. A node's rank MUST be greater than all elements of its DODAG
parent set.
6. When Neighbor Unreachability Detection (NUD), or an equivalent
mechanism, determines that a neighbor is no longer reachable, a
RPL node MUST NOT consider this node in the candidate neighbor
set when calculating and advertising routes until it determines
that it is again reachable. Routes through an unreachable
neighbor MUST be eliminated from the routing table.
These rules ensure that there is a consistent partial order on nodes
within the DODAG. As long as node ranks do not change, following the
above rules ensures that every node's route to a DODAG root is loop-
free, as rank decreases on each hop to the root. The OF can guide
candidate neighbor set and parent set selection, as discussed in
[I-D.ietf-roll-routing-metrics].
5.3.3. Neighbors and Parents across DODAG Iterations
The above rules govern a single DODAG iteration. The rules in this
section define how RPL operates when there are multiple DODAG
iterations:
5.3.3.1. DODAG Iteration
1. The tuple (RPLInstanceID, DODAGID, DODAGSequenceNumber) uniquely
defines a DODAG Iteration. Every element of a node's DODAG
parent set, as conveyed by the last heard DIO from each DODAG
parent, MUST belong to the same DODAG iteration. Elements of a
node's candidate neighbor set MAY belong to different DODAG
Iterations.
2. A node is a member of a DODAG iteration if every element of its
DODAG parent set belongs to that DODAG iteration, or if that node
is the root of the corresponding DODAG.
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3. A node MUST NOT send DIOs for DODAG iterations of which it is not
a member.
4. DODAG roots MAY increment the DODAGSequenceNumber that they
advertise and thus move to a new DODAG iteration. When a DODAG
root increments its DODAGSequenceNumber, it MUST follow the
conventions of Serial Number Arithmetic as described in
[RFC1982].
5. Within a given DODAG, a node that is a not a root MUST NOT
advertise a DODAGSequenceNumber higher than the highest
DODAGSequenceNumber it has heard. Higher is defined as the
greater-than operator in [RFC1982].
6. Once a node has advertised a DODAG iteration by sending a DIO, it
MUST NOT be member of a previous DODAG iteration of the same
DODAG (i.e. with the same RPLInstanceID, the same DODAGID, and a
lower DODAGSequenceNumber). Lower is defined as the less-than
operator in [RFC1982].
Within a particular implementation, a DODAG root may increment the
DODAGSequenceNumber periodically, at a rate that depends on the
deployment. In other implementations, loop detection may be
considered sufficient to solve routing issues, and the DODAG root may
increment the DODAGSequenceNumber only upon administrative
intervention. Another possibility is that nodes within the LLN have
some means by which they can signal detected routing inconsistencies
or suboptimalities to the DODAG root, in order to request an on-
demand DODAGSequenceNumber increment (i.e. request a global repair of
the DODAG).
When the DODAG parent set becomes empty on a node that is not a root,
(i.e. the last parent has been removed, causing the node to no longer
be associated with that DODAG), then the DODAG information should not
be suppressed until after the expiration of an implementation-
specific local timer in order to observe if the DODAGSequenceNumber
has been incremented, should any new parents appear for the DODAG.
As the DODAGSequenceNumber is incremented, a new DODAG Iteration
spreads outward from the DODAG root. Thus a parent that advertises
the new DODAGSequenceNumber can not possibly belong to the sub-DODAG
of a node that still advertises an older DODAGSequenceNumber. A node
may safely add such a parent, without risk of forming a loop, without
regard to its relative rank in the prior DODAG Iteration. This is
equivalent to jumping to a different DODAG.
As a node transitions to new DODAG Iterations as a consequence of
following these rules, the node will be unable to advertise the
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previous DODAG Iteration (prior DODAGSequenceNumber) once it has
committed to advertising the new DODAG Iteration.
During transition to a new DODAG Iteration, a node may decide to
forward packets via 'future parents' that belong to the same DODAG
(same RPLInstanceID and DODAGID), but are observed to advertise a
more recent (incremented) DODAGSequenceNumber.
5.3.3.2. DODAG Roots
1. A DODAG root that does not have connectivity to the set of
addresses described as application-level goals, MUST NOT set the
Grounded bit.
2. A DODAG root MUST advertise a rank of ROOT_RANK.
3. A node whose DODAG parent set is empty MAY become the DODAG root
of a floating DODAG. It MAY also set its DAGPreference such that
it is less preferred.
An LLN node that is a goal for the Objective Function is the root of
its own grounded DODAG, at rank ROOT_RANK.
In a deployment that uses a backbone link to federate a number of LLN
roots, it is possible to run RPL over that backbone and use one
router as a "backbone root". The backbone root is the virtual root
of the DODAG, and exposes a rank of BASE_RANK over the backbone. All
the LLN roots that are parented to that backbone root, including the
backbone root if it also serves as LLN root itself, expose a rank of
ROOT_RANK to the LLN, and are part of the same DODAG, coordinating
DODAGSequenceNumber and other DODAG root determined parameters with
the virtual root over the backbone.
5.3.3.3. DODAG Selection
The DODAGPreference (Prf) 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 DODAG while still meeting other optimization objectives,
then the node will generally seek to join the more preferred DODAG as
determined by the OF. All else being equal, it is left to the
implementation to determine which DODAG is most preferred, possibly
based on additional criteria beyond Prf and the OF.
5.3.3.4. Rank and Movement within a DODAG Iteration
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1. A node MUST NOT advertise a rank less than or equal to any member
of its parent set within the DODAG Iteration.
2. A node MAY advertise a rank lower than its prior advertisement
within the DODAG Iteration.
3. Let L be the lowest rank within a DODAG iteration that a given
node has advertised. Within the same DODAG Iteration, that node
MUST NOT advertise an effective rank higher than L +
DAGMaxRankIncrease. INFINITE_RANK is an exception to this rule:
a node MAY advertise an INFINITE_RANK at any time. (This
corresponds to a limited rank increase for the purpose of local
repair within the DODAG Iteration.)
4. A node MAY, at any time, choose to join a different DODAG within
a RPL Instance. Such a join has no rank restrictions, unless
that different DODAG is a DODAG Iteration of which that node has
previously been a member, in which case the rule of the previous
bullet (3) must be observed. Until a node transmits a DIO
indicating its new DODAG membership, it MUST forward packets
along the previous DODAG.
5. A node MAY, at any time after hearing the next
DODAGSequenceNumber Iteration advertised from suitable DODAG
parents, choose to migrate to the next DODAG Iteration within the
DODAG.
Conceptually, an implementation is maintaining a DODAG parent set
within the DODAG Iteration. Movement entails changes to the DODAG
parent set. Moving up does not present the risk to create a loop but
moving down might, so that operation is subject to additional
constraints.
When a node migrates to the next DODAG Iteration, the DODAG parent
and sibling sets need to be rebuilt for the new iteration. An
implementation could defer to migrate for some reasonable amount of
time, to see if some other neighbors with potentially better metrics
but higher rank announce themselves. Similarly, when a node jumps
into a new DODAG it needs to construct new DODAG parent/sibling sets
for this new DODAG.
When a node moves to improve its position, it must conceptually
abandon all DODAG parents and siblings with a rank larger than
itself. As a consequence of the movement it may also add new
siblings. Such a movement may occur at any time to decrease the
rank, as per the calculation indicated by the OF. Maintenance of the
parent and sibling sets occurs as the rank of candidate neighbors is
observed as reported in their DIOs.
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If a node needs to move down a DODAG that it is attached to, causing
the rank to increase, then it MAY poison its routes and delay before
moving as described in Section 5.3.3.5.
5.3.3.5. Poisoning a Broken Path
1. A node MAY poison, in order to avoid being used as an ancestor by
the nodes in its sub-DODAG, by advertising an effective rank of
INFINITE_RANK and resetting the associated DIO trickle timer to
cause this INFINITE_RANK to be announced promptly.
2. The node MAY advertise an effective rank of INFINITE_RANK for an
arbitrary number of DIO timer events, before announcing a new
rank.
3. As per Section 5.3.3.4, the node MUST advertise INFINITE_RANK
within the DODAG iteration in which it participates, if its
revision in rank would exceed the maximum rank increase.
An implementation may choose to employ this poisoning mechanism when
a node loses all of its current parents, i.e. the set of DODAG
parents becomes depleted, and it can not jump to an alternate DODAG.
An alternate mechanism is to form a floating DODAG.
The motivation for delaying announcement of the revised route through
multiple DIO events is to (i) increase tolerance to DIO loss, (ii)
allow time for the poisoning action to propagate, and (iii) to
develop an accurate assessment of its new rank. Such gains are
obtained at the expense of potentially increasing the delay before
portions of the network are able to re-establish upwards routes.
Path redundancy in the DODAG reduces the significance of either
effect, since children with alternate parents should be able to
utilize those alternates and retain their rank while the detached
parent re-establishes its rank.
Although an implementation may advertise INFINITE_RANK for the
purposes of poisoning, it is not expected to be equivalent to setting
the rank to INFINITE_RANK, and an implementation would likely retain
its rank value prior to the poisoning in some form, for purpose of
maintaining its effective position within (L + DAGMaxRankIncrease).
5.3.3.6. Detaching
1. A node unable to stay connected to a DODAG within a given DODAG
iteration MAY detach from this DODAG iteration. A node that
detaches becomes root of its own floating DODAG and SHOULD
immediately advertise this new situation in a DIO as an alternate
to poisoning.
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5.3.3.7. Following a Parent
1. If a node receives a DIO from one of its DODAG parents,
indicating that the parent has left the DODAG, that node SHOULD
stay in its current DODAG through an alternative DODAG parent, if
possible. It MAY follow the leaving parent.
A DODAG parent may have moved, migrated to the next DODAG Iteration,
or jumped to a different DODAG. A node should give some preference
to remaining in the current DODAG, if possible, but ought to follow
the parent if there are no other options.
5.3.4. DIO Message Communication
When an DIO message is received, 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 the sender of the DIO message is a member of the candidate
neighbor set, then the DIO is eligible for further processing.
5.3.4.1. DIO Message Processing
As DIO messages are received from candidate neighbors, the neighbors
may be promoted to DODAG parents by following the rules of DODAG
discovery as described in Section 5.3. When a node places a neighbor
into the DODAG parent set, the node becomes attached to the DODAG
through the new DODAG parent node.
The most preferred parent should be used to restrict which other
nodes may become DODAG parents. Some nodes in the DODAG parent set
may be of a rank less than or equal to the most preferred DODAG
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).
5.3.5. DIO Transmission
Each node maintains a timer, that governs when to multicast DIO
messages. This timer is a trickle timer, as detailed in
Section 5.3.5.1. The DIO Configuration Option includes the
configuration of a RPL Instance's trickle timer.
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o When a node detects or causes an inconsistency, it MUST reset the
trickle timer.
o When a node migrates to a new DODAG Iteration it MUST reset the
trickle timer to its minimum value
o When a node detects an inconsistency when forwarding a packet, as
detailed in Section 7.2, the node MUST reset the trickle timer.
o When a node receives a multicast DIS message, it MUST reset the
trickle timer to its minimum value.
o When a node receives a unicast DIS message, it MUST unicast a DIO
message in response, and the response MUST include the DODAG
Configuration Object. This provides a means that an interrogating
node may be guaranteed to receive the DODAG Configuration Object,
which otherwise might not be included at the option of the sender.
In this case the node SHOULD NOT reset the trickle timer.
o If a node is not a member of a DODAG, it MUST suppress
transmission of DIO messages.
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 DODAG (perhaps initially probing for a nearby DODAG with
an DIS message). Alternately, it MAY choose to root its own
floating DODAG and begin multicasting DIO messages using a default
trickle configuration. The second case may be advantageous if it
is desired for independent nodes to begin aggregating into
scattered floating DODAGs, in the absence of a grounded node, for
example in support of LLN installation and commissioning.
5.3.5.1. Trickle Timer for DIO Transmission
RPL treats the construction of a DODAG as a consistency problem, and
uses a trickle timer [Levis08] to control the rate of control
broadcasts.
For each DODAG that a node is part of (i.e. one DODAG per RPL
Instance), 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]
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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.
5.3.5.1.1. Resetting the Trickle Timer
The trickle timer for a DODAG is reset by:
1. Setting I_min and I_doublings to the values learned from the
DODAG root via a received DIO message.
2. Setting C to zero.
3. If I is not equal to I_min:
1. Setting I to I_min.
2. Setting T to a random value as described above.
3. Restarting the trickle timer to expire after a duration T
When a node learns about a DODAG through a DIO message, and makes the
decision to join this DODAG, it initializes the state of the trickle
timer by resetting the trickle timer and listening. Each time it
hears a redundant DIO message for this DODAG, it MAY increment C. The
exact determination of what constitutes a redundant DIO message is
left to an implementation; it could for example include DIOs that
advertise the same rank.
When the timer fires at time T, the node compares C to the redundancy
constant, DIORedundancyConstant. If C is less than that value, or if
the DIORedundancyConstant value is 0xFF, 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.
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5.3.5.1.2. Determination of Inconsistency
The trickle timer is reset whenever an inconsistency is detected
within the DODAG, for example:
o The node joins a new DODAG
o The node moves within a DODAG
o The node receives a DIO message from a DODAG parent that updates
the information learned from a prior DIO message for that DODAG
Parent
o A DODAG parent forwards a packet intended to move up, 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 DODAG parent has changed.
5.3.6. DODAG Selection
The DODAG selection is implementation and algorithm dependent. Nodes
SHOULD prefer to join DODAGs for RPLInstanceIDs 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 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 DODAGs MAY aggregate as much as
possible into larger DODAGs 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 DODAG parent.
5.4. Operation as a Leaf Node
In some cases a RPL node may attach to a DODAG as a leaf node only.
One example of such a case is when a node does not understand the RPL
Instance's OF. A leaf node does not extend DODAG connectivity but
still needs to advertise its presence using DIOs. A node operating
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as a leaf node must obey the following rules:
1. It MUST NOT transmit DIOs containing the DAG Metric Container.
2. Its DIOs must advertise a DAGRank of INFINITE_RANK.
3. It MAY transmit unicast DAOs as described in Section 6.2.
4. It MAY transmit multicast DAOs to the '1 hop' neighborhood as
described in Section 6.2.9.
5.5. Administrative Rank
In some cases it might be beneficial to adjust the rank advertised by
a node beyond that computed by the OF based on some implementation
specific policy and properties of the node. For example, 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.
5.6. 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 DODAG root of their own DODAGs. 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 left 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 DODAGs 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.
6. Downward Routes
This section describes how RPL discovers and maintains downward
routes. Messages containing the Destination Advertisement Object
(DAO), used to construct downward routes, are described. The
downward routes are necessary in support of P2MP flows, from the
DODAG roots toward the leaves. It specifies non-storing and storing
behavior of nodes with respect to DAO messaging and DAO routing table
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entries. Nodes, as according to their resources and the
implementation, may selectively store routing table entries learned
from DAO messages, or may instead propagate the DAO information
upwards while adding source routing information. A further
optimization is described whereby DAO messages may be used to
populate routing table entries for the '1-hop' neighbors, which may
be useful in some cases as a shortcut for P2P flows.
6.1. Destination Advertisement Object (DAO)
The Destination Advertisement Object (DAO) is used to propagate
destination information upwards along the DODAG.
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 | DAO Rank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID | Route Tag | Prefix Length | RRCount |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Prefix (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse Route Stack (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-option(s)...
+-+-+-+-+-+-+-+-+
Figure 11: The Destination Advertisement Object (DAO)
DAO Sequence: 16-bit unsigned integer. Incremented by the node that
owns the prefix for each new DAO message for that prefix.
DAO Rank: 16-bit unsigned integer indicating the DAO Rank associated
with the advertised Destination Prefix. The DAO Rank is
analogous to the Rank in the DIO message in that it may be used
to convey a relative distance to the Destination Prefix as
computed by the Objective Function in use over the DODAG. It
serves as a mechanism by which an ancestor node may order
alternate DAO paths.
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RPLInstanceID: 8-bit field indicating the topology instance
associated with the DODAG, as learned from the DIO.
Route Tag: 8-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: 8-bit unsigned integer. Number of valid leading bits
in the IPv6 Prefix.
RRCount: 8-bit unsigned integer. This counter is used to count the
number of entries in the Reverse Route Stack. A value of '0'
indicates that no Reverse Route Stack is present.
DAO Lifetime: 32-bit unsigned integer. The length of time in
seconds (relative to the time the packet is sent) that the
prefix is valid for route determination. A value of all one
bits (0xFFFFFFFF) represents infinity. A value of all zero
bits (0x00000000) indicates a loss of reachability.
Destination Prefix: Variable-length field identifying an IPv6
destination address, prefix, or multicast group. 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.
6.1.1. DAO Suboptions
The DAO message may optionally include a number of suboptions.
The DAO suboptions are in the same format as the DIO Suboptions
described in Section 6.1.1.
In particular, a DAO message may include a DAG Metric Container
suboption as described in Section 5.1.3.4. This suboption may be
present in implementations where the DAO Rank is insufficient to
optimize a path to the DAO Destination Prefix.
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6.2. Downward Route Discovery and Maintenance
6.2.1. Overview
Destination Advertisement operation produces DAO messages that flow
up the DODAG, provisioning downward routing state for destination
prefixes available in the sub-DODAG of the DODAG root, and possibly
other nodes. The routing state provisioned with this mechanism is in
the form of soft-state routing table entries. DAO messages are able
to record loose source routing information as by propagate up the
DODAG. This mechanism is flexible to support the provisioning of
paths which consist of fully specified source routes, piecewise
source routes, or hop-by-hop routes as according to the
implementation and the capabilities of the nodes.
Destination Advertisement may or may not be enabled over a DODAG
rooted at a DODAG root. This is an a priori configuration determined
by the implementation/deployment and not generally changed during the
operation of the RPL LLN.
When Destination Advertisement is enabled:
1. Some nodes in the LLN MAY store at least one routing table entry
for a particular destination learned from a DAO. Such a node is
termed a 'storing node', with respect to that particular
destination.
2. Some nodes are capable to store at least one routing table entry
for every unique destination observed from all DAOs that pass
through. Such a node is termed a 'fully storing node'.
3. DODAG roots nodes SHOULD be fully-storing nodes.
4. Other nodes in the DODAG are not required to store routing table
entries for any particular destinations observed in DAOs. Nodes
that do not store routing table entries from DAOs are termed
'non-storing nodes', with respect to a particular destination.
5. Non-storing nodes MUST participate in the construction of
piecewise source routes as they propagate the DAO message, as
described in Section 6.2.5.
6. Storing nodes MUST store any source route information received
from the DAO (RRStack) in the routing table entry entry. If a
node is not capable to do this then it must act as a non-storing
node with respect to that particular destination.
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7. Storing nodes MUST use piecewise source routes in order to
forward data across a non-storing region of the LLN. The source
routing mechanism is to be described in a companion
specification. (If a node is not capable to do this, then that
node MUST NOT operate as a storing node).
6.2.2. Mode of Operation
o DAO Operation may not be required for all use cases.
o Some applications may only need support for collection/upward/MP2P
flow with no acknowledgement/reciprocal traffic.
o Some DODAGs may not support DAO Operation, which could mean that
DAO Operation is wasteful overhead.
o As a special case, multicast DAO operation may be used to populate
'one-hop' neighborhood routing table entries, and is distinct from
the unicast DAO operation used to establish downward routes along
the DODAG.
1. The 'A' flag in the DIO as conveyed from the DODAG root serves to
enable/disable DAO operation over the entire DODAG. This flag
should be administratively provisioned a priori at the DODAG root
as a function of the implementation/deployment and not tend to
change.
2. When DAO Operation is disabled, a node SHOULD NOT emit DAOs.
3. When DAO Operation is disabled, a node MAY ignore received DAOs.
6.2.3. Destination Advertisement Parents
o Nodes will select a subset of their DODAG Parents to whom DAOs
will be sent
* This subset is the set of 'DAO Parents'
* Each DAO parent MUST be a DODAG Parent. (Not all DODAG parents
need to be DAO parents).
* Operation with more than DAO Parent requires consideration of
such issues as DAO fan-out and path diversity, to be elaborated
in a future version of this specification.
o The selection of DAO parents is implementation specific and may be
based on selecting the DODAG Parents that offer the best upwards
cost (as opposed to downwards or mixed), as determined by the
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metrics in use and the Objective Function.
o When DAO messages are unicast to the DAO Parent, the identity of
the DAO Parent (DODAGID x DAGSequenceNumber) combined with the
RPLInstanceID in the DAO message unambiguously associates the DAO
message, and thus the particular destination prefix, with a DODAG
Iteration.
o When DAO messages are unicast to the DAO Parent, the DAO Rank may
be updated as according to the implementation and Objective
Function in use to reflect the relative (aggregated) cost of
reaching the Destination Prefix through that DAO parent. As a
further extension, a DAO Suboption for the Metric Container may be
included.
6.2.4. Operation of DAO Storing Nodes
6.2.4.1. DAO Routing Table Entry
A DAO Routing Table Entry conceptually contains the following
elements:
o Advertising Neighbor Information
* IPv6 Addr
* Interface ID
o To which DAO Parents has this entry been reported
o Retry Counter
o Logical equivalent of DAO Content:
* DAO Sequence
* DAO Rank
* DAO Lifetime
* Route tag (used to prioritize which destination entries should
be stored)
* Destination Prefix (or Address or Mcast Group)
* RR Stack*
The DAO Routing Table Entry is logically associated with the
following states:
CONNECTED This entry is 'owned' by the node - it is manually
configured and is considered as a 'self' entry for DAO
Operation
REACHABLE This entry has been reported from a neighbor of the node.
This state includes the following substates:
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CONFIRMED This entry is active, newly validated, and
usable
PENDING This entry is active, awaiting validation, and
usable. A Retry Counter is associated with
this substate
UNREACHABLE This entry is being cleaned up. This entry may be
suppressed when the cleanup process is complete.
When an attempt is to be made to report the DAO entry to DAO Parents,
the DAO Entry record is logically marked to indicate that an attempt
has not yet been made for each parent. As the unicast attempts are
completed for each parent, this mark may be cleared. This mechanism
may serve to limit DAO entry updates for each parent to a subset that
needs to be reported.
6.2.4.1.1. DAO Routing Table Entry Management
+---------------------------------+
| |
| REACHABLE | +-------------+
| | | |
| +-----------+ | | CONNECTED |
(*)----------->| |-------+ | | |
| | Confirmed | | | +-------------+
| +-->| |---+ | |
| | +-----------+ | | |
| | | | |
| | | | |
| | | | |
| | +-----------+ | | | +-------------+
| | | |<--+ +-------->| |
| +---| Pending | | | UNREACHABLE |
| | |---------------->| |--->(*)
| +-----------+ | +-------------+
| |
+---------------------------------+
DAO Routing Table Entry FSM
6.2.4.1.1.1. Operation in the CONNECTED state
1. CONNECTED DAO entries are to be provisioned outside of the
context of RPL, e.g. through a management API. An implementation
SHOULD provide a means to provision/manage CONNECTED DAO entries,
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including whether they are to be redistributed in RPL.
6.2.4.1.1.2. Operation in the REACHABLE state
1. When a REACHABLE(*) entry times out, i.e. the DAO Lifetime has
elapsed, the entry MUST be placed into the UNREACHABLE state and
no-DAO SHOULD be scheduled to send to the node's DAO Parents.
2. When a no-DAO for a REACHABLE(*) entry is received with a newer
DAO Sequence Number, the entry MUST be placed into the
UNREACHABLE state and no-DAO SHOULD be scheduled to send to the
node's DAO Parents.
3. When a REACHABLE(*) entry is to be removed because NUD or
equivalent has determined that the next-hop neighbor is no longer
reachable, the entry MUST be placed into the UNREACHABLE state
and no-DAO SHOULD be scheduled to send to the node's DAO Parents.
4. When a REACHABLE(*) entry is to be removed because an associated
Forwarding Error has been returned by the next-hop neighbor, the
entry MUST be placed into the UNREACHABLE state and no-DAO SHOULD
be scheduled to send to the node's DAO Parents.
5. When a DAO (or no-DAO) for a REACHABLE(*) entry is received with
an older or unchanged DAO Sequence Number, then the DAO (or no-
DAO) SHOULD be ignored and the associated entry MUST NOT be
updated with the stale information.
6.2.4.1.1.2.1. REACHABLE(Confirmed)
1. When a DAO for a previously unknown (or UNREACHABLE) destination
is received and is to be stored, it MUST be entered into the
routing table in the REACHABLE(Confirmed) state, and a DAO SHOULD
be scheduled to send to the node's DAO Parents. Alternately the
node may behave as a non-storing node with respect to this
destination.
2. When a DAO for a REACHABLE(Confirmed) entry is received with a
newer DAO Sequence Number, the entry MUST be updated with the
logical equivalent of the DAO contents and a DAO SHOULD be
scheduled to send to the node's DAO Parents.
3. When a DAO for a REACHABLE(Confirmed) entry is expected, e.g.
because a DIO to request a DAO refresh is sent, then the DAO
entry MUST be placed in the REACHABLE(Pending) state and the
associated Retry Counter MUST be set to 0.
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6.2.4.1.1.2.2. REACHABLE(Pending)
1. When a DAO for a REACHABLE(Pending) entry is received with a
newer DAO Sequence Number, the entry MUST be updated with the
logical equivalent of the DAO contents and the entry MUST be
placed in the REACHABLE(Confirmed) state.
2. When a DAO for a REACHABLE(Pending) entry is expected, e.g.
because DAO has (again) been triggered with respect to that
neighbor, then the associated Retry Counter MUST be incremented.
3. When the associated Retry Counter for a REACHABLE(Pending) entry
reaches a maximum threshold, the entry MUST be placed into the
UNREACHABLE state and no-DAO SHOULD be scheduled to send to the
node's DAO Parents.
6.2.4.1.1.3. Operation in the UNREACHABLE state
1. An implementation SHOULD bound the time that the entry is
allocated in the UNREACHABLE state. Upon the equivalent expiry
of the related timer (RemoveTimer), the entry SHOULD be
suppressed.
2. While the entry is in the UNREACHABLE state a node SHOULD make a
reasonable attempt to report a no-DAO to each of the DAO parents.
3. When the node has completed an attempt to report a no-DAO to each
of the DAO parents, the entry SHOULD be suppressed.
6.2.5. Operation of DAO Non-storing Nodes
1. When a DAO is received from a child by a node who will not store
a routing table entry for the DAO, the node MUST schedule to pass
the DAO contents along to its DAO parents. Prior to passing the
DAO along, the node MUST process the DAO as follows, in order
that information necessary to construct a loose source route may
be accumulated within the DAO payload as it moves up the DODAG:
1. The most recent addition to the RRStack (the 'next waypoint')
is investigated to determine if the node already has a route
provisioned to the waypoint. If the node already has such a
route, then it is not necessary to add additional information
to the RRStack. The node SHOULD NOT modify the RRStack
further.
2. If the node does not have a route provisioned to the next
waypoint, then the node MUST append the address of the child
to the RRStack, and increment RRCount.
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6.2.6. Scheduling to Send DAO (or no-DAO)
1. An implementation SHOULD arrange to rate-limit the sending of
DAOs.
2. When scheduling to send a DAO, an implementation SHOULD
equivalently start a timer (DelayDAO) to delay sending the DAO.
If the DelayDAO timer is already running then the DAO may be
considered as already scheduled, and implementation SHOULD leave
the timer running at its present duration.
o When computing the delay before sending a DAO, in order to
increase the effectiveness of aggregation, an implementation MAY
allow time to receive DAOs from its sub-DODAG prior to emitting
DAOs to its DAO Parents.
* The scheduled delay in such cases may be, for example, such
that DAO_LATENCY/DAGRank(self_rank) <= DelayDAO < DAO_LATENCY/
DAGRank(parent_rank), where DAGRank() is defined as in
Section 3.6.2, such that nodes deeper in the DODAG may tend to
report DAO messages first before their parent nodes will report
DAO messages. Note that this suggestion is intended as an
optimization to allow efficient aggregation -- it is not
required for correct operation in the general case.
6.2.7. Triggering DAO Message from the Sub-DODAG
Triggering DAO messages from the Sub-DODAG occurs by using the
following control fields with the rules described below:
The DTSN field from the DIO is a sequence number that is part of the
mechanism to trigger DAO messages. The motivation to use a sequence
number is to provide some means of reliable signaling to the sub-
DODAG-- whereas a control flag that is activated for a short time may
be unobserved by the sub-DODAG if the triggering DIO messages are
lost, the DTSN increment may be observed later even if some DIO
messages have been lost since the sequence number increment.
The 'T' flag provides a way to signal the refresh of DAO information
over the entire DODAG iteration. Whereas a DTSN increment may only
trigger a DAO refresh as far as the nearest storing node (because a
storing node will not increment its own DTSN in response, as
described in the rules below), the assertion of the 'T' flag in
conjunction with an incremented DTSN will 'punch through' storing
nodes to elicit a DAO refresh from the entire DODAG Iteration.
The 'S' flag provides a way to signal to a sub-DODAG that there is at
least one non-root node somewhere in the set of DODAG ancestors,
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where that non-root node is a storing node. This allows for an
optimization-- when it is clear to a non-storing node that the root
node can be the only storing ancestor, then that node does not
necessarily need to trigger updates from its sub-DODAG when it
modifies its DAO parent set. The motivation here is that the root
node should be able to update its stored source routing information
for the affected sub-DODAG based only on receiving DAO information
concerning the link that changed. In the other case, when the 'S'
flag is set, the non-storing node does not have a means to determine
which DAO information may (or may not) need to be updated in the
intermediate storing node so it must trigger DAO messages in order to
update the intermediate storing node. Please note that some aspects
of the proper use of the 'S' flag remain under investigation.
Further examples of triggering DAO messages are contained in
Appendix B.
The control fields are used to trigger DAO messages as follows:
1. The DODAG root MUST clear the 'S' flag when it emits DIO
messages.
2. Non-root nodes that store routing table entries learned from
DAOs MUST set the 'S' flag when they emit DIO messages.
3. A node that has any DAO Parent with the 'S' flag set MUST also
set the 'S' flag when it emits DIO messages.
4. A node that has all DAO Parents with cleared 'S' flags, and does
not store routing table entries learned from DAOs, MUST clear
the 'S' flag when it emits DIO messages.
5. A DAO Trigger Sequence Number (DTSN) MUST be maintained by each
node per RPL Instance. The DTSN, in conjunction with the 'T'
flag from the DIO message, provides a means by which DAO
messages may be reliably triggered in the event of topology
change.
6. The DTSN MUST be advertised by the node in the DIO message.
7. A node keeps track of the DTSN that it has heard from the last
DIO from each of its DAO Parents. Note that there is one DTSN
maintained per DAO Parent- each DAO Parent may independently
increment it at will.
8. A node that is not a fully-storing node SHOULD increment its own
DTSN when it adds a new parent, that parent having the 'S' flag
set, to its DAO Parent set. It MAY defer advertising the
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increment as long as it has a DAO parent that already provides
adequate connectivity.
9. A node that is not a fully-storing node MUST increment its own
DTSN when it receives a DIO from a DAO Parent that contains a
newly incremented DTSN. (The newly incremented DTSN is detected
by comparing the value received in the DIO with the value last
recorded for that DAO parent).
10. A fully-storing node MUST increment its own DTSN when it
receives a DIO from a DAO Parent that contains a newly
incremented DTSN and a set 'T' flag.
11. When a storing or non-storing node joins a new DODAG iteration,
it SHOULD increment its DTSN as if the 'T' flag has been set.
12. DAO Transmission SHOULD be scheduled when a new parent is added
to the DAO Parent set.
13. A node that receives a newly incremented DTSN from a DAO Parent
MUST schedule a DAO transmission.
o When a node that is not fully-storing sees a DTSN increment, it
will increment its own DTSN. This will cause the DTSN increment
to extend down the DODAG to the first fully-storing node, which
will send its DAOs back up, rebuilding source routes information
along the way to the first node that incremented the DTSN, who
then may report the new DAO information to its new parent.
o When a fully-storing node sees a DTSN increment, it is caused to
reissue its entire set of routing table entries learned from DAOs
(or an aggregated subset thereof), but will not need to increment
its own DTSN. The 'DTSN increment wave' stops when it encounters
fully-storing nodes.
o When a fully-storing node sees a DTSN increment AND the 'T' flag
is set, it does increment its own DTSN as well. The 'T' flag
'punches through' all nodes, causing all routing tables in the
entire sub-DODAG to be refreshed.
6.2.8. Sending DAO Messages to DAO Parents
1. When storing nodes send DAO messages for stored entries the
RRStack SHOULD be cleared in the DAO message.
2. DAO Messages sent to DAO Parents MUST be unicast.
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* The IPv6 Source Address is the node sending the DAO message.
* The IPv6 Destination Address is DAO parent.
3. When the appointed time arrives (DelayDAO) for the transmission
of DAO messages (with jitter as appropriate) for the requested
entries, the implementation MAY aggregate the the entries into a
reduced numbers of DAOs to be reported to each parent, and
perform compression if possible.
4. Note: it is NOT RECOMMENDED that a DAO Transmission (No-DAO) be
scheduled when a DAO Parent is removed from the DAO Parent set.
6.2.9. Multicast Destination Advertisement Messages
A special case of DAO operation, distinct from unicast DAO operation,
is multicast DAO operation which may be used to populate '1-hop'
routing table entries.
1. A node MAY multicast a DAO message to the link-local scope all-
nodes multicast address FF02::1.
2. A multicast DAO message MUST be used only to advertise
information about self, i.e. prefixes directly connected to or
owned by this node, such as a multicast group that the node is
subscribed to or a global address owned by the node.
3. A multicast DAO message MUST NOT be used to relay connectivity
information learned (e.g. through unicast DAO) from another node.
4. Information obtained from a multicast DAO MAY be installed in the
routing table and MAY be propagated by a node in unicast DAOs.
5. A node MUST NOT perform any other DAO related processing on a
received multicast DAO, in particular a node MUST NOT perform the
actions of a DAO parent upon receipt of a multicast DAO.
o The multicast DAO may be used to enable direct P2P communication,
without needing the RPL routing structure to relay the packets.
o The multicast DAO does not presume any DODAG relationship between
the emitter and the receiver.
7. Packet Forwarding and Loop Avoidance/Detection
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7.1. Suggestions for 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 DODAG iteration that matches the RPLInstanceID
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 down the sub-
DODAG), then use that successor.
5. If there is a DODAG iteration offering a route to a prefix
matching the destination, then select one of those DODAG parents
as a successor.
6. If there is a DODAG parent offering a default route then select
that DODAG parent as a successor.
7. If there is a DODAG iteration offering a route to a prefix
matching the destination, but all DODAG parents have been tried
and are temporarily unavailable (as determined by the forwarding
procedure), then select a DODAG sibling as a successor.
8. Finally, if no DODAG siblings are available, the packet is
dropped. ICMP Destination Unreachable may be invoked. An
inconsistency is detected.
TTL MUST be decremented when forwarding. If the packet is being
forwarded via a sibling, then the TTL MAY be decremented more
aggressively (by more than one) to limit the impact of possible
loops.
Note that 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 up to an down flow,
such as switching from DIO routes to DAO routes as the destination is
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neared.
7.2. Loop Avoidance and 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 IPv6 flow label. The IPv6 flow label is defined in [RFC2460] and
its operation is further specified in [RFC3697]. For the purpose of
RPL operations, the flow label is constructed 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|O|S|R|F| SenderRank | RPLInstanceID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12: RPL Flow Label
Down 'O' bit: 1-bit flag indicating whether the packet is expected
to progress up or down. A router sets the 'O' bit when the
packet is expect to progress down (using DAO routes), and
resets it when forwarding towards the root of the DODAG
iteration. 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.
Forwarding-Error 'F' bit: 1-bit flag indicating that this node can
not forward the packet further towards the destination. The
'F' bit might be set by sibling that can not forward to a
parent a packet with the Sibling 'S' bit set, or by a child
node that does not have a route to destination for a packet
with the down 'O' bit set. A host MUST set the bit to 0.
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SenderRank: 8-bit field set to zero by the source and to
DAGRank(rank) by a router that forwards inside the RPL network.
(Note that the case where DAGRank(rank) does not fit into 8
bits is under investigation.)
RPLInstanceID: 8-bit field indicating the DODAG instance along which
the packet is sent.
7.2.1. Source Node Operation
A packet that is sourced at a node connected to a RPL network or
destined to a node connected to a RPL network MUST be issued with the
flow label zeroed out, but for the RPLInstanceID field.
If the source is aware of the RPLInstanceID that is preferred for the
flow, then it MUST set the RPLInstanceID field in the flow label
accordingly, otherwise it MUST set it to the RPL_DEFAULT_INSTANCE.
If a compression mechanism such as 6LoWPAN is applied to the packet,
the flow label MUST NOT be compressed even if it is set to all
zeroes.
7.2.2. Router Operation
7.2.2.1. Conformance to RFC 3697
[RFC3697] mandates that the Flow Label value set by the source MUST
be delivered unchanged to the destination node(s).
In order to restore the flow label to its original value, an RPL
router that delivers a packet to a destination connected to a RPL
network or that routes a packet outside the RPL network MUST zero out
all the fields but the RPLInstanceID field that must be delivered
without a change.
7.2.2.2. Instance Forwarding
Instance IDs are used to avoid loops between DODAGs from different
origins. DODAGs that constructed for antagonistic constraints might
contain paths that, if mixed together, would yield loops. Those
loops are avoided by forwarding a packet along the DODAG that is
associated to a given instance.
The RPLInstanceID is placed by the source in the flow label. This
RPLInstanceID MUST match the RPL Instance onto which the packet is
placed by any node, be it a host or router.
When a router receives a packet that specifies a given RPLInstanceID
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and the node can forward the packet along the DODAG associated to
that instance, then the router MUST do so and leave the RPLInstanceID
flag unchanged.
If any node can not forward a packet along the DODAG associated to
the RPLInstanceID in the flow label, then the node SHOULD discard the
packet.
7.2.2.3. DAG Inconsistency Loop Detection
The DODAG is inconsistent if 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 down) from a node of a higher rank.
the 'O' bit reset (for up) from a node of a lesser rank.
the 'S' bit set (to sibling) from a node of a different rank.
When the DODAG root increments the DODAGSequenceNumber a temporary
rank discontinuity may form between the next iteration and the prior
iteration, in particular if nodes are adjusting their rank in the
next iteration and deferring their migration into the next iteration.
A router that is still a member of the prior iteration may choose to
forward a packet to a (future) parent that is in the next iteration.
In some cases this could cause the parent to detect an inconsistency
because the rank-ordering in the prior iteration is not necessarily
the same as in the next iteration and the packet may be judged to not
be making forward progress. If the sending router is aware that the
chosen successor has already joined the next iteration, then the
sending router MUST update the SenderRank to INFINITE_RANK as it
forwards the packets across the discontinuity into the next DODAG
iteration in order to avoid a false detection of rank inconsistency.
One inconsistency along the path is not considered as a critical
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.
7.2.2.4. Sibling Loop Avoidance
When a packet is forwarded along siblings, it cannot be checked for
forward progress and may loop between siblings. Experimental
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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 then the router
SHOULD return the packet to the sibling that that passed it with the
Forwarding-Error 'F' bit set.
7.2.2.5. DAO Inconsistency Loop Detection and Recovery
A DAO inconsistency happens when router that has an down 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-DODAG.
In a general manner, a packet that goes down should never go up
again. If DAO inconsistency loop recovery is applied, then the
router SHOULD send the packet to the parent that passed it with the
Forwarding-Error 'F' bit set. Otherwise the router MUST silently
discard the packet.
7.2.2.6. Forward Path Recovery
Upon receiving a packet with a Forwarding-Error bit set, the node
MUST remove the routing states that caused forwarding to that
neighbor, clear the Forwarding-Error bit and attempt to send the
packet again. The packet may its way to an alternate neighbor. If
that alternate neighbor still has an inconsistent DAO state via this
node, the process will recurse, this node will set the Forwarding-
Error 'F' bit and the routing state in the alternate neighbor will be
cleaned up as well.
8. 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 up. Wherever the following text mentions
Multicast Listener Discovery (MLD), one can read MLDv2 ([RFC3810]) or
v3.
As is traditional, a listener uses a protocol such as MLD with a
router to register to a multicast group.
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Along the path between the router and the DODAG root, 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-DODAG 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
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
DODAG 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 down the
DODAG based on the multicast routing table entries installed from the
DAO message.
For a source inside the DODAG, the packet is passed to the preferred
parents, and if that fails then to the alternates in the DODAG. The
packet is also copied to all the registered children, except for the
one that passed the packet. Finally, if there is a listener in the
external infrastructure then the DODAG root has to further propagate
the packet into the external infrastructure.
As a result, the DODAG 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 DODAG is grounded or floating, the root can serve inner multicast
streams at all times.
9. Maintenance of Routing Adjacency
The selection of successors, along the default paths up along the
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DODAG, or along the paths learned from destination advertisements
down along the DODAG, 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.
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.
10. Guidelines for Objective Functions
An Objective Function (OF) allows for the selection of a DODAG to
join, and a number of peers in that DODAG as parents. The OF is used
to compute an ordered list of parents. The OF is also responsible to
compute the rank of the device within the DODAG iteration.
The Objective Function is indicated in the DIO message using an
Objective Code Point (OCP), as specified in
[I-D.ietf-roll-routing-metrics], and indicates the method that must
be used to construct the DODAG (e.g. "minimize the path cost using
the ETX metric and avoid 'Blue' links"). The Objective Code Points
are specified in [I-D.ietf-roll-routing-metrics],
[I-D.ietf-roll-of0], and related companion specifications.
Most Objective Functions are expected to follow the same abstract
behavior:
o The parent selection is triggered each time an event indicates
that a potential next hop information is updated. This might
happen upon the reception of a DIO message, a timer elapse, or a
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trigger indicating that the state of a candidate neighbor has
changed.
o An OF scans all the interfaces on the device. Although there may
typically be only one interface in most application scenarios,
there might be multiple of them and an interface might be
configured to be usable or not for RPL operation. An interface
can also be configured with a preference or dynamically learned to
be better than another by some heuristics that might be link-layer
dependent and are out of scope. Finally an interface might or not
match a required criterion for an Objective Function, for instance
a degree of security. As a result some interfaces might be
completely excluded from the computation, while others might be
more or less preferred.
o An OF scans all the candidate neighbors on the possible interfaces
to check whether they can act as a router for a DODAG. There
might be multiple of them and a candidate neighbor might need to
pass some validation tests before it can be used. In particular,
some link layers require experience on the activity with a router
to enable the router as a next hop.
o An OF computes self's rank by adding to the rank of the candidate
a value representing the relative locations of self and the
candidate in the DODAG iteration.
* The increase in rank must be at least MinHopRankIncrease.
(This prevents the creation of a path of sibling links
connecting a child with its parent.)
* To keep loop avoidance and metric optimization in alignment,
the increase in rank should reflect any increase in the metric
value. For example, with a purely additive metric such as ETX,
the increase in rank can be made proportional to the increase
in the metric.
* Candidate neighbors that would cause self's rank to increase
are not considered for parent selection
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 DODAG are ignored
* Candidate neighbors that are of greater rank than self are
ignored
* Candidate neighbors of an equal rank to self (siblings) are
ignored for parent selection
* Candidate neighbors of a lesser rank than self (non-siblings)
are preferred
11. RPL Constants and Variables
Following is a summary of RPL constants and variables.
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 DODAG root. ROOT_RANK has a value
of 1.
INFINITE_RANK This is the constant maximum for the rank.
INFINITE_RANK has a value of 0xFFFF.
RPL_DEFAULT_INSTANCE This is the RPLInstanceID that is used by this
protocol by a node without any overriding policy.
RPL_DEFAULT_INSTANCE has a value of 0.
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DEFAULT_DIO_INTERVAL_MIN TBD (To be determined)
DEFAULT_DIO_INTERVAL_DOUBLINGS TBD (To be determined)
DEFAULT_DIO_REDUNDANCY_CONSTANT TBD (To be determined)
DIO Timer One instance per DODAG that a node is a member of. Expiry
triggers DIO message transmission. Trickle timer with variable
interval in [0, DIOIntervalMin..2^DIOIntervalDoublings]. See
Section 5.3.5.1
DAG Sequence Number Increment Timer Up to one instance per DODAG
that the node is acting as DODAG root of. May not be supported
in all implementations. Expiry triggers revision of
DODAGSequenceNumber, causing a new series of updated DIO
message to be sent. Interval should be chosen appropriate to
propagation time of DODAG and as appropriate to application
requirements (e.g. response time vs. overhead).
DelayDAO Timer Up to one instance per DAO parent (the subset of
DODAG parents chosen to receive destination advertisements) per
DODAG. Expiry triggers sending of DAO message to the DAO
parent. See Section 6.2.6
RemoveTimer Up to one instance per DAO entry per neighbor (i.e.
those neighbors that have given DAO messages to this node as a
DODAG parent) Expiry triggers a change in state for the DAO
entry, setting up to do unreachable (No-DAO) advertisements or
immediately deallocating the DAO entry if there are no DAO
parents. See Section 6.2.4.1.1.3
12. 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.
12.1. Control of Function and Policy
12.1.1. 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 DODAG,
or to immediately root a transient DODAG and start sending multicast
DIO messages. A RPL implementation SHOULD allow configuring whether
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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 DODAGs, or should simply wait until it received DIO
messages from other nodes that are part of existing DODAGs.
12.1.2. DIO Base option
RPL specifies a number of protocol parameters.
A RPL implementation SHOULD allow configuring the following routing
protocol parameters, which are further described in Section 5.1.1:
DAGPreference
RPLInstanceID
DAGObjectiveCodePoint
DODAGID
Destination Prefixes
DIOIntervalDoublings
DIOIntervalMin
DIORedundancyConstant
DAG Root behavior: In some cases, a node may not want to permanently
act as a DODAG root if it cannot join a grounded DODAG. For
example a battery-operated node may not want to act as a DODAG
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 DODAG root for a configured period of time.
DODAG Table Entry Suppression A RPL implementation SHOULD provide
the ability to configure a timer after the expiration of which
logical equivalent of the DODAG table that contains all the
records about a DODAG is suppressed, to be invoked if the DODAG
parent set becomes empty.
12.1.3. 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 DODAG root
along the DODAG in DIO messages.
For each DODAG, a RPL implementation MUST allow for the monitoring of
the following parameters, further described in Section 5.3.5.1:
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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.
12.1.4. DAG Sequence Number Increment
A RPL implementation may allow by configuration at the DODAG root to
refresh the DODAG states by updating the DODAGSequenceNumber. A RPL
implementation SHOULD allow configuring whether or not periodic or
event triggered mechanism are used by the DODAG root to control
DODAGSequenceNumber change.
12.1.5. Destination Advertisement Timers
The following set of parameters of the DAO messages SHOULD be
configurable:
o The DelayDAO timer
o The Remove timer
12.1.6. Policy Control
DAG discovery enables nodes to implement different policies for
selecting their DODAG 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 DODAG, 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.
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12.1.7. 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.
12.2. 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.
12.3. 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 3.1, an implementation is expected to
maintain a set of data structures in support of DODAG discovery:
o The candidate neighbors data structure
o For each DODAG:
* A set of DODAG parents
12.3.1. 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.
12.3.2. Directed Acyclic Graph (DAG) Table
For each DAG, a RPL implementation is expected to keep track of the
following DODAG table values:
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o DODAGID
o DAGObjectiveCodePoint
o A set of Destination Prefixes offered upwards along the DODAG
o A set of DODAG Parents
o timer to govern the sending of DIO messages for the DODAG
o DODAGSequenceNumber
The set of DODAG 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 DODAG Parent
o A flag reporting if the Parent is a DAO Parent as described in
Section 6
12.3.3. 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
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12.3.4. 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
DODAG parent, e.g. if the DODAGID has changed.
A RPL implementation MAY log the reception of a malformed DIO message
along with the neighbor identification if avialable.
12.3.5. RPL Trickle Timers
A RPL implementation operating on a DODAG root MUST allow for the
configuration of the following trickle parameters:
o The DIOIntervalMin expressed in ms
o The DIOIntervalDoublings
o The DIORedundancyConstant
A RPL implementation MAY provide a counter reporting the number of
times an inconsistency (and thus the trickle timer has been reset).
12.4. 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.
12.5. Requirements on Other Protocols and Functional Components
RPL does not have any impact on the operation of existing protocols.
12.6. Impact on Network Operation
To be completed.
13. 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].
14. IANA Considerations
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14.1. RPL Control Message
The RPL Control Message is an ICMP information message type that is
to be used carry DODAG Information Objects, DODAG 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.
14.2. 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 | DODAG Information Solicitation | This document |
| 0x02 | DODAG Information Object | This document |
| 0x04 | Destination Advertisement Object | This document |
+------+----------------------------------+---------------+
RPL Control Codes
14.3. New Registry for the Control Field of the DIO Base
IANA is requested to create a registry for the Control field of the
DIO Base.
New fields may be allocated only by an IETF Consensus action. Each
field should be tracked with the following qualities:
o Bit number (counting from bit 0 as the most significant bit)
o Capability description
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o Defining RFC
Four groups are currently defined:
+-------+-----------------------------------------+---------------+
| Bit | Description | Reference |
+-------+-----------------------------------------+---------------+
| 0 | Grounded DODAG (G) | This document |
| 1 | Destination Advertisement Supported (A) | This document |
| 2 | Destination Advertisement Trigger (T) | This document |
| 3 | Destination Advertisements Stored (S) | This document |
| 5,6,7 | DODAG Preference (Prf) | This document |
+-------+-----------------------------------------+---------------+
DIO Base Flags
14.4. DODAG Information Object (DIO) Suboption
IANA is requested to create a registry for the DIO Base 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 |
+-------+------------------------------+---------------+
DODAG Information Option (DIO) Base Suboptions
15. Acknowledgements
The authors would like to acknowledge the review, feedback, and
comments from Emmanuel Baccelli, Dominique Barthel, Yusuf Bashir,
Phoebus Chen, Mathilde Durvy, Manhar Goindi, Mukul Goyal, Anders
Jagd, Quentin 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,
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and Arsalan Tavakoli, which have provided useful design
considerations to RPL.
16. 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
Thomas Heide Clausen
LIX, Ecole Polytechnique, France
Phone: +33 6 6058 9349
EMail: T.Clausen@computer.org
URI: http://www.ThomasClausen.org/
Philip Levis
Stanford University
358 Gates Hall, Stanford University
Stanford, CA 94305-9030
USA
Email: pal@cs.stanford.edu
Richard Kelsey
Ember Corporation
Boston, MA
USA
Phone: +1 617 951 1225
Email: kelsey@ember.com
Jonathan W. Hui
Arch Rock Corporation
501 2nd St. Ste. 410
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San Francisco, CA 94107
USA
Email: jhui@archrock.com
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
Stephen Dawson-Haggerty
UC Berkeley
Soda Hall, UC Berkeley
Berkeley, CA 94720
USA
Email: stevedh@cs.berkeley.edu
17. References
17.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
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17.2. Informative References
[I-D.ietf-bfd-base]
Katz, D. and D. Ward, "Bidirectional Forwarding
Detection", draft-ietf-bfd-base-11 (work in progress),
January 2010.
[I-D.ietf-manet-nhdp]
Clausen, T., Dearlove, C., and J. Dean, "Mobile Ad Hoc
Network (MANET) Neighborhood Discovery Protocol (NHDP)",
draft-ietf-manet-nhdp-11 (work in progress), October 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-09
(work in progress), January 2010.
[I-D.ietf-roll-home-routing-reqs]
Brandt, A. and J. Buron, "Home Automation Routing
Requirements in Low Power and Lossy Networks",
draft-ietf-roll-home-routing-reqs-11 (work in progress),
January 2010.
[I-D.ietf-roll-of0]
Thubert, P., "RPL Objective Function 0",
draft-ietf-roll-of0-01 (work in progress), February 2010.
[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-04 (work in progress),
December 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
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Sensor Networks", Communications of the ACM, v.51 n.7,
July 2008,
<http://portal.acm.org/citation.cfm?id=1364804>.
[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
August 1996.
[RFC3697] Rajahalme, J., Conta, A., Carpenter, B., and S. Deering,
"IPv6 Flow Label Specification", RFC 3697, March 2004.
[RFC3810] Vida, R. and L. Costa, "Multicast Listener Discovery
Version 2 (MLDv2) for IPv6", RFC 3810, June 2004.
[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.
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Appendix A. Requirements
A.1. Protocol Properties Overview
RPL demonstrates the following properties, consistent with the
requirements specified by the application-specific requirements
documents.
A.1.1. IPv6 Architecture
RPL is strictly compliant with layered IPv6 architecture.
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,
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[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.
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
B.1. DAO Operation When Only the Root Node Stores DAO Information
Consider the example of Figure 13. In this example only the root
node, (LBR*), will store DAO information. This is not known, nor is
it required to be known, to all nodes a priori. Rather, each node is
able to observe from the state of the 'S' flag that no ancestor, with
the exception of the root node, stores DAO information.
(LBR*)
/ \
/ \
/ \
(11) (12)
| |
| |
| |
(21) (22)
\
\
\
(31)
/ \
/ \
/ \
(41) (42)
: :
Figure 13: Only Root Node Stores DAOs
In this example:
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o The 'S' flag is cleared in DIO messages emitted by (LBR*), because
(LBR*) is the DODAG root.
o The 'S' flag is cleared in all DIO messages emitted by all other
nodes, because no other node stores DAO information.
o (LBR*) has learned from DAO messages how to reach node (31) with a
source route via {(11) (21)}.
o All source routes to nodes in the sub-DODAG of node (31),
including nodes (41), (42), and others will include the prefix
{(11) (21) (31)}
o Node (31) maintains a DTSN, (31).DTSN, that it will advertise in
DIO messages.
Suppose now that there is a topology change within the same DODAG
iteration, causing node (31) to evict node (21) as a DAO parent and
add node (22) as a DAO parent:
1. Node (31) will schedule a DAO transmission because it has added a
new node (22) to its DAO parent set.
2. Node (31) need not increment (31).DTSN at this event, because in
this example no DAO parents have the 'S' flag set. Specifically
this indicates to Node (31) that there are no intermediate
storing nodes that may need to be explicitly updated with DAO
information from it's sub-DODAG. Hence nodes (41), (42), and by
extension the sub-DODAG of node (31) will not subsequently
observe an incremented (31).DTSN and the sub-DODAG will not emit
DAOs.
3. A new flow of DAOs for node (31) reaches the (LBR*), updating the
source route information for node (31) to include the new path
{(12) (22)}.
4. (LBR*) may implicitly update all source routes that must transit
node (31), i.e. the sub-DODAG of node (31), to use the updated
source route prefix {(12) (22)} instead of {(11) (21)}.
Thus the use of the 'S' flag in the case where only the root node
stores DAO information has allowed an optimization whereby only a DAO
update for the node that changed its DAO parent set, (31), needs to
be sent to the DODAG root.
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B.2. DAO Operation When All Nodes Fully Store DAO Information
Consider the example of Figure 14. In this example all nodes will
fully store DAO information.
(LBR*)
/ \
/ \
/ \
(11*) (12*)
| |
| |
| |
(21*) (22*)
\
\
\
(31*)
/ \
/ \
/ \
(41*) (42*)
: :
Figure 14: All Nodes Store DAOs
In this example:
o The 'S' flag is cleared in DIO messages emitted by (LBR*), because
(LBR*) is the DODAG root.
o The 'S' flag is set in DIO messages emitted by all non-root nodes
because each non-root node stores DAO information.
o Source routing state is effectively not provisioned in this
example, because each node has been able to store hop-by-hop
routing state for each destination, possibly aggregated, as
learned from DAOs. For example, node (11*) will have learned and
stored information from a DAO to the effect that node (41*) is
routable through a next hop of node (21*). Node (12*) on the
other hand does not necessarily have a route provisioned to node
(41*).
Suppose now that there is a topology change within the same DODAG
iteration, causing node (31*) to evict node (21*) as a DAO parent and
add node (22*) as a DAO parent:
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1. Node (31*) will schedule a DAO transmission because it has added
a new node (22*) to its DAO parent set.
2. Node (31) need not increment (31).DTSN, because it is a fully
storing node and does not need to trigger DAO information from
its sub-DODAG.
3. Node (31) gives a DAO Update to node (22*). Presuming that node
(22*) has received the update, node (22*) will store the new
entries for routes including the sub-DODAG of node (31*),
including nodes (41*) and (42*). Node (22*) will schedule a DAO
transmission for the new entries.
4. Similarly, node (22*) updates node (12*) and node (12*) updates
(LBR*). Hop-by-hop routing state for the sub-DODAG of node (31*)
is now provisioned at nodes (12*) and (22*).
Thus the addition to the DAO Parent set at the fully storing node
(31*) does not elicit additional DAO-related traffic from its sub-
DODAG. The intermediate nodes along the 'new' downward path are
updated by DAO messages along the new path.
Suppose next that the DODAG root triggers a refresh of DAO
information over the same DODAG Iteration. (Note that the DODAG root
might also trigger a DAO refresh but allow other topology changes at
the same time by incrementing the DODAG Sequence Number to cause a
move to the next DODAG Iteration).:
1. (LBR*) will increment its DTSN and issue a DIO with the 'T' flag
set.
2. Nodes (11*) and (12*) will increment their own DTSNs in response
to observing in the DIO from LBR a new DTSN and the 'T' flag
being set. They will reset their trickle timers to cause the
issue of new DIOs with the 'T' flag set. These nodes will also
schedule a DAO transmission in response to observing a new DTSN
from their DAO Parent, (LBR*). (This DAO transmission may be
scheduled with a sufficient delay computed based on rank to allow
a chance for the sub-DODAGs of the nodes to report DAO messages
prior to the nodes reporting their own DAO information to (LBR*).
This is implementation specific and may allow a chance for DAO
aggregation.).
3. Node (21*) receives a DIO from node (11*) and observes the new
(11*).DTSN as well as the set 'T' flag. Node (21*) increments
its own DTSN, resets the trickle timer, and schedules a DAO
transmission.
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4. Similarly, as each node observes the incremented DTSN with the
'T' flag set from each of its parents, each node will increment
its own DTSN, reset the DIO trickle timer, and schedule a DAO
transmission.
Thus the entire DODAG iteration has been re-armed to send DAO
messages based on the (LBR*)'s assertion of the 'T' flag. Note that
normally a DTSN increment would cause no further action in a sub-
DODAG beyond the first fully storing node that is encountered, but
that in this case the 'T' flag effectively provides a means to 'punch
through' all fully storing nodes.
B.3. DAO Operation When Nodes Have Mixed Capabilities
Consider the example of Figure 15. In this example some nodes are
capable of storing DAO information and some are not.
(LBR*)
/ \
/ \
/ \
(11) (12*)
| |
| |
| |
(21) (22)
\
\
\
(31)
/ \
/ \
/ \
(41) (42*)
: :
Figure 15: Mixed Capability DAO Operation
In this example:
o The 'S' flag is cleared in DIO messages emitted by (LBR*), because
(LBR*) is the DODAG root.
o The 'S' flag is set in DIO messages emitted by (12*), because it
is a storing node.
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o The 'S' flag will be set in DIO messages emitted by nodes that
contain node (12*) (or more generally any non-root storing node)
as a DAO parent/ancestor. This indicates that somewhere along the
DAO path there is a non-root storing node that may need to have
its state updated (by a DAO refresh) in certain conditions.
Suppose that there is a topology change within the same DODAG
iteration, causing node (31) to add node (22) as a DAO parent:
1. Node (31) will schedule a DAO transmission because it has added a
new node (22) to its DAO parent set. Node (31) will increment
(31).DTSN because node (22) has set the 'S' flag in its DIO
messages. Node (31) will reset its DIO trickle timer.
2. Node (31)'s trickle timer will then expire and a DIO is issued
and received by node's (41) and (42*).
3. Node (41) is a non-storing node. It will increment (41).DTSN in
response to observing the increment in (31).DTSN, and reset its
trickle timer. This results finally in the reliable (thanks to
the DTSN) triggering of a DAO update from node (41)'s sub-DODAG.
4. As node (41) receives DAO updates from its sub-DODAG it updates
the DAOs with source routing information as necessary and passes
them on to node (31), along with its own (node (41)) DAO update.
5. Meanwhile, node (42*) is a fully storing node. It observes the
increment to (31).DTSN and schedules a DAO update. Node (42*)
does not need to increment (42*).DTSN, since it is a fully
storing node it does not need to solicit DAO updates from its
sub-DODAG in this case. At the scheduled time Node (42*)
reissues its DAO information to node (31).
6. Node (31) receives the DAO messages from its sub-DODAG, adds
source routing information as necessary, and issues DAO updates
to node (22).
7. Node (22) similarly receives DAO messages from node (31), updates
source routing information as necessary, and issues DAO updates
to node (12*).
8. The intermediate storing node (12*) has now received from DAO
messages the information necessary to provision routing state for
node (31) and its sub-DODAG. As downwards traffic is routed
through node (12*) it is able to consult source routing
information that was learned from the DAO messages as needed to
specify routes down the DAG across the non-storing nodes (22),
(31), ...
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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 upwards along the DODAG until a common ancestor
is reached and then flowing down, may not be suitable for all
application scenarios. A related scenario may occur when the down
paths setup along the DODAG by the destination advertisement
mechanism are not the most desirable downward paths for the specific
application scenario (in part because the DODAG 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. Destination Advertisement / DAO Fan-out
When DAO messages are relayed to more than one DODAG parent, in some
cases a situation may be created where a large number of DAO messages
conveying information about the same destination flow upwards 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 downwards
routes by sending DAO messages to more than one parent is under
investigation.
C.3. 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.4. 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.
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C.5. Managing Multiple Instances
A network may run multiple instances of RPL concurrently. Such a
network will require methods for assigning and otherwise managing
RPLInstanceIDs. This will likely be addressed in a separate
document.
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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