One document matched: draft-ietf-roll-rpl-03.xml
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<front>
<title abbrev="draft-ietf-roll-rpl-03">RPL: Routing Protocol for Low Power
and Lossy Networks</title>
<author fullname="Tim Winter" initials="T" role="editor" surname="Winter">
<organization></organization>
<address>
<email>wintert@acm.org</email>
</address>
</author>
<author fullname="Pascal Thubert" initials="P" role="editor"
surname="Thubert">
<organization abbrev="Cisco Systems">Cisco Systems</organization>
<address>
<postal>
<street>Village d'Entreprises Green Side</street>
<street>400, Avenue de Roumanille</street>
<street>Batiment T3</street>
<city>Biot - Sophia Antipolis</city>
<code>06410</code>
<country>FRANCE</country>
</postal>
<phone>+33 497 23 26 34</phone>
<email>pthubert@cisco.com</email>
</address>
</author>
<author fullname="ROLL Design Team" initials="" surname="ROLL Design Team">
<organization>IETF ROLL WG</organization>
<address>
<email>dtroll@external.cisco.com</email>
</address>
</author>
<date day="4" month="October" year="2009" />
<area>Routing Area</area>
<workgroup>Networking Working Group</workgroup>
<keyword>Draft</keyword>
<abstract>
<t>Low Power and Lossy Networks (LLNs) are made largely of constrained
nodes (with limited processing power, memory, and sometimes energy when
they are battery operated). These routers are interconnected by lossy
links, most of the time supporting only low data rates, that are usually
fairly 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
a large number of nodes, up to several dozens or hundreds or more nodes
in the network. These characteristics offer unique challenges to a
routing solution: the IETF ROLL Working Group has defined
application-specific routing requirements for a Low Power and Lossy
Network (LLN) routing protocol. This document specifies the Routing
Protocol for Low Power and Lossy Networks (RPL).</t>
</abstract>
<note title="Requirements Language">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref
target="RFC2119">RFC 2119</xref>.</t>
</note>
</front>
<middle>
<section title="Introduction">
<t>Low Power and Lossy Networks (LLNs) are made largely of constrained
nodes (with limited processing power, memory, and sometimes energy when
they are battery operated). These routers are interconnected by lossy
links, most of the time supporting only low data rates, that are usually
fairly 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
a large number of nodes, up to several dozens or hundreds or more nodes
in the network. These characteristics offer unique challenges to a
routing solution: the IETF ROLL Working Group has defined
application-specific routing requirements for a Low Power and Lossy
Network (LLN) routing protocol, specified in <xref
target="I-D.ietf-roll-building-routing-reqs"></xref>, <xref
target="I-D.ietf-roll-home-routing-reqs"></xref>, <xref
target="I-D.ietf-roll-indus-routing-reqs"></xref>, and <xref
target="RFC5548"></xref>. This document specifies the Routing Protocol
for Low Power and Lossy Networks (RPL).</t>
<section title="Design Principles">
<t>RPL was designed with the objective to meet the requirements
spelled out in <xref
target="I-D.ietf-roll-building-routing-reqs"></xref>, <xref
target="I-D.ietf-roll-home-routing-reqs"></xref>, <xref
target="I-D.ietf-roll-indus-routing-reqs"></xref>, and <xref
target="RFC5548"></xref>. Because those requirements are heterogeneous
and sometimes incompatible in nature, the approach is first taken to
design a protocol capable of supporting a core set of functionalities
corresponding to the intersection of the requirements. (Note: it is
intended that as this design evolves optional features may be added to
address some application specific requirements). This is a key
protocol design decision providing a granular approach in order to
restrict the core of the protocol to a minimal set of functionalities,
and to allow each instantiation of the protocol to be optimized in
terms of required code space. It must be noted that RPL is not
restricted to the aforementioned applications and is expected to be
used in other environments. All "MUST" application requirements that
cannot be satisfied by RPL will be specifically listed in the Appendix
A, accompanied by a justification.</t>
<t>The core set of functionalities is to be capable of operating in
the most severely constrained environments, with minimal requirements
for memory, energy, processing, communication, and other consumption
of limited resources from nodes. Trade-offs inherent in the
provisioning of protocol features will be exposed to the implementer
in the form of configurable parameters, such that the implementer can
further tweak and optimize the operation of RPL as appropriate to a
specific application and implementation. Finally, RPL is designed to
consult implementation specific policies to determine, for example,
the evaluation of routing metrics.</t>
<t>A set of companion documents to this specification will provide
further guidance in the form of applicability statements specifying a
set of operating points appropriate to the Building Automation, Home
Automation, Industrial, and Urban application scenarios.</t>
</section>
<section title="Expectations of Link Layer Behavior">
<t>This specification does not rely on any particular features of a
specific link layer technologies. It is anticipated that an
implementer should be able to operate RPL over a variety of different
low power wireless or PLC (Power Line Communication) link layer
technologies.</t>
<t>Implementers may find <xref target="RFC3819">RFC 3819</xref> a
useful reference when designing a link layer interface between RPL and
a particular link layer technology.</t>
</section>
</section>
<section anchor="Terminology" title="Terminology">
<t>The terminology used in this document is consistent with and
incorporates that described in `Terminology in Low power And Lossy
Networks' <xref target="I-D.ietf-roll-terminology"></xref>. The
terminology is extended in this document as follows:</t>
<t><list hangIndent="6" style="hanging">
<t hangText="Autonomous:">The ability of a routing protocol to
independently function without relying on any external influence or
guidance. Includes self-organization capabilities.</t>
<t hangText="DAG:">Directed Acyclic Graph. A directed graph having
the property that all edges are oriented in such a way that no
cycles exist. In the RPL context, all edges are contained in paths
oriented toward and terminating at a root node (a DAG root, or sink-
typically a Low Power and Lossy Network Border Router (LBR)).</t>
<t hangText="DAGID:">DAG Identifier. A globally unique identifier
for a DAG. All nodes who are part of a given DAG have knowledge of
the DAGID. This knowledge is used to identify peer nodes within the
DAG in order to coordinate DAG maintenance while avoiding loops.</t>
<t hangText="DAG parent:">A parent of a node within a DAG is one of
the immediate successors of the node on a path towards the DAG root.
For each DAGID that a node is a member of, the node will maintain a
set containing one or more DAG parents. If a node is a member of
multiple DAGs then it must conceptually maintain a set of DAG
parents for each DAGID.</t>
<t hangText="DAG sibling:">A sibling of a node within a DAG is
defined in this specification to be any neighboring node which is
located at the same rank (depth) within a DAG. Note that siblings
defined in this manner do not necessarily share a common parent. For
each DAG that a node is a member of, the node will maintain a set of
DAG siblings. If a node is a member of multiple DAGs then it must
conceptually maintain a set of DAG siblings for each DAG.</t>
<t hangText="DAG root:">A DAG root is a sink within the DAG. All
paths in the DAG terminate at a DAG root, and all DAG edges
contained in the paths terminating at a DAG root are oriented toward
the DAG root. There must be at least one DAG root per DAG, and in
some cases there may be more than one. In many use cases,
source-sink represents a dominant traffic flow, where the sink is a
DAG root or is located behind the DAG root. Maintaining routes
towards DAG roots is therefore a prominent functionality for
RPL.</t>
<t hangText="Grounded:">A DAG is grounded if it contains a DAG root
offering connectivity to an external routed infrastructure such as
the public Internet or a private core (non-LLN) IP network.</t>
<t hangText="Floating:">A DAG is floating if is not grounded. A
floating DAG is not expected to reach any additional external routed
infrastructure such as the public Internet or a private core
(non-LLN) IP network.</t>
<t hangText="Inward:">Inward refers to the direction from leaf nodes
towards DAG roots, following the orientation of the edges within the
DAG.</t>
<t hangText="Outward:">Outward refers to the direction from DAG
roots towards leaf nodes, going against the orientation of the edges
within the DAG.</t>
<t hangText="P2P:">Point-to-point. This refers to traffic exchanged
between two nodes.</t>
<t hangText="P2MP:">Point-to-Multipoint. This refers to traffic
between one node and a set of nodes. This is similar to the P2MP
concept in Multicast or MPLS Traffic Engineering (<xref
target="RFC4461"></xref> and <xref target="RFC4875"></xref>). A
common RPL use case involves P2MP flows from or through a DAG root
outward towards other nodes contained in the DAG.</t>
<t hangText="MP2P:">Multipoint-to-Point; used to describe a
particular traffic pattern. A common RPL use case involves MP2P
flows collecting information from many nodes in the DAG, flowing
inwards towards DAG roots. Note that a DAG root may not be the
ultimate destination of the information, but it is a common transit
node.</t>
<t hangText="OCP:">Objective Code Point. In RPL, the Objective Code
Point (OCP) indicates which routing metrics, optimization
objectives, and related functions are in use in a DAG. Instances of
the Objective Code Point are further described in <xref
target="I-D.ietf-roll-routing-metrics"></xref>.</t>
</list></t>
<t>Note that in this document, the terms `node' and `LLN router' are
used interchangeably.</t>
</section>
<section anchor="ProtocolModel" title="Protocol Model">
<t>The aim of this section is to describe RPL in the spirit of <xref
target="RFC4101"></xref>. Protocol details can be found in further
sections.</t>
<section title="Protocol Properties Overview">
<t>RPL demonstrates the following properties, consistent with the
requirements specified by the application-specific requirements
documents.</t>
<section title="IPv6 Architecture">
<t>RPL is strictly compliant with layered IPv6 architecture.</t>
<t>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).</t>
</section>
<section title="Typical LLN Traffic Patterns">
<t>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.</t>
<t>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).</t>
</section>
<section title="Constraint Based Routing">
<t>The RPL design supports constraint based routing, based on a set
of routing metrics. The routing metrics for links and nodes with
capabilities supported by RPL are specified in a companion document
to this specification, <xref
target="I-D.ietf-roll-routing-metrics"></xref>. RPL signals the
metrics and related objective functions in use in a particular
implementation by means of an Objective Code Point (OCP). Both the
routing metrics and the OCP help determine the construction of the
Directed Acyclic Graphs (DAG) using a distributed path computation
algorithm.</t>
<t>RPL supports the computation and installation of different paths
in support of and optimized for a set of application and
implementation specific constraints, as guided by an OCP. Traffic
may subsequently be directed along the appropriate constrained path
based on traffic marking within the IPv6 header. For more details on
the approach towards constraint-based routing, see <xref
target="ConstrainedLLNs"></xref>.</t>
</section>
</section>
<section title="Protocol Operation">
<t>A LLN deployment will consist of a number of nodes and a number of
edges (links) between them, whose characteristics will depend on
implementation and link layer (L2) specifics. Due to the nature of the
LLN environment the L2 links are expected to demonstrate a large
degree of variance as to their availability, quality, and other
related parameters. Certain links, demonstrating a viability above a
confidence threshold for particular node and link metrics, as based on
guidelines from <xref target="I-D.ietf-roll-routing-metrics"></xref>,
will be extracted from the L2 graph, and the resulting graph will be
used as the basis on which to operate the routing protocol. Note that
as the characteristics of the L2 topology vary over time the set of
viable links is to be updated and the routing protocol thus continues
to evaluate the LLN. In RPL this process happens in a distributed
manner, and from the perspective of a single node running RPL this
process results in a set of candidate neighbors, with associated node
and link metrics as well as confidence values.</t>
<t>Many of the dominant traffic flows in support of the LLN
application scenarios are MP2P flows (<xref
target="I-D.ietf-roll-building-routing-reqs"></xref>, <xref
target="I-D.ietf-roll-home-routing-reqs"></xref>, <xref
target="I-D.ietf-roll-indus-routing-reqs"></xref>, and <xref
target="RFC5548"></xref>). These flows are rooted at designated nodes
that have some application significance, such as providing
connectivity to an external routed infrastructure. The term "external"
is used top refer to the public Internet or a core private (non-LLN)
IP network. In support of this dominant flow RPL constructs Directed
Acyclic Graphs (DAGs) on top of the viable LLN topology, selecting and
orienting links among candidate neighbors toward DAG roots which root
the MP2P flows.</t>
<t>LLN nodes running RPL will construct Directed Acyclic Graphs (DAGs)
rooted at designated nodes that generally have some application
significance, such as providing connectivity to an external routed
infrastructure. The term "external" is used top refer to the public
Internet or a core private (non-LLN) IP network. This structure
provides the routing solution for the dominant MP2P traffic flows. The
DAG structure further provides each node potentially multiple
successors for MP2P flows, which may be used for, e.g., local route
repair or load balancing.</t>
<t>Nodes running RPL are able to further restrict the scope of the
routing problem by using the DAG as a reference topology. By
referencing a rank property that is related to the positions in the
DAG, nodes are able to determine their positions in a DAG relative to
each other. This information is used by RPL in part to construct rules
for movement relative to the DAG that endeavor to avoid loops. It is
important to note that the rank property is derived from metrics, and
not directly from the position in the DAG, as will be discussed
further.</t>
<t>As DAGs are organized, RPL will use a destination advertisement
mechanism to build up routing tables in support of outward P2MP
traffic flows. This mechanism, using the DAG as a reference,
distributes routing information across intermediate nodes (between the
DAG leaves and the root), guided along the DAG, such that the routes
toward destination prefixes in the outward direction may be set up. As
the DAG undergoes modification during DAG maintenance, the destination
advertisement mechanism can be triggered to update the outward routing
state.</t>
<t>A baseline support for P2P traffic in RPL is provided by the DAG,
as P2P traffic may flow inward along the DAG until a common parent is
reached who has stored an entry for the destination in its routing
table and is capable of directing the traffic outward along the
correct outward path. RPL also provides support for the trivial case
where a P2P destination may be a `one-hop' neighbor. In the present
specification RPL does not specify nor preclude any additional
mechanisms that may be capable to compute and install more optimal
routes into LLN nodes in support of arbitrary P2P traffic according to
some routing metric.</t>
<section title="DAG Construction">
<t>RPL constructs one or more DAGs, over gradients defined by
optimizing cost metrics along paths rooted at designated nodes.</t>
<t>The DAG construction algorithm is distributed; each node running
RPL invokes a set of DAG construction rules and objective functions
when considering its role with respect to neighboring nodes such
that the DAG structure emerges.</t>
<section title="IP Router Advertisement - DAG Information Option (RA-DIO)">
<t>The IPv6 Router Advertisement (RA) mechanism (as specified in
<xref target="RFC4861"></xref>) is used by RPL in order to build
and maintain a DAG.</t>
<t>The IPv6 RA message is augmented with a DAG Information Option
(DIO), forming an RA-DIO message, to convey information about the
DAG including:</t>
<t><list style="symbols">
<t>A DAGID used to identify the DAG as sourced from the DAG
root. The DAGID must be unique to a single DAG in the scope of
the LLN.</t>
<t>Objective Code Point (OCP) as described below.</t>
<t>Rank information used by nodes to determine their positions
in the DAG relative to each other. This is not a metric,
although its derivation is typically closely related to one or
more metrics as specified by the OCP. The rank information is
used to support loop avoidance strategies and in support of
ordering alternate successors when engaged in path
maintenance.</t>
<t>Sequence number originated from the DAG root, used to aid
in identification of dependent sub-DAGs and coordinate
topology changes in a manner so as to avoid loops.</t>
<t>Indications and configuration for the DAG, e.g. grounded or
floating, administrative preference, ...</t>
<t>A vector of path metrics, as further described in <xref
target="I-D.ietf-roll-routing-metrics"></xref>.</t>
<t>List of additional destination prefixes reachable inwards
along the DAG.</t>
</list></t>
<t>The RA messages are issued whenever a change is detected to the
DAG such that a node is able to determine that a region of the DAG
has become inconsistent. As the DAG stabilizes the period at which
RA messages occur is configured to taper off, reducing the
steady-state overhead of DAG maintenance. The periodic issue of RA
messages, along with the triggered RA messages in response to
inconsistency, is one feature that enables RPL to operate in the
presence of unreliable links.</t>
</section>
<section title="DAG Identification">
<t>Each DAG is identified by a particular identifier (DAGID) as
well as its supported optimization objectives and available
destination prefixes. The optimization objectives are conveyed as
an Objective Code Point (OCP) as described further below.
Available destination prefixes, which may include destinations
available beyond the DAG root, multicast destinations, or IPv6
node addresses, are advertised outwards along the DAG and
recipient nodes may then provision routing tables with entries
inwards towards the destinations. The RPL implementation at each
node will be provisioned by the application with sufficient
information to determine which objectives and destinations are
required, and thus the RPL implementation may determine which DAG
to join.</t>
<t>The decision for a node to join a DAG may be optimized
according to implementation specific policy functions on the node
as indicated by one or more specific OCP values. For example, a
node may be configured for one goal to optimize a bandwidth metric
(OCP-1), and with a parallel goal to optimize for a reliability
metric (OCP-2). Thus two DAGs, with two unique DAGIDs, may be
constructed and maintained in the LLN: DAG-1 would be optimized
according to OCP-1, whereas DAG-2 would be optimized according to
OCP-2. A node may then maintain independent sets of DAG parents
and related data structures for each DAG. Note that in such a case
traffic may directed along the appropriate constrained DAG based
on traffic marking within the IPv6 header. This specification will
focus on the case where the node only joins one DAG; further
elaboration on the proper operation of RPL in the presence of
multiple DAGs, including traffic marking and related rules, are to
be specified further in future revisions of this or companion
specifications.</t>
</section>
<section title="Grounded and Floating DAGs">
<t>Certain LLN nodes may offer connectivity to an external routed
infrastructure in support of an application scenario. These nodes
are designated `grounded', and may serve as the DAG roots of a
grounded DAG. DAGs that do not have a grounded DAG root are
floating DAGs. In either case routes may be provisioned toward the
DAG root, although in the floating case there is no expectation to
reach an external infrastructure. Some applications will include
permanent floating DAGs.</t>
</section>
<section title="Administrative Preference">
<t>An administrative preference may be associated with each DAG
root, and thereby each DAG, in order that some DAGs in the LLN may
be more preferred over other DAGs. For example, a DAG root that is
sinking traffic in support of a data collection application may be
configured by the application to be very preferred. A transient
DAG, e.g. a DAG that is only existing in support of DAG repair
until a permanent DAG is found, may be configured to be less
preferred. The administrative preference offers a way to engineer
the formation of the DAG in support of the application.</t>
</section>
<section title="Objective Code Point (OCP)">
<t>The OCP serves to convey and control the optimization
objectives in use within the DAG. The OCP is further specified in
<xref target="I-D.ietf-roll-routing-metrics"></xref>. Each
instance of an allocated OCP indicates:</t>
<t><list style="symbols">
<t>The set of metrics used within the DAG</t>
<t>The objective functions used for least cost path
determination.</t>
<t>The function used to compute DAG Rank</t>
<t>The functions used to accumulate metrics for propagation
within a RA-DIO message</t>
</list></t>
<t>For example, an objective code point might indicate that the
DAG is using the Expected Number of Transmissions (ETX) as a
metric, that the optimization goal is to minimize ETX, that DAG
Rank is equivalent to ETX, and that RA-DIO propagation entails
adding the advertised ETX of the most preferred parent to the ETX
of the link to the most preferred parent.</t>
<t>By using defined OCPs that are understood by all nodes in a
particular implementation, and by conveying them in the RA-DIO
message, RPL nodes may work to build optimized LLN using a variety
of application and implementation specific metrics and goals.</t>
<t>A default OCP, OCP 0, is specified with a well-defined default
behavior. OCP 0 is used to define RPL behaviors in the case where
a node encounters a RA-DIO message containing a code point that it
does not support.</t>
</section>
<section title="Distributed Algorithm Operation">
A high level overview of the distributed algorithm which constructs the DAG is as follows:
<t>
<list style="symbols">
<t>Some nodes may be initially provisioned to act as DAG
roots, either permanent or transient, with associated
preferences.</t>
<t>Nodes will maintain a data structure containing their
candidate (viable) neighbors, as based on guidelines in <xref
target="I-D.ietf-roll-routing-metrics" /> This data structure
will also track DAG information as learned from and associated
with each neighbor.</t>
<t>Nodes who are members of a DAG, including DAG roots, will
multicast RA-DIO messages as needed (when inconsistency is
detected), to their link-local neighbors. Nodes will also
respond to Router Solicitation (RS) messages.</t>
<t>Nodes who receive RA-DIO messages will take into
consideration several criteria when processing the extracted
DAG information. The node may discount the RA-DIO according to
loop avoidance rules based on rank as described further below.
Nodes will consider the information in the RA-DIO in order to
determine whether or not that candidate neighbor offers a
better attachment point to a DAG (which the node may or may
not be a member of) according to the implementation specific
optimization goals, OCP, and current metrics.</t>
<t>Nodes may join a new DAG or move within the current DAG, in
response to the information contained in the RA-DIO message,
and in accordance with loop avoidance rules described further
in this specification. For the successors within the DAG, a
node manages a set of DAG Parents. Joining, moving within, and
leaving the DAG is accomplished through managing this set
according to the rules specified by RPL.</t>
<t>As nodes join, move within, and leave DAGs they emit
updated RA-DIOs which are received and acted on by neighboring
nodes. When inconsistencies (such as caused by movement or
link loss) are detected within the DAG structure, RA-DIO
messages are emitted more frequently. When the DAG structure
becomes consistent, RA-DIO messages taper off.</t>
<t>As less preferred DAGs encounter more preferred DAGs that
offer equivalent or better optimization objectives, the nodes
in the less preferred DAGs may leave to join the more
preferred DAGs, finally leaving only the more preferred DAGs.
This is an illustration of the mechanism by which an
application may engineer DAG construction.</t>
<t>As the DAG construction operation proceeds, nodes
accumulate onto the DAG in progressively outward tiers,
centered around the DAG root.</t>
<t>The nodes provision routing table entries for the
destinations specified by the RA-DIO towards their DAG
Parents. Nodes may provision a DAG Parent as a default
gateway.</t>
</list>
</t>
</section>
<section anchor="DAGRank" title="DAG Rank">
<t>When nodes select DAG parents, they will select the most
preferred parent according to their implementation specific
objectives, using the cost metrics conveyed in the RA-DIO messages
along the DAG in conjunction with the related objective functions
as specified by the OCP.</t>
<t>Based on this selection, the metrics conveyed by the most
preferred DAG parent, the nodes own metrics and configuration, and
a related function defined by the OCP, a node will be able to
compute a value for its rank as a consequence of selecting a most
preferred DAG parent.</t>
<t>The rank value feeds back into the DAG parent selection
according to a loop-avoidance strategy. Once a DAG parent has been
added, and a rank value for the node within the DAG has been
computed, the nodes further options with regard to DAG parent
selection and movement within the DAG are restricted in favor of
loop avoidance.</t>
<t>It is important to note that the DAG Rank is not itself a
metric, although its value is derived from and influenced by the
use of metrics to select DAG parents and take up a position in the
DAG. In other words, routing metrics and OCP (not rank directly)
are used to determine the DAG structure and consequently the path
cost. The only aim of the rank is to inform loop avoidance as
explained hereafter. The computation of the DAG Rank MUST be done
in such a way so as to maintain the following properties for any
nodes M and N who are neighbors in the LLN:</t>
<t><list>
<t>For a node N, and its most preferred parent M, DAGRank(N)
> DAGRank(M) must hold. Further, all parents in the DAG
parent set must be of a rank less than self's DAGRank(N). In
other words, the rank presented by a node N MUST be greater
(deeper) than that presented by any of its parents. <list>
This mechanism serves to avoid loops. Node N will use one of its parents to relay its packet. If that parent of N were to be of a deeper rank, the traffic would make backwards progress and could result in a loop.
</list></t>
<t>If DAGRank(M) < DAGRank(N), then M is probably located
in a more preferred position than N in the DAG with respect to
the metrics and optimizations defined by the objective code
point. In any fashion, Node M may safely be a DAG parent for
Node N without risk of creating a loop. <list>
<t>For example, a Node M of rank 3 is likely located in a
more optimum position than a Node N of rank 5. A packet
directed inwards and forwarded from Node N to Node M will
always make forward progress with respect to the DAG
organization on that link; there is no risk of Node M at
rank 3 forwarding the packet back into Node N's sub-DAG at
rank of 5 or greater (which would be a sufficient
condition for a loop to occur).</t>
</list></t>
<t>If DAGRank(M) == DAGRank(N), then M and N are located
positions of relatively the same optimality within the DAG. In
some cases, Node M may be used as a successor by Node N, but
with related chance of creating a loop that must be detected
and broken by some other means. <list>
<t>If Node M is at rank 3 and node N is at rank 3, then
they are siblings; by definition Node M and N cannot be in
each others sub-DAG. They may then forward to each other
failing serviceable parents, making `sideways' progress
(but not reverse progress). If another sibling or more
gets involved there may then be some chance for 3 or more
way loops, which is the risk of sibling forwarding.</t>
</list></t>
<t>If DAGRank(M) > DAGRank(N), then node M is located in a
less preferred position than N in the DAG with respect to the
metrics and optimizations defined by the objective code point.
Further, Node (M) may in fact be in Node (N)'s sub-DAG. There
is no advantage to Node (N) selecting Node (M) as a DAG
parent, and such a selection may create a loop.<list>
<t>For example, if Node M is of rank 3 and Node N is of
rank 5, then by definition Node N is in a less optimum
position than Node N. Further, Node N at rank 5 may in
fact be in Node M's own sub-DAG, and forwarding a packet
directed inwards towards the DAG root from M to N will
result in backwards progress and possibly a loop.</t>
</list></t>
</list></t>
<t>As an example, the DAG Rank could be computed in such a way so
as to closely track ETX when the objective function is to minimize
ETX, or latency when the objective function is to minimize
latency, or in a more complicated way as appropriate to the
objective code point being used within the DAG.</t>
<t>The DAG rank is subsequently used to restrict the options a
node has for movement within the DAG and to coordinate movements
in order to avoid the creation of loops.</t>
</section>
<section title="Sub-DAG">
<t>The sub-DAG of a node is the set of other nodes of greater rank
in the DAG, and thus might use a path towards the DAG root that
contains this node. This is an important property that is
leveraged for loop avoidance- if a node has lesser rank then it is
not in the sub-DAG. (An arbitrary node with greater rank may or
may not be contained in the sub-DAG). Paths through siblings are
not contained in this set.</t>
<t>As a further illustration, consider the DAG examples in <xref
target="Examples"></xref>. Consider Node (24) in the DAG Example
depicted in <xref target="DAGExample"></xref>. In this example,
the sub-DAG of Node (24) is comprised of Nodes (34), (44), and
(45).</t>
<t>A frozen sub-DAG is a subset of nodes in the sub-DAG of a node
who have been informed of a change to the node, and choose to
follow the node in a manner consistent with the change, for
example in preparation for a coordinated move. Nodes in the
sub-DAG who hear of a change and have other options than to follow
the node do not have to become part of the frozen sub-DAG, for
example such a node may be able to remain attached to the original
DAG through a different DAG parent. A further example may be found
in <xref target="ExDAGMaintenance"></xref>.</t>
</section>
<section title="Moving up in a DAG">
<t>A node may safely move `up' in the DAG, causing its DAG rank to
decrease and moving closer to the DAG root without risking the
formation of a loop.</t>
</section>
<section title="Moving down in a DAG">
<t>A node may not consider to move `down' the DAG, causing its DAG
rank to increase and moving further from the DAG root. In the case
where a node looses connectivity to the DAG, it must first leave
the DAG before it may then rejoin at a deeper point. This allows
for the node to coordinate moving down, freezing its own sub-DAG
and poisoning stale routes to the DAG, and minimizing the chances
of re-attaching to its own sub-DAG thinking that it has found the
original DAG again. If a node where allowed to re-attach into its
own sub-DAG a loop would most certainly occur, and may not be
broken until a count-to-infinity process elapses. The procedure of
detaching before moving down eliminates the need to
count-to-infinity.</t>
</section>
<section title="DAG Jumps">
<t>A jump from one DAG to another DAG is attaching to a new DAGID,
in such a way that an old DAGID is replaced by the new DAGID. In
particular, when an old DAGID is left, all associated parents are
no longer feasible, and a new DAGID is joined.</t>
<t>When a node in a DAG follows a DAG parent, it means that the
DAG parent has changed its DAGID (e.g. by joining a new DAG) and
that the node updates its own DAGID in order to keep the DAG
parent.</t>
</section>
<section title="Floating DAGs for DAG Repair">
<t>A DAG may also be floating. Floating DAGs may be encountered,
for example, during coordinated reconfigurations of the network
topology wherein a node and its sub-DAG breaks off the DAG,
temporarily becomes a floating DAG, and reattaches to a grounded
DAG. (Such coordination endeavors to avoid the construction of
transient loops in the LLN).</t>
<t>A DAG, or a sub-DAG temporarily promoted to a DAG, may also
become floating because of a network element failure. If the DAG
parent set of the node becomes completely depleted, the node will
have detached from the DAG, and may, if so configured, become the
root of its own transient floating DAG with a less desirable
administrative preference (thus beginning the process of
establishing the frozen sub-DAG), and then may reattach to the
original DAG at a lower point if it is able (after hearing RA-DIO
messages from alternate attachment points).</t>
</section>
</section>
<section title="Destination Advertisement">
<t>As RPL constructs DAGs, nodes may provision routes toward
destinations advertised through RA-DIO messages through their
selected parents, and are thus able to send traffic inward along the
DAG by forwarding to their selected parents. However, this mechanism
alone is not sufficient to support P2MP traffic flowing outward
along the DAG from the DAG root toward nodes. A destination
advertisement mechanism is employed by RPL to build up routing state
in support of these outward flows. The destination advertisement
mechanism may not be supported in all implementations, as
appropriate to the application requirements. A DAG root that
supports using the destination advertisement mechanism to build up
routing state will indicate such in the RA-DIO message. A DAG root
that supports using the destination advertisement mechanism must be
capable of allocating enough state to store the routing state
received from the LLN.</t>
<section title="IPv6 Neighbor Advertisement - Destination Advertisement Option (NA-DAO)">
<t>An IPv6 Neighbor Advertisement Message with Destination
Advertisement Options (NA-DAO) is used to convey the destination
information inward along the DAG toward the DAG root.</t>
<t>The information conveyed in the NA-DAO message includes the
following:</t>
<t><list style="symbols">
<t>A lifetime and sequence counter to determine the freshness
of the destination advertisement.</t>
<t>Depth information used by nodes to determine how far away
the destination (the source of the destination advertisement)
is</t>
<t>Prefix information to identify the destination, which may
be a prefix, an individual host, or multicast listeners</t>
<t>Reverse Route information to record the nodes visited
(along the outward path) when the intermediate nodes along the
path cannot support storing state for Hop-By-Hop routing.</t>
</list></t>
</section>
<section title="Destination Advertisement Operation">
<t>As the DAG is constructed and maintained, nodes are capable to
emit NA-DAO messages to a subset, or all, of their DAG parents.
The selection of this subset is according to an implementation
specific policy.</t>
<t>As a special case, a node may periodically emit a link-local
multicast IPv6 NA-DAO message advertising its locally available
destination prefixes. This mechanism allows for the one-hop
neighbors of a node to learn explicitly of the prefixes on the
node, and in some application specific scenarios this is desirable
in support of provisioning a trivial `one-hop' route. In this
case, nodes who receive the multicast destination advertisement
may use it to provision the one-hop route only, and not engage in
any additional processing (so as not to engage the mechanisms used
by a DAG parent).</t>
<t>When a (unicast) NA-DAO message reaches a node capable of
storing routing state, the node extracts information from the
NA-DAO message and updates its local database with a record of the
NA-DAO message and who it was received from. When the node later
propagates NA-DAO messages, it selects the best (least depth)
information for each destination and conveys this information
again in the form of NA-DAO messages to a subset of its own DAG
parents. At this time the node may perform route aggregation if it
is able, thus reducing the overall number of NA-DAO messages.</t>
<t>When a (unicast) NA-DAO message reaches a node incapable of
storing additional state, the node must append the next-hop
address (from which neighbor the NA-DAO message was received) to a
Reverse Route Stack carried within the NA-DAO message. The node
then passes the NA-DAO message on to one or more of its DAG
parents without storing any additional state.</t>
<t>When a node that is capable of storing routing state encounters
a (unicast) NA-DAO message with a Reverse Route Stack that has
been populated, the node knows that the NA-DAO message has
traversed a region of nodes that did not record any routing state.
The node is able to detach and store the Reverse Route State and
associate it with the destination described by the NA-DAO message.
Subsequently the node may use this information to construct a
source route in order to bridge the region of nodes that are
unable to support Hop-By-Hop routing to reach the destination.</t>
<t>In this way the destination advertisement mechanism is able to
provision routing state in support of P2MP traffic flows outward
along the DAG, and as according to the available resources in the
network.</t>
<t>Further aggregations of NA-DAO messages prefix reachability
information by destinations are possible in order to support
additional scalability.</t>
<t>A further example of the operation of the destination
advertisement mechanism is available in <xref
target="DestinationAdvertisementExample"></xref></t>
</section>
</section>
</section>
<section title="Loop Avoidance and Stability">
<t>The goal of a guaranteed consistent and loop free global routing
solution for an LLN may not be practically achieved given the real
behavior and volatility of the underlying metrics. The trade offs to
achieve a stable approximation of global convergence may be too
restrictive with respect to the need of the LLN to react quickly in
response to the lossy environment. Globally the LLN may be able to
achieve a weak convergence, in particular as link changes are able to
be handled locally and result in minimal changes to global
topology.</t>
<t>RPL does not aim to guarantee loop free path selection, or strong
global convergence. In order to reduce control overhead, in particular
the expense of mechanisms such as count-to-infinity, RPL does try to
avoid the creation of loops when undergoing topology changes. Further
mechanisms to mitigate the impact of loops, such as loop detection
when forwarding, are under investigation.</t>
<section title="Greediness and Rank-based Instabilities">
<t>If a node is greedy and attempts to move deeper in the DAG,
beyond its most preferred parent, in order to increase the size of
the DAG parent set, then an instability can result. This is
illustrated in <xref target="Greedy"></xref>.</t>
<t>Suppose a node is willing to receive and process a RA-DIO
messages from a node in its own sub-DAG, and in general a node
deeper than it. In such cases a chance exists to create a feedback
loop, wherein two or more nodes continue to try and move in the DAG
in order to optimize against each other. In some cases this will
result in an instability. It is for this reason that RPL mandates
that a node never receive and process RA-DIO messages from deeper
nodes. This rule creates an `event horizon', whereby a node cannot
be influenced into an instability by the action of nodes that may be
in its own sub-DAG.</t>
<t>A further example of the consequences of greedy operation, and
instability related to processing RA-DIO messages from nodes of
greater rank, may be found in <xref
target="ExGreedyExample"></xref></t>
</section>
<section anchor="SectionDAGMerge" title="Merging DAGs">
<t>The merging of DAGs is coordinated in a way such as to try and
merge two DAGs cleanly, preserving as much DAG structure as
possible, and in the process effecting a clean merge with minimal
likelihood of forming transient DAG loops. The coordinated merge is
also intended to minimize the related control cost.</t>
<t>When a node, and perhaps a related frozen sub-DAG, jumps to a
different DAG, the move is coordinated by a set of timers (DAG Hop
timers). The DAG Hop timers allow the nodes who will attach closer
to the sink of the new DAG to `jump' first, and then drag dependent
nodes behind them, thus endeavoring to efficiently coordinate the
attachment of the frozen sub-DAG into the new DAG.</t>
<t>A further example of a DAG Merge operation may be found in <xref
target="ExDAGMerge"></xref></t>
</section>
<section title="DAG Loops">
<t>A DAG loop may occur when a node detaches from the DAG and
reattaches to a device in its prior sub-DAG that has missed the
whole detachment sequence and kept advertising the original DAG.
This may happen in particular when RA-DIO messages are missed. Use
of the DAG sequence number can eliminate this type of loop. If the
DAG sequence number is not in use, the protection is limited (it
depends on propagation of RA-DIO messages during DAG hop timer), and
temporary loops might occur. RPL will move to eliminate such a loop
as soon as a RA-DIO message is received from a parent that appears
to be going down, as the child has to detach from it immediately.
(The alternate choice of staying attached and following the parent
in its fall would have counted to infinity and led to detach as
well).</t>
<t>Consider node (24) in the DAG Example depicted in <xref
target="DAGExample"></xref>, and its sub-DAG nodes (34), (44), and
(45). An example of a DAG loop would be if node (24) were to detach
from the DAG rooted at (LBR), and nodes (34) and (45) were to miss
the detachment sequence. Subsequently, if the link (24)--(45) were
to become viable and node (24) heard node (45) advertising the DAG
rooted at (LBR), a DAG loop (45->34->24->45) may form if
node (24) attaches to node (45).</t>
</section>
<section title="DAO Loops">
<t>A DAO loop may occur when the parent has a route installed upon
receiving and processing a NA-DAO message from a child, but the
child has subsequently cleaned up the state. This loop happens when
a no-DAO was missed till a heartbeat cleans up all states. The DAO
loop is not explicitly handled by the current specification. Split
horizon, not forwarding a packet back to the node it came from, may
mitigate the DAO loop in some cases, but does not eliminate it.</t>
<t>Consider node (24) in the DAG Example depicted in <xref
target="DAGExample"></xref>. Suppose node (24) has received a DA
from node (34) advertising a destination at node (45). Subsequently,
if node (34) tears down the routing state for the destination and
node (24) did not hear a no-DAO message to clean up the routing
state, a DAO loop may exist. node (24) will forward traffic destined
for node (45) to node (34), who may then naively return it into a
loop (if split horizon is not in place). A more complicated DAO loop
may result if node (34) instead passes the traffic to it's sibling,
node (33), potentially resulting in a
(24->34->33->23->13->24) loop.</t>
</section>
<section title="Sibling Loops">
<t>Sibling loops occur when a group of siblings keep choosing
amongst themselves as successors such that a packet does not make
forward progress. The current draft limits those loops to some
degree by split horizon (do not send back to the same sibling) and
parent preference (always prefer parents vs. siblings).</t>
<t>Consider the DAG Example depicted in <xref
target="DAGExample"></xref>. Suppose that Node (32) and (34) are
reliable neighbors, and thus are siblings. Then, in the case where
Nodes (22), (23), and (24) are transiently unavailable, and with no
other guiding strategy, a sibling loop may exist, e.g.
(33->34->32->33) as the siblings keep choosing amongst each
other in an uncoordinated manner.</t>
</section>
</section>
<section title="Local and Temporary Routing Decision">
<t>Although implementation specific, it is worth noting that a node
may decide to implement some local routing decision based on some
metrics, as observed locally or reported in the RA-DIO message. For
example, the routing may reflect a set of successors (next-hop), along
with various aggregated metrics used to load balance the traffic
according to some local policy. Such decisions are local and
implementation specific.</t>
<t>Routing stability is crucial in a LLN: in the presence of unstable
links, the first option consists of removing the link from the DAG and
triggering a DAG recomputation across all of the nodes affected by the
removed link. Such a naive approach could unavoidably lead to frequent
and undesirable changes of the DAG, routing instability, and
high-energy consumption. The alternative approach adopted by RPL
relies on the ability to temporarily not use a link toward a successor
marked as valid, with no change on the DAG structure. If the link is
perceived as non-usable for some period of time (locally
configurable), this triggers a DAG recomputation, through the DAG
discovery mechanism further detailed in <xref
target="DAGDiscovery"></xref>, after reporting the link failure. Note
that this concept may be extended to take into account other link
characteristics: for the sake of illustration, a node may decide to
send a fixed number of packets to a particular successor (because of
limited buffering capability of the successor) before starting to send
traffic to another successor.</t>
<t>According to the local policy function, it is possible for the node
to order the DAG parent set from `most preferred' to `least
preferred'. By constructing such an ordered set, and by appending the
set with siblings, the node is able to construct an ordered list of
preferred next hops to assist in local and temporary routing
decisions. The use of the ordered list by a forwarding engine is
loosely constrained, and may take into account the dynamics of the
LLN. Further, a forwarding engine implementation may decide to perform
load balancing functions using hash-based mechanisms to avoid packet
re-ordering. Note however, that specific details of a forwarding
engine implementation are beyond the scope of this document.</t>
<t>These decisions may be local and/or temporary with the objective to
maintain the DAG shape while preserving routing stability.</t>
</section>
<!--
<section title="Scalability">
<t>As each node selects DAG parents according to implementation
specific objectives, RPL is able to dynamically partition an LLN
network into different regions, each anchored by a DAG root.
Multiple DAG roots may be deployed in accordance with an
implementation specific policy designed to limit the size of a
partition, e.g. for performance or other reasons.</t>
<t>A further example is illustrated in <xref
target="AdditionalExamples"></xref>.</t>
</section>
-->
<section title="Maintenance of Routing Adjacency">
<t>In order to relieve the LLN of the overhead of periodic keepalives,
RPL may employ an as-needed mechanism of NS/NA in order to verify
routing adjacencies just prior to forwarding data. Pending the outcome
of verifying the routing adjacency, the packet may either be forwarded
or an alternate next-hop may be selected.</t>
</section>
</section>
<section anchor="ConstrainedLLNs" title="Constraint Based Routing in LLNs">
<t>This aim of this section is to make a clear distinction between
routing metrics and constraints and define the term constraint based
routing as used in this document.</t>
<section title="Routing Metrics">
<t>Routing metrics are used by the routing protocol to compute the
shortest path according to one of more defined metrics. IGPs such as
IS-IS (<xref target="RFC5120"></xref>) and OSPF (<xref
target="RFC4915"></xref>) compute the shortest path according to a
Link State Data Base (LSDB) using link metrics configured by the
network administrator. Such metrics can represent the link bandwidth
(in which case the metric is usually inversely proportional to the
bandwidth), delay, etc. Note that in some cases the metric is a
polynomial function of several metrics defining different link
characteristics. The resulting shortest path cost is equal to the sum
(or multiplication) of the link metrics along the path: such metrics
are said to be additive or multiplicative metrics.</t>
<t>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
ECMP (Equal Cost Multiple Paths). The optimization of multiple metrics
is known as an NP complete problem and is sometimes supported by some
centralized path computation engine.</t>
<t>In the case of RPL, it is virtually impossible to define *the*
metric, or even a composite, that will fit it all:</t>
<t><list style="symbols">
<t>Some information apply when determining routes, other
information may apply only when forwarding packets along
provisioned routes.</t>
<t>Some values are aggregated hop-by-hop, others are triggers from
L2.</t>
<t>Some properties are very stable, others vary rapidly.</t>
<t>Some data are useful in a given scenario and useless in
another.</t>
<t>Some arguments are scalar, others statistical.</t>
</list></t>
<t>For that reason, the RPL protocol core is agnostic to the logic
that handles metrics. A node will be configured with some external
logic to use and prioritize certain metrics for a specific scenario.
As new heterogeneous devices are installed to support the evolution of
a network, or as networks form in a totally ad-hoc fashion, it will
happen that nodes that are programmed with antagonistic logics and
conflicting or orthogonal priorities end up participating in the same
network. It is thus recommended to use consistent parent selection
policy, as per Objective Code Points (OCP), to ensure consistent
optimized paths.</t>
<t>RPL is designed to survive and still operate, though in a somewhat
degraded fashion, when confronted to such heterogeneity. The key
design point is that each node is solely responsible for setting the
vector of metrics that it sources in the DAG, derived in part from the
metrics sourced from its preferred parent. As a result, the DAG is not
broken if another node makes its decisions in as antagonistic fashion,
though an end-to-end path might not fully achieve any of the
optimizations that nodes along the way expect. The default operation
specified in OCP 0 clarifies this point.</t>
</section>
<section title="Routing Constraints">
<t>A constraint is a link or a node characteristic that must be
satisfied by the computed path (using boolean values or lower/upper
bounds) and is by definition neither additive nor multiplicative.
Examples of links constraints are "available bandwidth",
"administrative values (e.g. link coloring)", "protected versus
non-protected links", "link quality" whereas a node constraint can be
the level of battery power, CPU processing power, etc.</t>
</section>
<section title="Constraint Based Routing">
<t>The notion of constraint based routing consists of finding the
shortest path according to some metrics satisfying a set of
constraints. A technique consists of first filtering out all links and
nodes that cannot satisfy the constraints (resulting in a
sub-topology) and then computing the shortest path.</t>
<?rfc subcompact="yes"?>
<t><list>
<t>Example 1:</t>
<list>
<t>Link Metric: Bandwidth</t>
<t>Link Constraint: Blue</t>
<t>Node Constraint: Mains-powered node</t>
</list>
<t />
<t>Objective function 1:</t>
<list>
<t>"Find the shortest path (path with lowest cost where the path
cost is the sum of all link costs (Bandwidth)) along the path
such that all links are colored `Blue' and that only traverses
Mains-powered nodes."</t>
</list>
<t />
<t />
<t />
<t>Example 2:</t>
<list>
<t>Link Metric: Delay</t>
<t>Link Constraint: Bandwidth</t>
</list>
<t />
<t>Objective function 2:</t>
<list>
<t>"Find the shortest path (path with lowest cost where the path
cost is the sum of all link costs (Delay)) along the path such
that all links provide at least X Bit/s of reservable
bandwidth."</t>
</list>
</list></t>
<?rfc subcompact="no"?>
</section>
</section>
<section anchor="SpecCore" title="RPL Protocol Specification">
<section anchor="DAGInformationOption" title="DAG Information Option">
<t>The DAG Information Option carries a number of metrics and other
information that allows a node to discover a DAG, select its DAG
parents, and identify its siblings while employing loop avoidance
strategies.</t>
<section anchor="DIOBaseOption"
title="DAG Information Option (DIO) base option">
<t>The DAG Information Option is a container option carried within
an IPv6 Router Advertisement message as defined in <xref
target="RFC4861"></xref>, which might contain a number of
suboptions. The base option regroups the minimum information set
that is mandatory in all cases.</t>
<t><figure anchor="DIObase" title="DIO Base Option">
<artwork><![CDATA[
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 | Length |G|D|A| 00000 | Sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAGPreference | BootTimeRandom |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| NodePref. | DAGRank | DAGDelay |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DIOIntDoubl. | DIOIntMin. | DAGObjectiveCodePoint |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PathDigest |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| DAGID |
+ +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t><list hangIndent="6" style="hanging">
<t hangText="Type:">8-bit unsigned identifying the DIO base
option. The suggested value is 140 to be confirmed by the
IANA.</t>
<t hangText="Length:">8-bit unsigned integer set to 4 when there
is no suboption. The length of the option (including the type
and length fields and the suboptions) in units of 8 octets.</t>
<t hangText="Flag Field:">Three flags are currently
defined:<list hangIndent="6" style="hanging">
<t hangText="Grounded (G):">The Grounded (G) flag is set
when the DAG root is offering connectivity to an external
routed infrastructure such as the Internet.</t>
<t hangText="Destination Advertisement Trigger (D):">The
Destination Advertisement Trigger (D) flag is set when the
DAG root or another node in the successor chain decides to
trigger the sending of destination advertisements in order
to update routing state for the outward direction along the
DAG, as further detailed in <xref
target="DestinationAdvertisement"></xref>. Note that the use
and semantics of this flag are still under
investigation.</t>
<t hangText="Destination Advertisement Supported (A) :">The
Destination Supported (A) bit is set when the DAG root is
capable to support the collection of destination
advertisement related routing state and enables the
operation of the destination advertisement mechanism within
the DAG.</t>
</list></t>
<t>Unassigned bits of the Flag Field are considered as reserved.
They MUST be set to zero on transmission and MUST be ignored on
receipt.</t>
<t hangText="Sequence Number:">8-bit unsigned integer set by the
DAG root, incremented according to a policy provisioned at the
DAG root, and propagated with no change outwards along the DAG.
Each increment SHOULD have a value of 1 and may cause a wrap
back to zero.</t>
<t hangText="DAGPreference:">8-bit unsigned integer set by the
DAG root to its preference and unchanged at propagation.
DAGPreference ranges from 0x00 (least preferred) to 0xFF (most
preferred). The default is 0 (least preferred). The DAG
preference provides an administrative mechanism to engineer the
self-organization of the LLN, for example indicating the most
preferred LBR. If a node has the option to join a more preferred
DAG while still meeting other optimization objectives, then the
node will seek to join the more preferred DAG.</t>
<t hangText="BootTimeRandom:">A random value computed at boot
time and recomputed in case of a duplication with another node.
The concatenation of the NodePreference and the BootTimeRandom
is a 32-bit extended preference that is used to resolve
collisions. It is set by each node at propagation time.</t>
<t hangText="NodePreference:">The administrative preference of
that LLN Node. Default is 0. 255 is the highest possible
preference. Set by each LLN Node at propagation time. Forms a
collision tiebreaker in combination with BootTimeRandom.</t>
<t hangText="DAGRank:">8-bit unsigned integer indicating the DAG
rank of the node sending the RA-DIO message. The DAGRank of the
DAG root is typically 1. DAGRank is further described in <xref
target="DAGDiscovery"></xref>.</t>
<t hangText="DAGDelay:">16-bit unsigned integer set by the DAG
root indicating the delay before changing the DAG configuration,
in TBD-units. A default value is TBD. It is expected to be an
order of magnitude smaller than the RA-interval. It is also
expected to be an order of magnitude longer than the typical
propagation delay inside the LLN.</t>
<t hangText="DIOIntervalDoublings:">8-bit unsigned integer.
Configured on the DAG root and used to configure the trickle
timer governing when RA-DIO message should be sent within the
DAG. DIOIntervalDoublings is the number of times that the
DIOIntervalMin is allowed to be doubled during the trickle timer
operation.</t>
<t hangText="DIOIntervalMin:">8-bit unsigned integer. Configured
on the DAG root and used to configure the trickle timer
governing when RA-DIO message should be sent within the DAG. The
minimum configured interval for the RA-DIO trickle timer in
units of ms is 2^DIOIntervalMin. For example, a DIOIntervalMin
value of 16ms is expressed as 4.</t>
<t hangText="DAGObjectiveCodePoint:">The DAG Objective Code
Point is used to indicate the cost metrics, objective functions,
and methods of computation and comparison for DAGRank in use in
the DAG. The DAG OCP is set by the DAG root. (Objective Code
Points are to be further defined in <xref
target="I-D.ietf-roll-routing-metrics"></xref>.</t>
<t hangText="PathDigest:">32-bit unsigned integer CRC, updated
by each LLN Node. This is the result of a CRC-32c computation on
a bit string obtained by appending the received value and the
ordered set of DAG parents at the LLN Node. DAG roots use a
'previous value' of zeroes to initially set the PathDigest. Used
to determine when something in the set of successor paths has
changed.</t>
<t hangText="DAGID:">128-bit unsigned integer which uniquely
identify a DAG. This value is set by the DAG root. The global
IPv6 address of the DAG root can be used, however. the DAGID
MUST be unique per DAG within the scope of the LLN. In the case
where a DAG root is rooting multiple DAGs the DAGID MUST be
unique for each DAG rooted at a specific DAG root.</t>
</list></t>
<t>The following values MUST NOT change during the propagation of
RA-DIO messages outwards along the DAG: Type, Length, G,
DAGPreference, DAGDelay and DAGID. All other fields of the RA-DIO
message are updated at each hop of the propagation.</t>
<section title="DAG Information Option (DIO) Suboptions">
<t>In addition to the minimum options presented in the base
option, several suboptions are defined for the RA-DIO message:</t>
<section title="Format">
<t><figure anchor="DIOsub" title="DIO Suboption Generic Format">
<artwork><![CDATA[
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Subopt. Type | Subopt Length | Suboption Data...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t><list hangIndent="6" style="hanging">
<t hangText="Suboption Type:">8-bit identifier of the type
of suboption. When processing a RA-DIO message containing a
suboption for which the Suboption Type value is not
recognized by the receiver, the receiver MUST silently
ignore the unrecognized option, continue to process the
following suboption, correctly handling any remaining
options in the message.</t>
<t hangText="Suboption Length:">8-bit unsigned integer,
representing the length in octets of the suboption, not
including the suboption Type and Length fields.</t>
<t hangText="Suboption Data:">A variable length field that
contains data specific to the option.</t>
</list></t>
<t>The following subsections specify the RA-DIO message
suboptions which are currently defined for use in the DAG
Information Option.</t>
<t>Implementations MUST silently ignore any RA-DIO message
suboptions options that they do not understand.</t>
<t>RA-DIO message suboptions may have alignment requirements.
Following the convention in IPv6, these options are aligned in a
packet such that multi-octet values within the Option Data field
of each option fall on natural boundaries (i.e., fields of width
n octets are placed at an integer multiple of n octets from the
start of the header, for n = 1, 2, 4, or 8).</t>
</section>
<section title="Pad1">
<t>The Pad1 suboption does not have any alignment requirements.
Its format is as follows:</t>
<t><figure anchor="DIOsubPad1" title="Pad 1">
<artwork><![CDATA[
0
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| Type = 0 |
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t>NOTE! the format of the Pad1 option is a special case - it
has neither Option Length nor Option Data fields.</t>
<t>The Pad1 option is used to insert one octet of padding in the
RA-DIO message to enable suboptions alignment. If more than one
octet of padding is required, the PadN option, described next,
should be used rather than multiple Pad1 options.</t>
</section>
<section title="PadN">
<t>The PadN option does not have any alignment requirements. Its
format is as follows:</t>
<t><figure anchor="DIOsubPadN" title="Pad N">
<artwork><![CDATA[
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Type = 1 | Subopt Length | Subopt Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
]]></artwork>
</figure></t>
<t>The PadN option is used to insert two or more octets of
padding in the RA-DIO message to enable suboptions alignment.
For N (N > 1) octets of padding, the Option Length field
contains the value N-2, and the Option Data consists of N-2
zero-valued octets. PadN Option data MUST be ignored by the
receiver.</t>
</section>
<section title="DAG Metric Container">
<t>The DAG Metric Container suboption may be aligned as
necessary to support its contents. Its format is as follows:</t>
<t><figure anchor="DIOsubLLNMetric" title="DAG Metric Container">
<artwork><![CDATA[
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
| Type = 2 | Container Len | DAG Metric Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
]]></artwork>
</figure></t>
<t>The DAG Metric Container is used to report aggregated path
metrics along the DAG. The DAG Metric Container may contain a
number of discrete node, link, and aggregate path metrics as
chosen by the implementer. The Container Length field contains
the length in octets of the DAG Metric Data. The order, content,
and coding of the DAG Metric Container data is as specified in
<xref target="I-D.ietf-roll-routing-metrics"></xref>.</t>
<t>The processing and propagation of the DAG Metric Container is
governed by implementation specific policy functions.</t>
</section>
<section title="Destination Prefix">
<t>The Destination Prefix suboption has an alignment requirement
of 4n+1. Its format is as follows:</t>
<t><figure anchor="DIOsubDestinationPrefix"
title="DAG Destination Prefix">
<artwork><![CDATA[
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 3 | Length | Prefix Length |Resvd|Prf|Resvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Prefix (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t>The Destination Prefix suboption is used when the DAG root,
or another node located inwards along the DAG on the path to the
DAG root, needs to indicate that it offers connectivity to
destination prefixes other than the default. This may be useful
in cases where more than one LBR is operating within the LLN and
offering connectivity to different administrative domains, e.g.
a home network and a utility network. In such cases, upon
observing the Destination Prefixes offered by a particular DAG,
a node MAY decide to join multiple DAGs in support of a
particular application.</t>
<t>The Length is coded as the length of the suboption in octets,
excluding the Type and Length fields.</t>
<t>The Prefix Length is an 8-bit unsigned integer that indicates
the number of leading bits in the destination prefix. Prf is the
Route Preference as in <xref target="RFC4191"></xref>. The
reserved fields MUST be set to zero on transmission and MUST be
ignored on receipt.</t>
<t>The Prefix Lifetime is a 32-bit unsigned integer representing
the length of time in seconds (relative to the time the packet
is sent) that the Destination Prefix is valid for route
determination. A value of all one bits (0xFFFFFFFF) represents
infinity. A value of all zero bits (0x00000000) indicates a loss
of reachability.</t>
<t>The Destination Prefix contains Prefix Length significant
bits of the destination prefix. The remaining bits of the
Destination Prefix, as required to complete the trailing octet,
are set to 0.</t>
<t>In the event that a RA-DIO message may need to specify
connectivity to more than one destination, the Destination
Prefix suboption may be repeated.</t>
</section>
</section>
</section>
</section>
<section anchor="ConceptualDataStructures"
title="Conceptual Data Structures">
<t>The RPL implementation MUST maintain the following conceptual data
structures in support of DAG discovery:</t>
<t><list style="symbols">
<t>A set of candidate neighbors</t>
<t>For each DAG:</t>
<list>
<t>A set of candidate DAG parents</t>
<t>A set of DAG parents (which are a subset of candidate DAG
parents and may be implemented as such)</t>
</list>
</list></t>
<section title="Candidate Neighbors Data Structure">
<t>The set of candidate neighbors is to be populated by neighbors
who are discovered by the neighbor discovery mechanism and further
qualified as statistically stable as per the mechanisms discussed in
<xref target="I-D.ietf-roll-routing-metrics"></xref>. The candidate
neighbors, and related metrics, should demonstrate
stability/reliability beyond a certain threshold, and it is
recommended that a local confidence value be maintained with respect
to the neighbor in order to track this. Implementations MAY choose
to bound the maximum size of the candidate neighbor set, in which
case a local confidence value will assist in ordering neighbors to
determine which ones should remain in the candidate neighbor set and
which should be evicted.</t>
<t>If Neighbor Unreachability Detection (NUD) determines that a
candidate neighbor is no longer reachable, then it shall be removed
from the candidate neighbor set. In the case that the candidate
neighbor has associated states in the DAG parent set or active DA
entries, then the removal of the candidate neighbor shall be
coordinated with tearing down these states. All provisioned routes
associated with the candidate neighbor should be removed.</t>
</section>
<section title="Directed Acyclic Graphs (DAGs) Data Structure">
<t>A DAG may be uniquely identified by within the LLN by its unique
DAGID. When a single device is capable to root multiple DAGs in
support of an application need for multiple optimization objectives
it is expected to produce a different and unique DAGID for each of
the multiple DAGs.</t>
<t>For each DAG that a node is, or may become, a member of, the
implementation MUST keep a DAG table with the following entries:</t>
<t><list style="symbols">
<t>DAGID</t>
<t>DAGObjectiveCodePoint</t>
<t>A set of Destination Prefixes offered inwards along the
DAG</t>
<t>A set of candidate DAG parents</t>
<t>A timer to govern the sending of RA-DIO messages for the
DAG</t>
<t>DAGSequenceNumber</t>
</list></t>
<t>When a DAG is discovered for which no DAG data structure is
instantiated, and the node wants to join (i.e. the neighbor is to
become a candidate DAG parent in the Held-Up state), then the DAG
data structure is instantiated.</t>
<t>When the candidate DAG parent set is depleted (i.e. the last
candidate DAG parent has timed out of the Held-Down state), then the
DAG data structure SHOULD be suppressed after the expiration of an
implementation-specific local timer. An implementation SHOULD delay
before deallocating the DAG data structure in order to observe that
the DAGSequenceNumber has incremented should any new candidate DAG
parents appear for the DAG.</t>
<section title="Candidate DAG Parents Structure">
<t>When the DAG is self-rooted, the set of candidate DAG parents
is empty.</t>
<t>In all other cases, for each candidate DAG parent in the set,
the implementation MUST keep a record of:</t>
<t>
<list style="symbols">
<t>a reference to the neighboring device which is the DAG
parent</t>
<t>a record of most recent information taken from the DAG
Information Object last processed from the candidate DAG
parent</t>
<t>a state associated with the role of the candidate as a
potential DAG parent {Current, Held-Up, Held-Down, Collision},
further described in <xref
target="CandidateParentStates" /></t>
<t>A DAG Hop Timer, if instantiated</t>
<t>A Held-Down Timer, if instantiated</t>
</list>
</t>
<section title="DAG Parents">
<t>Note that the subset of candidate DAG parents in the
`Current' state comprises the set of DAG parents, i.e. the nodes
actively acting as parents in the DAG.</t>
<t>DAG parents may be ordered, according to the OCP. When
ordering DAG parents, in consultation with the OCP, the most
preferred DAG parent may be identified. All current DAG parents
must have a rank less than or equal to that of the most
preferred DAG parent.</t>
<t>When nodes are added to or removed from the DAG parent set
the most preferred DAG parent may have changed and should be
reevaluated. Any nodes having a rank greater than self after
such a change must be placed in the Held-Down state and evicted
as per the procedures described in <xref
target="CandidateParentStates" /></t>
</section>
<t>An implementation may choose to keep these records as an
extension of the Default Router List (DRL).</t>
</section>
</section>
</section>
<section anchor="DAGDiscovery" title="DAG Discovery and Maintenance">
<t>DAG discovery locates the nearest sink, as determined according to
some metrics and constraints, and forms a Directed Acyclic Graph
towards that sink, by identifying a set of DAG parents. During this
process DAG discovery also identifies siblings, which may be used
later to provide additional path diversity towards the DAG root. DAG
discovery enables nodes to implement different policies for selecting
their DAG parents in the DAG by using implementation specific policy
functions. DAG discovery specifies a set of rules to be followed by
all implementations in order to ensure interoperation. DAG discovery
also standardizes the format that is used to advertise the most common
information that is used in order to select DAG parents.</t>
<t>One of these information, the DAG rank, is used by DAG discovery to
provide loop avoidance even if nodes implement different policies. The
DAG Rank is computed as specified by the Objective Code Point in use
by the DAG, demonstrating the properties described in <xref
target="DAGRank"></xref>. The rank should be computed in such a way so
as to provide a comparable basis with other nodes which may not use
the same metric at all.</t>
<t>The DAG discovery procedures take into account a number of factors,
including:</t>
<t><list style="symbols">
<t>RPL rules for loop avoidance based on rank</t>
<t>The OCP function</t>
<t>The advertised metrics</t>
<t>Local policy functions (e.g. a bounded number of candidate
neighbors).</t>
</list></t>
<section anchor="DAGDiscoveryRules" title="DAG Discovery Rules">
<t>In order to organize and maintain loopless structure, the DAG
discovery implementation in the nodes MUST obey to the following
rules and definitions:</t>
<t><list style="numbers">
<t>A node that does not have any DAG parents in a DAG is the
root of its own floating DAG. It's rank is 1. A node will end up
in that situation when it looses all of its current feasible
parents, i.e. the set of DAG parents becomes depleted. In that
case, the node SHOULD remember the DAGID and the sequence
counter of the last RA-DIO message from the lost parents for a
period of time which covers multiple RA-DIO messages. This is
done so that if the node does encounter another possible
attachment point to the lost DAGID within a period of time, the
node may observe a sequence counter change by comparing the
observed sequence counter to the last observed sequence counter
and thus verify that the new attachment point is a viable and
independent alternative to attach back to the lost DAGID.</t>
<t>A node that is attached to an infrastructure that does not
support RA-DIO messages, is the DAG root of its own grounded
DAG. It's rank is 1. (For example an LBR that is in
communication with a non-LLN router not running RPL).</t>
<t>A (non-LLN) router sending a RA messages without DIO is
considered a grounded infrastructure at rank 0. (For example, a
router that is in communication with an LLN node but not running
RPL such as a non-LLN public Internet router in communication
with an LBR)</t>
<t>The DAG root exposes the DAG in the RA-DIO message and nodes
propagate the RA-DIO message outwards along the DAG with the RAs
that they forward over their LLN links.</t>
<t>A node MAY move at any time, with no delay, within its DAG
when the move does not cause the node to increase its own DAG
rank, as per the rank calculation indicated by the OCP.</t>
<t>A node MUST NOT move outwards along a DAG that it is attached
to, causing the DAG rank to increase, except in a special case
where the node MAY choose to follow the last DAG parent in the
set of DAG parents. In the general case, if a node is required
to move such that it cannot stay within the DAG without a rank
increase, then it needs to first leave the DAG. In other words a
node that is already part of a DAG MAY move or follow a DAG
parent at any time and with no delay in order to be closer, or
stay as close, to the DAG root of its current DAG as it already
is, but may not move outwards. RAs received from other routers
located at lesser rank in the same DAG may be considered as
coming from candidate parents. RAs received from other routers
located at the same rank in the same DAG may be considered as
coming from siblings. Nodes MUST ignore RAs that are received
from other routers located at greater rank within the same
DAG.</t>
<t>A node may jump from its current DAG into any different DAG
if it is preferred for reasons of connectivity, configured
preference, free medium time, size, security, bandwidth, DAG
rank, or whatever metrics the LLN cares to use. A node may jump
at any time and to whatever rank it reaches in the new DAG, but
it may have to wait for a DAG Hop timer to elapse in order to do
so. This allows the new higher parts (closer to the sink) of the
DAG to move first, thus allowing stepped DAG reconfigurations
and limiting relative movements. A node SHOULD NOT join a
previous DAG (identified by its DAGID) unless the sequence
number in the RA-DIO message has incremented since the node left
that DAG. A newer sequence number indicates that the candidate
parents were not attached behind this node, as they kept getting
subsequent RA-DIO messages with new sequence numbers from the
same DAG. In the event that old sequence numbers (two or more
behind the present value) are encountered they are considered
stale and the corresponding parent SHOULD be removed from the
set.</t>
<t>If a node has selected a new set of DAG parents but has not
moved yet (because it is waiting for DAG Hop timer to elapse),
the node is unstable MUST NOT send RA-DIOs for that DAG.</t>
<t>If a node receives a RA-DIO from one of its DAG parents, and
if the parent contains a different DAGID, indicating that the
parent has left the DAG, and if the node can remain in the
current DAG through an alternate DAG parent, then the node
SHOULD remove the DAG parent which has joined the new DAG from
its DAG parent set and remain in the original DAG. If there is
no alternate parent for the DAG, then the node SHOULD follow
that parent into the new DAG.</t>
<t>When a node detects or causes a DAG inconsistency, as
described in <xref target="TrickleInconsistencies"></xref>, then
the node SHOULD send an unsolicited RA-DIO message to its
one-hop neighbors. The RA-DIO is updated to propagate the new
DAG information. Such an event MUST also cause the trickle timer
governing the periodic sending of RA-DIO messages to be
reset.</t>
<t>If a DAG parent increases its rank such that the node rank
would have to change, and if the node does not wish to follow
(e.g. it has alternate options), then the DAG parent SHOULD be
evicted from the DAG parent set. If the DAG parent is the last
in the DAG parent set, then the node SHOULD chose to follow
it.</t>
</list></t>
</section>
<section title="Reception and Processing of RA-DIO messages">
<t>When an RA-DIO message is received from a source device named
SRC, the receiving node must first determine whether or not the
RA-DIO message should be accepted for further processing, and
subsequently present the RA-DIO message for further processing if
eligible.</t>
<section title="Determination of Eligibility for DIO Processing">
<t>
<list>
<t>If the RA-DIO message is malformed, then the RA-DIO message
is not eligible for further processing and is silently
discarded. A RPL implementation MAY log the reception of a
malformed RA-DIO message.</t>
<t>If SRC is not a member of the candidate neighbor set, then
the RA-DIO is not eligible for further processing. (Further
evaluation/confidence of this neighbor is necessary)</t>
<t>If the RA-DIO message advertises a DAG that the node is
already a member of, then:</t>
<list>
<t>If the rank of SRC as reported in the RA-DIO message is
lesser than that of the node within the DAG, then the RA-DIO
message MUST be considered for further processing</t>
<t>If the rank of SRC as reported in the RA-DIO message is
equal to that of the node within the DAG, then SRC is marked
as a sibling and the RA-DIO message is not eligible for
further processing.</t>
<t>If the rank of SRC as reported in the RA-DIO message is
higher than that of the node within the DAG, and SRC is not
a DAG parent, then the RA-DIO message MUST NOT be considered
for further processing</t>
</list>
<t>If SRC is a DAG parent for any other DAG that the node is
attached to, then the RA-DIO message MUST be considered for
further processing (the DAG parent may have jumped).</t>
<t>If the RA-DIO message advertises a DAG that offers a better
(new or alternate) solution to an optimization objective
desired by the node, then the RA-DIO message MUST be
considered for further processing.</t>
</list>
</t>
</section>
<section title="Overview of RA-DIO Message Processing">
<t>
<list>
<t />
<t>If the received RA-DIO message is for a new/alternate
DAG:</t>
<list>
<t>Instantiate a data structure for the new/alternate DAG if
necessary</t>
<t>Place the neighbor in the candidate DAG parent set</t>
<t>If the node has sent an RA message within the risk window
as described in <xref target="DAGCollision" /> then perform
the collision detection described in <xref
target="DAGCollision" />. If a collision occurs, place the
candidate DAG parent in the collision state and do not
process the RA-DIO message any further as described in <xref
target="CandidateParentStates" />.</t>
<t>If the SRC node is also a DAG parent for another DAG that
the node is a member of, and if the new/alternate DAG
satisfies an equivalent optimization objective as the other
DAG, then the DAG parent is known to have jumped.</t>
<list>
<t>Remove SRC as a DAG parent from the other DAG (place it
in the held-down state)</t>
<t>If the other DAG is now empty of candidate parents,
then directly follow SRC into the new DAG by adding it as
a DAG parent in the Current state, else ignore the RA-DIO
message (do not follow the parent).</t>
</list>
<t>If the new/alternate DAG offers a better solution to the
optimization objectives, then prepare to jump: copy the DIO
information into the record for the candidate DAG parent,
place the candidate DAG parent into the Held-Up state, and
start the DAG Hop timer as per <xref
target="DAGHeldUp" />.</t>
</list>
<t>If the RA-DIO message is for a known/existing DAG:</t>
<list>
<t>Process the RA-DIO message as per the rules in <xref
target="DAGDiscovery" /></t>
</list>
</list>
</t>
</section>
<t>As candidate parents are identified, they may subsequently be
promoted to DAG parents by following the rules of DAG discovery as
described in <xref target="DAGDiscovery" />. When a node adds
another node to its set of candidate parents, the node becomes
attached to the DAG through the parent node.</t>
<t>In the DAG discovery implementation, the most preferred parent
should be used to restrict which other nodes may become DAG parents.
Some nodes in the DAG parent set may be of a rank less than or equal
to the most preferred DAG parent. (This case may occur, for example,
if an energy constrained device is at a lesser rank but should be
avoided as per an optimization objective, resulting in a more
preferred parent at a greater rank).</t>
</section>
<section title="RA-DIO Transmission">
<t>Each node maintains a timer that governs when to multicast RA
messages. This timer is implemented as a trickle timer operating
over a variable interval. Trickle timers are further detailed in
<xref target="TrickleImplementation"></xref>. The governing
parameters for the timer should be configured consistently across
the DAG, and are provided by the DAG root in the RA-DIO message. In
addition to periodic RA messages, each LLN node will respond to
Router Solicitation (RS) messages according to <xref
target="RFC4861"></xref>.</t>
<t><list style="symbols">
<t>When a node is unstable, because any DAG Hop timer is running
in preparation for a jump, then the node MUST NOT transmit
unsolicited RA-DIOs (i.e. the node will remain silent when the
timer expires).</t>
<t>When a node detects an inconsistency, it SHOULD reset the
interval of the trickle timer to a minimum value, causing RA
messages to be emitted more frequently as part of a strategy to
quickly correct the inconsistency. Such inconsistencies may be,
for example, an update to a key parameter (e.g. sequence number)
in the RA-DIO message or a loop detected when a node located
inwards along the DAG forwards traffic outwards. Inconsistencies
are further detailed in <xref
target="TrickleInconsistencies"></xref>.</t>
<t>When a node enters a mode of consistent operation within a
DAG, i.e. RA-DIO messages from its DAG parents are consistent
and no other inconsistencies are detected, it may begin to open
up the interval of the trickle timer towards a maximum value,
causing RAs to be emitted less frequently, thus reducing network
maintenance overhead and saving energy consumption (which is of
utmost importance for battery-operated nodes).</t>
<t>When a node is initialized, it MAY be configured to remain
silent and not multicast any RA messages until it has
encountered and joined a DAG (perhaps initially probing for a
nearby DAG with an RS message). Alternately, it may choose to
root its own floating DAG and begin multicasting RAs using a
default trickle configuration. The second case may be
advantageous if it is desired for independent nodes to begin
aggregating into scattered floating DAGs in the absence of a
grounded node, for example in support of LLN installation and
commissioning.</t>
</list></t>
<t>Note that if multiple DAG roots are participating in the same
DAG, i.e. offering RA-DIO messages with the same DAGID, then they
must coordinate with each other to ensure that their RA-DIO messages
are consistent when they emit RA-DIO messages. In particular the
Sequence number must be identical from each DAG root, regardless of
which of the multiple DAG roots issues the RA-DIO message, and
changes to the Sequence number should be issued at the same time.
The specific mechanism of this coordination, e.g. along a non-LLN
network between DAG roots, is beyond the scope of this
specification.</t>
</section>
<section anchor="TrickleImplementation"
title="Trickle Timer for RA Transmission">
<t>RPL treats the construction of a DAG as a consistency problem,
and uses a trickle timer <xref target="Levis08"></xref> to control
the rate of control broadcasts.</t>
<t>For each DAG that a node is part of, the node must maintain a
single trickle timer. The required state contains the following
conceptual items:</t>
<t><list hangIndent="6" style="hanging">
<t hangText="I:">The current length of the communication
interval</t>
<t hangText="T:">A timer with a duration set to a random value
in the range [I/2, I]</t>
<t hangText="C:">Redundancy Counter</t>
<t hangText="I_min:">The smallest communication interval in
milliseconds. This value is learned from the RA-DIO message as
(2^DIOIntervalMin)ms. The default value is
DEFAULT_DIO_INTERVAL_MIN.</t>
<t hangText="I_doublings:">The number of times I_min should be
doubled before maintaining a constant rate, i.e. I_max = I_min *
2^I_doublings. This value is learned from the RA-DIO message as
DIOIntervalDoublings. The default value is
DEFAULT_DIO_INTERVAL_DOUBLINGS.</t>
</list></t>
<section title="Resetting the Trickle Timer">
<t>The trickle timer for a DAGID is reset by:</t>
<t><list style="numbers">
<t>Setting I_min and I_doublings to the values learned from
the RA-DIO message.</t>
<t>Setting C to zero.</t>
<t>Setting I to I_min.</t>
<t>Setting T to a random value as described above.</t>
<t>Restarting the trickle timer to expire after a duration
T</t>
</list></t>
<t>When node learns about a DAG through a RA-DIO message and makes
the decision to join it, it initializes the state of the trickle
timer by resetting the trickle timer and listening. Each time it
hears a consistent RA for this DAG from a DAG parent, it MAY
increment C.</t>
<t>When the timer fires at time T, the node compares C to the
redundancy constant, DEFAULT_DIO_REDUNDANCY_CONSTANT. If C is less
than that value, the node generates a new RA and broadcasts 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.</t>
</section>
<section anchor="TrickleInconsistencies"
title="Determination of Inconsistency">
<t>The trickle timer is reset whenever an inconsistency is
detected within the DAG, for example:</t>
<t><list style="symbols">
<t>The node joins a new DAGID</t>
<t>The node moves within a DAGID</t>
<t>The node receives a modified RA-DIO message from a DAG
parent</t>
<t>A DAG parent forwards a packet intended to move inwards,
indicating an inconsistency and possible loop.</t>
<t>A metric communicated in the RA-DIO message is determined
to be inconsistent, as according to a implementation specific
path metric selection engine.</t>
<t>The rank of a DAG parent has changed.</t>
</list></t>
</section>
</section>
</section>
<section anchor="DAGHeartbeat" title="DAG Heartbeat">
<t>The DAG root makes the sole determination of when to revise the
DAGSequenceNumber by incrementing it upwards. When the
DAGSequenceNumber is increased an inconsistency results, causing
RA-DIO messages to be sent back outwards along the DAG to convey the
change. The degree to which this mechanism is relied on may be
determined by the implementation- on one hand it may serve as a
periodic heartbeat, refreshing the DAG states, and on the other hand
it may result in a constant steady-state control cost overhead which
is not desirable.</t>
<t>Some implementations may provide an administrative interface, such
as a command line, at the DAG root whereby the DAGSequenceNumber may
be caused to increment in response to some policy outside of the scope
of RPL.</t>
<t>Other implementations may make use of a periodic timer to
automatically increment the DAGSequenceNumber, resulting in a periodic
DAG Heartbeat at a rate appropriate to the application and
implementation.</t>
</section>
<section title="DAG Selection">
<t>The DAG selection is implementation and algorithm dependent. Nodes
SHOULD prefer to join DAGs advertising OCPs and destinations
compatible with their implementation specific objectives. In order to
limit erratic movements, and all metrics being equal, nodes SHOULD
keep their previous selection. Also, nodes SHOULD provide a means to
filter out a candidate parent whose availability is detected as
fluctuating, at least when more stable choices are available. Nodes
MAY place the failed candidate parent in a Hold Down mode that ensures
that the candidate parent will not be reused for a given period of
time.</t>
<t>When connection to a fixed network is not possible or preferable
for security or other reasons, scattered DAGs MAY aggregate as much as
possible into larger DAGs in order to allow connectivity within the
LLN.</t>
<t>A node SHOULD verify that bidirectional connectivity and adequate
link quality is available with a candidate neighbor before it
considers that candidate as a DAG parent.</t>
</section>
<section title="Administrative rank">
<t>When the DAG is formed under a common administration, or when a
node performs a certain role within a community, it might be
beneficial to associate a range of acceptable rank with that node. For
instance, a node that has limited battery should be a leaf unless
there is no other choice, and may then augment the rank computation
specified by the OCP in order to expose an exaggerated rank.</t>
</section>
<section anchor="CandidateParentStates"
title="Candidate DAG Parent States and Stability">
<t>Candidate DAG parents may or may not be eligible to act as DAG
parents depending on runtime conditions. The following states are
defined:</t>
<t><list hangIndent="12" style="hanging">
<t hangText="Current">This candidate parent is in the set of DAG
parents and may be used for forwarding traffic inward along the
DAG. When a candidate parent is placed into the Current state, or
taken out of the Current state, it is necessary to re-evaluate
which of the remaining DAG parents is the most preferred DAG
parent and its rank. At that time any remaining DAG parents of
greater rank than this node must be placed in the Held-Down state,
and the hold-down timer started, in order to be evicted as DAG
parents. In the same fashion, siblings must also be
reevaluated.</t>
<t hangText="Held-Up">This parent can not be used until the DAG
hop timer elapses.</t>
<t hangText="Held-Down">This candidate parent can not be used till
hold down timer elapses. At the end of the hold-down period, the
candidate is removed from the candidate DAG parent set, and may be
reinserted if it appears again with a RA-DIO message.</t>
<t hangText="Collision">This candidate parent can not be used till
its next RA-DIO message.</t>
</list></t>
<section anchor="DAGHeldUp" title="Held-Up">
<t>This state is managed by the DAG Hop timer, it serves 2
purposes:</t>
<t><list style="empty">
<t>Delay the reattachment of a sub-DAG that has been forced to
detach. This is not as safe as the use of the sequence, but
still covers that when a sub-DAG has detached, the RA-DIO
message that is initiated by the new DAG root has a chance to
spread outward along the sub-DAG, ideally forming a frozen
sub-DAG that is aware of the DAG change, such that two different
DAGs have formed prior to an attempted reattachment.</t>
<t>Limit RA-DIO message storms (control cost / churn) when two
DAGs collide/merge. The idea is that between the nodes from DAG
A that decide to move to DAG B, those that see the highest place
(closer to the DAG root) in DAG B will move first and advertise
their new locations before other nodes from DAG A actually
move.</t>
</list></t>
<t>A new DAG is discovered upon receiving a RA message with or
without a DIO. The node joins the DAG by selecting the source of the
RA message as a DAG parent (and possibly installing the DAG parent
as a default gateway). The node is then a member of the DAG and may
begin to multicast RA-DIO messages containing the DIO for the
DAG.</t>
<t>When a new DAG is discovered, the candidate parent that
advertises the new DAG is placed in a held up state for the duration
of a DAG Hop timer. If the resulting new set of DAG parents is more
preferable than the current one, or if the node is intending to
maintain a membership in the new DAG in addition to its current DAG,
the node expects to jump and becomes unstable.</t>
<t>A node that is unstable may discover other candidate parents from
the same new DAG during the instability phase. It needs to start a
new DAG Hop timer for all these. The first timer that elapses for a
given new DAG clears them all for that DAG, allowing the node to
jump to the highest position available in the new DAG.</t>
<t>The duration of the DAG Hop timer depends on the DAG Delay of the
new DAG and on the rank of candidate parent that triggers it:
(candidates rank + random) * candidate's DAG_delay (where 0 <=
random < 1). It is randomized in order to limit collisions and
synchronizations.</t>
</section>
<section title="Held-Down">
<t>When a neighboring node is 'removed' from the Default Router
List, it is actually held down for a hold down timer period, in
order to prevent flapping. This happens when a node disappears (upon
expiration timer).</t>
<t>When the hold down timer elapses, the node is removed from the
candidate DAG parent set.</t>
</section>
<section anchor="DAGCollision" title="Collision">
<t>A race condition occurs if 2 nodes send RA-DIO messages at the
same time and then attempt to join each other. This might happen,
for example, between nodes which act as DAG root of their own DAGs.
In order to detect the situation, LLN Nodes time stamp the sending
of RA-DIO message. Any RA-DIO message received within a short
link-layer-dependent period introduces a risk. To resolve the
collision, a 32bits extended preference is constructed from the
RA-DIO message by concatenating the NodePreference with the
BootTimeRandom.</t>
<t>A node that decides to add a candidate to its DAG parents will do
so between (candidate rank) and (candidate rank + 1) times the
candidate DAG Delay. But since a node is unstable as soon as it
receives the RA-DIO message from the desired candidate, it will
restrain from sending a RA-DIO message between the time it receives
the RA and the time it actually jumps. So the crossing of RA may
only happen during the propagation time between the candidate and
the node, plus some internal queuing and processing time within each
machine. It is expected that one DAG delay normally covers that
interval, but ultimately it is up to the implementation and the
configuration of the candidate parent to define the duration of risk
window.</t>
<t>There is risk of a collision when a node receives an RA, for
another candidate that is more preferable than the current
candidate, within the risk window. In the face of a potential
collision, the node with lowest extended preference processes the
RA-DIO message normally, while the router with the highest extended
preference places the other in collision state, does not start the
DAG hop timer, and does not become instable. It is expected that
next RAs between the two will not cross anyway.</t>
<t>For example, consider a case where two nodes are each rooting
their own transient floating DAGs and multicast RA-DIO messages
towards each other in a close enough interval that the RA-DIO
messages `cross'. Then each node may receive the RA-DIO message from
the other node, and in some scenario decide to join each others DAG.
RPL avoids this deadlock scenario via the collision mechanism
described above - after each node sends the RA-DIO message they will
enter the risk window. When the peer RA-DIO message is received in
the risk window, the nodes will calculate the extended preferences
as describe above and the node with the lowest extended preference
will proceed to process the RA-DIO message, while the other node
will defer, avoiding the deadlock scenario.</t>
</section>
<section title="Instability">
<t>A node is instable when it is prepared to shortly replace a set
of DAG parents in order to jump to a different DAGID. This happens
typically when the node has selected a more preferred candidate
parent in a different DAG and has to wait for the DAG hop timer to
elapse before adjusting the DAG parent set. Instability may also
occur when the entire current DAG parent set is lost and the next
best candidates are still held up. Instability is resolved when the
DAG hop timer of all the candidate(s) causing instability elapse.
Such candidates then change state to Current or Held- Down.</t>
<t>Instability is transient (in the order of DAG hop timers). When a
node is unstable, it MUST NOT send RAs with the DIO message. This
avoids loops when node A decides to attach to node B and node B
decides to attach to node A. Unless RAs cross (see Collision
section), a node receives RA-DIO messages from stable candidate
parents, which do not plan to attach to the node, so the node can
safely attach to them.</t>
</section>
</section>
<section title="Guidelines for Objective Code Points">
<section title="Objective Function">
<t>An Objective Function (OF) allows for the selection of a DAG to
join, and a number of peers in that DAG as parents. The OF is used
to compute an ordered list of parents and provides load balancing
guidance. The OF is also responsible to compute the rank of the
device within the DAG.</t>
<t>The Objective Function is specified in the RA-DIO message using
an objective code point (OCP) and indicates the objective function
that has been used to compute the DAG (e.g. "minimize the path cost
using the ETX metric and avoid `Blue' links"). The objective code
points are specified in <xref
target="I-D.ietf-roll-routing-metrics" />. This document specifies
the OCP 0, in support of default operation.</t>
<t>Most Objective Functions are expected to follow the same abstract
behavior:</t>
<list style="symbols">
<t>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 RA-DIO message, a timer elapse, or
a trigger indicating that the state of a candidate neighbor has
changed.</t>
<t>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.</t>
<t>The OF scans all the candidate neighbors on the possible
interfaces to check whether they can act as an attachment router
for a DAG. There might be multiple of them and a candidate
neighbor might need to pass some validation tests before it can be
used. In particular, some link layers require experience on the
activity with a router to enable the router as a next_hop.</t>
<t>The OF computes self's rank by adding the step of rank to that
candidate to the rank of that candidate. The step of rank is
estimated as follows:</t>
<list style="symbols">
<t>The step of rank might vary from 1 to 16.</t>
<list style="symbols">
<t>1 indicates a unusually good link, for instance a link
between powered devices in a mostly battery operated
environment.</t>
<t>4 indicates a `normal'/typical link, as qualified by the
implementation.</t>
<t>16 indicates a link that can hardly be used to forward any
packet, for instance a radio link with quality indicator or
expected transmission count that is close to the acceptable
threshold.</t>
</list>
<t>Candidate neighbors that would cause self's rank to increase
are ignored</t>
</list>
<t>Candidate neighbors that advertise an OF incompatible with the
set of OF specified by the policy functions are ignored.</t>
<t>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.</t>
<list style="symbols">
<t>If the routers are roughly equal for that relation then the
next test is attempted between the routers,</t>
<t>Else the best of the 2 becomes the current best parent and
the scan continues with the next candidate neighbor</t>
<t>Some OFs may include a test to compare the ranks that would
result if the node joined either router</t>
</list>
<t>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.</t>
<t>Other rounds of scans might be necessary to elect alternate
parents and siblings. In the next rounds:</t>
<list style="symbols">
<t>Candidate neighbors that are not in the same DAG are
ignored</t>
<t>Candidate neighbors that are of worse rank than self are
ignored</t>
<t>Candidate neighbors of a better rank than self (non-siblings)
are preferred</t>
</list>
</list>
</section>
<section title="Objective Code Point 0 (OCP 0)">
<t>Here follows the specification for the default Objective Function
corresponding to OCP codepoint 0. This is a very simple reference to
help design more complex Objective Functions. In particular, the
Objective Function described here does not use physical metrics as
described in <xref target="I-D.ietf-roll-routing-metrics"></xref>,
but are only based on abstract information from the RA-DIO message
such as rank and administrative preference.</t>
<t>This document specifies a default objective metric, called OF0,
and using the OCP 0. OF0 is the default objective function of RPL,
and can be used if allowed by the policy of the processing node when
no objective function is included in the RA-DIO message, or if the
OF indicated in the RA-DIO message is unknown to the node. If not
allowed, then the RA-DIO message is simply ignored and not processed
by the node.</t>
<section title="OCP 0 Objective Function (OF0)">
<t>OF0 favors the connectivity. That is, the Objective Function is
designed to find the nearest sink into a 'grounded' topology, and
if there is none then join any network per order of administrative
preference. The metric in use is the rank.</t>
<t>OF0 selects a preferred parent and a backup next_hop if one is
available. The backup next_hop might be a parent or a sibling. All
the traffic is routed via the preferred parent. When the link
conditions do not let a packet through to the preferred parent,
the packet is passed to the backup next_hop.</t>
<t>The step of rank is 4 for each hop.</t>
</section>
<section title="Selection of the Preferred Parent">
<t>As it scans all the candidate neighbors, OF0 keeps the parent
that is the best for the following criteria (in order):</t>
<t><list style="numbers">
<t>The interface must be usable and the administrative
preference (if any) applies first.</t>
<t>A candidate that would cause the node to augment the rank
in the current DAG is not considered.</t>
<t>A router that has been validated as usable, e.g. with a
local confidence that has exceeded some pre-configured
threshold, is better.</t>
<t>If none are grounded then a DAG with a more preferred
administrative preference is better.</t>
<t>A router that offers connectivity to a grounded DAG is
better.</t>
<t>A lesser resulting rank is better.</t>
<t>A DAG for which there is an alternate parent is better.
This check is optional. It is performed by computing the
backup next_hop while assuming that this router won.</t>
<t>The DAG that was in use already is preferred.</t>
<t>The router with a better router preference wins.</t>
<t>The preferred parent that was in use already is better.</t>
<t>A router that has announced a RA-DIO message more recently
is preferred.</t>
</list></t>
</section>
<section title="Selection of the Backup next_hop">
<list style="symbols">
<t>The interface must be usable and the administrative
preference (if any) applies first.</t>
<t>The preferred parent is ignored.</t>
<t>Candidate neighbors that are not in the same DAG are
ignored.</t>
<t>Candidate neighbors with a higher rank are ignored.</t>
<t>Candidate neighbors of a better rank than self (non-siblings)
are preferred.</t>
<t>A router that has been validated as usable, e.g. with a local
confidence that has exceeded some pre-configured threshold, is
better.</t>
<t>The router with a better router preference wins.</t>
<t>The backup next_hop that was in use already is better.</t>
</list>
</section>
</section>
</section>
<section anchor="DestinationAdvertisement"
title="Establishing Routing State Outward Along the DAG">
<t>The destination advertisement mechanism supports the dissemination
of routing state required to support traffic flows outward along the
DAG, from the DAG root toward nodes.</t>
<t>As a result of destination advertisement operation:</t>
<t><list style="symbols">
<t>DAG discovery establishes a DAG oriented toward a DAG root
using extended Neighbor Discovery RS/RA flows, along which inward
routes toward the DAG root are set up.</t>
<t>Destination advertisement extends Neighbor Discovery in order
to establish outward routes along the DAG. Such paths consist
of:</t>
<?rfc subcompact="yes"?>
<list style="symbols">
<t>Hop-By-Hop routing state within islands of `stateful'
nodes.</t>
<t>Source Routing `bridges' across nodes who do not retain
state.</t>
</list>
<?rfc subcompact="no"?>
</list></t>
<t>Destinations disseminated with the destination advertisement
mechanism may be prefixes, individual hosts, or multicast listeners.
The mechanism supports nodes of varying capabilities as follows:</t>
<t><list style="symbols">
<t>When nodes are capable of storing routing state, they may
inspect destination advertisements and learn hop-by-hop routing
state toward destinations by populating their routing tables with
the routes learned from nodes in their sub-DAG. In this process
they may also learn necessary piecewise source routes to traverse
regions of the LLN that do not maintain routing state. They may
perform route aggregation on known destinations before emitting
Destination Advertisements.</t>
<t>When nodes are incapable of storing routing state, they may
forward destination advertisements, recording the reverse route as
the go in order to support the construction of piecewise source
routes.</t>
</list></t>
<t>Nodes that are capable of storing routing state, and finally the
DAG roots, are able to learn which destinations are contained in the
sub-DAG below the node, and via which next-hop neighbors. The
dissemination and installation of this routing state into nodes allows
for Hop-By-Hop routing from the DAG root outwards along the DAG. The
mechanism is further enhance by supporting the construction of source
routes across stateless `gaps' in the DAG, where nodes are incapable
of storing additional routing state. An adaptation of this mechanism
allows for the implementation of loose-source routing.</t>
<t>A special case, the reception of a destination advertisement
addressed to a link-local multicast address, allows for a node to
learn destinations directly available from its one-hop neighbors.</t>
<t>A design choice behind advertising routes via destination
advertisements is not to synchronize the parent and children databases
along the DAG, but instead to update them regularly to recover from
the loss of packets. The rationale for that choice is time variations
in connectivity across unreliable links. If the topology can be
expected to change frequently, synchronization might be an excessive
goal in terms of exchanges and protocol complexity. The approach used
here results in a simple protocol with no real peering. The
destination advertisement mechanism hence provides for periodic
updates of the routing state, as cued by occasional RAs and other
mechanisms, similarly to other protocols such as RIP <xref
target="RFC2453"></xref>.</t>
<section title="Destination Advertisement Message Formats">
<section anchor="DAOptionMessage" title="DAO Option">
<t>RPL extends Neighbor Discovery <xref target="RFC4861"></xref>
and RFC4191 <xref target="RFC4191"></xref> to allow a node to
include a destination advertisement option, which includes prefix
information, in the Neighbor Advertisement (NA) messages. A prefix
option is normally present in RA messages only, but the NA is
augmented with this option in order to propagate destination
information inwards along the DAG. The option is named the
Destination Advertisement Option (DAO), and an NA message
containing this option may be referred to as a destination
advertisement, or NA-DAO. The RPL use of destination
advertisements allows the nodes in the DAG to build up routing
state for nodes contained in the sub-DAG in support of traffic
flowing outward along the DAG.</t>
<t><figure anchor="DAOption"
title="The Destination Advertisement Option (DAO)">
<artwork><![CDATA[
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 | Length | Prefix Length | RRCount |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Depth | Reserved | DAO Sequence |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse Route Stack (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t><list hangIndent="6" style="hanging">
<t hangText="Type:">8-bit unsigned identifying the Destination
Advertisement option. IANA had defined the IPv6 Neighbor
Discovery Option Formats registry. The suggested type value
for the Destination Advertisement Option carried within a NA
message is 141, to be confirmed by IANA.</t>
<t hangText="Length:">8-bit unsigned integer. The length of
the option (including the Type and Length fields) in units of
8 octets.</t>
<t hangText="Prefix Length:">Number of valid leading bits in
the IPv6 Prefix.</t>
<t hangText="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.</t>
<t hangText="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.</t>
<t hangText="Route Tag:">32-bit unsigned integer. The Route
Tag may be used to give a priority to prefixes that should be
stored. This may be useful in cases where intermediate nodes
are capable of storing a limited amount of routing state. The
further specification of this field and its use is under
investigation.</t>
<t hangText="DAO Depth:">Set to 0 by the node that owns the
prefix and first issues the NA-DAO message. Incremented by all
LLN nodes that propagate the NA-DAO message.</t>
<t hangText="Reserved:">8-bit unused field. The reserved field
MUST be set to zero on transmission and MUST be ignored on
receipt.</t>
<t hangText="DAO Sequence:">Incremented by the node that owns
the prefix for each new NA-DAO message for that prefix.</t>
<t hangText="Prefix:">Variable-length field containing an IPv6
address or a prefix of an IPv6 address. The Prefix Length
field contains the number of valid leading bits in the prefix.
The bits in the prefix after the prefix length (if any) are
reserved and MUST be set to zero on transmission and MUST be
ignored on receipt.</t>
<t hangText="Reverse Route Stack:">Variable-length field
containing a sequence of RRCount (possibly compressed) IPv6
addresses. A node who adds on to the Reverse Route Stack will
append to the list and increment the RRCount.</t>
</list></t>
</section>
</section>
<section title="Destination Advertisement Operation">
<section title="Overview">
<t>According to implementation specific policy, a subset or all of
the feasible parents in the DAG may be selected to receive prefix
information from the destination advertisement mechanism. This
subset of DAG parents shall be designated the set of DA
parents.</t>
<t>As NA-DAO messages for particular destinations move inwards
along the DAG, a sequence counter is used to guarantee their
freshness. The sequence counter is incremented by the source of
the NA-DAO message (the node that owns the prefix, or learned the
prefix via some other means), each time it issues a NA-DAO message
for its prefix. Nodes who receive the NA-DAO message and, if scope
allows, will be forwarding a NA-DAO message for the unmodified
destination inwards along the DAG, will leave the sequence number
unchanged. Intermediate nodes will check the sequence counter
before processing a NA-DAO message, and if the DAO is unchanged
(the sequence counter has not changed), then the NA-DAO message
will be discarded without additional processing. Further, if the
NA-DAO message appears to be out of synch (the sequence counter is
2 or more behind the present value) then the DAO state is
considered to be stale and may be purged, and the NA-DAO message
is discarded. A depth is also added for tracking purposes; the
depth is incremented at each hop as the NA-DAO message is
propagated up the DAG. Nodes who are storing routing state may use
the depth to determine which possible next-hops for the
destination are more optimal.</t>
<t>If destination advertisements are activated in the RA-DIO
message as indicated by the `D' bit, the node sends unicast
destination advertisements to its DA parents, and only accepts
unicast destination advertisements from any nodes but those
contained in the DA parent subset.</t>
<t>Every NA to a DA parent MAY contain one or more DAOs. Receiving
a RA-DIO message with the `D' destination advertisement bit set
from a DAG parent stimulates the sending of a delayed destination
advertisement back, with the collection of all known prefixes
(that is the prefixes learned via destination advertisements for
nodes lower in the DAG, and any connected prefixes). If the
Destination Advertisement Supported (A) bit is set in the RA-DIO
message for the DAG, then a destination advertisement is also sent
to a DAG parent once it has been added to the DA parent set after
a movement, or when the list of advertised prefixes has changed.
Destination advertisements may also be scheduled for sending when
the PathDigest of the RA-DIO message has changed, indicating that
some aspect of the inwards paths along the DAG has been
modified.</t>
<t>Destination advertisements may advertise positive (prefix is
present) or negative (removed) NA-DAO messages, termed as no-DAOs.
A no-DAO is stimulated by the disappearance of a prefix below.
This is discovered by timing out after a request (a RA-DIO
message) or by receiving a no-DAO. A no-DAO is a conveyed as a
NA-DAO message with a DAO Lifetime of 0.</t>
<t>A node who is capable of recording the state information
conveyed in a unicast NA-DAO message will do so upon receiving and
processing the NA-DAO message, thus building up routing state
concerning destinations below it in the DAG. If a node capable of
recording state information receives a NA-DAO message containing a
Reverse Route Stack, then the node knows that the NA-DAO message
has traversed one or more nodes that did not retain any routing
state as it traversed the path from the DAO source to the node.
The node may then extract the Reverse Route Stack and retain the
included state in order to specify Source Routing instructions
along the return path towards the destination. The node MUST set
the RRCount back to zero and clear the Reverse Route Stack prior
to passing the NA-DAO message information on.</t>
<t>A node who is unable to record the state information conveyed
in the NA-DAO message will append the next-hop address to the
Reverse Route Stack, increment the RRCount, and then pass the
destination advertisement on without recording any additional
state. In this way the Reverse Route Stack will contain a vector
of next hops that must be traversed along the reverse path that
the NA-DAO message has traveled. The vector will be ordered such
that the node closest to the destination will appear first in the
list. In such cases, if it is useful to the implementation to try
and build up redundant paths, the node may choose to convey the
destination advertisement to one or more DAG parents in order of
preference as guided by an implementation specific policy.</t>
<t>In some cases (called hybrid cases), some nodes along the path
a destination advertisement follows inward along the DAG may store
state and some may not. The destination advertisement mechanism
allows for the provisioning of routing state such that when a
packet is traversing outwards along the DAG, some nodes may be
able to directly forward to the next hop, and other nodes may be
able to specify a piecewise source route in order to bridge spans
of stateless nodes within the path on the way to the desired
destination.</t>
<t>In the case where no node is able to store any routing state as
destination advertisements pass by, and the DAG root ends up with
NA-DAO messages that contain a completely specified route back to
the originating node in the form of the inverted Reverse Route
Stack. A DAG root should not request (Destination Advertisement
Trigger) nor indicate support (Destination Advertisement
Supported) for destination advertisements if it is not able to
store the Reverse Route Stack information in this case.</t>
<t>The destination advertisement mechanism requires stateful nodes
to maintain lists of known prefixes. A prefix entry contains the
following abstract information:</t>
<t><list style="symbols">
<t>A reference to the ND entry that was created for the
advertising neighbor.</t>
<t>The IPv6 address and interface for the advertising
neighbor.</t>
<t>The logical equivalent of the full destination
advertisement information (including the prefixes, depth, and
Reverse Route Stack, if any).</t>
<t>A 'reported' Boolean to keep track whether this prefix was
reported already, and to which of the DA parents.</t>
<t>A counter of retries to count how many RA-DIO messages were
sent on the interface to the advertising neighbor without
reachability confirmation for the prefix.</t>
</list></t>
<t>Note that nodes may receive multiple information from different
neighbors for a specific destination, as different paths through
the DAG may be propagating information inwards along the DAG for
the same destination. A node who is recording routing state will
keep track of the information from each neighbor independently,
and when it comes time to propagate the NA-DAO message for a
particular prefix to the DA parents, then the DAO information will
be selected from among the advertising neighbors who offer the
least depth to the destination.</t>
<t>The destination advertisement mechanism stores the prefix
entries in one of 3 abstract lists; the Connected, the Reachable
and the Unreachable lists.</t>
<t>The Connected list corresponds to the prefixes owned and
managed by the local node.</t>
<t>The Reachable list contains prefixes for which the node keeps
receiving NA-DAO messages, and for those prefixes which have not
yet timed out.</t>
<t>The Unreachable list keeps track of prefixes which are no
longer valid and in the process of being deleted, in order to send
NA-DAO messages with zero lifetime (also called no-DAO) to the DA
parents.</t>
<section anchor="DATimers"
title="Destination Advertisement Timers">
<t>The destination advertisement mechanism requires 2 timers;
the DelayNA timer and the RemoveTimer.</t>
<t><list style="symbols">
<t>The DelayNA timer is armed upon a stimulation to send a
destination advertisement (such as a RA-DIO message from a
DA parent). When the timer is armed, all entries in the
Reachable list as well as all entries for Connected list are
set to not be reported yet for that particular DA
parent.</t>
<t>The DelayNA timer has a duration that is DEF_NA_LATENCY
divided by a multiple of the DAG rank of the node. The
intention is that nodes located deeper in the DAG should
have a shorter DelayNA timer, allowing NA-DAO messages a
chance to be reported from deeper in the DAG and potentially
aggregated along sub-DAGs before propagating further
inwards.</t>
<t>The RemoveTimer is used to clean up entries for which
NA-DAO messages are no longer being received from the
sub-DAG.</t>
<list style="symbols">
<t>When a RA-DIO message is sent that is requesting
destination advertisements, a flag is set for all DAO
entries in the routing table.</t>
<t>If the flag has already been set for a DAO entry, the
retry count is incremented.</t>
<t>If a NA-DAO message is received to confirm the entry,
the entry is refreshed and the flag and count may be
cleared.</t>
<t>If at least one entry has reached a threshold value and
the RemoveTimer is not running, the entry is considered to
be probably gone and the RemoveTimer is started.</t>
<t>When the RemoveTimer elapse, NA-DAO messages with
lifetime 0, i.e. no-DAOs, are sent to explicitly inform DA
parents that the entries who have reached the threshold
are no longer available, and the related routing states
may be propagated and cleaned up.</t>
</list>
<t>The RemoveTimer has a duration of min
(MAX_DESTROY_INTERVAL, RA_INTERVAL).</t>
</list></t>
</section>
</section>
<section title="Multicast Destination Advertisement messages">
<t>It is also possible for a node to multicast a NA-DAO message to
the link-local scope all-nodes multicast address FF02::1. This
message will be received by all node listening in range of the
emitting node. The objective is to enable direct P2P
communication, between destinations directly supported by
neighboring nodes, without needing the RPL routing structure to
relay the packets.</t>
<t>A multicast NA-DAO message MUST be used only to advertise
information about self, i.e. prefixes in the Connected list or
addresses owned by this node. This would typically be a multicast
group that this node is listening to or a global address owned by
this node, though it can be used to advertise any prefix owned by
this node as well. A multicast NA-DAO message is not used for
routing and does not presume any DAG relationship between the
emitter and the receiver; it MUST NOT be used to relay information
learned (e.g. information in the Reachable list) from another
node; information obtained from a multicast NA-DAO MAY be
installed in the routing table and MAY be propagated by a router
in unicast NA-DAOs.</t>
<t>A node receiving a multicast NA-DAO message addressed to
FF02::1 MAY install prefixes contained in the NA-DAO message in
the routing table for local use. Such a node MUST NOT perform any
other processing on the NA-DAO message (i.e. such a node does not
presume it is a DA parent).</t>
</section>
<section title="Unicast Destination Advertisement messages from child to parent">
<t>When sending a destination advertisement to a DA parent, a node
includes the DAOs for prefix entries not already reported (since
the last DA Trigger from an RA-DIO message) in the Reachable and
Connected lists, as well as no-DAOs for all the entries in the
Unreachable list. Depending on its policy and ability to retain
routing state, the receiving node SHOULD keep a record of the
reported NA-DAO message. If the NA-DAO message offers the best
route to the prefix as determined by policy and other prefix
records, the node SHOULD install a route to the prefix reported in
the NA-DAO message via the link local address of the reporting
neighbor and it SHOULD further propagate the information in a
NA-DAO message.</t>
<t>The RA-DIO message from the DAG root is used to synchronize the
whole DAG, including the periodic reporting of destination
advertisements back up the DAG. Its period is expected to vary,
depending on the configuration of the trickle timer that governs
the RAs.</t>
<t>When a node receives a RA-DIO message over an LLN interface
from a DA parent, the DelayNA is armed to force a full update.</t>
<t>When the node broadcasts a RA-DIO message on an LLN interface,
for all entries on that interface:</t>
<t><list style="symbols">
<t>If the entry is CONFIRMED, it goes PENDING with the retry
count set to 0.</t>
<t>If the entry is PENDING, the retry count is incremented. If
it reaches a maximum threshold, the entry goes ELAPSED If at
least one entry is ELAPSED at the end of the process: if the
Destroy timer is not running then it is armed with a
jitter.</t>
</list></t>
<t>Since the DelayNA timer has a duration that decreases with the
depth, it is expected to receive all NA-DAO messages from all
children before the timer elapses and the full update is sent to
the DA parents.</t>
<t>Once the RemoveTimer is elapsed, the prefix entry is scheduled
to be removed and moved to the Unreachable list if there are any
DA parents that need to be informed of the change in status for
the prefix, otherwise the prefix entry is cleaned up right away.
The prefix entry is removed from the Unreachable list when no more
DA parents need to be informed. This condition may be satisfied
when a no-DAO is sent to all current DA parents indicating the
loss of the prefix, and noting that in some cases parents may have
been removed from the set of DA parents.</t>
</section>
<section title="Other events">
<t>Finally, the destination advertisement mechanism responds to a
series of events, such as:</t>
<t><list style="symbols">
<t>Destination advertisement operation stopped: All entries in
the abstract lists are freed. All the routes learned from
NA-DAO messages are removed.</t>
<t>Interface going down: for all entries in the Reachable list
on that interface, the associated route is removed, and the
entry is scheduled to be removed.</t>
<t>Loss of routing adjacency: When the routing adjacency for a
neighbor is lost, as per the procedures described in <xref
target="MaintenanceRoutingAdjacency"></xref>, and if the
associated entries are in the Reachable list, the associated
routes are removed, and the entries are scheduled to be
destroyed.</t>
<t>Changes to DA parent set: all entries in the Reachable list
are set to not 'reported' and DelayNA is armed.</t>
</list></t>
</section>
<section title="Aggregation of prefixes by a node">
<t>There may be number of cases where a aggregation may be shared
within a group of nodes. In such a case, it is possible to use
aggregation techniques with destination advertisements and improve
scalability.</t>
<t>Other cases might occur for which additional support is
required:</t>
<t><list style="numbers">
<t>The aggregating node is attached within the sub-DAG of the
nodes it is aggregating for.</t>
<t>A node that is to be aggregated for is located somewhere
else within the DAG, not in the sub-DAG of the aggregating
node.</t>
<t>A node that is to be aggregated for is located somewhere
else in the LLN.</t>
</list></t>
<t>Consider a node M who is performing an aggregation, and a node
N who is to be a member of the aggregation group. A node Z
situated above the node M in the DAG, but not above node N, will
see the advertisements for the aggregation owned by M but not that
of the individual prefix for N. Such a node Z will route all the
packets for node N towards node M, but node M will have no route
to the node N and will fail to forward.</t>
<t>Additional protocols may be applied beyond the scope of this
specification to dynamically elect/provision an aggregating node
and groups of nodes eligible to be aggregated in order to provide
route summarization for a sub-DAG.</t>
</section>
<section title="Default Values">
<t>DEF_NA_LATENCY = To Be Determined</t>
<t>MAX_DESTROY_INTERVAL = To Be Determined</t>
</section>
</section>
</section>
<section title="Multicast Operation">
<t>This section describes further the multicast routing operations
over an IPv6 RPL network, and specifically how unicast NA-DAOs can be
used to relay group registrations inwards. Wherever the following text
mentions MLD, one can read MLDv2 or v3.</t>
<t>As is traditional, a listener uses a protocol such as MLD with a
router to register to a multicast group.</t>
<t>Along the path between the router and the root of the DAG, MLD
requests are mapped and transported as NA-DAO messages within the RPL
protocol; each hop coalesces the multiple requests for a same group as
a single NA-DAO message to the parent(s), in a fashion similar to
proxy IGMP, but recursively between child router and parent up to the
root.</t>
<t>A router might select to pass a listener registration NA-DAO
message to its preferred parent only, in which case multicast packets
coming back might be lost for all of its sub-DAG if the transmission
fails over that link. Alternatively the router might select to copy
additional parents as it would do for NA-DAO messages advertising
unicast destinations, in which case there might be duplicates that the
router will need to prune.</t>
<t>As a result, multicast routing states are installed in each router
on the way from the listeners to the root, enabling the root to copy a
multicast packet to all its children routers that had issued a NA-DAO
message including a DAO for that multicast group, as well as all the
attached nodes that registered over MLD.</t>
<t>For unicast traffic, it is expected that the grounded root of an
RPL DAG terminates RPL and MAY redistribute the RPL routes over the
external infrastructure using whatever routing protocol is used there.
For multicast traffic, the root MAY proxy MLD for all the nodes
attached to the RPL routers (this would be needed if the multicast
source is located in the external infrastructure). For such a source,
the packet will be replicated as it flows outwards along the DAG based
on the multicast routing table entries installed from the NA-DAO
message.</t>
<t>For a source inside the DAG, the packet is passed to the preferred
parents, and if that fails then to the alternates in the DAG. The
packet is also copied to all the registered children, except for the
one that passed the packet. Finally, if there is a listener in the
external infrastructure then the DAG root has to further propagate the
packet into the external infrastructure.</t>
<t>As a result, the DAG Root acts as an automatic proxy Rendez-vous
Point for the RPL network, and as source towards the Internet for all
multicast flows started in the RPL LLN. So regardless of whether the
root is actually attached to the Internet, and regardless of whether
the DAG is grounded or floating, the root can serve inner multicast
streams at all times.</t>
</section>
<section anchor="MaintenanceRoutingAdjacency"
title="Maintenance of Routing Adjacency">
<t>The selection of successors, along the default paths inward along
the DAG, or along the paths learned from destination advertisements
outward along the DAG, leads to the formation of routing adjacencies
that require maintenance.</t>
<t>In IGPs such as OSPF <xref target="RFC4915"></xref> or IS-IS <xref
target="RFC5120"></xref>, the maintenance of a routing adjacency
involves the use of Keepalive mechanisms (Hellos) or other protocols
such as BFD (<xref target="I-D.ietf-bfd-base"></xref>) and MANET
Neighborhood Discovery Protocol (NHDP <xref
target="I-D.ietf-manet-nhdp"></xref>). 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.</t>
<t>Thus RPL makes use of a different approach consisting of probing
the neighbor using a Neighbor Solicitation message (see <xref
target="RFC4861"></xref>). 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.</t>
</section>
<section anchor="PacketForwarding" title="Packet Forwarding">
<t>When forwarding a packet to a destination, precedence is given to
selection of a next-hop successor as follows:</t>
<t><list style="numbers">
<t>It is preferred to select a successor from a DAG who is
supporting an OCP and related optimization that maps to an
objective marked in the IPv6 header of the packet being
forwarded.</t>
<!--
<t>If the packet header contains any source routing directives
(TBD) then the highest precedence should be given to follow
the source routing directives.</t>
-->
<t>If a local administrative preference favors a route that has
been learned from a different routing protocol than RPL, then use
that successor.</t>
<t>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.</t>
<t>If there is an entry in the routing table matching the
destination that has been learned from a unicast destination
advertisement (e.g. the destination is located outwards along the
sub-DAG), then use that successor.</t>
<t>If there is a DAG offering a route to a prefix matching the
destination, then select one of those DAG parents as a
successor.</t>
<t>If there is a DAG offering a default route with a compatible
OCP, then select one of those DAG parents as a successor.</t>
<t>If there is a DAG offering a route to a prefix matching the
destination, but all DAG parents have been tried and are
temporarily unavailable (as determined by the forwarding
procedure), then select a DAG sibling as a successor.</t>
<t>Finally, if no DAG siblings are available, the packet is
dropped. ICMP Destination Unreachable may be invoked. An
inconsistency is detected.</t>
</list></t>
<t>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.</t>
<t>Note that the chosen successor MUST NOT be the neighbor who was the
predecessor of the packet (split horizon), except in the case where it
is intended for the packet to change from an inward to an outward
flow, such as switching from DIO routes to DAO routes as the
destination is neared.</t>
</section>
</section>
<section title="RPL Variables">
<t><list hangIndent="6" style="hanging">
<t hangText="DIO Timer">One instance per DAG that a node is a member
of. Expiry triggers RA-DIO message transmission. Trickle timer with
variable interval in [0, DIOIntervalMin..2^DIOIntervalDoublings].
See <xref target="TrickleImplementation"></xref></t>
<t hangText="DAG Hop Timer">Up to one instance per candidate DAG
parent in the `Held-Up' state per DAG that a node is going to jump
to. Expiry triggers candidate DAG parent to become a DAG parent in
the `Current' state, as well as cancellation of any other DAG Hop
timers associated with other DAG parents for that DAG. Duration is
computed based on the rank of the candidate DAG parent and DAG
delay, as (candidates rank + random) * candidate's DAG_delay (where
0 <= random < 1). See <xref target="DAGHeldUp"></xref>.</t>
<t hangText="Hold-Down Timer">Up to one instance per candidate DAG
parent in the `Held-Down' state per DAG. Expiry triggers the
eviction of the candidate DAG parent from the candidate DAG parent
set. The interval should be chosen as appropriate to prevent
flapping. See <xref target="CandidateParentStates"></xref>.</t>
<t hangText="DAG Heartbeat Timer">Up to one instance per DAG that
the node is acting as DAG root of. May not be supported in all
implementations. Expiry triggers revision of DAGSequenceNumber,
causing a new series of updated RA-DIO message to be sent. Interval
should be chosen appropriate to propagation time of DAG and as
appropriate to application requirements (e.g. response time vs.
overhead). See <xref target="DAGHeartbeat"></xref></t>
<t hangText="DelayNA Timer">Up to one instance per DA parent (the
subset of DAG parents chosen to receive destination advertisements)
per DAG. Expiry triggers sending of NA-DAO message to the DA parent.
The interval is to be proportional to DEF_NA_LATENCY/(node rank),
such that nodes of greater rank (further outward along the DAG)
expire first, coordinating the sending of NA-DAO messages to allow
for a chance of aggregation. See <xref target="DATimers"></xref></t>
<t hangText="DestroyTimer">Up to one instance per DA entry per
neighbor (i.e. those neighbors who have given NA-DAO messages to
this node as a DAG parent) Expiry triggers a change in state for the
DA entry, setting up to do unreachable (No-DAO) advertisements or
immediately deallocating the DA entry if there are no DA parents.
The interval is min(MAX_DESTROY_INTERVAL, RA_INTERVAL). See <xref
target="DATimers"></xref></t>
</list></t>
</section>
<section anchor="Manageability" title="Manageability Considerations">
<t>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.</t>
<section title="Control of Function and Policy">
<section title="Initialization Mode">
<t>When a node is first powered up, it may either choose to stay
silent and not send any multicast RA-DIO message until it has joined
a DAG, or to immediately root a transient DAG and start sending
multicast RA-DIO messages. A RPL implementation SHOULD allow
configuring whether the node should stay silent or should start
advertising RA-DIO messages.</t>
<t>Furthermore, the implementation SHOULD to allow configuring
whether or not the node should start sending an RS message as an
initial probe for nearby DAGs, or should simply wait until it
received RA messages from other nodes that are part of existing
DAGs.</t>
</section>
<section title="DIO Base option">
<t>RPL specifies a number of protocol parameters.</t>
<t>A RPL implementation SHOULD allow configuring the following
routing protocol parameters, which are further described in <xref
target="DIOBaseOption"></xref>:</t>
<t><list hangIndent="6" style="hanging">
<t hangText="DAGPreference"></t>
<t hangText="NodePreference"></t>
<t hangText="DAGDelay"></t>
<t hangText="DIOIntervalDoublings"></t>
<t hangText="DIOIntervalMin:"></t>
<t hangText="DAGObjectiveCodePoint"></t>
<t hangText="PathDigest"></t>
<t hangText="DAGID"></t>
<t hangText="Destination Prefixes"></t>
<t hangText="DAG Root behavior:">In some cases, a node may not
want to permanently act as a DAG root if it cannot join a
grounded DAG. For example a battery-operated node may not want
to act as a DAG root for a long period of time. Thus a RPL
implementation MAY support the ability to configure whether or
not a node could act as a DAG root for a configured period of
time.</t>
<t hangText="DAG Hop Timer:">A RPL implementation MUST provide
the ability to configure the value of the DAG Hop Timer,
expressed in ms.</t>
<t hangText="DAG Table Entry Suppression">A RPL implementation
SHOULD provide the ability to configure a timer after the
expiration of which the DAG table that contains all the records
about a DAG is suppressed, to be invoked if the DAG parent set
becomes empty.</t>
</list></t>
</section>
<section title="Trickle Timers">
<t>A RPL implementation makes use of trickle timer to govern the
sending of RA-DIO message. Such an algorithm is determined a by a
set of configurable parameters that are then advertised by the DAG
root along the DAG in RA-DIO messages.</t>
<t>For each DAG, a RPL implementation MUST allow for the monitoring
of the following parameters, further described in <xref
target="TrickleImplementation"></xref>:</t>
<t><list hangIndent="6" style="hanging">
<t hangText="I"></t>
<t hangText="T"></t>
<t hangText="C"></t>
<t hangText="I_min"></t>
<t hangText="I_doublings:"></t>
</list></t>
<t>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.</t>
</section>
<section title="DAG Heartbeat">
<t>A RPL implementation may allow by configuration at the DAG root
to refresh the DAG states by updating the DAGSequenceNumber. A RPL
implementation SHOULD allow configuring whether or not periodic or
event triggered mechanism are used by the DAG root to control
DAGSequenceNumber change.</t>
</section>
<section title="The Destination Advertisement Option">
<t>The following set of parameters of the NA-DAO messages SHOULD be
configurable:</t>
<t><list style="symbols">
<t>The DelayNA timer</t>
<t>The Remove timer</t>
</list></t>
</section>
<section title="Policy Control">
<t>DAG discovery enables nodes to implement different policies for
selecting their DAG parents.</t>
<t>A RPL implementation SHOULD allow configuring the set of
acceptable or preferred Objective Functions (OF) referenced by their
Objective Codepoints (OCPs) for a node to join a DAG, and what
action should be taken if none of a node's candidate neighbors
advertise one of the configured allowable Objective Functions.</t>
<t>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.</t>
<t>A RPL implementation SHOULD allow for the configuration of the
"Route Tag" field of the NA-DAO messages according to a set of rules
defined by policy.</t>
</section>
<section title="Data Structures">
<t>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.</t>
<t>A RPL implementation MAY provide an indicator on the size of the
candidate neighbor list.</t>
</section>
</section>
<section title="Information and Data Models">
<t>The information and data models necessary for the operation of RPL
will be defined in a separate document specifying the RPL SNMP
MIB.</t>
</section>
<section title="Liveness Detection and Monitoring">
<t>The aim of this section is to describe the various RPL mechanisms
specified to monitor the protocol.</t>
<t>As specified in <xref target="ConceptualDataStructures"></xref>, an
implementation must maintain a set of data structures in support of
DAG discovery:</t>
<t><list style="symbols">
<t>The candidate neighbors data structure</t>
<t>For each DAG:</t>
<list style="symbols">
<t>A set of candidate DAG parents</t>
<t>A set of DAG parents (which are a subset of candidate DAG
parents and may be implemented as such)</t>
</list>
</list></t>
<section title="Candidate Neighbor Data Structure">
<t>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.</t>
<t>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.</t>
</section>
<section title="Directed Acyclic Graph (DAG) Table">
<t>For each DAG, a RPL implementation MUST keep track of the
following DAG table values:</t>
<t><list style="symbols">
<t>DAGID</t>
<t>DAGObjectiveCodePoint</t>
<t>A set of Destination Prefixes offered inwards along the
DAG</t>
<t>A set of candidate DAG Parents</t>
<t>timer to govern the sending of RA-DIO messages for the
DAG</t>
<t>DAGSequenceNumber</t>
</list></t>
<t>The set of candidate DAG parents structure is itself a table with
the following entries:</t>
<t><list style="symbols">
<t>A reference to the neighboring device which is the DAG
parent</t>
<t>A record of most recent information taken from the DAG
Information Object last processed from the candidate DAG
Parent</t>
<t>a state associated with the role of the candidate as a
potential DAG Parent {Current, Held-Up, Held-Down, Collision},
further described in <xref
target="CandidateParentStates"></xref></t>
<t>A DAG Hop Timer, if instantiated</t>
<t>A Held-Down Timer, if instantiated</t>
<t>A flag reporting if the Parent is a DA Parent as described in
<xref target="DestinationAdvertisement"></xref></t>
</list></t>
</section>
<section title="Routing Table">
<t>To be completed.</t>
</section>
<section title="Other RPL Monitoring Parameters">
<t>A RPL implementation SHOULD provide a counter reporting the
number of a times the node has detected an inconsistency with
respect to a DAG parent, e.g. if the DAGID has changed.</t>
<t>A RPL implementation MAY log the reception of a malformed RA-DIO
message along with the neighbor identification if avialable.</t>
</section>
<section title="RPL Trickle Timers">
<t>A RPL implementation operating on a DAG root MUST allow for the
configuration of the following trickle parameters:</t>
<t><list style="symbols">
<t>The DIOIntervalMin expressed in ms</t>
<t>The DIOIntervalDoublings</t>
</list></t>
<t>A RPL implementation MAY provide a counter reporting the number
of times an inconsistency (and thus the trickle timer has been
reset).</t>
</section>
</section>
<section title="Verifying Correct Operation">
<t>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.</t>
</section>
<section title="Requirements on Other Protocols and Functional Components">
<t>RPL does not have any impact on the operation of existing
protocols.</t>
</section>
<section title="Impact on Network Operation">
<t>To be completed.</t>
</section>
</section>
<section anchor="Security" title="Security Considerations">
<t>Security Considerations for RPL are to be developed in accordance
with recommendations laid out in, for example, <xref
target="I-D.tsao-roll-security-framework"></xref>.</t>
</section>
<section anchor="IANA" title="IANA Considerations">
<section title="DAG Information Option (DIO) Base Option">
<t>The DAG Information Option is a container option carried within an
IPv6 Router Advertisement message as defined in <xref
target="RFC4861"></xref>, which might contain a number of suboptions.
The base option regroups the minimum information set that is mandatory
in all cases.</t>
<t>IANA had defined the IPv6 Neighbor Discovery Option Formats
registry. The suggested type value for the DAG Information Option
(DIO) Base Option is 140, to be confirmed by IANA.</t>
</section>
<section title="New Registry for the Flag Field of the DIO Base Option">
<t>IANA is requested to create a registry for the Flag field of the
DIO Base Option.</t>
<t>New bit numbers may be allocated only by an IETF Consensus action.
Each bit should be tracked with the following qualities:</t>
<t><list style="symbols">
<t>Bit number (counting from bit 0 as the most significant
bit)</t>
<t>Capability description</t>
<t>Defining RFC</t>
</list></t>
<t>Three flags are currently defined:</t>
<texttable title="DIO Base Option Flags">
<ttcol align="center">Bit</ttcol>
<ttcol align="left">Description</ttcol>
<ttcol align="left">Reference</ttcol>
<c>0</c>
<c>Grounded DAG</c>
<c>This document</c>
<c>1</c>
<c>Destination Advertisement Trigger</c>
<c>This document</c>
<c>2</c>
<c>Destination Advertisement Supported</c>
<c>This document</c>
</texttable>
</section>
<section title="DAG Information Option (DIO) Suboption">
<t>IANA is requested to create a registry for the DIO Base Option
Suboptions</t>
<texttable title="DAG Information Option (DIO) Base Option Suboptions">
<ttcol align="center">Value</ttcol>
<ttcol align="left">Meaning</ttcol>
<ttcol align="left">Reference</ttcol>
<c>0</c>
<c>Pad1 - DIO Padding</c>
<c>This document</c>
<c>1</c>
<c>PadN - DIO suboption padding</c>
<c>This document</c>
<c>2</c>
<c>DAG Metric Container</c>
<c>This Document</c>
<c>3</c>
<c>Destination Prefix</c>
<c>This Document</c>
</texttable>
</section>
<section title="Destination Advertisement Option (DAO) Option">
<t>The RPL protocol extends Neighbor Discovery <xref
target="RFC4861"></xref> and <xref target="RFC4191"></xref> to allow a
node to include a Destination Advertisement Option, which includes
prefix information in the Neighbor Advertisements messages. The
Neighbor Advertisement messages are augmented with the Destination
Advertisement Option (DAO).</t>
<t>IANA had defined the IPv6 Neighbor Discovery Option Formats
registry. The suggested type value for the Destination Advertisement
Option carried within a Neighbor Advertisement message is 141, to be
confirmed by IANA.</t>
</section>
<section title="Objective Code Point">
<t>This specification requests that an Objective Code Point registry,
as to be specified in <xref
target="I-D.ietf-roll-routing-metrics"></xref>, reserve the Objective
Code Point value 0x0000, for the purposes designated as OCP 0 in this
document.</t>
<t></t>
</section>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>The ROLL Design Team would like to acknowledge the review, feedback,
and comments from Dominique Barthel, Yusuf Bashir, Mathilde Durvy,
Manhar Goindi, Mukul Goyal, Quentin Lampin, Philip Levis, Jerry
Martocci, Alexandru Petrescu, and Don Sturek.</t>
<t>The ROLL Design Team would like to acknowledge the guidance and input
provided by the ROLL Chairs, David Culler and JP Vasseur.</t>
<t>The ROLL Design Team would like to acknowledge prior contributions of
Robert Assimiti, Mischa Dohler, Julien Abeille, Ryuji Wakikawa, Teco
Boot, Patrick Wetterwald, Bryan Mclaughlin, Carlos J. Bernardos, Thomas
Watteyne, Zach Shelby, Caroline Bontoux, Marco Molteni, Billy Moon, and
Arsalan Tavakoli, which have provided useful design considerations to
RPL.</t>
</section>
<section title="Contributors">
<figure>
<artwork><![CDATA[
JP Vasseur
Cisco Systems, Inc
11, Rue Camille Desmoulins
Issy Les Moulineaux, 92782
France
Email: jpv@cisco.com
Jonathan W. Hui
Arch Rock Corporation
501 2nd St. Ste. 410
San Francisco, CA 94107
USA
Email: jhui@archrock.com
Thomas Heide Clausen
LIX, Ecole Polytechnique, France
Phone: +33 6 6058 9349
EMail: T.Clausen@computer.org
URI: http://www.ThomasClausen.org/
Richard Kelsey
Ember Corporation
Boston, MA
USA
Phone: +1 617 951 1225
Email: kelsey@ember.com
Stephen Dawson-Haggerty
UC Berkeley
Soda Hall, UC Berkeley
Berkeley, CA 94720
USA
Email: stevedh@cs.berkeley.edu
Kris Pister
Dust Networks
30695 Huntwood Ave.
Hayward, 94544
USA
Email: kpister@dustnetworks.com
Anders Brandt
Zensys, Inc.
Emdrupvej 26
Copenhagen, DK-2100
Denmark
Email: abr@zen-sys.com
]]></artwork>
</figure>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
</references>
<references title="Informative References">
<?rfc include='reference.I-D.draft-ietf-roll-building-routing-reqs-07.xml'?>
<?rfc include='reference.I-D.draft-ietf-roll-home-routing-reqs-08.xml'?>
<?rfc include='reference.I-D.draft-ietf-roll-indus-routing-reqs-06.xml'?>
<?rfc include="reference.RFC.5548"?>
<?rfc include='reference.I-D.ietf-roll-terminology.xml'?>
<?rfc include='reference.I-D.ietf-roll-routing-metrics.xml'?>
<?rfc include='reference.I-D.tsao-roll-security-framework.xml'?>
<?rfc include="reference.RFC.2453"?>
<?rfc include="reference.RFC.3819"?>
<?rfc include="reference.RFC.4101"?>
<?rfc include="reference.RFC.4191"?>
<?rfc include="reference.RFC.4461"?>
<?rfc include="reference.RFC.4861"?>
<?rfc include="reference.RFC.4875"?>
<?rfc include="reference.RFC.4915"?>
<?rfc include="reference.RFC.5120"?>
<?rfc include="reference.I-D.draft-ietf-bfd-base-09.xml"?>
<?rfc include="reference.I-D.draft-ietf-manet-nhdp-10.xml"?>
<reference anchor="Levis08"
target="http://portal.acm.org/citation.cfm?id=1364804">
<front>
<title abbrev="Levis08">The Emergence of a Networking Primitive in
Wireless Sensor Networks</title>
<author fullname="Philip Levis" initials="P." surname="Levis">
<organization></organization>
</author>
<author fullname="Eric Brewer" initials="E." surname="Brewer">
<organization></organization>
</author>
<author fullname="David Culler" initials="D." surname="Culler">
<organization></organization>
</author>
<author fullname="David Gay" initials="D." surname="Gay">
<organization></organization>
</author>
<author fullname="Samuel Madden" initials="S." surname="Madden">
<organization></organization>
</author>
<author fullname="Neil Patel" initials="N." surname="Patel">
<organization></organization>
</author>
<author fullname="Joe Polastre" initials="J." surname="Polastre">
<organization></organization>
</author>
<author fullname="Scott Shenker" initials="S." surname="Shenker">
<organization></organization>
</author>
<author fullname="Robert Szewczyk" initials="R." surname="Szewczyk">
<organization></organization>
</author>
<author fullname="Alec Woo" initials="A." surname="Woo">
<organization></organization>
</author>
<date month="July" year="2008" />
</front>
<seriesInfo name="Communications of the ACM," value="v.51 n.7" />
<format target="http://portal.acm.org/citation.cfm?id=1364804"
type="HTML" />
</reference>
</references>
<section anchor="Requirements" title="Deferred Requirements">
<t>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.</t>
</section>
<section anchor="Examples" title="Examples">
<t>Consider the example LLN physical topology in <xref
target="LLNExample"></xref>. In this example the links depicted are all
usable L2 links. Suppose that all links are equally usable, and that the
implementation specific policy function is simply to minimize hops. This
LLN physical topology then yields the DAG depicted in <xref
target="DAGExample"></xref>, where the links depicted are the edges
toward DAG parents. This topology includes one DAG, rooted by an LBR
node (LBR) at rank 1. The LBR node will issue RAs containing DIO, as
governed by a trickle timer. Nodes (11), (12), (13), have selected (LBR)
as their only parent, attached to the DAG at rank 2, and periodically
advertise RA-DIO multicasts. Node (22) has selected (11) and (12) in its
DAG parent set, and advertises itself at rank 3. Node (22) thus has a
set of DAG parents {(11), (12)} and siblings {((21), (23)}.</t>
<figure anchor="LLNExample" title="Example LLN Topology">
<artwork><![CDATA[
(LBR)
/ | \
.---` | `----.
/ | \
(11)------(12)------(13)
| \ | \ | \
| `----. | `----. | `----.
| \| \| \
(21)------(22)------(23) (24)
| /| /| |
| .----` | .----` | |
| / | / | |
(31)------(32)------(33)------(34)
| /| \ | \ | \
| .----` | `----. | `----. | `----.
| / | \| \| \
.--------(41) (42) (43)------(44)------(45)
/ / /| \ | \
.----` .----` .----` | `----. | `----.
/ / / | \| \
(51)------(52)------(53)------(54)------(55)------(56)
]]></artwork>
<postamble>Note that the links depicted represent the usable L2
connectivity available in the LLN. For example, Node (31) can
communicate directly with its neighbors, Nodes (21), (22), (32), and
(41). Node (31) cannot communicate directly with any other nodes, e.g.
(33), (23), (42). In this example these links offer bidirectional
communication, and `bad' links are not depicted.</postamble>
</figure>
<figure anchor="DAGExample" title="Example DAG">
<artwork><![CDATA[
(LBR)
/ | \
.---` | `----.
/ | \
(11) (12) (13)
| \ | \ | \
| `----. | `----. | `----.
| \| \| \
(21) (22) (23) (24)
| /| /| |
| .----` | .----` | |
| / | / | |
(31) (32) (33) (34)
| /| \ | \ | \
| .----` | `----. | `----. | `----.
| / | \| \| \
.--------(41) (42) (43) (44) (45)
/ / /| \ | \
.----` .----` .----` | `----. | `----.
/ / / | \| \
(51) (52) (53) (54) (55) (56)
]]></artwork>
<postamble>Note that the links depicted represent directed links in
the DAG overlaid on top of the physical topology depicted in <xref
target="LLNExample"></xref>. As such, the depicted edges represent the
relationship between nodes and their DAG parents, wherein all depicted
edges are directed and oriented `up' on the page toward the DAG root
(LBR). The DAG may provide default routes within the LLN, and serves
as the foundation on which RPL builds further routing structure, e.g.
through the destination advertisement mechanism.</postamble>
</figure>
<section title="Moving Down a DAG">
<t>Consider node (56) in the example of <xref
target="LLNExample"></xref>. In the unmodified example, node (56) is
at rank 6 with one DAG parent, {(43)}, and one sibling (55). Suppose,
for example, that node (56) wished to expand its DAG parent set to
contain node (55), as {(43), (55)}. Such a change would require node
(56) to detach from the DAG, to defer reattachment until a loop
avoidance algorithm has completed, and to then reattach to the DAG
with {(43), (55)} as it's DAG parents. When node (56) detaches from
the DAG, it is able to act as the root of its own floating DAG and
establish its frozen sub-DAG (which is empty). Node (56) can then
observe that Node (55) is still attached to the original DAG, that its
sequence number is able to increment, and deduce that Node (55) is
safely not behind Node (56). There is then little change for a loop,
and Node (56) may safely reattach to the DAG, with parents {(43),
(55)}. At reattachment time, node (56) would present itself with a
rank deeper than that of its deepest DAG parent (node (55) at rank 6),
rank 7.</t>
</section>
<section title="Link Removed">
<t>Consider the example of <xref target="LLNExample"></xref> when link
(13)-(24) goes down.</t>
<t><list style="symbols">
<t>Node (24) will detach and become the root of its own floating
DAG</t>
<t>Node (34) will learn that its DAG parent is now part of its own
floating DAG, will consider that it can remain a part of the DAG
rooted at node (LBR) via node (33), and will initiate procedures
to detach from DAG (LBR) in order to re-attach at a lower
rank.</t>
<t>Node (45) will similarly make preparations to remain attached
to the DAG rooted at (LBR) by detaching from Node (34) and
re-attaching at a lower rank to node (44).</t>
<t>Node (34) will complete re-attachment to Node (33) first, since
it is able to attach closer to the root of the DAG.</t>
<t>Node (45) will cancel plans to detach/reattach, keep node (34)
as a DAG parent, and update its dependent rank accordingly.</t>
<t>Node (45) may now anyway add node (44) to its set of DAG
parents, as such an addition does not require any modification to
its own rank.</t>
<t>Node (24) will observe that it may reattach to the DAG rooted
at node (LBR) by selecting node (34) as its DAG parent, thus
reversing the relationship that existed in the initial state.</t>
</list></t>
</section>
<section title="Link Added">
<t>Consider the example of <xref target="LLNExample"></xref> when link
(12)-(42) appears.</t>
<t><list style="symbols">
<t>Node (42) will see a chance to get closer to the LBR by adding
(12) to its set of DAG parents, {(32), (12)}</t>
<t>Node (42) may be content to leave its advertised rank at 5,
reflecting a rank deeper than its deepest parent (32).</t>
<t>Node (42) may now choose to remain where it is, with two
parents {(12), (32)}. Should there be a reason for Node (42) to
evict Node (32) from its set of DAG parents, Node (42) would then
advertise itself at rank 2, thus moving up the DAG. In this case,
Node (53), (54), and (55) may similarly follow and advertise
themselves at rank 3.</t>
</list></t>
</section>
<section title="Node Removed">
<t>Consider the example of <xref target="LLNExample"></xref> when node
(41) disappears.</t>
<t><list style="symbols">
<t>Node (51) and (52) will now have empty DAG parent sets and be
detached from the DAG rooted by (LBR), advertising themselves as
the root of their own floating DAGs.</t>
<t>Node (52) would observe a chance to reattach to the DAG rooted
at (LBR) by adding Node (53) to its set of DAG parents, after an
appropriate delay to avoid creating loops. Node (52) will then
advertise itself in the DAG rooted at (LBR) at rank 7.</t>
<t>Node (51) will then be able to reattach to the DAG rooted at
(LBR) by adding Node (52) to its set of DAG parents and
advertising itself at rank 8.</t>
</list></t>
</section>
<section title="New LBR Added">
<t>Consider the example of <xref target="LLNExample"></xref> when a
new LBR, (LBR2) appears, with connectivity (LBR2)-(52),
(LBR2)-(53).</t>
<t><list style="symbols">
<t>Nodes (52) and Node (53) will see a chance to join a new DAG
rooted at (LBR2) with a rank of 2. Node (52) and (53) may take
this chance immediately, as there is no risk of forming loops when
joining a DAG that has never before been encountered. Note that
the nodes may choose to join the new DAG rooted at (LBR2) if and
only if (LBR2) offers more optimum properties in line with the
implementation specific local policy.</t>
<t>Nodes (52) and (53) begin to send RA-DIO messages advertising
themselves at rank 2 in the DAGID (LBR2).</t>
<t>Nodes (51), (41), (42), and (54) may then choose to join the
new DAG at rank 3, possibly to get closer to the DAG root. Note
that in a more advanced case, these nodes also remain members of
the DAG rooted at (LBR), for example in support of different
constraints for different types of traffic.</t>
<t>Node (55) may then join the new DAG at rank 4, possibly to get
closer to the DAG root.</t>
<t>The remaining nodes may choose to remain in their current
positions within the DAG rooted at node (LBR), since there is no
clear advantage to be gained by moving to DAG (LBR2).</t>
</list></t>
</section>
<section anchor="DestinationAdvertisementExample"
title="Destination Advertisement">
<t>Consider the example DAG depicted in <xref
target="DAGExample"></xref>. Suppose that Nodes (22) and (32) are
unable to record routing state. Suppose that Node (42) is able to
perform prefix aggregation on behalf of Nodes (53), (54), and
(55).</t>
<t><list style="symbols">
<t>Node (53) would send a NA-DAO message to Node (42), indicating
the availability of destination (53).</t>
<t>Node (54) and Node (55) would similarly send NA-DAO messages to
Node (42) indicating their own destinations.</t>
<t>Node (42) would collect and store the routing state for
destinations (53), (54), and (55).</t>
<t>In this example, Node (42) may then be capable of representing
destinations (42), (53), (54), and (55) in the aggregation
(42').</t>
<t>Node (42) sends a NA-DAO message advertising destination (42')
to Node 32.</t>
<t>Node (32) does not want to maintain any routing state, so it
adds onto to the Reverse Route Stack in the NA-DAO message and
passes it on to Node (22) as (42'):[(42)]. It may send a separate
NA-DAO message to indicate destination (32).</t>
<t>Node (22) does not want to maintain any routing state, so it
adds on to the Reverse Route Stack in the NA-DAO message and
passes it on to Node (12) as (42'):[(42), (32)]. It also relays
the NA-DAO message containing destination (32) to Node 12 as
(32):[(32)], and finally may send a NA-DAO message for itself
indicating destination (22).</t>
<t>Node (12) is capable to maintain routing state again, and
receives the NA-DAO messages from Node (22). Node (12) then
learns:</t>
<?rfc subcompact="yes"?>
<list style="symbols">
<t>Destination (22) is available via Node (22)</t>
<t>Destination (32) is available via Node (22) and the piecewise
source route to (32)</t>
<t>Destination (42') is available via Node (22) and the
piecewise source route to (32), (42').</t>
</list>
<?rfc subcompact="no"?>
<t>Node (12) sends NA-DAO messages to (LBR), allowing (LBR) to
learn routes to the destinations (12), (22), (32), and (42').
(42), (53), (54), and (55) are available via the aggregation
(42'). It is not necessary for Node (12) to propagate the
piecewise source routes to (LBR).</t>
</list></t>
</section>
<section anchor="ExDAGParentSelection"
title="Example: DAG Parent Selection">
<t>For example, suppose that a node (N) is not attached to any DAG,
and that it is in range of nodes (A), (B), (C), (D), and (E). Let all
nodes be configured to use an OCP which defines a policy such that ETX
is to be minimized and paths with the attribute `Blue' should be
avoided. Let the rank computation indicated by the OCP simply reflect
the ETX aggregated along the path. Let the links between node (N) and
its neighbors (A-E) all have an ETX of 1 (which is learned by node (N)
through some implementation specific method). Let node (N) be
configured to send IPv6 Router Solicitation (RS) messages to probe for
nearby DAGs.</t>
<t><list style="symbols">
<t>Node (N) transmits a Router Solicitation.</t>
<t>Node (B) responds. Node (N) investigates the RA-DIO message,
and learns that Node (B) is a member of DAGID 1 at rank 4, and not
`Blue'. Node (N) takes note of this, but is not yet confident.</t>
<t>Similarly, Node (N) hears from Node (A) at rank 9, Node (C) at
rank 5, and Node (E) at rank 4.</t>
<t>Node (D) responds. Node (D) has a RA-DIO message that indicates
that it is a member of DAGID 1 at rank 2, but it carries the
attribute `Blue'. Node (N)'s policy function rejects Node (D), and
no further consideration is given.</t>
<t>This process continues until Node (N), based on implementation
specific policy, builds up enough confidence to trigger a decision
to join DAGID 1. Let Node (N) determine its most preferred parent
to be Node (E).</t>
<t>Node (N) adds Node (E) (rank 4) to its set of DAG parents for
DAGID 1. Following the mechanisms specified by the OCP, and given
that the ETX is 1 for the link between (N) and (E), Node (N) is
now at rank 5 in DAGID 1.</t>
<t>Node (N) adds Node (B) (rank 4) to its set of DAG parents for
DAGID 1.</t>
<t>Node (N) is a sibling of Node (C), both are at rank 5.</t>
<t>Node (N) may now forward traffic intended for the default
destination inward along DAGID 1 via nodes (B) and (E). In some
cases, e.g. if nodes (B) and (E) are tried and fail, node (N) may
also choose to forward traffic to its sibling node (C), without
making inward progress but with the intention that node (C) or a
following successor can make inward progress. Should Node (C) not
have a viable parent, it should never send the packet back to Node
(N) (to avoid a 2-node loop).</t>
</list></t>
</section>
<section anchor="ExDAGMaintenance" title="Example: DAG Maintenance">
<figure anchor="DAGMaintenance" title="DAG Maintenance">
<artwork><![CDATA[
: : :
: : :
(A) (A) (A)
|\ | |
| `-----. | |
| \ | |
(B) (C) (B) (C) (B)
| | \
| | `-----.
| | \
(D) (D) (C)
|
|
|
(D)
-1- -2- -3-
]]></artwork>
</figure>
<t>Consider the example depicted in <xref
target="DAGMaintenance"></xref>-1. In this example, Node (A) is
attached to a DAG at some rank d. Node (A) is a DAG parent of Nodes
(B) and (C). Node (C) is a DAG parent of Node (D). There is also an
undirected sibling link between Nodes (B) and (C).</t>
<t>In this example, Node (C) may safely forward to Node (A) without
creating a loop. Node (C) may not safely forward to Node (D),
contained within it's own sub-DAG, without creating a loop. Node (C)
may forward to Node (B) in some cases, e.g. the link (C)->(A) is
temporarily unavailable, but with some chance of creating a loop (e.g.
if multiple nodes in a set of siblings start forwarding `sideways' in
a cycle) and requiring the intervention of additional mechanisms to
detect and break the loop.</t>
<t>Consider the case where Node (C) hears a RA-DIO message from a Node
(Z) at a lesser rank and superior position in the DAG than node (A).
Node (C) may safely undergo the process to evict node (A) from its DAG
parent set and attach directly to Node (Z) without creating a loop,
because its rank will decrease.</t>
<t>Now consider the case where the link (C)->(A) becomes nonviable,
and node (C) must move to a deeper rank within the DAG:</t>
<t><list style="symbols">
<t>Node (C) must first detach from the DAG by removing Node (A)
from its DAG parent set, leaving an empty DAG parent set. Node (C)
becomes the root of its own floating, less preferred, DAG.</t>
<t>Node (D), hearing a modified RA-DIO message from Node (C),
follows Node (C) into the floating DAG. This is depicted in <xref
target="DAGMaintenance"></xref>-2. In general, any node with no
other options in the sub-DAG of Node (C) will follow Node (C) into
the floating DAG, maintaining the structure of the sub-DAG.</t>
<t>Node (C) hears a RA-DIO message from Node (B) and determines it
is able to rejoin the grounded DAG by reattaching at a deeper rank
to Node (B). Node (C) starts a DAG Hop timer to coordinate this
move.</t>
<t>The timer expires and Node (C) adds Node (B) to its DAG parent
set. Node (C) has now safely moved deeper within the grounded DAG
without creating any loops. Node (D), and any other sub-DAG of
Node (C), will hear the modified RA-DIO message sourced from Node
(C) and follow Node (C) in a coordinated manner to reattach to the
grounded DAG. The final DAG is depicted in <xref
target="DAGMaintenance"></xref>-3</t>
</list></t>
</section>
<section anchor="ExGreedyExample"
title="Example: Greedy Parent Selection and Instability">
<figure anchor="Greedy" title="Greedy DAG Parent Selection">
<artwork><![CDATA[
(A) (A) (A)
|\ |\ |\
| `-----. | `-----. | `-----.
| \ | \ | \
(B) (C) (B) \ | (C)
\ | | /
`-----. | | .-----`
\| |/
(C) (B)
-1- -2- -3-
]]></artwork>
</figure>
<t>Consider the example depicted in <xref target="Greedy"></xref>. A
DAG is depicted in 3 different configurations. A usable link between
(B) and (C) exists in all 3 configurations. In <xref
target="Greedy"></xref>-1, Node (A) is a DAG parent for Nodes (B) and
(C), and (B)--(C) is a sibling link. In <xref
target="Greedy"></xref>-2, Node (A) is a DAG parent for Nodes (B) and
(C), and Node (B) is also a DAG parent for Node (C). In <xref
target="Greedy"></xref>-3, Node (A) is a DAG parent for Nodes (B) and
(C), and Node (C) is also a DAG parent for Node (B).</t>
<t>If a RPL node is too greedy, in that it attempts to optimize for an
additional number of parents beyond its preferred parent, then an
instability can result. Consider the DAG illustrated in <xref
target="Greedy"></xref>-1. In this example, Nodes (B) and (C) may most
prefer Node (A) as a DAG parent, but are operating under the greedy
condition that will try to optimize for 2 parents.</t>
<t>When the preferred parent selection causes a node to have only one
parent and no siblings, the node may decide to insert itself at a
slightly higher rank in order to have at least one sibling and thus an
alternate forwarding solution. This does not deprive other nodes of a
forwarding solution and this is considered acceptable greediness.</t>
<t><list style="symbols">
<t>Let <xref target="Greedy"></xref>-1 be the initial
condition.</t>
<t>Suppose Node (C) first is able to leave the DAG and rejoin at a
lower rank, taking both Nodes (A) and (B) as DAG parents as
depicted in <xref target="Greedy"></xref>-2. Now Node (C) is
deeper than both Nodes (A) and (B), and Node (C) is satisfied to
have 2 DAG parents.</t>
<t>Suppose Node (B), in its greediness, is willing to receive and
process a RA-DIO message from Node (C) (against the rules of RPL),
and then Node (B) leaves the DAG and rejoins at a lower rank,
taking both Nodes (A) and (C) as DAG parents. Now Node (B) is
deeper than both Nodes (A) and (C) and is satisfied with 2 DAG
parents.</t>
<t>Then Node (C), because it is also greedy, will leave and rejoin
deeper, to again get 2 parents and have a lower rank then both of
them.</t>
<t>Next Node (B) will again leave and rejoin deeper, to again get
2 parents</t>
<t>And again Node (C) leaves and rejoins deeper...</t>
<t>The process will repeat, and the DAG will oscillate between
<xref target="Greedy"></xref>-2 and <xref
target="Greedy"></xref>-3 until the nodes count to infinity and
restart the cycle again.</t>
<t>This cycle can be averted through mechanisms in RPL: <list>
<t>Nodes (B) and (C) stay at a rank sufficient to attach to
their most preferred parent (A) and don't go for any deeper
(worse) alternate parents (Nodes are not greedy)</t>
<t>Nodes (B) and (C) do not process RA-DIO messages from nodes
deeper than themselves (because such nodes are possibly in
their own sub-DAGs)</t>
</list></t>
</list></t>
</section>
<section anchor="ExDAGMerge" title="Example: DAG Merge">
<figure anchor="DAGMerge" title="Merging DAGs">
<artwork><![CDATA[
:
:
(A) (D)
| |
| |
| |
(B) (E)
| |
| |
| |
(C) (F)
]]></artwork>
</figure>
<t>Consider the example depicted in <xref target="DAGMerge"></xref>.
Nodes (A), (B), and (C) are part of some larger grounded DAG, where
Node (A) is at a rank of d, Node (B) at d+1, and Node (C) at d+2. The
DAG comprised of Nodes (D), (E), and (F) is a floating, less
preferred, DAG, with Node (D) as the DAG root. This floating DAG may
have been formed, for example, in the absence of a grounded DAG or
when Node (D) had to detach from a grounded DAG and (E) and (F)
followed. All nodes are using compatible objective code points.</t>
<t>Nodes (D), (E), and (F) would rather join the more preferred
grounded DAG if they are able than to remain in the less preferred
floating DAG.</t>
<t>Next, let links (C)--(D) and (A)--(E) become viable. The following
sequence of events may then occur in a typical case:</t>
<t><list style="symbols">
<t>Node (D) will receive and process a RA-DIO message from Node
(C) on link (C)--(D). Node (D) will consider Node (C) a candidate
neighbor and process the RA-DIO message since Node (C) belongs to
a different DAG (different DAGID) than Node (D). Node (D) will
note that Node (C) is in a grounded DAG at rank d+2, and will
begin the process to join the grounded DAG at rank d+3. Node (D)
will start a DAG Hop timer, logically associated with the grounded
DAG at Node (C), to coordinate the jump. The DAG Hop timer will
have a duration proportional to d+2.</t>
<t>Similarly, Node (E) will receive and process a RA-DIO message
from Node (A) on link (A)--(E). Node (E) will consider Node (A) a
candidate neighbor, will note that Node (A) is in a grounded DAG
at rank d, and will begin the process to join the grounded DAG at
rank d+1. Node (E) will start a DAG Hop timer, logically
associated with the grounded DAG at Node (A), to coordinate the
jump. The DAG Hop timer will have a duration proportional to
d.</t>
<t>Node (F) takes no action, for Node (F) has observed nothing new
to act on.</t>
<t>Node (E)'s DAG Hop timer for the grounded DAG at Node (A)
expires first. Node (E), upon the DAG Hop timer expiry, removes
Node (D) as its parent, thus emptying the DAG parent set for the
floating DAG, and leaving the floating DAG. Node (E) then jumps to
the grounded DAG by entering Node (A) into the set of DAG parents
for the grounded DAG. Node (E) is now in the grounded DAG at rank
d+1. Node (E), by jumping into the grounded DAG, has created an
inconsistency by changing its DAGID, and will begin to emit RA-DIO
messages more frequently.</t>
<t>Node (F) will receive and process a RA-DIO message from Node
(E). Node (F) will observe that Node (E) has changed its DAGID and
will directly follow Node (E) into the grounded DAG. Node (F) is
now a member of the grounded DAG at rank d+2. Note that any
additional sub-DAG of Node (E) would continue to join into the
grounded DAG in this coordinated manner.</t>
<t>Node (D) will receive a RA-DIO message from Node (E). Since
Node (E) is now in a different DAG, Node (D) may process the
RA-DIO message from Node (E). Node (D) will observe that, via node
(E), it could attach to the grounded DAG at rank d+2. Node (D)
will start another DAG Hop timer, logically associated with the
grounded DAG at Node (E), with a duration proportional to d+1.
Node (D) now is running two DAG hop timers, one which was started
with duration proportional to d+1 and one proportional to d+2.</t>
<t>Generally, Node (D) will expire the timer associated with the
jump to the grounded DAG at node (E) first. Node (D) may then jump
to the grounded DAG by entering Node (E) into its DAG parent set
for the grounded DAG. Node (D) is now in the grounded DAG at rank
d+2.</t>
<t>In this way RPL has coordinated a merge between the more
preferred grounded DAG and the less preferred floating DAG, such
that the nodes within the two DAGs come together in a generally
ordered manner, avoiding the formation of loops in the
process.</t>
</list></t>
</section>
</section>
<section anchor="AdditionalExamples" title="Additional Examples">
<t>Consider the expanded example LLN physical topology in <xref
target="LLNExample2"></xref>. In this example an additional LBR is
added. Suppose that all nodes are configured with an implementation
specific policy function that aims to minimize the number of hops, and
that both LBRs are configured to root different DAGIDs. We may now walk
through the formation of the two DAGs.</t>
<figure anchor="LLNExample2" title="Expanded LLN Topology">
<artwork><![CDATA[
(LBR) (LBR2)
/ | \ / \
.---` | `----. / \
/ | \ | |
(11)------(12)------(13) (14) (15)
| \ | \ | \ | /|
| `----. | `----. | `----. | .----` |
| \| \| \| / |
(21)------(22)------(23) (24) (25)
| /| /| | / /
| .----` | .----` | .-----]|[------` /
| / | / | / | /
(31)------(32)------(33)------(34)-----`
| /| \ | \ | \
| .----` | `----. | `----. | `----.
| / | \| \| \
.--------(41) (42) (43)------(44)------(45)
/ / /| \ | \
.----` .----` .----` | `----. | `----.
/ / / | \| \
(51)------(52)------(53)------(54)------(55)------(56)
]]></artwork>
</figure>
<figure anchor="DAGStep1" title="DAG Construction Step 1">
<artwork><![CDATA[
(LBR) (LBR2)
/ | \ / \
.---` | `----. / \
/ | \ | |
(11) (12) (13) (14) (15)
(21) (22) (23) (24) (25)
(31) (32) (33) (34)
(41) (42) (43) (44) (45)
(51) (52) (53) (54) (55) (56)
]]></artwork>
</figure>
<figure anchor="DAGStep2" title="DAG Construction Step 2">
<artwork><![CDATA[
(LBR) (LBR2)
/ | \ / \
.---` | `----. / \
/ | \ | |
(11) (12) (13) (14) (15)
| \ | \ | | /|
| `----. | `----. | | .----` |
| \| \| | / |
(21) (22) (23) (24) (25)
(31) (32) (33) (34)
(41) (42) (43) (44) (45)
(51) (52) (53) (54) (55) (56)
]]></artwork>
</figure>
<figure anchor="DAGStep3" title="DAG Construction Step 3">
<artwork><![CDATA[
(LBR) (LBR2)
/ | \ / \
.---` | `----. / \
/ | \ | |
(11) (12) (13) (14) (15)
| \ | \ | | /|
| `----. | `----. | | .----` |
| \| \| | / |
(21) (22) (23) (24) (25)
| /| / | / /
| .----` | .----` .-----]|[------` /
| / | / / | /
(31) (32) (33) (34)-----`
(41) (42) (43) (44) (45)
(51) (52) (53) (54) (55) (56)
]]></artwork>
</figure>
<figure anchor="DAGStep4" title="DAG Construction Step 4">
<artwork><![CDATA[
(LBR) (LBR2)
/ | \ / \
.---` | `----. / \
/ | \ | |
(11) (12) (13) (14) (15)
| \ | \ | | /|
| `----. | `----. | | .----` |
| \| \| | / |
(21) (22) (23) (24) (25)
| /| / | / /
| .----` | .----` .-----]|[------` /
| / | / / | /
(31) (32) (33) (34)-----`
| /| | \ | \
| .----` | | `----. | `----.
| / | | \| \
(41) (42) (43) (44) (45)
(51) (52) (53) (54) (55) (56)
]]></artwork>
</figure>
<figure anchor="DAGStep5" title="DAG Construction Step 5">
<artwork><![CDATA[
(LBR) (LBR2)
/ | \ / \
.---` | `----. / \
/ | \ | |
(11) (12) (13) (14) (15)
| \ | \ | | /|
| `----. | `----. | | .----` |
| \| \| | / |
(21) (22) (23) (24) (25)
| /| / | / /
| .----` | .----` .-----]|[------` /
| / | / / | /
(31) (32) (33) (34)-----`
| /| | \ | \
| .----` | | `----. | `----.
| / | | \| \
.--------(41) (42) (43) (44) (45)
/ / /| | \
.----` .----` .----` | | `----.
/ / / | | \
(51) (52) (53) (54) (55) (56)
]]></artwork>
</figure>
</section>
<section anchor="TODO" title="Outstanding Issues">
<t>This section enumerates some outstanding issues that are to be
addressed in future revisions of the RPL specification.</t>
<section title="Additional Support for P2P Routing">
<t>In some situations the baseline mechanism to support arbitrary P2P
traffic, by flowing inward along the DAG until a common parent is
reached and then flowing outward, may not be suitable for all
application scenarios. A related scenario may occur when the outward
paths setup along the DAG by the destination advertisement mechanism
are not be the most desirable outward paths for the specific
application scenario (in part because the DAG links may not be
symmetric). It may be desired to support within RPL the discovery and
installation of more direct routes `across' the DAG. Such mechanisms
need to be investigated.</t>
</section>
<section title="Loop Detection">
<t>It is under investigation to complement the loop avoidance
strategies provided by RPL with a loop detection mechanism that may be
employed when traffic is forwarded.</t>
</section>
<section title="Destination Advertisement / DAO Fan-out">
<t>When NA-DAO messages are relayed to more than one DAG parent, in
some cases a situation may be created where a large number of NA-DAO
messages conveying information about the same destination flow inward
along the DAG. It is desirable to bound/limit the
multiplication/fan-out of NA-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.</t>
</section>
<section title="Source Routing">
<t>In support of nodes who maintain minimal routing state, and to make
use of the collection of piecewise source routes from the destination
advertisement mechanism, there needs to be some investigation of a
mechanism to specify, attach, and follow source routes for packets
traversing the LLN.</t>
</section>
<section title="Address / Header Compression">
<t>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.</t>
</section>
</section>
</back>
</rfc>
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