One document matched: draft-ietf-roll-rpl-04.xml
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<front>
<title abbrev="draft-ietf-roll-rpl-04">RPL: IPv6 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>rpl-authors@external.cisco.com</email>
</address>
</author>
<date day="26" 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 a class of network in which
both the routers and their interconnect are constrained: LLN routers
typically operate with constraints on (any subset of) processing power,
memory and energy (battery), and their interconnects are characterized
by (any subset of) high loss rates, low data rates and instability. LLNs
are comprised of anything from a few dozen and up to thousands of LLN
routers, and support point-to- point traffic (between devices inside the
LLN), point-to-multipoint traffic (from a central control point to a
subset of devices inside the LLN) and multipoint-to- point traffic (from
devices inside the LLN towards a central control point). This document
specifies the IPv6 Routing Protocol for LLNs (RPL), which provides a
mechanism whereby multipoint-to-point traffic from devices inside the
LLN towards a central control point, as well as point-to-multipoint
traffic from the central control point to the devices inside the LLN, is
supported. Support for point-to-point traffic is also available.</t>
</abstract>
</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, typically time supporting only low data rates, that are usually
unstable with relatively low packet delivery rates. Another
characteristic of such networks is that the traffic patterns are not
simply unicast, but in many cases point-to-multipoint or
multipoint-to-point. Furthermore such networks may potentially comprise
up to thousands of nodes. These characteristics offer unique challenges
to a routing solution: the IETF ROLL Working Group has defined
application-specific routing requirements for a Low power and Lossy
Network (LLN) routing protocol, specified in <xref
target="I-D.ietf-roll-building-routing-reqs"></xref>, <xref
target="I-D.ietf-roll-home-routing-reqs"></xref>, <xref
target="RFC5673"></xref>, and <xref target="RFC5548"></xref>. This
document specifies the IPv6 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="RFC5673"></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
implementation of the protocol to be optimized in terms of, e.g.,
minimizing required code space and use of limited computation
resources.</t>
<t>Multiple instances of the protocol can be operated at the same time
in order to serve different and potentially antagonistic constraints.
Instances run independently of one another with no required
interaction. A node might participate to multiple instances and route
independently along the associated topologies. This specification
defines only the protocol operation for the node within one instance.
Consideration is given to default behavior that enables future
extensions for the multiple instances and related policies.</t>
<t>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 Type">
<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
link layers, including but not limited to low power wireless or PLC
(Power Line Communication) 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 key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in <xref
target="RFC2119">RFC 2119</xref>.</t>
<t>This document requires readers to be familiar with the terminology
described in `Terminology in Low power And Lossy Networks' <xref
target="I-D.ietf-roll-terminology"></xref>.</t>
<t><list hangIndent="6" style="hanging">
<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 one or more root nodes (a DAG
root, or sink- typically a Low power and Lossy Network Border Router
(LBR)). For the purpose of this document, the term DAG is often used
to refer to a DAG Iteration as defined below.</t>
<t hangText="DAG Instance:">A DAG Instance is a set of possibly
multiple Destination Oriented DAGs. A network may have more than one
DAG Instance, and a RPL router can participate to multiple DAG
instances. Each DAG Instance operates independently of other DAG
Instances. This document describes operation within a single DAG
instance.</t>
<t hangText="InstanceID:">Unique identifier of a DAG Instance.</t>
<t hangText="Destination Oriented DAG:">A DAG rooted at a single
destination, which is a node with no outgoing edges. The tuple
(InstanceID, DAGID) uniquely identifies a Destination Oriented DAG.
In the RPL context, a router can can belong to at most one
Destination Oriented DAG per DAG Instance.</t>
<t hangText="DAGID:">The identifier of a DAG root. The DAGID must be
unique within the scope of a DAG Instance in the LLN.</t>
<t hangText="DAG Iteration:">The DAG that results from the iterative
process that reshapes the Destination Oriented DAG upon a
stimulation by the root.</t>
<t hangText="DAGSequenceNumber:">A sequential counter that is
incremented by the root to form a new Iteration of a DAG. A DAG
Iteration is identified uniquely by the (InstanceID, DAGID,
DAGSequenceNumber) tuple.</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.</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 within a DAG. Note that siblings defined in
this manner do not necessarily share a common parent.</t>
<t hangText="DAG root:">A DAG root is a node within the DAG that has
no outgoing edges. Because the graph is acyclic, by definition all
DAGs must have at least one DAG root and all paths terminate at a
DAG root.</t>
<t hangText="Sub-DAG">The sub-DAG of a node is the set of other
nodes in the DAG that might use a path towards the DAG root that
contains the node. Nodes in the sub-DAG of a node have a greater
rank (although not all nodes of greater rank are in the
sub-DAG).</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="OCP:">Objective Code Point. The Objective Code Point is
used to indicate which Objective Function is in use in a DAG. The
Objective Code Point is further described in <xref
target="I-D.ietf-roll-routing-metrics"></xref>.</t>
<t hangText="OF:">Objective Function. The Objective Function (OF)
defines which routing metrics, optimization objectives, and related
functions are in use in a DAG. The Objective Function is further
described in <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 anchor="ProtocolOverview" title="Overview">
<section title="Topology Instance and Objectives">
<t>A topology instance of RPL exists over the scope of an LLN in
support of a particular application, or service, and is optimized
according to a certain objective, as determined by an Objective
Function (OF), and may be characterized by certain destination
prefixes as well. A topology instance, or DAG Instance, may be
administratively associated with an InstanceID.</t>
<t>A single topology instance may comprise:</t>
<t><list style="symbols">
<t>a single Destination Oriented DAG with a single DAG root
<list>
<t>For example, a DAG optimized to minimize latency rooted
at a single centralized lighting controller in a home
automation application.</t>
</list></t>
<t>multiple uncoordinated Destination Oriented DAGs with
independent DAG roots (differing DAGIDs) <list>
<t>For example, multiple data collection points in an urban
data collection application that do not have an always-on
backbone suitable to coordinate to form a single DAG, and
further use the formation of multiple DAGs as a means to
dynamically and autonomously partition the network.</t>
</list></t>
<t>a single Destination Oriented DAG with multiple DAG roots
coordinating over some backbone <list>
<t>For example, multiple border routers operating with a
reliable backbone, e.g. in support of a 6LowPAN application,
that are capable to act as logically equivalent sinks to the
same DAG.</t>
</list></t>
<t>a combination of one of the above as suited to some
application scenario</t>
</list></t>
<t>The exact deployment scenario is determined as appropriate to the
application and capabilities of the LLN nodes. What is suitable for
one deployment may not be possible or necessary for another.</t>
<t>Traffic is bound to a specific DAG Instance by a marking in the
flow label of the IPv6 header. Traffic originating in support of a
particular application may be tagged to follow an appropriate
instance, for example to follow paths optimized for low latency or
low energy. The provisioning or automated discovery of a mapping
between an InstanceID and a type or service of application traffic
is beyond the scope of this specification.</t>
<t>Conceptually a node running RPL may capable to maintain a
membership in multiple DAG Instances in support of different
application services and/or optimization objectives. For example,
one instance may optimize for minimizing latency and a separate
orthogonal instance may optimize for minimizing energy. This
scenario does introduce some additional considerations, for example
loop avoidance and default routing behavior. These considerations
are beyond the scope of this specification and are intended to be
elaborated on in a future revision of this or a companion
specification. As such, this specification will deal exclusively
with the scenario where a node implements RPL in support of a single
DAG Instance.</t>
</section>
<section title="Multipoint-to-Point Traffic Flows and DAGs">
<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="RFC5673"></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 to refer to the public
Internet or a core private (non-LLN) IP network.</t>
<t>LLN nodes running RPL will construct Directed Acyclic Graphs
(DAGs) rooted at DAG roots, which may be naturally designated
according to their application significance. This structure provides
the routing solution for the dominant MP2P traffic flows. The DAG
structure further provides each node potentially multiple successors
for MP2P flows, which may be used for, e.g., local route repair or
load balancing.</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 (<xref
target="DAGRank"></xref>).</t>
</section>
<section title="Point-to-Multipoint Traffic Flows">
<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>
</section>
<section title="Point-to-Point Traffic Flows">
<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 that has stored an entry for the destination in its
routing table and is capable of directing the traffic outward along
the correct outward path. RPL also provides support for the trivial
case where a P2P destination may be a `one-hop' neighbor. In the
present document RPL does not specify nor preclude any additional
mechanisms that may be capable to compute and install more optimal
routes into LLN nodes in support of arbitrary P2P traffic according
to some routing metric.</t>
</section>
</section>
<section title="Protocol Operation">
<section title="DAG Construction">
<section title="DAG Information Object (DIO)">
<t>A DAG Information Object is defined and used by RPL in order to
build and maintain a DAG. This document defines an ICMPv6 Message
Type RPL Control Message, which is capable to carry the DIO. The
DIO message conveys information about the DAG, including:</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 Function identified by an 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.</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 set of path metrics and constraints, 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 DIO messages are issued whenever a change is detected to
the DAG such that a node is able to determine that a region of the
DAG has become inconsistent. As the DAG stabilizes the period at
which RA messages occur is configured to taper off, reducing the
steady-state overhead of DAG maintenance. The periodic issue of
DIO messages, along with the triggered DIO messages in response to
inconsistency, is one feature that enables RPL to operate in the
presence of unreliable links.</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 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 Function (OF)">
<t>The Objective Function (OF) conveys and controls the
optimization objectives in use within the DAG. The Objective
Function is indicated by an Objective Code Point (OCP), and is
further specified in <xref
target="I-D.ietf-roll-routing-metrics"></xref>. Each instance of
an allocated OF indicates:</t>
<t><list style="symbols">
<t>The set of metrics used within the DAG</t>
<t>The method used for least cost path determination.</t>
<t>The method used to compute DAG Rank</t>
<t>The methods used to prepare metrics for propagation within
a DIO message</t>
</list></t>
<t>By using defined OCPs that are understood by all nodes in a
particular implementation, and by conveying them in the DIO
message, RPL nodes may work to build optimized LLN using a variety
of application and implementation specific metrics and goals.</t>
<t>A default OF, OF0 (designated by OCP value of 0x0000), is
specified with a well-defined default behavior. OF0 may be used to
define RPL behaviors in the case where a node encounters a DIO
message containing a code point that it does not support, if
allowed by policy.</t>
</section>
<section title="Distributed Algorithm Operation">
<t>A high level overview of the distributed algorithm which
constructs the DAG is as follows:</t>
<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 determined by the
implementation. This data structure will also track DAG
information as learned from and associated with each
neighbor.</t>
<t>Nodes that are members of a DAG, including DAG roots, will
multicast DIO messages as needed (when inconsistency is
detected), to their link-local neighbors. Nodes will also
respond to DIS messages.</t>
<t>Nodes that receive DIO messages may either discard the DIO
based on several criteria, including rank-based loop avoidance
rules, or process the DIO to maintain a position in an
existing DAG or improve a position as according to an
Objective Function (OF) and current path cost.</t>
<t>Nodes manage a set of DAG Parents according to the rules
specified by RPL. This set is also augmented to include DAG
siblings.</t>
<t>DIO messages may be emitted more or less frequently as a
function of DAG consistency.</t>
<t>As less preferred DAGs encounter more preferred DAGs that
offer equivalent or better optimization objectives for the
same InstanceID, the nodes in the less preferred DAGs may
leave to join the more preferred DAGs, finally leaving only
the more preferred DAGs. This is an illustration of the
mechanism by which an application may engineer DAG
construction.</t>
<t>The nodes provision routing table entries for the
destinations specified by the DIO towards their DAG Parents.
Nodes may provision a DAG Parent as a default gateway.</t>
</list></t>
</section>
</section>
<section title="Destination Advertisement">
<t>As RPL constructs DAGs, nodes may provision routes toward
destinations advertised through DIO messages through their selected
parents, and are thus able to send traffic inward along the DAG by
forwarding to their selected parents. However, this mechanism alone
is not sufficient to support P2MP traffic flowing outward along the
DAG from the DAG root toward nodes. A destination advertisement
mechanism is employed by RPL to build up routing state in support of
these outward flows. The destination advertisement mechanism may not
be supported in all implementations, as appropriate to the
application requirements. A DAG root that supports using the
destination advertisement mechanism to build up routing state will
indicate such in the DIO message. A DAG root that supports using the
destination advertisement mechanism must be capable of allocating
enough state to store the routing state received from the LLN.</t>
<section title="Destination Advertisement Object (DAO)">
<t>A Destination Advertisement Object is defined and used by RPL
in order to convey the destination information inward along the
DAG toward the DAG root. This document defines an ICMPv6 Message
Type RPL Control Message, which is capable to carry the DAO. The
information conveyed in the DAO message includes the
following:</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 DAO messages to a subset of their DAG parents.</t>
<section title="`One-Hop' Neighbors">
<t>As a special case, a node may periodically emit a link-local
multicast IPv6 DAO message advertising its locally available
destination prefixes. This mechanism allows for the one-hop
neighbors of a node to learn explicitly of the prefixes on the
node, and in some application specific scenarios this is
desirable in support of provisioning a trivial `one-hop' route.
In this case, nodes that receive the multicast destination
advertisement may use it to provision the one-hop route only,
and not engage in any additional processing (so as not to engage
the mechanisms used by a DAG parent).</t>
</section>
<section title="Operation in Support of Stateful Nodes">
<t>When a (unicast) DAO message reaches a node capable of
storing routing state, the node extracts information from the
DAO message and updates its local database with a record of the
DAO message and the neighbor that it was received from. When the
node later propagates DAO messages, it selects the best (least
depth) information for each destination and conveys this
information again in the form of DAO messages to a subset of its
own DAG parents. At this time the node may perform route
aggregation if it is able, thus reducing the overall number of
DAO messages.</t>
</section>
<section title="Operation in Support of Stateless Nodes">
<t>When a (unicast) DAO message reaches a node incapable of
storing additional state, the node must append the next-hop
address (from which neighbor the DAO message was received) to a
Reverse Route Stack carried within the DAO message. The node
then passes the DAO message on to one or more of its DAG parents
without storing any additional state.</t>
<t>When a node that is capable of storing routing state
encounters a (unicast) DAO message with a Reverse Route Stack
that has been populated, the node knows that the DAO message has
traversed a region of nodes that did not record any routing
state. The node is able to detach and store the Reverse Route
State and associate it with the destination described by the DAO
message. Subsequently the node may use this information to
construct a source route in order to bridge the region of nodes
that are unable to support Hop-By-Hop routing to reach the
destination.</t>
</section>
<section title="Additional Considerations">
<t>Further aggregations of DAO messages prefix reachability
information by destinations are possible in order to support
additional scalability.</t>
<t>A special case of an DAO message, termed a `no-DAO', may be
used to tear down the routing state that has been established by
the destination advertisement mechanism in case of, e.g.,
unreachability or other events that affect the outward routing
state.</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>
<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.</t>
<t>RPL includes rank-based mechanisms for detecting loops to ensure
that packets make forward progress within the DAG and trigger DAG
repair if necessary.</t>
<section title="Greediness and Rank-based Instabilities">
<t>Once a node has joined a DAG, RPL disallows certain behaviors,
including greediness, in order to prevent resulting instabilities in
the DAG.</t>
<t>If a node is allowed to be greedy and attempts to move deeper in
the DAG, beyond its most preferred parent, in order to increase the
size of the DAG parent set, then an instability can result. This is
illustrated in <xref target="Greedy"></xref>.</t>
<t>Suppose a node is willing to receive and process a DIO messages
from a node in its own sub-DAG, and in general a node deeper than
it. In such cases a chance exists to create a feedback loop, wherein
two or more nodes continue to try and move in the DAG in order to
optimize against each other. In some cases this will result in an
instability. It is for this reason that RPL mandates that a node
never receive and process DIO messages from deeper nodes. This rule
creates an `event horizon', whereby a node cannot be influenced into
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 DIO messages from nodes of greater
rank, may be found in <xref target="ExGreedyExample"></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. This may happen in
particular when DIO messages are missed. Strict use of the DAG
sequence number can eliminate this type of loop.</t>
</section>
<section title="DAO Loops">
<t>A DAO loop may occur when the parent has a route installed upon
receiving and processing a DAO message from a child, but the child
has subsequently cleaned up the state. This loop happens when a
no-DAO was missed till a heartbeat cleans up all states. RPL
includes loop detection mechanisms that may mitigate the impact of
DAO loops and trigger their repair.</t>
<t>In the case where stateless DAO operation is used, i.e. source
routing specifies the outwards routes, then DAO Loops should not
occur on the stateless portions of the path.</t>
</section>
<section title="Sibling Loops">
<t>Sibling loops could occur if a group of siblings kept choosing
amongst themselves as successors such that a packet does not make
forward progress. This specification limits the number of times that
sibling forwarding may be used at a given rank to prevent sibling
loops.</t>
</section>
</section>
</section>
<section anchor="ConstrainedLLNs"
title="Routing Metrics and Constraints Used By RPL">
<t>Routing metrics are used by routing protocols to compute the shortest
paths. Interior Gateway Protocols (IGPs) such as IS-IS (<xref
target="RFC5120"></xref>) and OSPF (<xref target="RFC4915"></xref>) use
static link metrics. Such link metrics can simply reflect the bandwidth
or can also be computed according to a polynomial function of several
metrics defining different link characteristics; in all cases they are
static metrics. Some routing protocols support more than one metric: in
the vast majority of the cases, one metric is used per (sub)topology.
Less often, a second metric may be used as a tie-breaker in the presence
of Equal Cost Multiple Paths (ECMP). The optimization of multiple
metrics is known as an NP complete problem and is sometimes supported by
some centralized path computation engine.</t>
<t>In contrast, LLNs do require the support of both static and dynamic
metrics. Furthermore, both link and node metrics are required. In the
case of RPL, it is virtually impossible to define one metric, or even a
composite, that will satisfy all use cases.</t>
<t>In addition, RPL supports constrained-based routing where constraints
may be applied to link and nodes. If a link or a node does not satisfy a
required constraint, it is `pruned' from the candidate list thus leading
to a constrained shortest path.</t>
<t>The set of supported link/node constraints and metrics is specified
in <xref target="I-D.ietf-roll-routing-metrics"></xref>.</t>
<t>The role of the Objective Function is to advertise routing metrics
and constraints in addition to the objectives used to compute the
(constrained) shortest path.</t>
<t><list hangIndent="11" style="hanging">
<t hangText="Example 1:">Shortest path: path offering the shortest
end-to-end delay</t>
<t hangText="Example 2:">Constrained shortest path: the path that
does traverse any battery-operated node and that optimizes the path
reliability</t>
</list></t>
</section>
<section anchor="SpecCore" title="RPL Protocol Specification">
<section anchor="RPLMessages" title="RPL Messages">
<section anchor="RPLControlMessage" title="ICMPv6 RPL Control Message">
<t>This document defines the RPL Control Message, a new ICMPv6
message. The RPL Control Message has the following general format,
in accordance with <xref target="RFC4443"></xref>:</t>
<t><figure anchor="ICMPFormat" title="RPL Control Message">
<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 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ Message Body +
| |
]]></artwork>
</figure></t>
<t>The RPL Control message is an ICMPv6 information message with a
requested Type of 155.</t>
<t>The Code will be used to identify RPL Control Messages as
follows:</t>
<t><list style="symbols">
<t>0x01: DAG Information Solicitation (<xref
target="DAGInformationSolicitation"></xref>)</t>
<t>0x02: DAG Information Object (<xref
target="DAGInformationObject"></xref>)</t>
<t>0x04: Destination Advertisement Object (<xref
target="DestinationAdvertisementObject"></xref>)</t>
</list></t>
</section>
<section anchor="DAGInformationSolicitation"
title="DAG Information Solicitation (DIS)">
<t>The DAG Information Solicitation (DIS) message may be used to
solicit a DAG Information Object from a RPL node. Its use is
analogous to that of a Router Solicitation; a node may use DIS to
probe its neighborhood for nearby DAGs. The DAG Information
Solicitation carries no additional message body.</t>
</section>
<section anchor="DAGInformationObject"
title="DAG Information Object (DIO)">
<t>The DAG Information Object carries a number of metrics and other
information that allows a node to discover a DAG, select its DAG
parents, and identify its siblings while employing loop avoidance
strategies.</t>
<section anchor="DIOBaseOption" title="DIO Base Option">
<t>The DIO Base Option is a container option, which is always
present, and might contain a number of suboptions. The base option
regroups the minimum information set that is mandatory in all
cases.</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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|G|D|A|0|0| Prf | Sequence | InstanceID | DAGRank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
+ +
| DAGID |
+ +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t><list hangIndent="6" style="hanging">
<t hangText="Control Field:">The DAG Control Field is
currently allocated as follows: <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>
<t hangText="DAGPreference (Prf):">3-bit unsigned integer
set by the DAG root to its preference and unchanged at
propagation. DAGPreference ranges from 0x00 (least
preferred) to 0x07 (most preferred). The default is 0
(least preferred). The DAG preference provides an
administrative mechanism to engineer the self-organization
of the LLN, for example indicating the most preferred LBR.
If a node has the option to join a more preferred DAG
while still meeting other optimization objectives, then
the node will generally seek to join the more preferred
DAG as determined by the OF.</t>
</list></t>
<t>Unassigned bits of the Control 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="InstanceID:">8-bit field indicating the topology
instance associated with the DAG, as provisioned at the DAG
root.</t>
<t hangText="DAGRank:">8-bit unsigned integer indicating the
DAG rank of the node sending the DIO message. The DAGRank of
the DAG root is ROOT_RANK. DAGRank is further described in
<xref target="DAGDiscovery"></xref>.</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
DIO messages outwards along the DAG:<?rfc subcompact="yes"?><list>
<t>Grounded (G)</t>
<t>Destination Advertisement Supported (A)</t>
<t>DAGPreference (Prf)</t>
<t>Sequence</t>
<t>InstanceID</t>
<t>DAGID</t>
</list><?rfc subcompact="no"?>All other fields of the DIO
message may be updated at each hop of the propagation.</t>
<section title="DIO Suboptions">
<t>In addition to the minimum options presented in the base
option, several suboptions are defined for the DIO message:</t>
<section title="Format">
<t><figure anchor="DIOsub"
title="DIO Suboption Generic Format">
<artwork><![CDATA[
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Subopt. Type | Subopt Length | Subopt Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
]]></artwork>
</figure></t>
<t><list hangIndent="6" style="hanging">
<t hangText="Suboption Type:">8-bit identifier of the type
of suboption. When processing a DIO message containing a
suboption for which the Suboption Type value is not
recognized by the receiver, the receiver MUST silently
ignore the unrecognized option, continue to process the
following suboption, correctly handling any remaining
options in the message.</t>
<t hangText="Suboption Length:">16-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 DIO message
suboptions which are currently defined for use in the DAG
Information Object.</t>
<t>Implementations MUST silently ignore any DIO message
suboptions options that they do not understand.</t>
<t>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 or two octets of
padding in the DIO message to enable suboptions alignment. If
more than two octets of padding is required, the PadN option,
described next, should be used rather than multiple Pad1
options.</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 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Type = 1 | Subopt Length | Subopt Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
]]></artwork>
</figure></t>
<t>The PadN option is used to insert three or more octets of
padding in the DIO message to enable suboptions alignment. For
N (N > 2) octets of padding, the Option Length field
contains the value N-3, and the Option Data consists of N-3
zero-valued octets. PadN Option data MUST be ignored by the
receiver.</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 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
| Type = 2 | Container Length | DAG Metric Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
]]></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 does not have any
alignment requirements. 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 |Resvd|Prf|Resvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | |
+-+-+-+-+-+-+-+-+ |
| 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>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 Prefix Length is an 16-bit unsigned integer that
indicates the number of leading bits in the destination
prefix.</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 DIO message may need to specify
connectivity to more than one destination, the Destination
Prefix suboption may be repeated.</t>
</section>
<section title="DAG Timer Configuration">
<t>The DAG Timer Configuration suboption does not have any
alignment requirements. Its format is as follows:</t>
<t><figure anchor="DIOsubDAGTimerConfig"
title="DAG Timer Configuration">
<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 = 4 | Length | DIOIntDoubl. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DIOIntMin. |
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t>The DAG Timer Configuration suboption is used to distribute
configuration information for DAG Timer Operation through the
DAG. The information communicated in this suboption is
generally static and unchanging within the DAG, therefore it
is not necessary to include in every DIO. This suboption MAY
be included periodically by the DAG Root, and SHOULD be
included in response to a unicast request, e.g. a DAG
Information Solicitation (DIS) message.</t>
<t>The Length is coded as 2.</t>
<t>DIOIntervalDoublings is an 8-bit unsigned integer.
Configured on the DAG root and used to configure the trickle
timer governing when DIO message should be sent within the
DAG. DIOIntervalDoublings is the number of times that the
DIOIntervalMin is allowed to be doubled during the trickle
timer operation.</t>
<t>DIOIntervalMin is an 8-bit unsigned integer. Configured on
the DAG root and used to configure the trickle timer governing
when DIO message should be sent within the DAG. The minimum
configured interval for the DIO trickle timer in units of ms
is 2^DIOIntervalMin. For example, a DIOIntervalMin value of
16ms is expressed as 4.</t>
</section>
</section>
</section>
</section>
<section anchor="DestinationAdvertisementObject"
title="Destination Advertisement Object (DAO)">
<t>The Destination Advertisement Object (DAO) is used to propagate
destination information inwards along the DAG. The RPL use of the
DAO allows the nodes in the DAG to build up routing state for nodes
contained in the sub-DAG in support of traffic flowing outward along
the DAG.</t>
<t><figure anchor="DAObject"
title="The Destination Advertisement Object (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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Sequence | InstanceID | DAO Rank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Route Tag |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | RRCount | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| Prefix (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse Route Stack (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t><list hangIndent="6" style="hanging">
<t hangText="DAO Sequence:">Incremented by the node that owns
the prefix for each new DAO message for that prefix.</t>
<t hangText="InstanceID:">8-bit field indicating the topology
instance associated with the DAG, as learned from the DIO.</t>
<t hangText="DAO Rank:">Set by the node that owns the prefix and
first issues the DAO message to its rank.</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="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="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 that adds on to the Reverse Route Stack will
append to the list and increment the RRCount.</t>
</list></t>
</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 DAG parents and siblings</t>
</list>
</list></t>
<section title="Candidate Neighbors Data Structure">
<t>The set of candidate neighbors is to be populated by neighbors
that are discovered by the neighbor discovery mechanism and further
qualified as statistically stable as per the mechanisms discussed in
<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>At a given point of time, a DAG Iteration is uniquely identified
by the tuple (DagID, InstanceID, DAGSequenceNumber) where a change
in the sequence denotes the iteration of a given DAG over time. When
a single device is capable to root multiple DAGs in support of an
application need for multiple optimization objectives it MUST
produce a different and unique (DagID, InstanceID) pair for each of
the multiple DAGs.</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>InstanceID</t>
<t>DAGID</t>
<t>DAGSequenceNumber</t>
<t>DAG Metric Container, including DAGObjectiveCodePoint</t>
<t>A set of Destination Prefixes offered inwards along the
DAG</t>
<t>A set of DAG parents and siblings</t>
<t>A timer to govern the sending of DIO messages for the DAG</t>
</list>
</t>
<t>When a DAG is discovered for which no DAG data structure is
instantiated, and the node wants to join, then the DAG data
structure is instantiated.</t>
<t>When the DAG parent set is depleted (i.e. the last DAG is
removed), then the DAG data structure SHOULD be suppressed after the
expiration of an implementation-specific local timer. An
implementation SHOULD delay before deallocating the DAG data
structure in order to observe that the DAGSequenceNumber has
incremented should any new DAG parents appear for the DAG.</t>
<section title="DAG Parents/Siblings Structure">
<t>When the DAG is self-rooted, the set of DAG parents/siblings is
empty.</t>
<t>In all other cases, for each node 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 or sibling</t>
<t>a record of most recent information taken from the DAG
Information Object last processed from the DAG parent</t>
</list>
</t>
<t>DAG parents may be ordered, according to the OF. When ordering
DAG parents, in consultation with the OF, the most preferred DAG
parent may be identified. All current DAG parents must have a rank
less than self. All current DAG siblings must have a rank equal to
self.</t>
<t>When nodes are added to or removed from the DAG set the most
preferred DAG parent may have changed. The role of all the nodes
in the list should be reevaluated. In particular, any nodes having
a rank greater than self after such a change must be evicted from
the set.</t>
</section>
<t>An implementation may choose to keep these records as an
extension of the Default Router List (DRL).</t>
</section>
</section>
<section anchor="DAGRank" title="DAG Rank">
<t>Based on the selection of DAG Parents, the metrics conveyed by the
most preferred DAG parent, the nodes own metrics and configuration,
and a related function defined by the OF, a node will be able to
compute a value for its rank as a consequence of selecting a most
preferred DAG parent.</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. The
only aim of the rank is to inform loop avoidance and detection.</t>
<t>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 that are
neighbors in the LLN:</t>
<t><list hangIndent="8" style="hanging">
<t hangText="DAGRank(M) is less than DAGRank(N):">In this case, M
is probably located in a more preferred position than N in the DAG
with respect to the metrics and optimizations defined by the
objective code point. In any fashion, Node M may safely be a DAG
parent for Node N without risk of creating a loop. Further, for a
node N, all parents in the DAG parent set must be of rank less
than self's DAGRank(N). In other words, the rank presented by a
node N MUST be greater (deeper) than that presented by any of its
parents.<!--<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 hangText="DAGRank(M) equals DAGRank(N):">In this case M and N
are located positions of relatively the same optimality within the
DAG. In some cases, Node M may be used as a successor by Node N,
but with related chance of creating a loop that must be detected
and broken by some other means.<!-- <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 hangText="DAGRank(M) is greater than DAGRank(N):">In this case,
then node M is located in a less preferred position than N in the
DAG with respect to the metrics and optimizations defined by the
objective code point. Further, Node (M) may in fact be in Node
(N)'s sub-DAG. There is a higher risk to Node (N) selecting Node
(M) as a DAG parent, as such a selection may create a loop.<!--<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>
</section>
<section anchor="DAGDiscovery" title="DAG Discovery and Maintenance">
<t>DAG discovery locates the nearest sink (aka root), as determined
according to some metrics and constraints, and forms a Directed
Acyclic Graph towards that sink, by identifying a set of DAG parents.
During this process DAG discovery also identifies siblings, which may
be used later to provide additional path diversity towards the DAG
root. DAG discovery enables nodes to implement different policies for
selecting their DAG parents in the DAG by using implementation
specific policy functions. DAG discovery specifies a set of rules to
be followed by all implementations in order to ensure interoperation.
DAG discovery also standardizes the format that is used to advertise
the most common information that is used in order to select DAG
parents.</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 OF 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 DAGs and ranks</t>
<t>The Objective 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>
<section anchor="DAGDiscoveryRulesDAD" title="DAGs">
<t><list style="numbers">
<t>DAG discovery instantiates LLN topologies that are each
optimized for specific constraints and goals. A topology
assumes the shape of a DAG, and a DAG Instance is uniquely
identified by its instanceID.</t>
<t>For reasons of scalability and operations of the protocol,
a DAG Instance is partitioned into a set of DAGs rooted at a
destination, aka Destination Oriented DAGs. A destination is
uniquely identified by a DAGID so a DAG rooted at a
destination is uniquely identified by the pair (InstanceID,
DAGID).</t>
<t>A Destination Oriented DAG is periodically reconstructed
from the root, by incrementing a DAGSequenceNumber. An
Iteration of a Destination Oriented DAG is thus uniquely
identified by the tuple (InstanceID, DAGID,
DAGSequenceNumber). Through this document, the graph formed by
this iterative process is referred to as the DAG Iteration, or
in short, the DAG.</t>
<t>The rank is defined within the scope of a DAG Iteration as
an abstract coordinate to compare the relative position of
nodes and ensure forward progress of the traffic.</t>
<t>A node MUST belong at most to one DAG Iteration per
InstanceID and MUST select all its parents and siblings within
that same DAG Iteration.</t>
</list></t>
</section>
<section anchor="DAGDiscoveryRulesSeq" title="DAG Sequence Number">
<t><list style="numbers">
<t>The DAGSequenceNumber is incremented by the root and
flooded through DIOs.</t>
<t>The root floods a new DAGSequenceNumber periodically, at a
rate that depends on the deployment. This rate can be set to 0
if other methods such as loop detection are considered
sufficient to solve the routing issues in that deployment.</t>
<t>The root MAY also flood a new DAGSequenceNumber on-demand.
The details of the mechanism to signal the root to do so are
to be specified in a future revision of this document.</t>
<t>A parent that advertises the new DAGSequenceNumber can not
possibly belong to the sub-DAG of a node that still advertises
an older DAGSequenceNumber. The node MAY thus attach to that
parent regardless of the relative rank, and this situation is
equivalent to jumping onto a different Destination Oriented
DAG.</t>
<t>Thus, as a new DAGSequenceNumber spreads, a new DAG
Iteration forms that supersedes the previous one. During a
DAGSequenceNumber transition, a node MAY decide to forward
packets via 'future parents' that belong to the same
Destination Oriented DAG (same InstanceID and DagID), but a
more recent (incremented) DAGSequenceNumber.</t>
</list></t>
</section>
<section anchor="DAGDiscoveryRulesRoot" title="DAG Root">
<t><list style="numbers">
<t>A node that does not have any DAG parent MAY become the
root of its own floating DAG. It's rank is ROOT_RANK.</t>
<t>A (non-LLN) router is considered connected to a grounded
infrastructure at rank BASE_RANK. A LLN node that is attached
to such an infrastructure router is the DAG root of its own
grounded DAG. It's rank is ROOT_RANK.</t>
<t>In a deployment that uses a backbone link to federate a
number of LLN roots, it is possible to run RPL over the
backbone and use one router as a backbone root. The backbone
root exposes a rank of BASE_RANK over the backbone. All the
LLN roots that are parented to that backbone root, including
the backbone root if it also serves as LLN root, expose a rank
of ROOT_RANK over the LLN and act as multiple roots for a same
DAG, coordinated by the backbone root.</t>
<t>The DAG root exposes the DAG in the DIO message and LLN
nodes propagate the DIO message outwards along the DAG.</t>
</list></t>
</section>
<section anchor="DAGDiscoveryRulesMove" title="Moving Inside a DAG">
<t><list style="numbers">
<t>A node moves when it changes its parent selection within
the same DAG Iteration. When a node moves (within its DAG) in
a fashion that cause its rank to decrease, the node MUST
abandon all parents and siblings with a rank larger than self,
and MAY adopt as siblings nodes with the same rank.</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 OF.</t>
<t>A node MUST NOT move outwards along a DAG that it is
attached to, causing the DAG rank to increase. If a node
cannot stay within the DAG without a rank increase, then it
MUST poison its routes as described in <xref
target="DAGDiscoveryRulesdetach"></xref>.</t>
<t>When DIO messages are received from other routers located
at lesser rank in the same DAG, those routers are eligible for
consideration as DAG parents. DIO messages received from other
routers located at the same rank in the same DAG may be
considered as coming from siblings. DIO messages that are
received from other routers located at greater rank within the
same DAG might cause greedy behaviors and loops; such a DIO is
ignored unless: <list style="numbers">
<t>The DIO comes from an existing parent or sibling; in
which case that parent must be removed.</t>
<t>The DIO comes from a node that has better OF ratings
than any parent known at this point; in that case, this
potential parent MAY be remembered in order to jump at a
better position when the next sequence is flooded.</t>
</list></t>
</list></t>
</section>
<section anchor="DAGDiscoveryRulesJump"
title="Jumping Onto Another DAG">
<t><list style="numbers">
<t>A node jumps when it performs a new parent selection
whereby its DAG Iteration changes within the same DAG
Instance. When a node jumps onto a new DAG Iteration, it MUST
abandon all parents and siblings from its previous
position.</t>
<t>A node MAY jump from its current DAG onto any other DAG
that provides service for the same InstanceID if it is
preferred by the OF, for example for reasons such as
connectivity, configured preference, free medium time, size,
security, bandwidth, DAG rank, or whatever metrics the LLN
uses. This is allowed regardless of the rank that the node
reaches in the new DAG.</t>
<!-- TBD turn this into an implementation note?
<t>If a node has selected a new set of DAG parents but has not
jumped yet (because it is waiting for DAG Hop timer to
elapse), the node is UNSTABLE and MUST NOT send DIOs for that
DAG.</t>
-->
<t>A node that jumps should attempt to transmit all the
packets received as part of the previous DAG along the
previous DAG. In other words, it should switch the parent set
only after the outstanding packet queue of packets received
prior to announcing the jump is exhausted.</t>
<t>Jumping back onto a previous DAG <!--within a short period of time -->
is equivalent to moving inside that DAG and obeys the same
rules. <!--That period of time must be sufficient for DIOs that advertise the
previous situation to have spread so that the probability is high
that the node does not jump back into its own sub-DAG. -->
To satisfy this, a node detaching from a DAG SHOULD remember
its DAG as identified by the tuple (InstanceID, DagID,
DAGSequenceNumber) as well as its rank within that DAG for
long as that DAG exists.<!-- TBD In practice, the node needs not maintain its history information
forever, but for such a period of time as it can be expected
that the risk of forming loops is reduced sufficiently by the
propagation of the route poisoning described in
<xref target="DAGDiscoveryRulesdetach"/>. --></t>
</list></t>
</section>
<section anchor="DAGDiscoveryRulesdetach"
title="Poisoning a Broken Path">
<t><list style="numbers">
<t>A node SHOULD poison its inwards routes when it looses all
of its current feasible parents, i.e. the set of DAG parents
becomes depleted, and it can not jump onto an alternate
DAG.</t>
<t>In order to poison its inwards routes, a node MAY stay at
its position within its DAG (that is maintain its InstanceID,
DagID, DAGSequenceNumber and Rank) but it SHOULD immediately
advertise a rank of INFINITE_RANK in a DIO so as to force all
its children to remove it from their parent list and try an
alternate path. The node SHOULD then wait for a new DAG
Iteration (DAGSequenceNumber increment) before resuming its
operation in the same Destination Oriented DAG.<!-- For the same
constraints, the node might not be able to support the DAG Hop
Timer and it MAY move or jump at anytime and without a
wait.--></t>
<t>Alternatively, a node MAY detach from its DAG. A node that
detaches becomes root of its own floating DAG and MUST
immediately advertise its new situation in a DIO.</t>
<t>Either way, the route poisoning will recursively be flooded
throughout the impacted sub-DAG as children lose their last
parent in the original DAG.</t>
<t>The loss of a DIO message may interrupt the flooding. This
can be compensated by cheer repetition through the trickle
algorithm. If that also fails, packet loops will be prevented
by the detection mechanism described in <xref
target="loopdetect"></xref>.</t>
</list></t>
</section>
<section anchor="DAGDiscoveryRulesfollow" title="Following a Parent">
<t><list style="numbers">
<t>If a node that receives a DIO from one of its DAG parents
indicating that the parent has left the DAG, it may either
follow that parent or stay in its current DAG through an
alternate DAG parent if that is possible.<!-- The node may follow
its parent without a wait in order to prevent packet loss.--></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 anchor="DAGDiscoveryRulesincons" title="DAG Inconsistency">
<t><list style="numbers">
<t>When a node detects or causes a DAG inconsistency, as
described in <xref target="TrickleInconsistencies"></xref>,
then the node SHOULD send an unsolicited DIO message to its
one-hop neighbors. The DIO is updated to propagate the new DAG
information. Such an event MUST also cause the trickle timer
governing the periodic sending of DIO messages to be
reset.</t>
</list></t>
</section>
</section>
<section title="Reception and Processing of DIO messages">
<t>When an DIO message is received from a source device named SRC,
the receiving node must first determine whether or not the DIO
message should be accepted for further processing, and subsequently
present the DIO message for further processing if eligible.</t>
<t>
<list style="numbers">
<t>If the DIO message is malformed, then the DIO message is not
eligible for further processing and is silently discarded. A RPL
implementation MAY log the reception of a malformed DIO
message.</t>
<t>If SRC is not a member of the candidate neighbor set, then
the DIO is not eligible for further processing. (Further
evaluation/confidence of this neighbor is necessary)</t>
<t>If the DIO message advertises a DAG that the node is already
a member of, then:</t>
<list style="symbols">
<t>If the rank of SRC as reported in the DIO message is lesser
than that of the node within the DAG, then the DIO message
MUST be considered for further processing.</t>
<t>If the rank of SRC as reported in the DIO message is equal
to that of the node within the DAG, then SRC is marked as a
sibling and the DIO message is not eligible for further
processing.</t>
<t>If the rank of SRC as reported in the DIO message is higher
than that of the node within the DAG, and SRC is not a DAG
parent, then the DIO message MUST NOT be considered for
further processing</t>
</list>
<t>Even if not processed further, information from a DIO might
be remembered for instance if SRC is preferable to the current
parents per the OF selection process.</t>
<t>If SRC is a DAG parent for any other DAG that the node is
attached to, then the DIO message MUST be considered for further
processing (the DAG parent may have jumped).</t>
<t>If the DIO message advertises a DAG that offers a better (new
or alternate) solution to an optimization objective desired by
the node, then the DIO message MUST be considered for further
processing.</t>
</list>
</t>
<section title="Overview of DIO Message Processing">
<t>
<list>
<t>If the received DIO message is for a new/alternate DAG:</t>
<list>
<t>If the node has sent an DIO message within the risk
window as described in <xref target="DAGCollision" /> then a
collision has occurred; do not process the DIO message any
further.</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 is the
same InstanceID 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</t>
<t>If the other DAG is now empty of candidate parents,
then prepare to directly follow SRC into the new DAG by
adding it as a DAG parent for the new DAG, else ignore the
DIO message (do not follow the parent).</t>
</list>
<t>Instantiate a data structure for the new/alternate DAG if
necessary</t>
<t>If the new/alternate DAG offers a better solution to the
optimization objectives, then jump: copy the DIO information
place the neighbor into the DAG parent set.</t>
</list>
<t>If the DIO message is for a known/existing DAG:</t>
<list>
<t>Process the DIO message as per the rules in <xref
target="DAGDiscovery" /></t>
</list>
</list>
</t>
</section>
<t>As DIO messages are received from candidate neighbors, the
neighbors may be promoted to DAG parents by following the rules of
DAG discovery as described in <xref target="DAGDiscovery" />. When a
node places a neighbor into the DAG Parent set, the node becomes
attached to the DAG through the new 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="DIO Transmission">
<t>Each node maintains a timer that governs when to multicast DIO
messages. This timer is implemented as a trickle timer operating
over a variable interval. Trickle timers are further detailed in
<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 DIO message. In
addition to periodic DIO messages, each node may respond to a DIS
message with a DIO message.</t>
<t><list style="symbols">
<t>When a node detects an inconsistency, it SHOULD reset the
interval of the trickle timer to a minimum value, causing DIO
messages to be emitted more frequently as part of a strategy to
quickly correct the inconsistency. Such inconsistencies may be,
for example, an update to a key parameter (e.g. sequence number)
in the DIO message or a loop detected when a node located
inwards along the DAG forwards traffic outwards. Inconsistencies
are further detailed in <xref
target="TrickleInconsistencies"></xref>.</t>
<t>When a node enters a mode of consistent operation within a
DAG, i.e. DIO messages from its DAG parents are consistent and
no other inconsistencies are detected, it may begin to open up
the interval of the trickle timer towards a maximum value,
causing DIO messages to be emitted less frequently, thus
reducing network maintenance overhead and saving energy
consumption.</t>
<t>When a node is initialized, it MAY be configured to remain
silent and not multicast any DIO messages until it has
encountered and joined a DAG (perhaps initially probing for a
nearby DAG with an DIS message). Alternately, it may choose to
root its own floating DAG and begin multicasting DIO messages
using a default trickle configuration. The second case may be
advantageous if it is 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 DIO messages with the same DAGID, then they must
coordinate with each other to ensure that their DIO messages are
consistent when they emit DIO messages. In particular the Sequence
number must be identical from each DAG root, regardless of which of
the multiple DAG roots issues the DIO message, and changes to the
Sequence number should be issued at the same time. The specific
mechanism of this coordination, e.g. along a non-LLN network between
DAG roots, is beyond the scope of this specification.</t>
</section>
<section anchor="TrickleImplementation"
title="Trickle Timer for DIO 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 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 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 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 DIO message and makes
the decision to join it, it initializes the state of the trickle
timer by resetting the trickle timer and listening. Each time it
hears a consistent DIO message for this DAG from a DAG parent, it
MAY increment C.</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 DIO message and
multicasts it. When the communication interval I expires, the node
doubles the interval I so long as it has previously doubled it
fewer than I_doubling times, resets C, and chooses a new T
value.</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 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 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="DAGSequenceIncrement"
title="DAG Sequence Number Increment">
<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 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 iteration at a rate appropriate to the application and
implementation. Other automated mechanisms to determine
DAGSequenceNumber increments are also possible as appropriate to a
deployment.</t>
</section>
<section title="DAG Selection">
<t>The DAG selection is implementation and algorithm dependent. Nodes
SHOULD prefer to join DAGs for InstanceIDs advertising OCPs and
destinations compatible with their implementation specific objectives.
In order to limit erratic movements, and all metrics being equal,
nodes SHOULD keep their previous selection. Also, nodes SHOULD provide
a means to filter out a candidate parent whose availability is
detected as fluctuating, at least when more stable choices are
available.</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 OF in order to expose an exaggerated rank.</t>
</section>
<section anchor="DAGCollision" title="Collision">
<t>A race condition occurs if 2 nodes send DIO messages at the same
time and then attempt to join each other. This might happen, for
example, between nodes which act as DAG root of their own DAGs. In
order to detect the situation, LLN Nodes time stamp the sending of DIO
message. Any DIO message received within a short link-layer-dependent
period introduces a risk. It is up to the implementation to define the
duration of the risk window.</t>
<t>There is risk of a collision when a node receives and processes a
DIO within the risk window. For example, it may occur that two nodes
are associated with different DAGs and near-simultaneously send DIO
messages, which are received and processed by both, and possibly
result in both nodes simultaneously deciding to attach to each other.
As a remedy, in the face of a potential collision, as determined by
receiving a DIO within the risk window, the DIO message is not
processed. It is expected that subsequent DIOs would not cross.</t>
</section>
<section title="Guidelines for Objective Functions">
<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. The OF is also responsible to
compute the rank of the device within the DAG.</t>
<t>The Objective Function is specified in the DIO message within a
DAG Metric Container using an Objective Code Point (OCP), as
specified in <xref target="I-D.ietf-roll-routing-metrics" />, and
indicates the method that must be used to compute the DAG (e.g.
"minimize the path cost using the ETX metric and avoid `Blue'
links"). The Objective Code Points are specified in <xref
target="I-D.ietf-roll-routing-metrics" />. This document specifies
an Objective Function, OF0, in support of default operation. In the
case where the DIO does not include an OCP specification in the DAG
Metric Container, OF0 MAY be presumed.</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 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>An OF scans all the candidate neighbors on the possible
interfaces to check whether they can act as a router for a DAG.
There might be multiple of them and a candidate neighbor might
need to pass some validation tests before it can be used. In
particular, some link layers require experience on the activity
with a router to enable the router as a next hop.</t>
<t>An OF computes self's rank by adding the step of rank to that
candidate to the rank of that candidate. The step of rank is
computed by estimating the link as follows:</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 greater rank than self are
ignored</t>
<t>Candidate neighbors of an equal rank to self (siblings) are
ignored</t>
<t>Candidate neighbors of a lesser rank than self (non-siblings)
are preferred</t>
</list>
</list>
</section>
<section title="Objective Function 0 (OF0)">
<t>This document specifies a default objective function, called OF0,
indicated by an OCP value of 0x0000. OF0 is the default objective
function of RPL, and can be used if allowed by the policy of the
processing node when the OF indicated in the DIO message is unknown
to the node. If not allowed, then the DIO message is simply ignored
and not processed by the node. OF0 is notable in that it does not
use physical metrics as described in <xref
target="I-D.ietf-roll-routing-metrics"></xref>, but is only based on
abstract information from the DIO message such as rank and
administrative preference.</t>
<t>OF0 favors connectivity. That is, the Objective Function is
designed to find the nearest sink into a 'grounded' topology, and if
there is none then join any network per order of administrative
preference. The metric in use is the rank.</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 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 any administrative
preference associated with the interface 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 (DAGPreference) 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 preferred parent that was in use already is better.</t>
<t>A router that has announced a 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
along which inward routes toward the DAG root are set up.</t>
<t>Destination advertisement establishes 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 that 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 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 DAO messages for particular destinations move inwards along
the DAG, a sequence counter is used to guarantee their freshness.
The sequence counter is incremented by the source of the DAO
message (the node that owns the prefix, or learned the prefix via
some other means), each time it issues a DAO message for its
prefix. Nodes that receive the DAO message and, if scope allows,
will be forwarding a DAO message for the unmodified destination
inwards along the DAG, will leave the sequence number unchanged.
Intermediate nodes will check the sequence counter before
processing a DAO message, and if the DAO is unchanged (the
sequence counter has not changed), then the DAO message will be
discarded without additional processing. Further, if the DAO
message appears to be out of synch (the sequence counter is 2 or
more behind the present value) then the DAO state is considered to
be stale and may be purged, and the DAO message is discarded. A
depth is also added for tracking purposes; the depth is
incremented at each hop as the DAO message is propagated up the
DAG. Nodes that 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 DIO message
as indicated by the `D' bit, the node sends unicast destination
advertisements to one of its DA parents, that is selected as most
favored for incoming outwards traffic. The node only accepts
unicast destination advertisements from any nodes but those
contained in the DA parent subset.</t>
<t>Receiving a DIO message with the `D' destination advertisement
bit set from a DAG parent stimulates the sending of a delayed
destination advertisement back, with the collection of all known
prefixes (that is the prefixes learned via destination
advertisements for nodes lower in the DAG, and any connected
prefixes). If the Destination Advertisement Supported (A) bit is
set in the DIO message for the DAG, then a destination
advertisement is also sent to a DAG parent once it has been added
to the DA parent set after a movement, or when the list of
advertised prefixes has changed.</t>
<t>A node that modifies its DAG Parent set may set the `D' bit in
subsequent DIO propagation in order to trigger destination
advertisements to be updated to its DAG Parents and other inward
nodes on the DAG. Additional recommendations and guidelines
regarding the use of this mechanism are still under consideration
and will be elaborated in a future revision of this
specification.</t>
<t>Destination advertisements may advertise positive (prefix is
present) or negative (removed) DAO messages, termed as no-DAOs. A
no-DAO is stimulated by the disappearance of a prefix below. This
is discovered by timing out after a request (a DIO message) or by
receiving a no-DAO. A no-DAO is a conveyed as a DAO message with a
DAO Lifetime of ZERO_LIFETIME.</t>
<t>A node that is capable of recording the state information
conveyed in a unicast DAO message will do so upon receiving and
processing the DAO message, thus building up routing state
concerning destinations below it in the DAG. If a node capable of
recording state information receives a DAO message containing a
Reverse Route Stack, then the node knows that the DAO message has
traversed one or more nodes that did not retain any routing state
as it traversed the path from the DAO source to the node. The node
may then extract the Reverse Route Stack and retain the included
state in order to specify Source Routing instructions along the
return path towards the destination. The node MUST set the RRCount
back to zero and clear the Reverse Route Stack prior to passing
the DAO message information on.</t>
<t>A node that is unable to record the state information conveyed
in the DAO message will append the next-hop address to the Reverse
Route Stack, increment the RRCount, and then pass the destination
advertisement on without recording any additional state. In this
way the Reverse Route Stack will contain a vector of next hops
that must be traversed along the reverse path that the DAO message
has traveled. The vector will be ordered such that the node
closest to the destination will appear first in the list. In such
cases, if it is useful to the implementation to try and build up
redundant paths, the node may choose to convey the destination
advertisement to one or more DAG parents in order of preference as
guided by an implementation specific policy.</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
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 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 that is recording routing state will
keep track of the information from each neighbor independently,
and when it comes time to propagate the DAO message for a
particular prefix to the DA parents, then the DAO information will
be selected from among the advertising neighbors who offer the
least depth to the destination.</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 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
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 DelayDAO timer and the RemoveTimer.</t>
<t><list style="symbols">
<t>The DelayDAO timer is armed upon a stimulation to send a
destination advertisement (such as a DIO message from a DA
parent). When the timer is armed, all entries in the
Reachable list as well as all entries for Connected list are
set to not be reported yet for that particular DA
parent.</t>
<t>The DelayDAO timer has a duration that is DEF_DAO_LATENCY
divided by a multiple of the DAG rank of the node. The
intention is that nodes located deeper in the DAG should
have a shorter DelayDAO timer, allowing DAO messages a
chance to be reported from deeper in the DAG and potentially
aggregated along sub-DAGs before propagating further
inwards.</t>
<t>The RemoveTimer is used to clean up entries for which DAO
messages are no longer being received from the sub-DAG.</t>
<list style="symbols">
<t>When a 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 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, DAO messages with lifetime
0, i.e. no-DAOs, are sent to explicitly inform DA parents
that the entries which have reached the threshold are no
longer available, and the related routing states may be
propagated and cleaned up.</t>
</list>
<t>The RemoveTimer has a duration of min
(MAX_DESTROY_INTERVAL, TBD(DIO Trickle Timer Interval)).</t>
</list></t>
</section>
</section>
<section title="Multicast Destination Advertisement Messages">
<t>It is also possible for a node to multicast a DAO message to
the link-local scope all-nodes multicast address FF02::1. This
message will be received by all node listening in range of the
emitting node. The objective is to enable direct P2P
communication, between destinations directly supported by
neighboring nodes, without needing the RPL routing structure to
relay the packets.</t>
<t>A multicast DAO message MUST be used only to advertise
information about self, i.e. prefixes in the Connected list or
addresses owned by this node. This would typically be a multicast
group that this node is listening to or a global address owned by
this node, though it can be used to advertise any prefix owned by
this node as well. A multicast DAO message is not used for routing
and does not presume any DAG relationship between the emitter and
the receiver; it MUST NOT be used to relay information learned
(e.g. information in the Reachable list) from another node;
information obtained from a multicast DAO MAY be installed in the
routing table and MAY be propagated by a router in unicast
DAOs.</t>
<t>A node receiving a multicast DAO message addressed to FF02::1
MAY install prefixes contained in the DAO message in the routing
table for local use. Such a node MUST NOT perform any other
processing on the DAO message (i.e. such a node does not presume
it is a DA parent).</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 DIO message) in the Reachable and
Connected lists, as well as no-DAOs for all the entries in the
Unreachable list. Depending on its policy and ability to retain
routing state, the receiving node SHOULD keep a record of the
reported DAO message. If the DAO message offers the best route to
the prefix as determined by policy and other prefix records, the
node SHOULD install a route to the prefix reported in the DAO
message via the link local address of the reporting neighbor and
it SHOULD further propagate the information in a DAO message.</t>
<t>The DIO message from the DAG root is used to synchronize the
whole DAG, including the periodic reporting of destination
advertisements back up the DAG. Its period is expected to vary,
depending on the configuration of the trickle timer that governs
the RAs.</t>
<t>When a node receives a DIO message over an LLN interface from a
DA parent, the DelayDAO is armed to force a full update.</t>
<t>When the node broadcasts a 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
RemoveTimer is not running then it is armed with a jitter.</t>
</list></t>
<t>Since the DelayDAO timer has a duration that decreases with the
depth, it is expected to receive all DAO messages from all
children before the timer elapses and the full update is sent to
the DA parents.</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 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 DelayDAO 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 that is performing an aggregation, and a node
N that is to be a member of the aggregation group. A node Z
situated above the node M in the DAG, but not above node N, will
see the 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>
</section>
<section anchor="loopdetect" title="Loop Detection">
<t>RPL loop avoidance mechanisms are kept simple and designed to
minimize churn and states. Loops may form for a number of reasons,
from control packet loss to sibling forwarding. RPL includes a
reactive loop detection technique that protects from meltdown and
triggers repair of broken paths.</t>
<t>RPL loop detection uses information that is placed into the packet
in the flow label. It assumes that the flow label may be overloaded
for this purpose. The flow label is constructed as follows: <t>
<figure anchor="flowlabel" title="RPL Flow Label">
<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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|O|S|R|D| SenderRank | InstanceID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure>
</t></t>
<t><list hangIndent="6" style="hanging">
<t hangText="Outwards 'O' bit:">1-bit flag indicating whether the
packet is expected to progress inwards or outwards. A router sets
the 'O' bit when the packet is expect to progress outwards (using
DAO routes), and resets it when forwarding towards the root of the
DAG. A host MUST set the bit to 0.</t>
<t hangText="Sibling 'S' bit:">1-bit flag indicating whether the
packet has been forwarded via a sibling at the present rank, and
denotes a risk of a sibling loop. A host sets the bit to 0.</t>
<t hangText="Rank-Error 'R' bit:">1-bit flag indicating whether a
rank error was detected. A rank error is detected when there is a
mismatch in the relative ranks and the direction as indicated in
the 'O' bit. A host MUST set the bit to 0.</t>
<t hangText="DAO-Error 'D' bit:">1-bit flag indicating whether a
DAO error was detected. An undetected DAO error would have
resulted in an inward to outward transition that is not expected
with this spec. A host MUST set the bit to 0.</t>
<t hangText="SenderRank:">8-bit field indicating the rank of the
sender. A host MUST set the rank to INFINITE_RANK. A router MUST
place its own rank in the flow label when forwarding.</t>
<t hangText="InstanceID:">8-bit field indicating the DAG instance
along which the packet is sent.</t>
</list></t>
<section title="Host Basic Operation">
<t>It is expected that a host that does not participate to RPL in
any fashion is configured to set the flow label to all zeroes in its
outgoing packets. The host MAY send a packet to any router
regardless of the DAG and RPL operations at large.</t>
<t>A host that participates to RPL SHOULD zero out all the flags,
and it MUST set the sender rank to INFINITE_RANK. If the host can
map a flow to a given InstanceID then it MUST set the flow label
accordingly. Forwarding rules are the same for this host and a
router, and are described in the next section.</t>
</section>
<section title="Instance Forwarding">
<t>Instance IDs is used to avoid loops between DAGs from different
origins. DAGs that constructed for antagonistic constraints might
contain paths that, if mixed together, would yield loops. Those
loops are avoided by forwarding a packet along the DAG that is
associated to a given instance.</t>
<t>The InstanceID is placed by the source in the flow label. It is
not meaningful if the packet has the flow label set to all zeroes.
Otherwise it MUST match the DAG instance onto which the packet is
placed by any node, be it a host or router.</t>
<t>When a router receives a packet that is flagged with a given
instance ID and the node can forward the packet along the DAG
associated to that instance, then the router MUST do so and leave
the instance ID flag unchanged.</t>
<t>If any node can not forward a packet along the DAG associated to
the instance ID in the flow label, then the node MAY either change
the InstanceID to match a DAG that it is using for this packet or
discard the packet. That decision is based on a policy.</t>
<t>The default policy is as follows: if the node can forward along
the DAG associated to the instance RPL_DEFAULT_INSTANCE then it
should do so. Otherwise it should drop the packet.</t>
</section>
<section title="DAG Inconsistency Loop Detection">
<t>The DAG is inconsistent is the direction of a packet does not
match the rank relationship. A receiver detects an inconsistency if
it receives a packet with either: <list>
<t>the 'O' bit set (to outwards) from a node of a higher
rank.</t>
<t>the 'O' bit reset (for inwards) from a node of a lesser
rank.</t>
<t>the 'S' bit set (to sibling) from a node of a different
rank.</t>
</list></t>
<t>The propagation of a new sequence creates local inconsistencies.
In particular, it is possible for a router to forward a packet to a
future parent (same instance, same DAGID, higher sequence) without a
loop, regardless of the rank of that parent. In that case, the
sending router MUST present itself as a host on the future DAG and
use a rank of INFINITE_RANK as it forwards the packets via a future
parent to avoid a false positive.</t>
<t>One inconsistency along the path is not considered as a critical
error and the packet may continue. But a second detection along the
path of a same packet should not occur and the packet is
dropped.</t>
<t>This process is controlled by the Rank-Error bit in the Flow
Label. When an inconsistency, is detected on a packet, if the
Rank-Error bit was not set then the Rank-Error bit is set. If it was
set the packet is discarded and the trickle timer is reset.</t>
</section>
<section title="Sibling Loop Avoidance">
<t>When a packet is forwarded along siblings, it cannot be checked
for forward progress and may loop between siblings. Experimental
evidence has shown that one sibling hop can be very useful but is
generally sufficient to avoid loops. Based on that evidence, this
specification enforces the simple rule that a packet may not make 2
sibling hops in a row.</t>
<t>When a host issues a packet or when a router forwards a packet to
a non sibling, the Sibling bit in the packet must be reset. When a
router forwards to a sibling: if the Sibling bit was not set then
the Sibling bit is set. If the Sibling bit was set then the packet
is discarded. This does not denote a graph inconsistency but
indicates that a new graph should probably be formed with a new
sequence.</t>
</section>
<section title="DAO Inconsistency Loop Detection and Recovery">
<t>A DAO inconsistency happens when router that has an outwards DAO
route via a child that is a remnant from an obsolete state that is
not matched in the child. With DAO inconsistency loop recovery, a
packet can be used to recursively explore and cleanup the obsolete
DAO states along a sub-DAG.</t>
<t>In a general manner, a packet that goes outwards should never go
inwards again. So rather than routing inwards a packet with the
Outwards bit set, the router MUST discard the packet. If DAO
inconsistency loop recovery is applied, then the router SHOULD send
the packet to the parent that passed it with the DAO-Error bit
set.</t>
<t>Upon a packet with a DAO bit set, the parent MUST remove the
routing states that caused forwarding to that child, clear DAO-Error
bit and send the packet again. The packet will make its way either
to an alternate child or inwards to a parent. If that parent still
has an inconsistent DAO state via self, the process will recurse and
that state will be cleaned up as well.</t>
</section>
</section>
<section title="Multicast Operation">
<t>This section describes further the multicast routing operations
over an IPv6 RPL network, and specifically how unicast DAOs can be
used to relay group registrations inwards. Wherever the following text
mentions MLD, one can read MLDv2 or v3.</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 DAO messages within the RPL
protocol; each hop coalesces the multiple requests for a same group as
a single DAO message to the parent(s), in a fashion similar to proxy
IGMP, but recursively between child router and parent up to the
root.</t>
<t>A router might select to pass a listener registration DAO message
to its preferred parent only, in which case multicast packets coming
back might be lost for all of its sub-DAG if the transmission fails
over that link. Alternatively the router might select to copy
additional parents as it would do for DAO messages advertising unicast
destinations, in which case there might be duplicates that the router
will need to prune.</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 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 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 Rendezvous
Point for the RPL network, and as source towards the Internet for all
multicast flows started in the RPL LLN. So regardless of whether the
root is actually attached to the Internet, and regardless of whether
the DAG is grounded or floating, the root can serve inner multicast
streams at all times.</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>In the scope of this specification, it is preferred to select a
successor from a DAG that matches the InstanceID marked in the
IPv6 header of the packet being forwarded.</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 parent offering a default route then select
that DAG parent 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 that was
the predecessor of the packet (split horizon), except in the case
where it is intended for the packet to change from an inward to an
outward flow, such as switching from DIO routes to DAO routes as the
destination is neared.</t>
</section>
</section>
<section title="RPL Constants and Variables">
<t><list hangIndent="6" style="hanging">
<t hangText="ZERO_LIFETIME">This is the special value of a lifetime
that indicates immediate death and removal. ZERO_LIFETIME has a
value of 0.</t>
<t hangText="BASE_RANK">This is the rank for a virtual root that
might be used to coordinate multiple roots. BASE_RANK has a value of
0.</t>
<t hangText="ROOT_RANK">This is the rank for a DAG root. ROOT_RANK
has a value of 1.</t>
<t hangText="INFINITE_RANK">This is the constant maximum for the
rank. INFINITE_RANK has a value of 0xFF.</t>
<t hangText="RPL_DEFAULT_INSTANCE">This is the instance ID that is
used by this protocol by a node without a policy to know any better.
RPL_DEFAULT_INSTANCE has a value of 0.</t>
<t hangText="DEFAULT_DIO_INTERVAL_MIN">To be determined</t>
<t hangText="DEFAULT_DIO_INTERVAL_DOUBLINGS">To be determined</t>
<t hangText="DEF_DAO_LATENCY">To be determined</t>
<t hangText="MAX_DESTROY_INTERVAL">To be determined</t>
<t hangText="DIO Timer">One instance per DAG that a node is a member
of. Expiry triggers DIO message transmission. Trickle timer with
variable interval in [0, DIOIntervalMin..2^DIOIntervalDoublings].
See <xref target="TrickleImplementation"></xref></t>
<t hangText="DAG Sequence Number Increment Timer">Up to one instance
per DAG that the node is acting as DAG root of. May not be supported
in all implementations. Expiry triggers revision of
DAGSequenceNumber, causing a new series of updated DIO message to be
sent. Interval should be chosen appropriate to propagation time of
DAG and as appropriate to application requirements (e.g. response
time vs. overhead). See <xref
target="DAGSequenceIncrement"></xref></t>
<t hangText="DelayDAO Timer">Up to one instance per DA parent (the
subset of DAG parents chosen to receive destination advertisements)
per DAG. Expiry triggers sending of DAO message to the DA parent.
The interval is to be proportional to DEF_DAO_LATENCY/(node rank),
such that nodes of greater rank (further outward along the DAG)
expire first, coordinating the sending of DAO messages to allow for
a chance of aggregation. See <xref target="DATimers"></xref></t>
<t hangText="RemoveTimer">Up to one instance per DA entry per
neighbor (i.e. those neighbors that have given DAO messages to this
node as a DAG parent) Expiry triggers a change in state for the DA
entry, setting up to do unreachable (No-DAO) advertisements or
immediately deallocating the DA entry if there are no DA parents.
The interval is min(MAX_DESTROY_INTERVAL, TBD(DIO Trickle Timer
Interval)). See <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 DIO message until it has joined a
DAG, or to immediately root a transient DAG and start sending
multicast DIO messages. A RPL implementation SHOULD allow
configuring whether the node should stay silent or should start
advertising DIO messages.</t>
<t>Furthermore, the implementation SHOULD to allow configuring
whether or not the node should start sending an DIS message as an
initial probe for nearby DAGs, or should simply wait until it
received RA messages from other nodes that are part of existing
DAGs.</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>
<?rfc subcompact="yes"?>
<t><list hangIndent="6" style="hanging">
<t hangText="DAGPreference"></t>
<t hangText="InstanceID"></t>
<t hangText="DAGObjectiveCodePoint"></t>
<t hangText="DAGID"></t>
<t hangText="Destination Prefixes"></t>
<t hangText="DIOIntervalDoublings"></t>
<t hangText="DIOIntervalMin"></t>
<t></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></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>
<?rfc subcompact="no"?>
</section>
<section title="Trickle Timers">
<t>A RPL implementation makes use of trickle timer to govern the
sending of DIO message. Such an algorithm is determined a by a set
of configurable parameters that are then advertised by the DAG root
along the DAG in DIO messages.</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 Sequence Number Increment">
<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="Destination Advertisement Timers">
<t>The following set of parameters of the DAO messages SHOULD be
configurable:</t>
<t><list style="symbols">
<t>The DelayDAO 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 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 DAG parents</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 DAG Parents</t>
<t>timer to govern the sending of DIO messages for the DAG</t>
<t>DAGSequenceNumber</t>
</list></t>
<t>The set of 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 DAG Parent</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>For each route provisioned by RPL operation, a RPL implementation
MUST keep track of the following:</t>
<t><list style="symbols">
<t>Destination Prefix</t>
<t>Destination Prefix Length</t>
<t>Lifetime Timer</t>
<t>Next Hop</t>
<t>Next Hop Interface</t>
<t>Flag indicating that the route was provisioned from one
of:<list>
<t>Unicast DAO message</t>
<t>DIO message</t>
<t>Multicast DAO message</t>
</list></t>
</list></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 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="RPL Control Message">
<t>The RPL Control Message is an ICMP information message type that is
to be used carry DAG Information Objects, DAG Information
Solicitations, and Destination Advertisement Objects in support of RPL
operation.</t>
<t>IANA has defined a ICMPv6 Type Number Registry. The suggested type
value for the RPL Control Message is 155, to be confirmed by IANA.</t>
</section>
<section title="New Registry for RPL Control Codes">
<t>IANA is requested to create a registry, RPL Control Codes, for the
Code field of the ICMPv6 RPL Control Message.</t>
<t>New codes may be allocated only by an IETF Consensus action. Each
code should be tracked with the following qualities:</t>
<t><list style="symbols">
<t>Code</t>
<t>Description</t>
<t>Defining RFC</t>
</list></t>
<t>Three codes are currently defined:</t>
<texttable title="RPL Control Codes">
<ttcol align="center">Code</ttcol>
<ttcol align="left">Description</ttcol>
<ttcol align="left">Reference</ttcol>
<c>0x01</c>
<c>DAG Information Solicitation</c>
<c>This document</c>
<c>0x02</c>
<c>DAG Information Object</c>
<c>This document</c>
<c>0x04</c>
<c>Destination Advertisement Object</c>
<c>This document</c>
</texttable>
</section>
<section title="New Registry for the Control Field of the DIO Base Option">
<t>IANA is requested to create a registry for the Control 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>Four groups 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>
<c>5,6,7</c>
<c>DAG Preference</c>
<c>This document</c>
</texttable>
</section>
<section title="DAG Information Object (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>
<c>4</c>
<c>DAG Timer Configuration</c>
<c>This Document</c>
</texttable>
</section>
<section title="Objective Code Point for the Default Objective Function OF0">
<t>This specification specifies the Default Objective Function (called
OF0) for which the OCP field of the OF object, as defined in <xref
target="I-D.ietf-roll-routing-metrics"></xref>, is equal to 0x0000</t>
<texttable title="OCP Allocation">
<ttcol align="center">Value</ttcol>
<ttcol align="left">Meaning</ttcol>
<ttcol align="left">Reference</ttcol>
<c>0</c>
<c>OF0</c>
<c>This document</c>
</texttable>
</section>
</section>
<section anchor="Acknowledgements" title="Acknowledgements">
<t>The authors would like to acknowledge the review, feedback, and
comments from Emmanuel Baccelli, Dominique Barthel, Yusuf Bashir,
Mathilde Durvy, Manhar Goindi, Mukul Goyal, Anders Jagd, Quentin Lampin,
Jerry Martocci, Alexandru Petrescu, and Don Sturek.</t>
<t>The authors would like to acknowledge the guidance and input provided
by the ROLL Chairs, David Culler and JP Vasseur.</t>
<t>The authors would like to acknowledge prior contributions of Robert
Assimiti, Mischa Dohler, Julien Abeille, Ryuji Wakikawa, Teco Boot,
Patrick Wetterwald, Bryan Mclaughlin, Carlos J. Bernardos, Thomas
Watteyne, Zach Shelby, Caroline Bontoux, Marco Molteni, Billy Moon, and
Arsalan Tavakoli, which have provided useful design considerations to
RPL.</t>
</section>
<section title="Contributors">
<t>RPL is the result of the contribution of the following members of the
ROLL Design Team, including the editors, and additional contributors as
listed below:</t>
<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
Philip Levis
Stanford University
358 Gates Hall, Stanford University
Stanford, CA 94305-9030
USA
Email: pal@cs.stanford.edu
Stephen Dawson-Haggerty
UC Berkeley
Soda Hall, UC Berkeley
Berkeley, CA 94720
USA
Email: stevedh@cs.berkeley.edu
Kris Pister
Dust Networks
30695 Huntwood Ave.
Hayward, 94544
USA
Email: kpister@dustnetworks.com
Anders Brandt
Zensys, Inc.
Emdrupvej 26
Copenhagen, DK-2100
Denmark
Email: abr@zen-sys.com
]]></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.RFC.5673'?>
<?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.4443"?>
<?rfc include="reference.RFC.4861"?>
<?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="Requirements">
<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).
Conceptually, multiple instances of RPL can be used to send traffic
along different topology instances, the construction of which is
governed by different Objective Functions (OF). Details of RPL
operation in support of multiple instances are beyond the scope of
the present specification.</t>
</section>
<section title="Constraint Based Routing">
<t>The RPL design supports constraint based routing, based on a set
of routing metrics and constraints. The routing metrics and
constraints for links and nodes with capabilities supported by RPL
are specified in a companion document to this specification, <xref
target="I-D.ietf-roll-routing-metrics"></xref>. RPL signals the
metrics, constraints, and related Objective Functions (OFs) in use
in a particular implementation by means of an Objective Code Point
(OCP). Both the routing metrics, constraints, and the OF help
determine the construction of the Directed Acyclic Graphs (DAG)
using a distributed path computation algorithm.</t>
</section>
</section>
<section 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>
<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 DIO messages, as governed
by a trickle timer. Nodes (11), (12), (13), have selected (LBR) as their
only parent, attached to the DAG at rank 2, and periodically multicast
DIOs. Node (22) has selected (11) and (12) in its DAG parent set, and
advertises itself at rank 3. Node (22) thus has a set of DAG parents
{(11), (12)} and siblings {((21), (23)}.</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="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 / expose infinite rank</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 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 DAO message to Node (42), indicating the
availability of destination (53).</t>
<t>Node (54) and Node (55) would similarly send 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 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 DAO message and passes
it on to Node (22) as (42'):[(42)]. It may send a separate 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 DAO message and passes
it on to Node (12) as (42'):[(42), (32)]. It also relays the DAO
message containing destination (32) to Node 12 as (32):[(32)], and
finally may send a DAO message for itself indicating destination
(22).</t>
<t>Node (12) is capable to maintain routing state again, and
receives the 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 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 RPL DIS messages to probe for nearby DAGs.</t>
<t><list style="symbols">
<t>Node (N) transmits a RPL DIS message.</t>
<t>Node (B) responds. Node (N) investigates the DIO message, and
learns that Node (B) is a member of DAGID 1 at rank 4, and not
`Blue'. Node (N) takes note of this, but is not yet confident.</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 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 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)
may become the root of its own floating, less preferred, DAG.</t>
<t>Node (D), hearing a modified 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 DIO message with an incremented
DAGSequenceNumber from Node (B) and determines it is able to
rejoin the grounded DAG by reattaching at a deeper rank to Node
(B). Node (C) adds Node (B) to its DAG parent set. Node (C) has
now safely moved deeper within the grounded DAG without creating
any loops.</t>
<t>Node (D), and any other sub-DAG of Node (C), will hear the
modified DIO message sourced from Node (C) and follow Node (C) in
a coordinated manner to reattach to the grounded DAG. The final
DAG is depicted in <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 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 DIO messages from nodes
deeper than themselves (because such nodes are possibly in
their own sub-DAGs)</t>
</list></t>
</list></t>
</section>
</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 DAO messages are relayed to more than one DAG parent, in some
cases a situation may be created where a large number of DAO messages
conveying information about the same destination flow inward along the
DAG. It is desirable to bound/limit the multiplication/fan-out of DAO
messages in this manner. Some aspects of the Destination Advertisement
mechanism remain under investigation, such as behavior in the face of
links that may not be symmetric.</t>
<t>In general, the utility of providing redundancy along outwards
routes by sending DAO messages to more than one parent is under
investigation.</t>
<t>The use of suitable triggers, such as the `D' bit, to trigger DA
operation within an affected sub-DAG, is under investigation. Further,
the ability to limit scope of the affected depth within the sub-DAG is
under investigation (e.g. if a stateful node can proxy for all nodes
`behind' it, then there may be no need to propagate the triggered `D'
bit further).</t>
</section>
<section title="Source Routing">
<t>In support of nodes that maintain minimal routing state, and to
make use of the collection of piecewise source routes from the
destination advertisement mechanism, there needs to be some
investigation of a mechanism to specify, attach, and follow source
routes for packets traversing the LLN.</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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