One document matched: draft-ietf-roll-rpl-06.xml
<?xml version="1.0" encoding="US-ASCII"?>
<!DOCTYPE rfc SYSTEM "rfc2629.dtd">
<?rfc toc="yes"?>
<?rfc tocompact="yes"?>
<?rfc tocdepth="3"?>
<?rfc tocindent="yes"?>
<?rfc symrefs="yes"?>
<?rfc sortrefs="yes"?>
<?rfc comments="yes"?>
<?rfc inline="yes"?>
<?rfc compact="yes"?>
<?rfc subcompact="no"?>
<?rfc authorship="yes"?>
<?rfc tocappendix="yes"?>
<rfc category="std" docName="draft-ietf-roll-rpl-06" ipr="trust200902">
<front>
<title abbrev="draft-ietf-roll-rpl-06">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="03" month="February" year="2010" />
<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) consist of largely of constrained
nodes (with limited processing power, memory, and sometimes energy when
they are battery operated). These routers are interconnected by lossy
links, typically supporting only low data rates, that are usually
unstable with relatively low packet delivery rates. Another
characteristic of such networks is that the traffic patterns are not
simply unicast, but in many cases point-to-multipoint or
multipoint-to-point. Furthermore such networks may potentially comprise
up to thousands of nodes. These characteristics offer unique challenges
to a routing solution: the IETF ROLL Working Group has defined
application-specific routing requirements for a Low power and Lossy
Network (LLN) routing protocol, specified in <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. As the RPL design evolves optional
features may be added to address some application specific
requirements. This is a key protocol design decision providing a
granular approach in order to restrict the core of the protocol to a
minimal set of functionalities, and to allow each implementation of
the protocol to be optimized differently. All "MUST" application
requirements that cannot be satisfied by RPL will be specifically
listed in the Appendix A, accompanied by a justification.</t>
<t>A network may run multiple instances of RPL concurrently. Each such
instance may serve different and potentially antagonistic constraints
or performance criteria. This document defines how a single instance
operates.</t>
<t>RPL is a generic protocol that is to be deployed by instantiating
the generic operation described in this document with a specific
objective function (OF) (which ties together metrics, constraints, and
an optimization objective) to realize a desired objective in a given
environment.</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>RPL does not rely on any particular features of a specific link
layer technology. RPL is designed to be able to operate over a variety
of different link layers, including but not limited to, low power
wireless or PLC (Power Line Communication) technologies.</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>Additionally, this document uses terminology from <xref
target="I-D.ietf-roll-terminology"></xref>, and introduces the following
terminology: <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. All edges are contained in paths oriented toward and
terminating at one or more root nodes.</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="Destination Oriented DAG (DODAG):">A DAG rooted at a
single destination, i.e. at a single DAG root (the DODAG root) with
no outgoing edges.</t>
<t hangText="DODAG root:">A DODAG root is the DAG root of a
DODAG.</t>
<t hangText="Rank:">The rank of a node in a DAG identifies the nodes
position with respect to a DODAG root. The farther away a node is
from a DODAG root, the higher is the rank of that node. The rank of
a node may be a simple topological distance, or may more commonly be
calculated as a function of other properties as described later.</t>
<t hangText="DODAG parent:">A parent of a node within a DODAG is one
of the immediate successors of the node on a path towards the DODAG
root. The DODAG parent of a node will have a lower rank than the
node itself. (See <xref target="RankComparison"></xref>).</t>
<t hangText="DODAG sibling:">A sibling of a node within a DODAG is
defined in this specification to be any neighboring node which is
located at the same rank within a DODAG. Note that siblings defined
in this manner do not necessarily share a common DODAG parent. (See
<xref target="RankComparison"></xref>).</t>
<t hangText="Sub-DODAG">The sub-DODAG of a node is the set of other
nodes in the DODAG that might use a path towards the DODAG root that
contains that node. Nodes in the sub-DODAG of a node have a greater
rank than that node itself (although not all nodes of greater rank
are necessarily in the sub-DODAG of that node). (See <xref
target="RankComparison"></xref>).</t>
<t hangText="DODAGID:">The identifier of a DODAG root. The DODAGID
must be unique within the scope of a RPL Instance in the LLN.</t>
<t hangText="DODAG Iteration:">A specific sequence number iteration
("version") of a DODAG with a given DODAGID.</t>
<t hangText="RPL Instance:">A set of possibly multiple DODAGs. A
network may have more than one RPL Instance, and a RPL node can
participate in multiple RPL Instances. Each RPL Instance operates
independently of other RPL Instances. This document describes
operation within a single RPL Instance. In RPL, a node can belong to
at most one DODAG per RPL Instance. The tuple (RPLInstanceID,
DODAGID) uniquely identifies a DODAG.</t>
<t hangText="RPLInstanceID:">Unique identifier of a RPL
Instance.</t>
<t hangText="DODAGSequenceNumber:">A sequential counter that is
incremented by the root to form a new Iteration of a DODAG. A DODAG
Iteration is identified uniquely by the (RPLInstanceID, DODAGID,
DODAGSequenceNumber) tuple.</t>
<t hangText="Up:">Up refers to the direction from leaf nodes towards
DODAG roots, following the orientation of the edges within the
DODAG.</t>
<t hangText="Down:">Down refers to the direction from DODAG roots
towards leaf nodes, going against the orientation of the edges
within the DODAG.</t>
<t hangText="Objective Code Point (OCP):">An identifier, used to
indicate which Objective Function is in use for forming a DODAG. The
Objective Code Point is further described in <xref
target="I-D.ietf-roll-routing-metrics"></xref>.</t>
<t hangText="Objective Function (OF):">Defines which routing
metrics, optimization objectives, and related functions are in use
in a DODAG. The Objective Function is further described in <xref
target="I-D.ietf-roll-routing-metrics"></xref>.</t>
<t hangText="Goal:">The Goal is a host or set of hosts that satisfy
a particular application objective / OF. Whether or not a DODAG can
provide connectivity to a goal is a property of the DODAG. For
example, a goal might be a host serving as a data collection point,
or a gateway providing connectivity to an external
infrastructure.</t>
<t hangText="Grounded:">A DODAG is said to be grounded, when the
root can reach the Goal of the objective function.</t>
<t hangText="Floating:">A DODAG is floating if is not Grounded. A
floating DODAG is not expected to reach the Goal defined for the
OF.</t>
</list></t>
<t>As they form networks, LLN devices often mix the roles of 'host' and
'router' when compared to traditional IP networks. In this document,
'host' refers to an LLN device that can generate but does not forward
RPL traffic, 'router' refers to an LLN device that can forward as well
as generate RPL traffic, and 'node' refers to any RPL device, either a
host or a router.</t>
</section>
<section anchor="ProtocolModel" title="Protocol Overview">
<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="UpwardTopology" title="Topology">
<t>This section describes how the basic RPL topologies, and the rules
by which these are constructed, i.e. the rules governing DODAG
formation.</t>
<section anchor="TopologyIdentifiers" title="Topology Identifiers">
<t>RPL uses four identifiers to track and control the topology:
<list style="symbols">
<t>The first is a RPLInstanceID. A RPLInstanceID identifies a
set of one or more DODAGs. All DODAGs in the same RPL Instance
use the same OF. A network may have multiple RPLInstanceIDs,
each of which defines an independent set of DODAGs, which may be
optimized for different OFs and/or applications. The set of
DODAGs identified by a RPLInstanceID is called a RPL
Instance.</t>
<t>The second is a DODAGID. The scope of a DODAGID is a RPL
Instance. The combination of RPLInstanceID and DODAGID uniquely
identifies a single DODAG in the network. A RPL Instance may
have multiple DODAGs, each of which has an unique DODAGID.</t>
<t>The third is a DODAGSequenceNumber. The scope of a
DODAGSequenceNumber is a DODAG. A DODAG is sometimes
reconstructed from the DODAG root, by incrementing the
DODAGSequenceNumber. The combination of RPLInstanceID, DODAGID,
and DODAGSequenceNumber uniquely identifies a DODAG
Iteration.</t>
<t>The fourth is rank. The scope of rank is a DODAG Iteration.
Rank establishes a partial order over a DODAG Iteration,
defining individual node positions with respect to the DODAG
root.</t>
</list></t>
</section>
<section title="DODAG Information">
<t>For each DODAG that a node is, or may become, a member of, the
implementation should conceptually keep track of the following
information for each DODAG. The data structures described in this
section are intended to illustrate a possible implementation to aid
in the description of the protocol, but are not intended to be
normative.</t>
<t><list style="symbols">
<t>RPLInstanceID</t>
<t>DODAGID</t>
<t>DODAGSequenceNumber</t>
<t>DAG Metric Container, including DAGObjectiveCodePoint</t>
<t>A set of Destination Prefixes offered by the DODAG root and
available via paths upwards along the DODAG</t>
<t>A set of DODAG parents</t>
<t>A set of DODAG siblings</t>
<t>A timer to govern the sending of DIO messages</t>
</list></t>
</section>
</section>
<section title="Instances, DODAGs, and DODAG Iterations">
<t>Each RPL Instance constructs a routing topology optimized for a
certain Objective Function (OF). A RPL Instance may provide routes to
certain destination prefixes, reachable via the DODAG roots. A single
RPL Instance contains one or more Destination Oriented DAG (DODAG)
roots. These roots may operate independently, or may coordinate over a
non-LLN backchannel.</t>
<t>Each root has a unique identifier, the DODAGID<!--, such that nodes can
identify the DODAG root-->.</t>
<t>A RPL Instance may comprise:</t>
<t><list style="symbols">
<t>a single DODAG with a single root <list>
<t>For example, a DODAG optimized to minimize latency rooted
at a single centralized lighting controller in a home
automation application.</t>
</list></t>
<t>multiple uncoordinated DODAGs with independent roots (differing
DODAGIDs) <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 DODAG, and
further use the formation of multiple DODAGs as a means to
dynamically and autonomously partition the network.</t>
</list></t>
<t>a single DODAG with a single virtual root coordinating LLN
sinks (with the same DODAGID) over some non-LLN 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 DODAG.</t>
</list></t>
<t>a combination of one of the above as suited to some application
scenario.</t>
</list></t>
<t>Traffic is bound to a specific RPL Instance by a marking in the
flow label of the IPv6 header. Traffic originating in support of a
particular application may be tagged to follow an appropriate RPL
instance which enables certain (path) properties, for example to
follow paths optimized for low latency or low energy. The provisioning
or automated discovery of a mapping between a RPLInstanceID and a type
or service of application traffic is beyond the scope of this
specification.</t>
<t>An example of a RPL Instance comprising a number of DODAGs is
depicted in <xref target="figInstance"></xref>. A DODAG Iteration (two
"versions" of the same DODAG) is depicted in <xref
target="figDODAGIteration"></xref>.</t>
<figure anchor="figInstance" title="RPL Instance">
<artwork><![CDATA[
+----------------------------------------------------------------+
| |
| +--------------+ |
| | | |
| | (R1) | (R2) (Rn) |
| | / \ | /| \ / | \ |
| | / \ | / | \ / | \ |
| | (A) (B) | (C) | (D) ... (F) (G) (H) |
| | /|\ |\ | / | |\ | | | |
| | : : : : : | : (E) : : : : : |
| | | / \ |
| +--------------+ : : |
| DODAG |
| |
+----------------------------------------------------------------+
RPL Instance
]]></artwork>
</figure>
<figure anchor="figDODAGIteration" title="DODAG Iteration">
<artwork><![CDATA[
+----------------+ +----------------+
| | | |
| (R1) | | (R1) |
| / \ | | / |
| / \ | | / |
| (A) (B) | \ | (A) |
| /|\ |\ | ------\ | /|\ |
| : : (C) : : | \ | : : (C) |
| | / | \ |
| | ------/ | \ |
| | / | (B) |
| | | |\ |
| | | : : |
| | | |
+----------------+ +----------------+
Sequence N Sequence N+1
]]></artwork>
</figure>
</section>
<section title="Traffic Flows">
<section title="Multipoint-to-Point Traffic">
<t>Multipoint-to-Point (MP2P) is a dominant traffic flow in many LLN
applications (<xref
target="I-D.ietf-roll-building-routing-reqs"></xref>, <xref
target="I-D.ietf-roll-home-routing-reqs"></xref>, <xref
target="RFC5673"></xref>, <xref target="RFC5548"></xref>). The
destinations of MP2P flows are designated nodes that have some
application significance, such as providing connectivity to the
larger Internet or core private IP network. RPL supports MP2P
traffic by allowing MP2P destinations to be reached via DODAG
roots.</t>
</section>
<section title="Point-to-Multipoint Traffic">
<t>Point-to-multipoint (P2MP) is a traffic pattern required by
several LLN applications (<xref
target="I-D.ietf-roll-building-routing-reqs"></xref>, <xref
target="I-D.ietf-roll-home-routing-reqs"></xref>, <xref
target="RFC5673"></xref>, <xref target="RFC5548"></xref>). RPL
supports P2MP traffic by using a destination advertisement mechanism
that provisions routes toward destination prefixes and away from
roots. Destination advertisements can update routing tables as the
underlying DODAG topology changes.</t>
</section>
<section title="Point-to-Point Traffic">
<t>RPL DODAGs provide a basic structure for point-to-point (P2P)
traffic. For a RPL network to support P2P traffic, a root must be
able to route packets to a destination. Nodes within the network may
also have routing tables to destinations. A packet flows towards a
root until it reaches an ancestor that has a known route to the
destination.</t>
<t>RPL also supports the case where a P2P destination is a 'one-hop'
neighbor.</t>
<t>RPL neither specifies nor precludes additional mechanisms for
computing and installing more optimal routes to support arbitrary
P2P traffic.</t>
</section>
</section>
<section title="Upward Routes and DODAG Construction">
<t>RPL provisions routes up towards DODAG roots, forming a DODAG
optimized according to the Objective Function (OF) in use. RPL nodes
construct and maintain these DODAGs through exchange of DODAG
Information Object (DIO) messages. Undirected links between siblings
are also identified during this process, which can be used to provide
additional diversity.</t>
<section title="DAG Information Object (DIO)">
<t>A DIO identifies the RPL Instance, the DODAGID, the values used
to compute the RPL Instance's objective function, and the present
DODAG Sequence Number. It can also include additional routing and
configuration information. The DIO includes a measure derived from
the position of the node within the DODAG, the rank, which is used
for nodes to determine their positions relative to each other and to
inform loop avoidance/detection procedures. RPL exchanges DIO
messages to establish and maintain routes.</t>
<t>RPL adapts the rate at which nodes send DIO messages. When a
DODAG is detected to be inconsistent or needs repair, RPL sends DIO
messages more frequently. As the DODAG stabilizes, the DIO message
rate tapers off, reducing the maintenance cost of a steady and
well-working DODAG.</t>
<t>This document defines an ICMPv6 Message Type "RPL Control
Message", which is capable of carrying a DIO.</t>
</section>
<section title="DAG Repair">
<t>RPL supports global repair over the DODAG. A DODAG Root may
increment the DODAG Sequence Number, thereby initiating a new DODAG
iteration. This institutes a global repair operation, revising the
DODAG and allowing nodes to choose an arbitrary new position within
the new DODAG iteration.</t>
<t>RPL supports mechanisms which may be used for local repair within
the DODAG iteration. The DIO message specifies the necessary
parameters as configured from the DODAG root. Local repair options
include the allowing a node, upon detecting a loss of connectivity
to a DODAG it is a member of, to:</t>
<t><list style="symbols">
<t>Poison its sub-DODAG by advertising an effective rank of
INFINITY to its sub-DODAG, OR detach and form a floating DODAG
in order to preserve inner connectivity within its
sub-DODAG.</t>
<t>Move down within the DODAG iteration (i.e. increase its rank)
in a limited manner, no further than a bound configured by the
DODAG root via the DIO so as not to count all the way to
infinity. Such a move may be undertaken after waiting an
appropriate poisoning interval, and should allow the node to
restore connectivity to the DODAG Iteration, if at all
possible.</t>
</list></t>
</section>
<section title="Grounded and Floating DODAGs">
<t>DODAGs can be grounded or floating. A grounded DODAG offers
connectivity to to a goal. A floating DODAG offers no such
connectivity, and provides routes only to nodes within the DODAG.
Floating DODAGs may be used, for example, to preserve inner
connectivity during repair.</t>
</section>
<section title="Administrative Preference">
<t>An implementation/deployment may specify that some DODAG roots
should be used over others through an administrative preference.
Administrative preference offers a way to control traffic and
engineer DODAG formation in order to better support application
requirements or needs.</t>
</section>
<section title="Objective Function (OF)">
<t>The Objective Function (OF) implements the optimization
objectives of route selection within the RPL Instance. The OF is
identified by an Objective Code Point (OCP) within the DIO, and its
specification also indicates the metrics and constraints in use. The
OF also specifies the procedure used to compute rank within a DODAG
iteration. Further details may be found in <xref
target="I-D.ietf-roll-routing-metrics"></xref>, <xref
target="I-D.ietf-roll-of0"></xref>, and related companion
specifications.</t>
<t>By using defined OFs that are understood by all nodes in a
particular deployment, and by referencing these in the DIO message,
RPL nodes may work to build optimized LLN routes using a variety of
application and implementation specific metrics and goals.</t>
<t>In the case where a node is unable to encounter a suitable RPL
Instance using a known Objective Function, it may be configured to
join a RPL Instance using an unknown Objective Function - but in
that case only acting as a leaf node.</t>
</section>
<section title="Distributed Algorithm Operation">
<t>A high level overview of the distributed algorithm which
constructs the DODAG is as follows:</t>
<t><list style="symbols">
<t>Some nodes are configured to be DODAG roots, with associated
DODAG configuration.</t>
<t>Nodes advertise their presence, affiliation with a DODAG,
routing cost, and related metrics by sending link-local
multicast DIO messages.</t>
<t>Nodes may adjust the rate at which DIO messages are sent in
response to stability or detection of routing
inconsistencies.</t>
<t>Nodes listen for DIOs and use their information to join a new
DODAG, or to maintain an existing DODAG, as according to the
specified Objective Function and rank-based loop avoidance
rules.</t>
<t>Nodes provision routing table entries, for the destinations
specified by the DIO, via their DODAG parents in the DODAG
iteration. Nodes may provision a DODAG parent as a default
gateway.</t>
<t>Nodes may identify DODAG siblings within the DODAG iteration
to increase path diversity.</t>
<t>Using DIOs, and possibly information in data packets, RPL
nodes detect possible routing loops. When a RPL node detects a
possible routing loop, it may adapt its DIO transmission rate to
apply a local repair to the topology.<!-- This process and its
limitations is discussed in greater detail in TBD-FIXREF.--></t>
</list></t>
</section>
</section>
<section title="Downward Routes and Destination Advertisement">
<t>RPL constructs and maintains DODAGs with DIO messages to establish
upward routes: it uses Destination Advertisement Object (DAO) messages
to establish downward routes along the DODAG as well as other routes.
DAO messages are an optional feature for applications that require
P2MP or P2P traffic. DIO messages advertise whether destination
advertisements are enabled within a given DODAG.</t>
<section title="Destination Advertisement Object (DAO)">
<t>A Destination Advertisement Object (DAO) conveys destination
information upwards along the DODAG so that a DODAG root (an other
intermediate nodes) can provision downward routes. A DAO message
includes prefix information to identify destinations, a capability
to record routes in support of source routing, and information to
determine the freshness of a particular advertisement.</t>
<t>Nodes that are capable of maintaining routing state may aggregate
routes from DAO messages that they receive before transmitting a DAO
message. Nodes that are not capable of maintaining routing state may
attach a next-hop address to the Reverse Route Stack contained
within the DAO message. The Reverse Route Stack is subsequently used
to generate piecewise source routes over regions of the LLN that are
incapable of storing downward routing state.</t>
<t>A special case of the DAO message, termed a no-DAO, is used to
clear downward routing state that has been provisioned through DAO
operation.</t>
<t>This document defines an ICMPv6 Message Type "RPL Control
Message", which is capable of carrying a DAO.</t>
<section title="'One-Hop' Neighbors">
<t>In addition to sending DAOs toward DODAG roots, RPL nodes may
occasionally emit a link-local multicast DAO message advertising
available destination prefixes. This mechanism allow provisioning
a trivial 'one-hop' route to local neighbors.</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 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 metric, that will satisfy all use cases.</t>
<t>In addition, RPL supports constrained-based routing where
constraints may be applied to both link and nodes. If a link or a node
does not satisfy a required constraint, it is 'pruned' from the
candidate list, thus leading to a constrained shortest path.</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 specify which routing
metrics and constraints are in use, and how these are used, in
addition to the objectives used to compute the (constrained) shortest
path.</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 not traverse any battery-operated node and that optimizes the
path reliability</t>
</list></t>
<section title="Loop Avoidance">
<t>RPL guarantees neither loop free path selection nor strong global
convergence. In order to reduce control overhead, however, such as
the cost of the count-to-infinity problem, RPL avoids creating loops
when undergoing topology changes. Furthermore, RPL includes
rank-based mechanisms for detecting loops when they do occur. RPL
uses this loop detection to ensure that packets make forward
progress within the DODAG iteration and trigger repairs when
necessary.</t>
<section title="Greediness and Rank-based Instabilities">
<t>Once a node has joined a DODAG iteration, RPL disallows certain
behaviors, including greediness, in order to prevent resulting
instabilities in the DODAG iteration.</t>
<t>If a node is allowed to be greedy and attempts to move deeper
in the DODAG iteration, beyond its most preferred parent, in order
to increase the size of the parent set, then an instability can
result. 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-DODAG, and in general a node deeper
than itself. In this case, a possibility exists that a feedback
loop is created, wherein two or more nodes continue to try and
move in the DODAG iteration while attempting to optimize against
each other. In some cases, this will result in instability. It is
for this reason that RPL limits the cases where a node may process
DIO messages from deeper nodes to some forms of local repair. This
approach creates an 'event horizon', whereby a node cannot be
influenced beyond some limit into an instability by the action of
nodes that may be in its own sub-DODAG.</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="DODAG Loops">
<t>A DODAG loop may occur when a node detaches from the DODAG and
reattaches to a device in its prior sub-DODAG. This may happen in
particular when DIO messages are missed. Strict use of the DAG
sequence number can eliminate this type of loop, but this type of
loop may possibly be encountered when using some local repair
mechanisms.</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 and persists until all state has been cleaned
up. RPL includes loop detection mechanisms that may mitigate the
impact of DAO loops and trigger their repair.</t>
<t>In the case where stateless DAO operation is used, i.e. source
routing specifies the down routes, then DAO Loops should not occur
on the stateless portions of the path.</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, in order to
prevent sibling loops.</t>
</section>
</section>
<section anchor="DAGRank" title="Rank Properties">
<t>The rank of a node is a scalar representation of the location of
that node within a DODAG iteration. The rank is used to avoid and
detect loops, and as such must demonstrate certain properties. The
exact calculation of the rank is left to the Objective Function, and
may depend on parents, link metrics, and the node configuration and
policies.</t>
<t>The rank is not a cost metric, although its value can be derived
from and influenced by metrics. The rank has properties of its own
that are not necessarily those of all metrics: <list hangIndent="8"
style="hanging">
<t hangText="Type:">Rank is an abstract scalar. Some metrics are
boolean (e.g. grounded), others are statistical and better
expressed as a tuple like an expected value and a variance. Some
OCPs use not one but a set of metrics bound by a piece of
logic.</t>
<t hangText="Function:">Rank is the expression of a relative
position within a DODAG iteration with regard to neighbors and
is not necessarily a good indication or a proper expression of a
distance or a cost to the root.</t>
<t hangText="Stability:">The stability of the rank determines
the stability of the routing topology. Some dampening or
filtering might be applied to keep the topology stable, and thus
the rank does not necessarily change as fast as some physical
metrics would. A new DODAG iteration would be a good opportunity
to reconcile the discrepancies that might form over time between
metrics and ranks within a DODAG iteration.</t>
<t hangText="Granularity:">Rank is coarse grained. A fine
granularity would prevent the selection of siblings.</t>
<t hangText="Properties:">Rank is strictly monotonic, and can be
used to validate a progression from or towards the root. A
metric, like bandwidth or jitter, does not necessarily exhibit
this property.</t>
<t hangText="Abstract:">Rank does not have a physical unit, but
rather a range of increment per hop that varies from 1 (best) to
16 (worst), where the assignment of each value is to be
determined by the implementation.</t>
</list></t>
<t>The rank value feeds into DODAG parent selection, according to
the RPL loop-avoidance strategy. Once a parent has been added, and a
rank value for the node within the DODAG has been advertised, the
nodes further options with regard to DODAG parent selection and
movement within the DODAG are restricted in favor of loop
avoidance.</t>
<section anchor="RankComparison" title="Rank Comparison">
<t>Rank may be thought of as a fixed point number, where the
position of the decimal point is determined by MinHopRankIncrease.
The integer portion of the Rank is determined by
floor(Rank/MinHopRankIncrease).</t>
<t>MinHopRankIncrease is provisioned at the DODAG Root and
propagated in the DIO message. For efficient implementation the
MinHopRankIncrease SHOULD be a power of 2. An implementation may
configure a value MinHopRankIncrease as appropriate to balance
between the loop avoidance logic of RPL (i.e. selection of
eligible parents and siblings) and the metrics in use.</t>
<t>A node A has a rank less than the rank of a node B if
floor(Rank(A) / MinHopRankIncrease) is less than floor (Rank(B) /
MinHopRankIncrease).</t>
<t>A node A has a rank equal to the rank of a node B if
floor(Rank(A) / MinHopRankIncrease) is equal to floor (Rank(B) /
MinHopRankIncrease).</t>
<t>A node A has a rank greater than the rank of a node B if
floor(Rank(A) / MinHopRankIncrease) is greater than floor (Rank(B)
/ MinHopRankIncrease).</t>
</section>
<section title="Rank Relationships">
<t>The computation of the rank MUST be done in such a way so as to
maintain the following properties for any nodes M and N that are
neighbors in the LLN:</t>
<t><list hangIndent="8" style="hanging">
<t hangText="DAGRank(M) is less than DAGRank(N):">In this
case, the position of M is closer to the DODAG root than the
position of N. Node M may safely be a DODAG parent for Node N
without risk of creating a loop. Further, for a node N, all
parents in the DODAG parent set must be of rank less than
DAGRank(N). In other words, the rank presented by a node N
MUST be greater than that presented by any of its parents.</t>
<t hangText="DAGRank(M) equals DAGRank(N):">In this case the
positions of M and N within the DODAG and with respect to the
DODAG root are similar (identical). In some cases, Node M may
be used as a successor by Node N, which however entails the
probability of creating a loop (which must be detected and
resolved by some other means).</t>
<t hangText="DAGRank(M) is greater than DAGRank(N):">In this
case, the position of M is farther from the DODAG root than
the position of N. Further, Node M may in fact be in the
sub-DODAG of Node N. If node N selects node M as DODAG parent
there is a risk to create a loop.</t>
</list></t>
<t>As an example, the rank could be computed in such a way so as
to closely track ETX when the objective function is to minimize
ETX, or latency when the objective function is to minimize
latency, or in a more complicated way as appropriate to the
objective code point being used within the DODAG.</t>
</section>
</section>
</section>
</section>
<!-- <section anchor="SpecCore" title="Base Protocol Specification">
<t>This section describes the format of RPL control messages.</t>
-->
<section anchor="RPLControlMessage" title="ICMPv6 RPL Control Message">
<t>This document defines the RPL Control Message, a new ICMPv6 message.
In accordance with <xref target="RFC4443"></xref>, the RPL Control
Message has the following format:</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 field identifies the type of RPL Control Message. This
document defines three types:</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> -->
<section anchor="UpwardRoutes" title="Upward Routes">
<t>This section describes how RPL discovers and maintains upward routes.
It describes DODAG Information Objects (DIOs), the messages used to
discover and maintain these routes. It specifies how RPL generates and
responds to DIOs. It also describes DAG Information Solicitation (DIS)
messages, which are used to trigger DIO transmissions.</t>
<section anchor="DAGInformationObject"
title="DODAG Information Object (DIO)">
<t>The DODAG Information Object carries information that allows a node
to discover a RPL Instance, learn its configuration parameters, select
a DODAG parent set, and maintain the upward routing topology.</t>
<section anchor="DIOBase" title="DIO Base Format">
<t>DIO Base is an always-present container option in a DIO message.
Every DIO MUST include a DIO Base.</t>
<t><figure anchor="DIObase" title="DIO Base">
<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|A|T|S|0| Prf | Sequence | Rank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID | DTSN | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
| |
+ +
| DODAGID |
+ +
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | sub-option(s)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t><list hangIndent="6" style="hanging">
<t hangText="Control Field:">The DAG Control Field has three
flags and one field: <list hangIndent="6" style="hanging">
<t hangText="Grounded (G):">The Grounded (G) flag indicates
whether the upward routes this node advertises provide
connectivity to the set of addresses which are
application-defined goals. If the flag is set, the DODAG is
grounded and provides such connectivity. If the flag is
cleared, the DODAG is floating and may not provide such
connectivity.</t>
<t hangText="Destination Advertisement Supported (A):">The
Destination Advertisement Supported (A) bit indicates
whether the root of this DODAG can collect and use downward
route state. The flag is set when nodes in the network are
to exchange destination advertisements messages to build
downward routes (<xref target="DownwardRoutes"></xref>).<!-- and cross-routes (<xref target="CrossRoutes"></xref>)-->
The flag is cleared when the DODAG maintains only upward
routes.</t>
<t hangText="Destination Advertisement Trigger (T):">The
Destination Advertisement Trigger (T) flag is used to
trigger a complete refresh of downward routes. The details
of this process are described in <xref
target="DownwardRoutes"></xref>.</t>
<t hangText="Destination Advertisements Stored (S):">The
Destination Advertisements Stored (S) flag is used to
indicate that a non-root ancestor is storing routing table
entries learned from DAO messaging. The meaning and further
use of this flag is described in <xref
target="DownwardRoutes"></xref>.</t>
<t hangText="DODAGPreference (Prf):">A 3-bit unsigned
integer that defines how preferable the root of this DODAG
is compared to other DODAG roots within the DODAG.
DAGPreference ranges from 0x00 (least preferred) to 0x07
(most preferred). The default is 0 (least preferred). <xref
target="DAGDiscovery"></xref> describes how DAGPreference
affects DIO processing.</t>
</list></t>
<t>Unassigned bits of the Control Field are reserved. They MUST
be set to zero on transmission and MUST be ignored on
reception.</t>
<t hangText="Sequence Number:">8-bit unsigned integer set by the
DODAG root. <xref target="DAGDiscovery"></xref> describes the
rules for sequence numbers and how they affect DIO
processing.</t>
<t hangText="Rank:">8-bit unsigned integer indicating the DODAG
rank of the node sending the DIO message. <xref
target="DAGDiscovery"></xref> describes how Rank is set and how
it affects DIO processing.</t>
<t hangText="RPLInstanceID:">8-bit field set by the DODAG root
that indicates which RPL Instance the DODAG is part of.</t>
<t
hangText="Destination Advertisement Trigger Sequence Number (DTSN):">8-bit
unsigned integer set by the node issuing the DIO message. The
Destination Advertisement Trigger Sequence Number (DTSN) flag is
used as part of the procedure to maintain downward routes. The
details of this process are described in <xref
target="DownwardRoutes"></xref>.</t>
<t hangText="DODAGID:">128-bit unsigned integer set by a DODAG
root which uniquely identifies a DODAG. Possibly derived from
the IPv6 address of the DODAG root.</t>
</list></t>
</section>
<section anchor="DIOBaseRules" title="DIO Base Rules">
<t><list style="numbers">
<t>If the 'A' flag of a DIO Base is cleared, the 'T' flag MUST
also be cleared.</t>
<t>For the following DIO Base fields, a node that is not a DODAG
root MUST advertise the same values as its preferred DODAG
parent (defined in <xref target="parentset"></xref>). Therefore,
if a DODAG root does not change these values, every node in a
route to that DODAG root eventually advertises the same values
for these fields. These fields are: <?rfc subcompact="yes"?><list>
<t>Grounded (G)</t>
<t>Destination Advertisement Supported (A)</t>
<t>Destination Advertisement Trigger (T)</t>
<t>DAGPreference (Prf)</t>
<t>Sequence</t>
<t>RPLInstanceID</t>
<t>DODAGID</t>
</list><?rfc subcompact="no"?></t>
<t>A node MAY update the following fields at each hop: <?rfc subcompact="yes"?><list>
<t>DAGRank</t>
<t>DTSN</t>
</list><?rfc subcompact="no"?></t>
<t>The DODAGID field each root sets MUST be unique within the
RPL Instance.</t>
</list></t>
</section>
<section anchor="DIOSuboptions" title="DIO Suboptions">
<t>This section describes the format of DIO suboptions and the five
suboptions this document defines: Pad 1, Pad N, DAG Metric
Container, DAG Destination Prefix, and DAG Configuration.</t>
<section title="DIO Suboption Format">
<t>The Pad N, DAG Metric Container, DAG Destination Prefix, and
DAG Configuration suboptions all follow this format: <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.</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>When processing a DIO message containing a suboption for which
the Suboption Type value is not recognized by the receiver, the
receiver MUST silently ignore the unrecognized option and continue
to process the following suboption, correctly handling any
remaining options in the message.</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 anchor="DIOMetricContainer" title="Metric Container">
<t>The Metric Container suboption may be aligned as necessary to
support its contents. Its format is as follows:</t>
<t><figure anchor="DIOsubLLNMetric" title="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 | Subopt Length | Metric Data
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - -
]]></artwork>
</figure></t>
<t>The Metric Container is used to report aggregated path metrics
along the DODAG. The Metric Container may contain a number of
discrete node, link, and aggregate path metrics as chosen by the
implementer. The Suboption Length field contains the length in
octets of the Metric Data. The order, content, and coding of the
Metric Container data is as specified in <xref
target="I-D.ietf-roll-routing-metrics"></xref>.</t>
<!-- Ticket #23: This behavior is controlled by the metric draft
and OCP is conveyed in the OCP object
<t>The Metric Container MUST include the value for the Objective
Code Point in use for the RPL Instance.</t>
-->
<t>The processing and propagation of the 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 | Subopt Length |Resvd|Prf|Resvd|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Prefix Length | |
+-+-+-+-+-+-+-+-+ |
| Destination Prefix (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t>The Destination Prefix suboption is used when the DODAG root,
or another node located upwards along the DODAG on the path to the
DODAG 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 DODAG, a node MAY
decide to join multiple DODAGs in support of a particular
application.</t>
<t>The Suboption 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. The lifetime is initially set by the node that owns
the prefix and denotes the valid lifetime for that prefix (similar
to AdvValidLifetime <xref target="RFC4861"></xref>). The value
might be reduced by the originator and/or en-route nodes that will
not provide connectivity for the whole valid lifetime. A value of
all one bits (0xFFFFFFFF) represents infinity. A value of all zero
bits (0x00000000) indicates a loss of reachability.</t>
<t>The Prefix Length is an 8-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="DODAG Configuration">
<t>The DODAG Configuration suboption does not have any alignment
requirements. Its format is as follows:</t>
<t><figure anchor="DIOsubDAGConfig" title="DODAG 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. | DIORedun. | MaxRankInc | MinHopRankInc |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t>The DODAG Configuration suboption is used to distribute
configuration information for DODAG Operation through the DODAG.
The information communicated in this suboption is generally static
and unchanging within the DODAG, therefore it is not necessary to
include in every DIO. This suboption MAY be included occasionally
by the DODAG Root, and MUST be included in response to a unicast
request, e.g. a DODAG Information Solicitation (DIS) message.</t>
<t>The Length is coded as 5.</t>
<t>DIOIntervalDoublings is an 8-bit unsigned integer, configured
on the DODAG root and used to configure the trickle timer (see
<xref target="TrickleImplementation"></xref> for details on
trickle timers) governing when DIO message should be sent within
the DODAG. DIOIntervalDoublings is the number of times that the
DIOIntervalMin is allowed to be doubled during the trickle timer
operation.</t>
<t>DIOIntervalMin is an 8-bit unsigned integer, configured on the
DODAG root and used to configure the trickle timer governing when
DIO message should be sent within the DODAG. The minimum
configured interval for the DIO trickle timer in units of ms is
2^DIOIntervalMin. For example, a DIOIntervalMin value of 16ms is
expressed as 4.</t>
<t>DIORedundancyConstant is an 8-bit unsigned integer used to
configure suppression of DIO transmissions. DIORedundancyConstant
is the minimum number of relevant incoming DIOs required to
suppress a DIO transmission. If the value is 0xFF then the
suppression mechanism is disabled.</t>
<t>MaxRankInc, 8-bit unsigned integer, is the DAGMaxRankIncrease.
This is the allowable increase in rank in support of local repair.
If DAGMaxRankIncrease is 0 then this mechanism is disabled.</t>
<t>MinHopRankInc, 8-bit unsigned integer, is the
MinHopRankIncrease as described in <xref
target="RankComparison"></xref>.</t>
</section>
</section>
</section>
<section anchor="DAGInformationSolicitation"
title="DODAG Information Solicitation (DIS)">
<t>The DODAG Information Solicitation (DIS) message may be used to
solicit a DODAG Information Object from a RPL node. Its use is
analogous to that of a Router Solicitation; a node may use DIS to
probe its neighborhood for nearby DODAGs. The DODAG Information
Solicitation carries no additional message body. <xref
target="DIOTransmission"></xref> describes how nodes respond to a
DIS.</t>
</section>
<section anchor="DAGDiscovery"
title="Upward Route Discovery and Maintenance">
<t>Upward route discovery allows a node to join a DODAG by discovering
neighbors that are members of the DODAG and identifying a set of
parents. The exact policies for selecting neighbors and parents is
implementation-dependent. This section specifies the set of rules
those policies must follow for interoperability.</t>
<section anchor="RPLInstance" title="RPL Instance">
<t><list>
<t>A RPLInstanceID MUST be unique across an LLN.</t>
<t>A node MAY belong to multiple RPL Instances.</t>
</list></t>
<t>Within a given LLN, there may be multiple, logically independent
RPL instances. This document describes how a single instance
behaves.</t>
</section>
<section anchor="parentset"
title="Neighbors and Parents within a DODAG Iteration">
<t>RPL's upward route discovery algorithms and processing are in
terms of three logical sets of link-local nodes. First, the
candidate neighbor set is a subset of the nodes that can be reached
via link-local multicast. The selection of this set is
implementation-dependent and OF-dependent. Second, the parent set is
a restricted subset of the candidate neighbor set. Finally, the
preferred parent, a set of size one, is an element of the parent set
that is the preferred next hop in upward routes.</t>
<t>More precisely: <list style="numbers">
<t>The DODAG parent set MUST be a subset of the candidate
neighbor set.</t>
<t>A DODAG root MUST have a DODAG parent set of size zero.</t>
<t>A node that is not a DODAG root MAY maintain a DODAG parent
set of size greater than or equal to one.</t>
<t>A node's preferred DODAG parent MUST be a member of its DODAG
parent set.</t>
<t>A node's rank MUST be greater than all elements of its DODAG
parent set.</t>
<t>When Neighbor Unreachability Detection (NUD), or an
equivalent mechanism, determines that a neighbor is no longer
reachable, a RPL node MUST NOT consider this node in the
candidate neighbor set when calculating and advertising routes
until it determines that it is again reachable. Routes through
an unreachable neighbor MUST be eliminated from the routing
table.</t>
</list></t>
<t>These rules ensure that there is a consistent partial order on
nodes within the DODAG. As long as node ranks do not change,
following the above rules ensures that every node's route to a DODAG
root is loop-free, as rank decreases on each hop to the root. The OF
can guide candidate neighbor set and parent set selection, as
discussed in <xref
target="I-D.ietf-roll-routing-metrics"></xref>.</t>
</section>
<section anchor="DAGDiscoveryRules"
title="Neighbors and Parents across DODAG Iterations">
<t>The above rules govern a single DODAG iteration. The rules in
this section define how RPL operates when there are multiple DODAG
iterations:</t>
<section anchor="DAGDiscoveryRulesSeq" title="DODAG Iteration">
<t><list style="numbers">
<t>The tuple (RPLInstanceID, DODAGID, DODAGSequenceNumber)
uniquely defines a DODAG Iteration. Every element of a node's
DODAG parent set, as conveyed by the last heard DIO from each
DODAG parent, MUST belong to the same DODAG iteration.
Elements of a node's candidate neighbor set MAY belong to
different DODAG Iterations.</t>
<t>A node is a member of a DODAG iteration if every element of
its DODAG parent set belongs to that DODAG iteration, or if
that node is the root of the corresponding DODAG.</t>
<t>A node MUST NOT send DIOs for DODAG iterations of which it
is not a member.</t>
<t>DODAG roots MAY increment the DODAGSequenceNumber that they
advertise and thus move to a new DODAG iteration. When a DODAG
root increments its DODAGSequenceNumber, it MUST follow the
conventions of Serial Number Arithmetic as described in <xref
target="RFC1982"></xref>.</t>
<t>Within a given DODAG, a node that is a not a root MUST NOT
advertise a DODAGSequenceNumber higher than the highest
DODAGSequenceNumber it has heard. Higher is defined as the
greater-than operator in <xref target="RFC1982"></xref>.</t>
<t>Once a node has advertised a DODAG iteration by sending a
DIO, it MUST NOT be member of a previous DODAG iteration of
the same DODAG (i.e. with the same DODAGID and a lower
DODAGSequenceNumber). Lower is defined as the less-than
operator in <xref target="RFC1982"></xref>.</t>
</list></t>
<t>Within a particular implementation, a DODAG root may increment
the DODAGSequenceNumber periodically, at a rate that depends on
the deployment. In other implementations, loop detection may be
considered sufficient to solve routing issues, and the DODAG root
may increment the DODAGSequenceNumber only upon administrative
intervention. Another possibility is that nodes within the LLN
have some means by which they can signal detected routing
inconsistencies or suboptimalities to the DODAG root, in order to
request an on-demand DODAGSequenceNumber increment (i.e. request a
global repair of the DODAG).</t>
<t>When the DODAG parent set is depleted on a node that is not a
root, (i.e. the last parent is removed), then the DODAG
information should not be suppressed until after the expiration of
an implementation-specific local timer in order to observe if the
DODAGSequenceNumber has been incremented, should any new parents
appear for the DODAG.</t>
<t>As the DODAGSequenceNumber is incremented, a new DODAG
Iteration spreads outward from the DODAG root. Thus a parent that
advertises the new DODAGSequenceNumber can not possibly belong to
the sub-DODAG of a node that still advertises an older
DODAGSequenceNumber. A node may safely add such a parent, without
risk of forming a loop, without regard to its relative rank in the
prior DODAG Iteration. This is equivalent to jumping to a
different DODAG.</t>
<t>As a node transitions to new DODAG Iterations as a consequence
of following these rules, the node will be unable to advertise the
previous DODAG Iteration (prior DODAGSequenceNumber) once it has
committed to advertising the new DODAG Iteration.</t>
<t>During transition to a new DODAG Iteration, a node may decide
to forward packets via 'future parents' that belong to the same
DODAG (same RPLInstanceID and DODAGID), but are observed to
advertise a more recent (incremented) DODAGSequenceNumber.</t>
</section>
<section anchor="DAGDiscoveryRulesRoot" title="DODAG Roots">
<t><list style="numbers">
<t>A DODAG root that does not have connectivity to the set of
addresses described as application-level goals, MUST NOT set
the Grounded bit.</t>
<t>A DODAG root MUST advertise a rank of ROOT_RANK.</t>
<t>A node whose DODAG parent set is empty MAY become the DODAG
root of a floating DODAG. It MAY also set its DAGPreference
such that it is less preferred.</t>
</list></t>
<t>An LLN node that is a goal for the Objective Function is the
root of its own grounded DODAG, at rank 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 that backbone and use
one router as a "backbone root". The backbone root is the virtual
root of the DODAG, and exposes a rank of BASE_RANK over the
backbone. All the LLN roots that are parented to that backbone
root, including the backbone root if it also serves as LLN root
itself, expose a rank of ROOT_RANK to the LLN, and are part of the
same DODAG, coordinating DODAGSequenceNumber and other DODAG root
determined parameters with the virtual root over the backbone.</t>
</section>
<section anchor="DAGSelection" title="DODAG Selection">
<t>The DODAGPreference (Prf) provides an administrative mechanism
to engineer the self-organization of the LLN, for example
indicating the most preferred LBR. If a node has the option to
join a more preferred DODAG while still meeting other optimization
objectives, then the node will generally seek to join the more
preferred DODAG as determined by the OF.</t>
</section>
<section anchor="DAGDiscoveryRulesMove"
title="Rank and Movement within a DODAG Iteration">
<t><list style="numbers">
<t>A node MUST NOT advertise a rank less than or equal to any
member of its parent set within the DODAG Iteration.</t>
<t>A node MAY advertise a rank lower than its prior
advertisement within the DODAG Iteration<!-- TBD incorrect i.e.: (i.e. a node MUST NOT
move to an earlier DODAG Iteration)-->.</t>
<t>Let L be the lowest rank within a DODAG iteration that a
given node has advertised. Within the same DODAG Iteration,
that node MUST NOT advertise an effective rank higher than L +
DAGMaxRankIncrease. INFINITE_RANK is an exception to this
rule: a node MAY advertise an INFINITE_RANK at any time. (This
corresponds to a limited rank increase for the purpose of
local repair within the DODAG Iteration.)</t>
<t>A node MAY, at any time, choose to join a different DODAG
within a RPL Instance. Such a join has no rank restrictions,
unless that different DODAG is a DODAG Iteration of which that
node has previously been a member, in which case the rule of
the previous bullet (3) must be observed. Until a node
transmits a DIO indicating its new DODAG membership, it MUST
forward packets along the previous DODAG.</t>
<t>A node MAY, at any time after hearing the next
DODAGSequenceNumber Iteration advertised from suitable DODAG
parents, choose to migrate to the next DODAG Iteration within
the DODAG.</t>
</list></t>
<t>Conceptually, an implementation is maintaining a DODAG parent
set within the DODAG Iteration. Movement entails changes to the
DODAG parent set. Moving up does not present the risk to create a
loop but moving down might, so that operation is subject to
additional constraints.</t>
<t>When a node migrates to the next DODAG Iteration, the DODAG
parent and sibling sets need to be rebuilt for the new iteration.
An implementation could defer to migrate for some reasonable
amount of time, to see if some other neighbors with potentially
better metrics but higher rank announce themselves. Similarly,
when a node jumps into a new DODAG it needs to construct new DODAG
parent/sibling sets for this new DODAG.</t>
<t>When a node moves to improve its position, it must conceptually
abandon all DODAG parents and siblings with a rank larger than
itself. As a consequence of the movement it may also add new
siblings. Such a movement may occur at any time to decrease the
rank, as per the calculation indicated by the OF. Maintenance of
the parent and sibling sets occurs as the rank of candidate
neighbors is observed as reported in their DIOs.</t>
<t>If a node needs to move down a DODAG that it is attached to,
causing the rank to increase, then it MAY poison its routes and
delay before moving as described in <xref
target="DAGDiscoveryRulesPoison"></xref>.</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>
-->
</section>
<section anchor="DAGDiscoveryRulesPoison"
title="Poisoning a Broken Path">
<t><list style="numbers">
<t>A node MAY poison, in order to avoid being used as an
ancestor by the nodes in its sub-DODAG, by advertising an
effective rank of INFINITE_RANK and resetting the associated
DIO trickle timer to cause this INFINITE_RANK to be announced
promptly.</t>
<t>The node MAY advertise an effective rank of INFINITE_RANK
for an arbitrary number of DIO timer events, before announcing
a new rank.</t>
<t>As per <xref target="DAGDiscoveryRulesMove"></xref>, the
node MUST advertise INFINITE_RANK within the DODAG iteration
in which it participates, if its revised rank would exceed the
maximum rank increase.</t>
</list></t>
<t>An implementation may choose to employ this poisoning mechanism
when a node loses all of its current parents, i.e. the set of
DODAG parents becomes depleted, and it can not jump to an
alternate DODAG. An alternate mechanism is to form a floating
DODAG.</t>
<t>The motivation for delaying announcement of the revised route
through multiple DIO events is to (i) increase tolerance to DIO
loss, (ii) allow time for the poisoning action to propagate, and
(iii) to develop an accurate assessment of its new rank. Such
gains are obtained at the expense of potentially increasing the
delay before portions of the network are able to re-establish
upwards routes. Path redundancy in the DODAG reduces the
significance of either effect, since children with alternate
parents should be able to utilize those alternates and retain
their rank while the detached parent re-establishes its rank.</t>
<t>Although an implementation may advertise INFINITE_RANK for the
purposes of poisoning, it is not expected to be equivalent to
setting the rank to INFINITE_RANK, and an implementation would
likely retain its rank value prior to the poisoning in some form,
for purpose of maintaining its effective position within (L +
DAGMaxRankIncrease).</t>
</section>
<section anchor="DAGDiscoveryRulesdetach" title="Detaching">
<t><list style="numbers">
<t>A node unable to stay connected to a DODAG within a given
DODAG iteration MAY detach from this DODAG iteration. A node
that detaches becomes root of its own floating DODAG and
SHOULD immediately advertise this new situation in a DIO as an
alternate to poisoning.</t>
</list></t>
</section>
<section anchor="DAGDiscoveryRulesfollow" title="Following a Parent">
<t><list style="numbers">
<t>If a node receives a DIO from one of its DODAG parents,
indicating that the parent has left the DODAG, that node
SHOULD stay in its current DODAG through an alternative DODAG
parent, if possible. It MAY follow the leaving parent.</t>
</list></t>
<t>A DODAG parent may have moved, migrated to the next DODAG
Iteration, or jumped to a different DODAG. A node should give some
preference to remaining in the current DODAG, if possible, but
ought to follow the parent if there are no other options.</t>
</section>
</section>
<section title="DIO Message Communication">
<t>When an DIO message is received, the receiving node must first
determine whether or not the DIO message should be accepted for
further processing, and subsequently present the DIO message for
further processing if eligible.</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 the sender of the DIO message is a member of the candidate
neighbor set, then the DIO is eligible for further
processing.</t>
</list></t>
<section title="DIO Message Processing">
<!--
<t>
<list>
<t>Process the DIO message as per the rules in <xref
target="DAGDiscovery" /></t>
</list>
</t>
-->
<t>As DIO messages are received from candidate neighbors, the
neighbors may be promoted to DODAG parents by following the rules
of DODAG discovery as described in <xref
target="DAGDiscovery"></xref>. When a node places a neighbor into
the DODAG parent set, the node becomes attached to the DODAG
through the new DODAG parent node.</t>
<t>The most preferred parent should be used to restrict which
other nodes may become DODAG parents. Some nodes in the DODAG
parent set may be of a rank less than or equal to the most
preferred DODAG parent. (This case may occur, for example, if an
energy constrained device is at a lesser rank but should be
avoided as per an optimization objective, resulting in a more
preferred parent at a greater rank).</t>
</section>
</section>
<section anchor="DIOTransmission" title="DIO Transmission">
<t>Each node maintains a timer, that governs when to multicast DIO
messages. This timer is a trickle timer, as detailed in <xref
target="TrickleImplementation"></xref>. The DIO Configuration Option
includes the configuration of a RPL Instance's trickle timer.</t>
<t><list style="symbols">
<t>When a node detects or causes an inconsistency, it MUST reset
the interval of the trickle timer to its minimum value.</t>
<t>When a node migrates to a new DODAG Iteration it MUST reset
the trickle timer to its minimum value</t>
<t>When a node detects an inconsistency when forwarding a
packet, as detailed in <xref target="loopdetect"></xref>, the
node MUST reset the trickle timer to its minimum value.</t>
<t>When a node receives a multicast DIS message, it MUST reset
the trickle timer to its minimum value.</t>
<t>When a node receives a unicast DIS message, it MUST unicast a
DIO message in response, and MUST include the DODAG
Configuration Object. In this case the node SHOULD NOT reset the
trickle timer.</t>
<t>If a node is not a member of a DODAG, it MUST suppress
transmission of DIO messages.</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 DODAG (perhaps initially probing for a
nearby DODAG with an DIS message). Alternately, it MAY choose to
root its own floating DODAG and begin multicasting DIO messages
using a default trickle configuration. The second case may be
advantageous if it is desired for independent nodes to begin
aggregating into scattered floating DODAGs, in the absence of a
grounded node, for example in support of LLN installation and
commissioning.</t>
</list></t>
<!--
<t>Note that if multiple DAG roots are participating in the same
DAG, i.e. offering DIO messages with the same DODAGID, 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 anchor="TrickleImplementation"
title="Trickle Timer for DIO Transmission">
<t>RPL treats the construction of a DODAG as a consistency
problem, and uses a trickle timer <xref target="Levis08"></xref>
to control the rate of control broadcasts.</t>
<t>For each DODAG that a node is part of (i.e. one DODAG per RPL
Instance), the node must maintain a single trickle timer. The
required state contains the following conceptual items:</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 DODAG is reset by:</t>
<t><list style="numbers">
<t>Setting I_min and I_doublings to the values learned from
the DODAG root via a received 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 a node learns about a DODAG through a DIO message, and
makes the decision to join this DODAG, it initializes the state
of the trickle timer by resetting the trickle timer and
listening. Each time it hears a redundant DIO message for this
DODAG, it MAY increment C. The exact determination of what
constitutes a redundant DIO message is left to an
implementation; it could for example include DIOs that advertise
the same rank.</t>
<t>When the timer fires at time T, the node compares C to the
redundancy constant, DIORedundancyConstant. If C is less than
that value, or if the DIORedundancyConstant value is 0xFF, the
node generates a new DIO message and multicasts it. When the
communication interval I expires, the node doubles the interval
I so long as it has previously doubled it fewer than I_doubling
times, resets C, and chooses a new T value.</t>
</section>
<!-- TBD redundant -->
<section anchor="TrickleInconsistencies"
title="Determination of Inconsistency">
<t>The trickle timer is reset whenever an inconsistency is
detected within the DODAG, for example:</t>
<t><list style="symbols">
<t>The node joins a new DODAG</t>
<t>The node moves within a DODAG</t>
<t>The node receives a modified DIO message from a DODAG
parent</t>
<t>A DODAG parent forwards a packet intended to move up,
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 DODAG parent has changed.</t>
</list></t>
</section>
</section>
</section>
<section title="DODAG Selection">
<t>The DODAG selection is implementation and algorithm dependent.
Nodes SHOULD prefer to join DODAGs for RPLInstanceIDs advertising
OCPs and destinations compatible with their implementation specific
objectives. In order to limit erratic movements, and all metrics
being equal, nodes SHOULD keep their previous selection. Also, nodes
SHOULD provide a means to filter out a parent whose availability is
detected as fluctuating, at least when more stable choices are
available.</t>
<t>When connection to a fixed network is not possible or preferable
for security or other reasons, scattered DODAGs MAY aggregate as
much as possible into larger DODAGs in order to allow connectivity
within the LLN.</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 DODAG parent.</t>
</section>
</section>
<section title="Operation as a Leaf Node">
<t>In some cases a RPL node may attach to a DODAG as a leaf node only.
One example of such a case is when a node does not understand the RPL
Instance's OF. A leaf node does not extend DODAG connectivity but
still needs to advertise its presence using DIOs. A node operating as
a leaf node must obey the following rules:</t>
<t><list style="numbers">
<t>It MUST NOT transmit DIOs containing the DAG Metric
Container.</t>
<t>Its DIOs must advertise a DAGRank of INFINITE_RANK.</t>
<t>It MAY transmit unicast DAOs as described in <xref
target="DownwardDiscovery"></xref>.</t>
<t>It MAY transmit multicast DAOs to the '1 hop' neighborhood as
described in <xref target="MulticastDAO"></xref>.</t>
</list></t>
</section>
<section title="Administrative Rank">
<!-- TBD should this go with `Guidelines for OF?' -->
<t>In some cases it might be beneficial to adjust the rank advertised
by a node beyond that computed by the OF based on some implementation
specific policy and properties of the node. For example, a node that
has limited battery should be a leaf unless there is no other choice,
and may then augment the rank computation specified by the OF in order
to expose an exaggerated rank.</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 DODAG root of their own DODAGs. In
order to detect the situation, LLN Nodes time stamp the sending of DIO
message. Any DIO message received within a short link-layer-dependent
period introduces a risk. It left to the implementation to define the
duration of the risk window.</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 DODAGs and near-simultaneously send DIO
messages, which are received and processed by both, and possibly
result in both nodes simultaneously deciding to attach to each other.
As a remedy, in the face of a potential collision, as determined by
receiving a DIO within the risk window, the DIO message is not
processed. It is expected that subsequent DIOs would not cross.</t>
</section>
</section>
<section anchor="DownwardRoutes" title="Downward Routes">
<t>This section describes how RPL discovers and maintains downward
routes. Messages containing the Destination Advertisement Object (DAO),
used to construct downward routes, are described. The downward routes
are necessary in support of P2MP flows, from the DODAG roots toward the
leaves. It specifies non-storing and storing behavior of nodes with
respect to DAO messaging and DAO routing table entries. Nodes, as
according to their resources and the implementation, may selectively
store routing table entries learned from DAO messages, or may instead
propagate the DAO information upwards while adding source routing
information. A further optimization is described whereby DAO messages
may be used to populate routing table entries for the '1-hop' neighbors,
which may be useful in some cases as a shortcut for P2P flows.</t>
<section anchor="DestinationAdvertisementObject"
title="Destination Advertisement Object (DAO)">
<t>The Destination Advertisement Object (DAO) is used to propagate
destination information upwards along the DODAG.</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 | DAO Rank |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| RPLInstanceID | Route Tag | Prefix Length | RRCount |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| DAO Lifetime |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Destination Prefix (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reverse Route Stack (Variable Length) |
. .
. .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| sub-option(s)...
+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t><list hangIndent="6" style="hanging">
<t hangText="DAO Sequence:">16-bit unsigned integer. Incremented
by the node that owns the prefix for each new DAO message for that
prefix.</t>
<t hangText="DAO Rank:">16-bit unsigned integer indicating the DAO
Rank associated with the advertised Destination Prefix. The DAO
Rank is analogous to the Rank in the DIO message in that it may be
used to convey a relative distance to the Destination Prefix as
computed by the Objective Function in use over the DODAG. It
serves as a mechanism by which an ancestor node may order
alternate DAO paths.</t>
<t hangText="RPLInstanceID:">8-bit field indicating the topology
instance associated with the DODAG, as learned from the DIO.</t>
<t hangText="Route Tag:">8-bit unsigned integer. The Route Tag may
be used to give a priority to prefixes that should be stored. This
may be useful in cases where intermediate nodes are capable of
storing a limited amount of routing state. The further
specification of this field and its use is under
investigation.</t>
<t hangText="Prefix Length:">8-bit unsigned integer. Number of
valid leading bits in the IPv6 Prefix.</t>
<t hangText="RRCount:">8-bit unsigned integer. This counter is
used to count the number of entries in the Reverse Route Stack. A
value of '0' indicates that no Reverse Route Stack is present.</t>
<t hangText="DAO Lifetime:">32-bit unsigned integer. The length of
time in seconds (relative to the time the packet is sent) that the
prefix is valid for route determination. A value of all one bits
(0xFFFFFFFF) represents infinity. A value of all zero bits
(0x00000000) indicates a loss of reachability.</t>
<t hangText="Destination Prefix:">Variable-length field
identifying an IPv6 destination address, prefix, or multicast
group. The Prefix Length field contains the number of valid
leading bits in the prefix. The bits in the prefix after the
prefix length (if any) are reserved and MUST be set to zero on
transmission and MUST be ignored on receipt.</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 anchor="DAOSuboptions" title="DAO Suboptions">
<t>The DAO message may optionally include a number of
suboptions.</t>
<t>The DAO suboptions are in the same format as the DIO Suboptions
described in <xref target="DAOSuboptions"></xref>.</t>
<t>In particular, a DAO message may include a DAG Metric Container
suboption as described in <xref target="DIOMetricContainer"></xref>.
This suboption may be present in implementations where the DAO Rank
is insufficient to optimize a path to the DAO Destination
Prefix.</t>
</section>
</section>
<section anchor="DownwardDiscovery"
title="Downward Route Discovery and Maintenance">
<section title="Overview">
<t>Destination Advertisement operation produces DAO messages that
flow up the DODAG, provisioning downward routing state for
destination prefixes available in the sub-DODAG of the DODAG root,
and possibly other nodes. The routing state provisioned with this
mechanism is in the form of soft-state routing table entries. DAO
messages are able to record loose source routing information as by
propagate up the DODAG. This mechanism is flexible to support the
provisioning of paths which consist of fully specified source
routes, piecewise source routes, or hop-by-hop routes as according
to the implementation and the capabilities of the nodes.</t>
<t>Destination Advertisement may or may not be enabled over a DODAG
rooted at a DODAG root. This is an a priori configuration determined
by the implementation/deployment and not generally changed during
the operation of the RPL LLN.</t>
<t>When Destination Advertisement is enabled:</t>
<t><list style="numbers">
<t>Some nodes in the LLN MAY store at least one routing table
entry for a particular destination learned from a DAO. Such a
node is termed a 'storing node', with respect to that particular
destination.</t>
<t>Some nodes are capable to store at least one routing table
entry for every unique destination observed from all DAOs that
pass through. Such a node is termed a 'fully storing node'.</t>
<t>DODAG roots nodes SHOULD be fully-storing nodes.</t>
<t>Other nodes in the DODAG are not required to store routing
table entries for any particular destinations observed in DAOs.
Nodes that do not store routing table entries from DAOs are
termed 'non-storing nodes', with respect to a particular
destination.</t>
<t>Non-storing nodes MUST participate in the construction of
piecewise source routes as they propagate the DAO message, as
described in <xref target="DAONonStoring"></xref>.</t>
<t>Storing nodes MUST store any source route information
received from the DAO (RRStack) in the routing table entry
entry. If a node is not capable to do this then it must act as a
non-storing node with respect to that particular
destination.</t>
<t>Storing nodes MUST use piecewise source routes in order to
forward data across a non-storing region of the LLN. The source
routing mechanism is to be described in a companion
specification. (If a node is not capable to do this, then that
node MUST NOT operate as a storing node).</t>
</list></t>
</section>
<section title="Mode of Operation">
<t><list style="symbols">
<t>DAO Operation may not be required for all use cases.</t>
<t>Some applications may only need support for
collection/upward/MP2P flow with no acknowledgement/reciprocal
traffic.</t>
<t>Some DODAGs may not support DAO Operation, which could mean
that DAO Operation is wasteful overhead.</t>
<t>As a special case, multicast DAO operation may be used to
populate 'one-hop' neighborhood routing table entries, and is
distinct from the unicast DAO operation used to establish
downward routes along the DODAG.</t>
</list></t>
<t><list style="numbers">
<t>The 'A' flag in the DIO as conveyed from the DODAG root
serves to enable/disable DAO operation over the entire DODAG.
This flag should be administratively provisioned a priori at the
DODAG root as a function of the implementation/deployment and
not tend to change.</t>
<t>When DAO Operation is disabled, a node SHOULD NOT emit
DAOs.</t>
<t>When DAO Operation is disabled, a node MAY ignore received
DAOs.</t>
</list></t>
</section>
<section title="Destination Advertisement Parents">
<t><list style="symbols">
<t>Nodes will select a subset of their DODAG Parents to whom
DAOs will be sent<list style="symbols">
<t>This subset is the set of 'DAO Parents'</t>
<t>Each DAO parent MUST be a DODAG Parent. (Not all DODAG
parents need to be DAO parents).</t>
<t>Operation with more than DAO Parent requires
consideration of such issues as DAO fan-out and path
diversity, to be elaborated in a future version of this
specification.</t>
</list></t>
<t>The selection of DAO parents is implementation specific and
may be based on selecting the DODAG Parents that offer the best
upwards cost (as opposed to downwards or mixed), as determined
by the metrics in use and the Objective Function.</t>
<t>When DAO messages are unicast to the DAO Parent, the identity
of the DAO Parent (DODAGID x DAGSequenceNumber) combined with
the RPLInstanceID in the DAO message unambiguously associates
the DAO message, and thus the particular destination prefix,
with a DODAG Iteration.</t>
<t>When DAO messages are unicast to the DAO Parent, the DAO Rank
may be updated as according to the implementation and Objective
Function in use to reflect the relative (aggregated) cost of
reaching the Destination Prefix through that DAO parent. As a
further extension, a DAO Suboption for the Metric Container may
be included.</t>
</list></t>
</section>
<section title="Operation of DAO Storing Nodes">
<section title="DAO Routing Table Entry">
<?rfc subcompact="yes"?>
<t>A DAO Routing Table Entry conceptually contains the following
elements:</t>
<t><list style="symbols">
<t>Advertising Neighbor Information <list style="symbols">
<t>IPv6 Addr</t>
<t>Interface ID</t>
</list></t>
<t>To which DAO Parents has this entry been reported</t>
<t>Retry Counter</t>
<t>Logical equivalent of DAO Content: <list style="symbols">
<t>DAO Sequence</t>
<t>DAO Rank</t>
<t>DAO Lifetime</t>
<t>Route tag (used to prioritize which destination entries
should be stored)</t>
<t>Destination Prefix (or Address or Mcast Group)</t>
<t>RR Stack*</t>
</list></t>
</list></t>
<?rfc subcompact="no"?>
<t>The DAO Routing Table Entry is logically associated with the
following states:</t>
<t><list hangIndent="12" style="hanging">
<t hangText="CONNECTED">This entry is 'owned' by the node - it
is manually configured and is considered as a 'self' entry for
DAO Operation</t>
<t hangText="REACHABLE">This entry has been reported from a
neighbor of the node. This state includes the following
substates: <list hangIndent="10" style="hanging">
<t hangText="CONFIRMED">This entry is active, newly
validated, and usable</t>
<t hangText="PENDING">This entry is active, awaiting
validation, and usable. A Retry Counter is associated with
this substate</t>
</list></t>
<t hangText="UNREACHABLE">This entry is being cleaned up. This
entry may be suppressed when the cleanup process is
complete.</t>
</list></t>
<t>When an attempt is to be made to report the DAO entry to DAO
Parents, the DAO Entry record is logically marked to indicate that
an attempt has not yet been made for parent. As the unicast
attempts are completed for each parent, this mark may be cleared.
This mechanism may serve to limit DAO entry updates for each
parent to a subset that needs to be reported.</t>
<section title="DAO Routing Table Entry Management">
<figure title="DAO Routing Table Entry FSM">
<artwork><![CDATA[
+---------------------------------+
| |
| REACHABLE | +-------------+
| | | |
| +-----------+ | | CONNECTED |
(*)----------->| |-------+ | | |
| | Confirmed | | | +-------------+
| +-->| |---+ | |
| | +-----------+ | | |
| | | | |
| | | | |
| | | | |
| | +-----------+ | | | +-------------+
| | | |<--+ +-------->| |
| +---| Pending | | | UNREACHABLE |
| | |---------------->| |--->(*)
| +-----------+ | +-------------+
| |
+---------------------------------+
]]></artwork>
</figure>
<section title="Operation in the CONNECTED state">
<t><list style="numbers">
<t>CONNECTED DAO entries are to be provisioned outside of
the context of RPL, e.g. through a management API. An
implementation SHOULD provide a means to provision/manage
CONNECTED DAO entries, including whether they are to be
redistributed in RPL.</t>
</list></t>
</section>
<section title="Operation in the REACHABLE state">
<t><list style="numbers">
<t>When a REACHABLE(*) entry times out, the entry MUST be
placed into the UNREACHABLE state and no-DAO SHOULD be
scheduled to send to the node's DAO Parents. (TBD
MUST?)</t>
<t>When a no-DAO for a REACHABLE(*) entry is received with
a newer DAO Sequence Number, the entry MUST be placed into
the UNREACHABLE state and no-DAO SHOULD be scheduled to
send to the node's DAO Parents.</t>
<t>When a REACHABLE(*) entry is to be removed because NUD
or equivalent has determined that the next-hop neighbor is
no longer reachable, the entry MUST be placed into the
UNREACHABLE state and no-DAO SHOULD be scheduled to send
to the node's DAO Parents.</t>
<t>When a REACHABLE(*) entry is to be removed because an
associated Forwarding Error has been returned by the
next-hop neighbor, the entry MUST be placed into the
UNREACHABLE state and no-DAO SHOULD be scheduled to send
to the node's DAO Parents.</t>
<t>When a DAO (or no-DAO) for a REACHABLE(*) entry is
received with an older or unchanged DAO Sequence Number,
then the DAO (or no-DAO) SHOULD be ignored and the
associated entry MUST NOT be updated with the stale
information.</t>
</list></t>
<section title="REACHABLE(Confirmed)">
<t><list style="numbers">
<t>When a DAO for a previously unknown (or UNREACHABLE)
destination is received and is to be stored, it MUST be
entered into the routing table in the
REACHABLE(Confirmed) state. Alternately the node may
behave as a non-storing node with respect to this
destination.</t>
<t>When a DAO for a REACHABLE(Confirmed) entry is
received with a newer DAO Sequence Number the entry MUST
be updated with the logical equivalent of the DAO
contents.</t>
<t>When a DAO for a REACHABLE(Confirmed) entry is
expected, e.g. because a DIO to request a DAO refresh is
sent, then the DAO entry MUST be placed in the
REACHABLE(Pending) state and the associated Retry
Counter MUST be set to 0.</t>
</list></t>
</section>
<section title="REACHABLE(Pending)">
<t><list style="numbers">
<t>When a DAO for a REACHABLE(Pending) entry is received
with a newer DAO Sequence Number, the entry MUST be
updated with the logical equivalent of the DAO contents
and the entry MUST be placed in the REACHABLE(Confirmed)
state.</t>
<t>When a DAO for a REACHABLE(Pending) entry is
expected, e.g. because DAO has (again) been triggered
with respect to that neighbor, then the associated Retry
Counter MUST be incremented.</t>
<t>When a the associated Retry Counter for a
REACHABLE(Pending) entry reaches a maximum threshold,
the entry MUST be placed into the UNREACHABLE state and
no-DAO SHOULD be scheduled to send to the node's DAO
Parents.</t>
</list></t>
</section>
</section>
<section anchor="DAOUnreachable"
title="Operation in the UNREACHABLE state">
<t><list style="numbers">
<t>An implementation SHOULD bound the time that the entry
is allocated in the UNREACHABLE state. Upon the equivalent
expiry of the related timer (RemoveTimer), the entry
SHOULD be suppressed.</t>
<t>While the entry is in the UNREACHABLE state a node
SHOULD make a reasonable attempt to report a no-DAO to
each of the DAO parents.</t>
<t>When the node has completed an attempt to report a
no-DAO to each of the DAO parents, the entry SHOULD be
suppressed.</t>
</list></t>
</section>
</section>
</section>
</section>
<section anchor="DAONonStoring"
title="Operation of DAO Non-storing Nodes">
<t><list style="numbers">
<t>When a DAO is received from a child by a node who will not
store a routing table entry for the DAO, the node MUST schedule
to pass the DAO contents along to its DAO parents. Prior to
passing the DAO along, the node MUST process the DAO as follows,
in order that information necessary to construct a loose source
route may be accumulated within the DAO payload as it moves up
the DODAG:<list style="numbers">
<t>The most recent addition to the RRStack (the 'next
waypoint') is investigated to determine if the node already
has a route provisioned to the waypoint. If the node already
has such a route, then it is not necessary to add additional
information to the RRStack. The node SHOULD NOT modify the
RRStack further.</t>
<t>If the node does not have a route provisioned to the next
waypoint, then the node MUST append the address of the child
to the RRStack, and increment RRCount.</t>
</list></t>
</list></t>
</section>
<section anchor="ScheduleDAO"
title="Scheduling to Send DAO (or no-DAO)">
<t><list style="numbers">
<t>An implementation SHOULD arrange to rate-limit the sending of
DAOs.</t>
<t>When scheduling to send a DAO, an implementation SHOULD
equivalently start a timer (DelayDAO) to delay sending the DAO.
If the timer has already been armed then the DAO may be
considered as already scheduled, and implementation SHOULD leave
the timer running at its present duration.</t>
</list></t>
<t><list style="symbols">
<t>In order to increase the effectiveness of aggregation, an
implementation MAY allow time to receive no-DAOs from its
sub-DODAG prior to emitting DAOs to its DAO Parents. <list
style="symbols">
<t>The scheduled delay in such cases may be, for example,
such that DAO_LATENCY/f(self_rank) <= delay <
DAO_LATENCY/f(parent_rank), where f(rank) is
floor(rank/MinHopRankIncrease), such that nodes deeper in
the DODAG may tend to report DAO messages first before their
parent nodes will report DAO messages. Note that this
suggestion is intended as an optimization to allow efficient
aggregation -- it is not required for correct operation in
the general case.</t>
</list></t>
</list></t>
</section>
<section title="Triggering DAO Message from the Sub-DODAG">
<t>Note: The DIO is modified to add a 'S' flag, which is used to
indicate if a non-root ancestor storing routing table entries
learned from DAOs. This allows an optimization in the case where
ONLY the root node is storing such routing table entries, then it is
not necessary for an intermediate node to trigger DAO messages from
its sub-DODAG when it changes its DAO Parent.</t>
<t><list style="numbers">
<t>The DODAG root MUST clear the 'S' flag when it emits DIO
messages.</t>
<t>Non-root nodes that store routing table entries learned from
DAOs MUST set the 'S' flag when they emit DIO messages.</t>
<t>A node that has any DAO Parent with the 'S' flag set MUST
also set the 'S' flag when it emits DIO messages.</t>
<t>A node that has all DAO Parents with cleared 'S' flags MUST
clear the 'S' flag when it emits DIO messages.</t>
<t>A DAO Trigger Sequence Number (DTSN) MUST be maintained by
each node per RPL Instance. The DTSN, in conjunction with the
'T' flag from the DIO message, provides a means by which DAO
messages may be reliably triggered in the event of topology
change.</t>
<t>The DTSN MUST be advertised by the node in the DIO
message.</t>
<t>A node keeps track of the DTSN that it has heard from the
last DIO from each of its DAO Parents. Note that there is one
DTSN maintained per DAO Parent-- each DAO Parent may
independently increment it at will. (TBD A change to DTSN does
not indicate DAG inconsistency?).</t>
<t>A node that is not a fully-storing node SHOULD increment its
own DTSN when it adds a new parent, that parent having the 'S'
flag set, to its DAO Parent set. It MAY defer advertising the
increment as long as it has a DAO parent that already provides
adequate connectivity.</t>
<t>A node that is not a fully-storing node MUST increment its
own DTSN when it receives a DIO from a DAO Parent that contains
a newly incremented DTSN. (The newly incremented DTSN is
detected by comparing the value received in the DIO with the
value last recorded for that DAO parent).</t>
<t>A fully-storing node MUST increment its own DTSN when it
receives a DIO from a DAO Parent that contains a newly
incremented DTSN and a set 'T' flag.</t>
<t>When a storing or non-storing node joins a new DODAG
iteration, it SHOULD increment its DTSN as if the 'T' flag has
been set.</t>
<t>DAO Transmission SHOULD be scheduled when a new parent is
added to the DAO Parent set.</t>
<t>A node that receives a newly incremented DTSN from a DAO
Parent MUST schedule a DAO transmission.</t>
</list></t>
<t><list style="symbols">
<t>When a node that is not fully-storing sees a DTSN increment,
it will increment its own DTSN. This will cause the DTSN
increment to extend down the DODAG to the first fully-storing
node, which will send its DAOs back up, rebuilding source routes
information along the way to the first node that incremented the
DTSN, who then may report the new DAO information to its new
parent.</t>
<t>When a fully-storing node sees a DTSN increment, it is caused
to reissue its entire set of routing table entries learned from
DAOs (or an aggregated subset thereof), but will not need to
increment its own DTSN. The 'DTSN increment wave' stops when it
encounters fully-storing nodes.</t>
<t>When a fully-storing node sees a DTSN increment AND the 'T'
flag is set, it does increment its own DTSN as well. The 'T'
flag 'punches through' all nodes, causing all routing tables in
the entire sub-DODAG to be refreshed.</t>
</list></t>
</section>
<section title="Sending DAO Messages to DAO Parents">
<t><list style="numbers">
<t>When storing nodes send DAO messages for stored entries the
RRStack SHOULD be cleared in the DAO message.</t>
<t>DAO Messages sent to DAO Parents MUST be unicast.<list
style="symbols">
<t>The IPv6 Source Address is the node sending the DAO
message.</t>
<t>The IPv6 Destination Address is DAO parent.</t>
</list></t>
<t>When the appointed time arrives (DelayDAO) for the
transmission of DAO messages (with jitter as appropriate) for
the requested entries, the implementation MAY aggregate the the
entries into a reduced numbers of DAOs to be reported to each
parent, and perform compression if possible.</t>
<t>Note: it is NOT RECOMMENDED that a DAO Transmission (No-DAO)
be scheduled when a DAO Parent is removed from the DAO Parent
set.</t>
</list></t>
</section>
<section anchor="MulticastDAO"
title="Multicast Destination Advertisement Messages">
<t>A special case of DAO operation, distinct from unicast DAO
operation, is multicast DAO operation which may be used to populate
'1-hop' routing table entries.</t>
<t><list style="numbers">
<t>A node MAY multicast a DAO message to the link-local scope
all-nodes multicast address FF02::1.</t>
<t>A multicast DAO message MUST be used only to advertise
information about self, i.e. prefixes directly connected to or
owned by this node, such as a multicast group that the node is
subscribed to or a global address owned by the node.</t>
<t>A multicast DAO message MUST NOT be used to relay
connectivity information learned (e.g. through unicast DAO) from
another node.</t>
<t>Information obtained from a multicast DAO MAY be installed in
the routing table and MAY be propagated by a node in unicast
DAOs.</t>
<t>A node MUST NOT perform any other DAO related processing on a
received multicast DAO, in particular a node MUST NOT perform
the actions of a DAO parent upon receipt of a multicast DAO.</t>
</list></t>
<t><list style="symbols">
<t>The multicast DAO may be used to enable direct P2P
communication, without needing the RPL routing structure to
relay the packets.</t>
<t>The multicast DAO does not presume any DODAG relationship
between the emitter and the receiver.</t>
</list></t>
</section>
</section>
</section>
<!--
<section anchor="CrossRoutes" title="Cross-Routes"></section>
-->
<section anchor="forwarding"
title="Packet Forwarding and Loop Avoidance/Detection">
<section anchor="PacketForwarding"
title="Suggestions for 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 DODAG iteration that matches the RPLInstanceID
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 down the
sub-DODAG), then use that successor.</t>
<t>If there is a DODAG iteration offering a route to a prefix
matching the destination, then select one of those DODAG parents
as a successor.</t>
<t>If there is a DODAG parent offering a default route then select
that DODAG parent as a successor.</t>
<t>If there is a DODAG iteration offering a route to a prefix
matching the destination, but all DODAG parents have been tried
and are temporarily unavailable (as determined by the forwarding
procedure), then select a DODAG sibling as a successor.</t>
<t>Finally, if no DODAG 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 up to an down
flow, such as switching from DIO routes to DAO routes as the
destination is neared.</t>
</section>
<section anchor="loopdetect" title="Loop Avoidance and 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 IPv6 flow label. The IPv6 flow label is defined in <xref
target="RFC2460"></xref> and its operation is further specified in
<xref target="RFC3697"></xref>. For the purpose of RPL operations, the
flow label is constructed as follows:</t>
<t><figure anchor="flowlabel" title="RPL Flow Label">
<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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|O|S|R|F| SenderRank | RPLInstanceID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
]]></artwork>
</figure></t>
<t><list hangIndent="6" style="hanging">
<t hangText="Down 'O' bit:">1-bit flag indicating whether the
packet is expected to progress up or down. A router sets the 'O'
bit when the packet is expect to progress down (using DAO routes),
and resets it when forwarding towards the root of the DODAG
iteration. A host MUST set the bit to 0.</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="Forwarding-Error 'F' bit:">1-bit flag indicating that
this node can not forward the packet further towards the
destination. The 'F' bit might be set by sibling that can not
forward to a parent a packet with the Sibling 'S' bit set, or by a
child node that does not have a route to destination for a packet
with the down 'O' bit set. A host MUST set the bit to 0.</t>
<t hangText="SenderRank:">8-bit field set to zero by the source
and to its rank by a router that forwards inside the RPL
network.</t>
<t hangText="RPLInstanceID:">8-bit field indicating the DODAG
instance along which the packet is sent.</t>
</list></t>
<section title="Source Node Operation">
<t>A packet that is sourced at a node connected to a RPL network or
destined to a node connected to a RPL network MUST be issued with
the flow label zeroed out, but for the RPLInstanceID field.</t>
<t>If the source is aware of the RPLInstanceID that is preferred for
the flow, then it MUST set the RPLInstanceID field in the flow label
accordingly, otherwise it MUST set it to the
RPL_DEFAULT_INSTANCE.</t>
<t>If a compression mechanism such as 6LoWPAN is applied to the
packet, the flow label MUST NOT be compressed even if it is set to
all zeroes.</t>
</section>
<section title="Router Operation">
<section title="Conformance to RFC 3697">
<t><xref target="RFC3697"></xref> mandates that the Flow Label
value set by the source MUST be delivered unchanged to the
destination node(s).</t>
<t>In order to restore the flow label to its original value, an
RPL router that delivers a packet to a destination connected to a
RPL network or that routes a packet outside the RPL network MUST
zero out all the fields but the RPLInstanceID field that must be
delivered without a change.</t>
</section>
<section title="Instance Forwarding">
<t>Instance IDs are used to avoid loops between DODAGs from
different origins. DODAGs that constructed for antagonistic
constraints might contain paths that, if mixed together, would
yield loops. Those loops are avoided by forwarding a packet along
the DODAG that is associated to a given instance.</t>
<t>The RPLInstanceID is placed by the source in the flow label.
This RPLInstanceID MUST match the RPL Instance onto which the
packet is placed by any node, be it a host or router.</t>
<t>When a router receives a packet that is flagged with a given
RPLInstanceID and the node can forward the packet along the DODAG
associated to that instance, then the router MUST do so and leave
the RPLInstanceID flag unchanged.</t>
<t>If any node can not forward a packet along the DODAG associated
to the RPLInstanceID in the flow label, then the node SHOULD
discard the packet.</t>
</section>
<section title="DAG Inconsistency Loop Detection">
<t>The DODAG is inconsistent if the direction of a packet does not
match the rank relationship. A receiver detects an inconsistency
if it receives a packet with either: <list>
<t>the 'O' bit set (to down) from a node of a higher rank.</t>
<t>the 'O' bit reset (for up) 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>When the DODAG root increments the DODAGSequenceNumber a
temporary rank discontinuity may form between the next iteration
and the prior iteration, in particular if nodes are adjusting
their rank in the next iteration and deferring their migration
into the next iteration. A router that is still a member of the
prior iteration may choose to forward a packet to a (future)
parent that is in the next iteration. In some cases this could
cause the parent to detect an inconsistency because the
rank-ordering in the prior iteration is not necessarily the same
as in the next iteration and the packet may be judged to not be
making forward progress. If the sending router is aware that the
chosen successor has already joined the next iteration, then the
sending router MUST update the SenderRank to INFINITE_RANK as it
forwards the packets across the discontinuity into the next DODAG
iteration in order to avoid a false detection of rank
inconsistency.</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 DODAGID,
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 DODAG iteration 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
then the router SHOULD return the packet to the sibling that that
passed it with the Forwarding-Error 'F' bit set.</t>
</section>
<section title="DAO Inconsistency Loop Detection and Recovery">
<t>A DAO inconsistency happens when router that has an down DAO
route via a child that is a remnant from an obsolete state that is
not matched in the child. With DAO inconsistency loop recovery, a
packet can be used to recursively explore and cleanup the obsolete
DAO states along a sub-DODAG.</t>
<t>In a general manner, a packet that goes down should never go up
again. So rather than routing up a packet with the down 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 Forwarding-Error 'F' bit set.</t>
</section>
<section title="Forward Path Recovery">
<t>Upon receiving a packet with a Forwarding-Error bit set, the
node MUST remove the routing states that caused forwarding to that
neighbor, clear the Forwarding-Error bit and attempt to send the
packet again. The packet may its way to an alternate neighbor. If
that alternate neighbor still has an inconsistent DAO state via
this node, the process will recurse, this node will set the
Forwarding-Error 'F' bit and the routing state in the alternate
neighbor will be cleaned up as well.</t>
</section>
</section>
</section>
</section>
<section title="Multicast Operation">
<t>This section describes further the multicast routing operations over
an IPv6 RPL network, and specifically how unicast DAOs can be used to
relay group registrations up. 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 DODAG root, MLD requests
are mapped and transported as DAO messages within the RPL protocol; each
hop coalesces the multiple requests for a same group as a single DAO
message to the parent(s), in a fashion similar to proxy IGMP, but
recursively between child router and parent up to the root.</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-DODAG if the transmission fails over
that link. Alternatively the router might select to copy additional
parents as it would do for DAO messages advertising unicast
destinations, in which case there might be duplicates that the router
will need to prune.</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
DODAG terminates RPL and MAY redistribute the RPL routes over the
external infrastructure using whatever routing protocol is used there.
For multicast traffic, the root MAY proxy MLD for all the nodes attached
to the RPL routers (this would be needed if the multicast source is
located in the external infrastructure). For such a source, the packet
will be replicated as it flows down the DODAG based on the multicast
routing table entries installed from the DAO message.</t>
<t>For a source inside the DODAG, the packet is passed to the preferred
parents, and if that fails then to the alternates in the DODAG. The
packet is also copied to all the registered children, except for the one
that passed the packet. Finally, if there is a listener in the external
infrastructure then the DODAG root has to further propagate the packet
into the external infrastructure.</t>
<t>As a result, the DODAG Root acts as an automatic proxy Rendezvous
Point for the RPL network, and as source towards the Internet for all
multicast flows started in the RPL LLN. So regardless of whether the
root is actually attached to the Internet, and regardless of whether the
DODAG is grounded or floating, the root can serve inner multicast
streams at all times.</t>
</section>
<section anchor="MaintenanceRoutingAdjacency"
title="Maintenance of Routing Adjacency">
<t>The selection of successors, along the default paths up along the
DODAG, or along the paths learned from destination advertisements down
along the DODAG, 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 title="Guidelines for Objective Functions">
<t>An Objective Function (OF) allows for the selection of a DODAG to
join, and a number of peers in that DODAG as parents. The OF is used to
compute an ordered list of parents. The OF is also responsible to
compute the rank of the device within the DODAG iteration.</t>
<t>The Objective Function is indicated in the DIO message using an
Objective Code Point (OCP), as specified in <xref
target="I-D.ietf-roll-routing-metrics"></xref>, and indicates the method
that must be used to compute the DODAG (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"></xref>,
<xref target="I-D.ietf-roll-of0"></xref>, and related companion
specifications.</t>
<t>Most Objective Functions are expected to follow the same abstract
behavior: <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 DODAG.
There might be multiple of them and a candidate neighbor might need
to pass some validation tests before it can be used. In particular,
some link layers require experience on the activity with a router to
enable the router as a next hop.</t>
<t>An OF computes self's rank by adding to the rank of the candidate
a value representing the relative locations of self and the
candidate in the DODAG iteration.<list style="symbols">
<t>The increase in rank must be at least MinHopRankIncrease.
(This prevents the creation of a path of sibling links
connecting a child with its parent.)</t>
<t>To keep loop avoidance and metric optimization in alignment,
the increase in rank should reflect any increase in the metric
value. For example, with a purely additive metric such as ETX,
the increase in rank can be made proportional to the increase in
the metric.</t>
<t>Candidate neighbors that would cause self's rank to increase
are ignored</t>
</list></t>
<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. <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>
<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: <list style="symbols">
<t>Candidate neighbors that are not in the same DODAG 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></t>
</list></t>
</section>
<section title="RPL Constants and Variables">
<t>Following is a summary of RPL constants and variables. Some default
values are to be determined in companion applicability statements.</t>
<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 DODAG 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 RPLInstanceID that is
used by this protocol by a node without any overriding policy.
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="DEFAULT_DIO_REDUNDANCY_CONSTANT">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 DODAG 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 DODAG that the node is acting as DODAG root of. May not be
supported in all implementations. Expiry triggers revision of
DODAGSequenceNumber, causing a new series of updated DIO message to
be sent. Interval should be chosen appropriate to propagation time
of DODAG and as appropriate to application requirements (e.g.
response time vs. overhead).</t>
<t hangText="DelayDAO Timer">Up to one instance per DAO parent (the
subset of DODAG parents chosen to receive destination
advertisements) per DODAG. Expiry triggers sending of DAO message to
the DAO parent. See <xref target="ScheduleDAO"></xref></t>
<t hangText="RemoveTimer">Up to one instance per DAO entry per
neighbor (i.e. those neighbors that have given DAO messages to this
node as a DODAG parent) Expiry triggers a change in state for the
DAO entry, setting up to do unreachable (No-DAO) advertisements or
immediately deallocating the DAO entry if there are no DAO parents.
See <xref target="DAOUnreachable"></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
DODAG, or to immediately root a transient DODAG 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 DODAGs, or should simply wait until it
received DIO messages from other nodes that are part of existing
DODAGs.</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="DIOBase"></xref>:</t>
<?rfc subcompact="yes"?>
<t><list hangIndent="6" style="hanging">
<t hangText="DAGPreference"></t>
<t hangText="RPLInstanceID"></t>
<t hangText="DAGObjectiveCodePoint"></t>
<t hangText="DODAGID"></t>
<t hangText="Destination Prefixes"></t>
<t hangText="DIOIntervalDoublings"></t>
<t hangText="DIOIntervalMin"></t>
<t hangText="DIORedundancyConstant"></t>
<t></t>
<t hangText="DAG Root behavior:">In some cases, a node may not
want to permanently act as a DODAG root if it cannot join a
grounded DODAG. For example a battery-operated node may not want
to act as a DODAG root for a long period of time. Thus a RPL
implementation MAY support the ability to configure whether or
not a node could act as a DODAG root for a configured period of
time.</t>
<t></t>
<t hangText="DODAG Table Entry Suppression">A RPL implementation
SHOULD provide the ability to configure a timer after the
expiration of which logical equivalent of the DODAG table that
contains all the records about a DODAG is suppressed, to be
invoked if the DODAG parent set becomes empty.</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 DODAG
root along the DODAG in DIO messages.</t>
<t>For each DODAG, a RPL implementation MUST allow for the
monitoring of the following parameters, further described in <xref
target="TrickleImplementation"></xref>:</t>
<?rfc subcompact="yes"?>
<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>
<?rfc subcompact="no"?>
<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 DODAG root
to refresh the DODAG states by updating the DODAGSequenceNumber. A
RPL implementation SHOULD allow configuring whether or not periodic
or event triggered mechanism are used by the DODAG root to control
DODAGSequenceNumber change.</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 DODAG 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 DODAG, 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="UpwardTopology"></xref>, an
implementation is expected to maintain a set of data structures in
support of DODAG discovery:</t>
<t><list style="symbols">
<t>The candidate neighbors data structure</t>
<t>For each DODAG: <list style="symbols">
<t>A set of DODAG parents</t>
</list></t>
</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 is expected to keep track of
the following DODAG table values:</t>
<t><list style="symbols">
<t>DODAGID</t>
<t>DAGObjectiveCodePoint</t>
<t>A set of Destination Prefixes offered upwards along the
DODAG</t>
<t>A set of DODAG Parents</t>
<t>timer to govern the sending of DIO messages for the DODAG</t>
<t>DODAGSequenceNumber</t>
</list></t>
<t>The set of DODAG 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 DODAG Parent</t>
<t>A flag reporting if the Parent is a DAO Parent as described
in <xref target="DownwardRoutes"></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 DODAG parent, e.g. if the DODAGID 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 DODAG 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>
<t>The DIORedundancyConstant</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">
<t>IANA is requested to create a registry for the Control field of the
DIO Base.</t>
<t>New fields may be allocated only by an IETF Consensus action. Each
field 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 Flags">
<ttcol align="center">Bit</ttcol>
<ttcol align="left">Description</ttcol>
<ttcol align="left">Reference</ttcol>
<c>0</c>
<c>Grounded DODAG (G)</c>
<c>This document</c>
<c>1</c>
<c>Destination Advertisement Supported (A)</c>
<c>This document</c>
<c>2</c>
<c>Destination Advertisement Trigger (T)</c>
<c>This document</c>
<c>3</c>
<c>Destination Advertisements Stored (S)</c>
<c>This document</c>
<c>5,6,7</c>
<c>DODAG Preference (Prf)</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
Suboptions</t>
<texttable title="DAG Information Option (DIO) Base 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>
<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
Thomas Heide Clausen
LIX, Ecole Polytechnique, France
Phone: +33 6 6058 9349
EMail: T.Clausen@computer.org
URI: http://www.ThomasClausen.org/
Philip Levis
Stanford University
358 Gates Hall, Stanford University
Stanford, CA 94305-9030
USA
Email: pal@cs.stanford.edu
Richard Kelsey
Ember Corporation
Boston, MA
USA
Phone: +1 617 951 1225
Email: kelsey@ember.com
Jonathan W. Hui
Arch Rock Corporation
501 2nd St. Ste. 410
San Francisco, CA 94107
USA
Email: jhui@archrock.com
Kris Pister
Dust Networks
30695 Huntwood Ave.
Hayward, 94544
USA
Email: kpister@dustnetworks.com
Anders Brandt
Zensys, Inc.
Emdrupvej 26
Copenhagen, DK-2100
Denmark
Email: abr@zen-sys.com
Stephen Dawson-Haggerty
UC Berkeley
Soda Hall, UC Berkeley
Berkeley, CA 94720
USA
Email: stevedh@cs.berkeley.edu
]]></artwork>
</figure>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
<?rfc include="reference.RFC.2460"?>
</references>
<references title="Informative References">
<?rfc include='reference.I-D.ietf-roll-building-routing-reqs.xml'?>
<?rfc include='reference.I-D.ietf-roll-home-routing-reqs.xml'?>
<?rfc include='reference.RFC.3697'?>
<?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.ietf-roll-of0.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.RFC.1982"?>
<?rfc include="reference.I-D.ietf-bfd-base.xml"?>
<?rfc include="reference.I-D.ietf-manet-nhdp.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 DODAG depicted in <xref
target="DAGExample"></xref>, where the links depicted are the edges
toward DODAG 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 DODAG at rank 2, and periodically multicast
DIOs. Node (22) has selected (11) and (12) in its DODAG parent set, and
advertises itself at rank 3. Node (22) thus has a set of DODAG 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 DODAG 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 DODAG parents, wherein all
depicted edges are directed and oriented 'up' on the page toward the
DODAG root (LBR). The DODAG 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 anchor="DestinationAdvertisementExample"
title="Destination Advertisement">
<t>Consider the example DODAG 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: DODAG 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 DODAGID 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 DODAGID 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 enough confidence to trigger a decision to
join DODAGID 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 DODAG parents for
DODAGID 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 DODAGID 1.</t>
<t>Node (N) adds Node (B) (rank 4) to its set of DODAG parents for
DODAGID 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 upwards along DODAGID 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 upwards progress but with the intention that node (C) or a
following successor can make upwards 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: DODAG 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 DODAG at some rank d. Node (A) is a DODAG parent of
Nodes (B) and (C). Node (C) is a DODAG 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-DODAG, 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 DODAG 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 DODAG by removing Node (A)
from its DODAG parent set, leaving an empty DODAG 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-DODAG of Node (C) will follow Node (C)
into the floating DAG, maintaining the structure of the
sub-DODAG.</t>
<t>Node (C) hears a DIO message with an incremented
DODAGSequenceNumber from Node (B) and determines it is able to
rejoin the grounded DODAG by reattaching at a deeper rank to Node
(B). Node (C) adds Node (B) to its DODAG parent set. Node (C) has
now safely moved deeper within the grounded DODAG without creating
any loops.</t>
<t>Node (D), and any other sub-DODAG 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
DODAG 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 DODAG 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
DODAG 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 DODAG parent for Nodes (B)
and (C), and (B)--(C) is a sibling link. In <xref
target="Greedy"></xref>-2, Node (A) is a DODAG parent for Nodes (B)
and (C), and Node (B) is also a DODAG parent for Node (C). In <xref
target="Greedy"></xref>-3, Node (A) is a DODAG parent for Nodes (B)
and (C), and Node (C) is also a DODAG 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 DODAG illustrated in <xref
target="Greedy"></xref>-1. In this example, Nodes (B) and (C) may most
prefer Node (A) as a DODAG 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 DODAG and rejoin at
a lower rank, taking both Nodes (A) and (B) as DODAG 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 DODAG 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 DODAG and rejoins at a lower rank,
taking both Nodes (A) and (C) as DODAG 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 DODAG 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-DODAGs)</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 upwards along the DODAG until a common ancestor is
reached and then flowing down, may not be suitable for all application
scenarios. A related scenario may occur when the down paths setup
along the DODAG by the destination advertisement mechanism are not be
the most desirable downward paths for the specific application
scenario (in part because the DODAG links may not be symmetric). It
may be desired to support within RPL the discovery and installation of
more direct routes 'across' the DAG. Such mechanisms need to be
investigated.</t>
</section>
<!-- DONE (pending Extension Header vs. Flow Label)
<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 DODAG parent, in
some cases a situation may be created where a large number of DAO
messages conveying information about the same destination flow upwards
along the DAG. It is desirable to bound/limit the
multiplication/fan-out of DAO messages in this manner. Some aspects of
the Destination Advertisement mechanism remain under investigation,
such as behavior in the face of links that may not be symmetric.</t>
<t>In general, the utility of providing redundancy along downwards
routes by sending DAO messages to more than one parent is under
investigation.</t>
<t>The use of suitable triggers, such as the 'T' flag, to trigger DA
operation within an affected sub-DODAG, is under investigation.
Further, the ability to limit scope of the affected depth within the
sub-DODAG 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 'T' flag 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 title="Managing Multiple Instances">
<t>A network may run multiple instances of RPL concurrently. Such a
network will require methods for assigning and otherwise managing
RPLInstanceIDs. This will likely be addressed in a separate
document.</t>
</section>
</section>
</back>
</rfc>
| PAFTECH AB 2003-2026 | 2026-04-23 03:06:23 |