One document matched: draft-ietf-roll-of0-08.xml
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<rfc category="std" docName="draft-ietf-roll-of0-08" ipr="trust200902">
<front>
<title abbrev="draft-ietf-roll-of0">RPL Objective Function 0</title>
<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>
<date />
<area>Routing Area</area>
<workgroup>ROLL</workgroup>
<keyword>Draft</keyword>
<abstract>
<t>The Routing Protocol for Low Power and Lossy Networks
defines a generic Distance Vector protocol for Low Power and
Lossy Networks. That generic protocol requires a specific
Objective Function to establish a desired routing topology.
This specification defines a basic Objective Function that
operates solely with the protocol elements defined in the
generic protocol specification.
</t>
</abstract>
<note title="Requirements Language">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in <xref
target="RFC2119">RFC 2119</xref>.</t>
</note>
</front>
<middle>
<section title="Introduction">
<t>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="RFC5548"></xref>,
<xref target="RFC5673"></xref>,
<xref target="RFC5826"></xref>, and
<xref target="RFC5867"></xref>.
</t>
<t>
<xref target="I-D.ietf-roll-rpl"> The Routing Protocol for Low Power and Lossy Networks </xref>
was designed as a generic core that is agnostic to metrics and that is adapted to a given problem
using Objective Functions (OF). This separation of Objective Functions from
the core protocol specification allows RPL to adapt to meet the different optimization
criteria the wide range of use cases requires.</t>
<t>RPL forms Destination Oriented Directed Acyclic Graphs (DODAGs) within instances of the protocol.
Each instance is associated with an Objective Function that is designed to solve the problem that is
addressed by that instance.</t>
<t>An Objective Function selects the DODAG version that a device joins, and a number
of neighbor routers within that version as parents or feasible successors.
The OF generates the Rank of the device, that represents an abstract distance to the root within the DODAG.
In turn, the Rank is used by the generic RPL core to enable a degree of loop avoidance
and verify forward progression towards a destination, as specified in <xref target="I-D.ietf-roll-rpl"/>.</t>
<t>The Objective Function 0 (OF0) corresponds to the Objective Code Point 0 (OCP0).
OF0 only requires the information in the RPL DIO base container, such as
Rank and the DODAGPreference field that describes an administrative preference
<xref target="I-D.ietf-roll-rpl"/>.
The Rank of a node is obtained by adding a normalized scalar Rank-increase
to the Rank of a selected preferred parent.
OF0 uses a unit of Rank-increase of 0x100 so that Rank value can be stored in one octet.
This allows up to at least 28 hops even when default settings are used and
each hop has the worst Rank-increase of 9.</t>
<!-- t>
How the link properties are transformed into a Step-of-Rank for a given hop
depends on the link type and on the implementation. It can be as simple as an administrative cost,
but might also derive from a statistical metric with some hysteresis.</t -->
<t>
Since there is no default OF or metric container in the RPL main specification, it might
happen that, unless given two implementations follow a same guidance for a specific problem
or environment, those implementations will not support a common OF with which they could interoperate.
OF0 is designed to be used as a common denominator between all generic implementations.
This is why it is very abstract as to how the link properties are transformed into a Rank-increase
and leaves that responsibility to implementation;
rather, OF0 enforces normalized values for the Rank-increase of a normal link and its acceptable range,
as opposed to formulating the details of the its computation.
This is also why OF0 ignores metric containers.
</t>
<!-- t>
Indeed, it is the general design in RPL that the metrics are passed from parent to children in
a specific container and that the OF will derive the Rank from the natural metric.
The separation of Rank and metrics avoids a loss of information as the various metrics are propagated down the DAG.
This specification can be used when the link properties that are considered are
such that they can be turned in a scalar Step-of-Rank in a reversible fashion and the resulting
Step-of-Rank is additive over multiple hops. </t -->
</section>
<section anchor="Terminology" title="Terminology">
<t>The terminology used in this document is consistent with and
incorporates that described in `Terminology in Low power And Lossy
Networks' <xref target="I-D.ietf-roll-terminology"></xref>
and <xref target="I-D.ietf-roll-rpl"/>.</t>
<t>The term feasible successor is used to refer to a neighbor that can possibly be used as
a next-hop for upwards traffic following the loop avoidance and forwarding rules that
the nodes implements and that
are defined outside of this specification, in particular in the RPL specification.
</t>
</section>
<!--
<section title="Objective Functions">
<t>An Objective Function (OF) allows for the selection of a DAG to
join, and a number of peers in that DAG as parents. The OF is used
to compute an ordered list of parents and provides load balancing
guidance. The OF is also responsible to compute the Rank of the
device within the DAG.</t>
<t><xref target="I-D.ietf-roll-rpl"/> defines the operation of RPL
and the generic interfaces that all OFs need to implement. OF0
does not require additional headers and is implemented using
only the information in the DIO Base.</t>
<t>The Objective Function is specified in the RA-DIO message using
an objective code point (OCP) and indicates the objective function
that has been used to compute the DAG (e.g. "minimize the path cost
using the ETX metric and avoid `Blue' links"). The objective code
points are specified in <xref
target="I-D.ietf-roll-routing-metrics" />. This document specifies
the OCP 0, in support of default operation.</t>
<t>OFO follows the same abstract behavior specified in < xref
target="I-D.ietf-roll-rpl" />: </t>
<list style="symbols">
<t>The parent selection is triggered each time an event indicates
that a potential next_hop information is updated. This might
happen upon the reception of a RA-DIO message, a timer elapse, or
a trigger indicating that the state of a candidate neighbor has
changed.</t>
<t>An OF scans all the interfaces on the device. Although there
may typically be only one interface in most application scenarios,
there might be multiple of them and an interface might be
configured to be usable or not for RPL operation. An interface can
also be configured with a preference or dynamically learned to be
better than another by some heuristics that might be link-layer
dependent and are out of scope. Finally an interface might or not
match a required criterion for an Objective Function, for instance
a degree of security. As a result some interfaces might be
completely excluded from the computation, while others might be
more or less preferred.</t>
<t>The OF scans all the candidate neighbors on the possible
interfaces to check whether they can act as an attachment router
for a DAG. There might be multiple of them and a candidate
neighbor might need to pass some validation tests before it can be
used. In particular, some link layers require experience on the
activity with a router to enable the router as a next_hop.</t>
<t>The OF computes self's Rank by adding the Step-of-Rank to that
candidate to the Rank of that candidate. The Step-of-Rank is
estimated as follows:</t>
<list style="symbols">
<t>The Step-of-Rank might vary from MINIMUM_STEP_OF_RANK to
MAXIMUM_STEP_OF_RANK.
</t>
<list style="symbols">
<t>MINIMUM_STEP_OF_RANK indicates a unusually good link, for instance a link
between powered devices in a mostly battery operated
environment.</t>
<t>DEFAULT_STEP_OF_RANK indicates a `normal'/typical link, as qualified by the
implementation.</t>
<t>MAXIMUM_STEP_OF_RANK indicates a link that can hardly be used to forward any
packet, for instance a radio link with quality indicator or
expected transmission count that is close to the acceptable
threshold.</t>
</list>
<t>Candidate neighbors that would cause self's Rank to increase
are ignored</t>
</list>
<t>Candidate neighbors that advertise an OF incompatible with the
set of OF specified by the policy functions are ignored.</t>
<t>As it scans all the candidate neighbors, the OF keeps the
current best parent and compares its capabilities with the current
candidate neighbor. The OF defines a number of tests that are
critical to reach the Objective. A test between the routers
determines an order relation.</t>
<list style="symbols">
<t>If the routers are roughly equal for that relation then the
next test is attempted between the routers,</t>
<t>Else the best of the 2 becomes the current best parent and
the scan continues with the next candidate neighbor</t>
<t>Some OFs may include a test to compare the Ranks that would
result if the node joined either router</t>
</list>
<t>When the scan is complete, the preferred parent is elected and
self's Rank is computed as the preferred parent Rank plus the step
in Rank with that parent.</t>
<t>Other rounds of scans might be necessary to elect alternate
parents and siblings. In the next rounds:</t>
<list style="symbols">
<t>Candidate neighbors that are not in the same DAG are
ignored</t>
<t>Candidate neighbors that are of worse Rank than self are
ignored</t>
<t>Candidate neighbors of a better Rank than self (non-siblings)
are preferred</t>
</list>
</list>
</section>
-->
<!--
<t>This document specifies a default objective metric, called OF0,
and using the OCP 0. OF0 is the default objective function of RPL,
and can be used if allowed by the policy of the processing node when
no objective function is included in the RA-DIO message, or if the
OF indicated in the RA-DIO message is unknown to the node. If not
allowed, then the RA-DIO message is simply ignored and not processed
by the node.</t>
-->
<section title="Objective Function 0 Overview">
<t>The core RPL specification describes constraints on how nodes select
potential parents, called a parent set, from their neighbors.
All parents are feasible successors for upgoing traffic (towards
the root). Additionally, RPL allows the use of nodes in a
subsequent version of a same DODAG as feasible successors, in which
case this node acts as a leaf in the subsequent DODAG version.
Further specifications might extend the set of feasible successors,
for instance to nodes of a same Rank, aka siblings.
</t>
<t> The Goal of the OF0 is for a node to join a DODAG version that
offers connectivity to a specific set of nodes or to
a larger routing infrastructure.
For the purpose of OF0, Grounded thus means that the root
provides such connectivity. How that connectivity is asserted
and maintained is out of scope.</t>
<t>Objective Function 0 is designed to find the nearest Grounded
root. This can be achieved if the Rank of a node represents closely
its distance to the root. This need is balanced with the
other need of maintaining some path diversity.</t>
<t> In the absence of a Grounded root, LLN inner connectivity
is still desirable and floating DAGs will form, rooted at the
nodes with the highest administrative preference.</t>
<t>OF0 selects a preferred parent and a backup feasible successor if one is
available. All the upward traffic is normally routed via the preferred parent.
When the link conditions do not let an upward packet through the preferred parent,
the packet is passed to the backup feasible successor.</t>
<t>OF0 assigns a Step-of-Rank to each link to another node that it monitors.
The exact method for computing the Step-of-Rank is implementation-dependent.
<!-- This Step-of-Rank can be augmented by a configurable Stretch-of-Rank
to enable the node to obtain an alternate feasible successor.
A configurable Rank-factor can be used to multiply the effect of the
resulting value in the Rank-increase computation. If the Rank-increase
is computed from a Step-of-Rank Sp to a given parent,
a Stretch-of-Rank Sr, and a Rank-factor F, then the increase I is:
I = F * (Sp + Sr).
-->
</t>
<t>One trivial OF0 implementation might compute the Step-of-Rank from
as a classical administrative cost that is assigned to the link.
Using a metric similar to hop count implies that the OF0 implementation
only considers neighbors with good enough connectivity, for instance
neighbors that are reachable over an ethernet link, or a WIFI link in
infrastructure mode.</t>
<t>In most wireless networks, a Rank that is analogous to an unweighted hop count
favors paths with long distance links and poor connectivity properties.
Other link properties such as the expected transmission count metric (ETX)
<xref target="DeCouto03"/> should be used instead to compute the Step-of-Rank.
For instance, <xref target="I-D.ietf-roll-minrank-hysteresis-of">
the Minimum Rank Objective Function with Hysteresis </xref>
provides guidance on how link cost can be computed and on how
hysteresis can improve Rank stability.</t>
<t>
An implementation MAY allow to stretch the Step-of-Rank with a Stretch-of-Rank
up to no more than MAXIMUM_RANK_STRETCH in order to enable the selection of a
feasible successor in order to maintain some path diversity.
The use of a Stretch-of-Rank augments the apparent distance
from the node to the root and distorts the DODAG; it should be used with
care so as to avoid instabilities due to greedy behaviours.
</t>
<t>
The Step-of-Rank is expressed in units of MINIMUM_STEP_OF_RANK.
As a result, the least significant octet in the RPL Rank is not used.
The default Step-of-Rank is DEFAULT_STEP_OF_RANK for each hop.
An implementation MUST maintain the stretched Step-of-Rank between
MINIMUM_STEP_OF_RANK and MAXIMUM_STEP_OF_RANK, which allows to reflect a
large variation of link quality.</t>
<t>The gap between MINIMUM_STEP_OF_RANK and MAXIMUM_RANK_STRETCH
may not be sufficient in every case to strongly distinguish links of different
types or categories in order to favor, say, powered over battery-operated or wired
over wireless, within a same DAG. An implementation SHOULD allow a configurable factor
called Rank-factor and to apply the factor on all links and peers.
</t>
<t>An implementation MAY recognizes sub-categories of peers and links,
such as different MAC types, in which case it SHOULD be able to configure a more
specific Rank-factor to those categories.
The Rank-factor SHOULD be set between MINIMUM_RANK_FACTOR and MAXIMUM_RANK_FACTOR.
The Step-of-Rank Sp that is computed for that link s multipled by the
Rank-factor Rf and then possibly stretched by a Stretch-of-Rank Sr.
The resulting Rank-increase Ri is added
to the Rank of preferred parent R(P) to obtain that of this node R(N):
</t>
<t>R(N) = R(P) + Ri where Ri = Rf*Sp + Sr.
</t>
<t>Optionally, the administrative preference of a root MAY be configured
to supercede the goal to reach Grounded root. In that case, nodes will
associate to the root with the highest preference available, regardless
of whether that root is Grounded or not. Compared to a deployment with
a multitude of Grounded roots that would result in a same multitude of DODAGs,
such a configuration may result in possibly less but larger DODAGs, as many as
roots configured with the highest priority in the reachable vincinity.</t>
</section>
<section title="Selection of the Preferred Parent">
<t>As it scans all the candidate neighbors, OF0 keeps the parent
that is the best for the following criteria (in order):
<list style="numbers">
<t><xref target="I-D.ietf-roll-rpl"/> spells out the generic
rules for a node to reparent and in particular the boundaries
to augment its Rank within a DODAG version.
A candidate that would not satisfy those rules MUST NOT be considered.</t>
<t>An implementation should validate a router prior to selecting it
as preferred. This validation process is implementation and link type
dependent, and is out of scope. A router that has been validated
is preferrable.</t>
<t>When multiple interfaces are available,
a policy might be locally configured to prioritize them and that policy applies first;
that is a router on a higher order interface is preferable.</t>
<t>In the absence of a Grounded DODAG version, the router with a higher
administrative preference SHOULD be preferred. Optionally, this selection applies
regardless of whether the DODAG is Grounded or not.</t>
<t>A router that offers connectivity to a grounded DODAG version SHOULD be
preferred over one that does not.</t>
<t>When comparing 2 routers that belong to the same DODAG, a router that
offers connectivity to the freshest version SHOULD be preferred.</t>
<t>The parent that causes the lesser resulting Rank for this node SHOULD be preferred.</t>
<t>A DODAG version for which there is an alternate parent SHOULD be preferred.
This check is optional. It is performed by computing the
backup feasible successor while assuming that the router that is currently examined
is finally selected as preferred parent.</t>
<!-- t>The DODAG version that was in use already SHOULD be preferred.</t -->
<t>The preferred parent that was in use already SHOULD be preferred.</t>
<t>A router that has announced a DIO message more recently SHOULD be preferred.</t>
</list>
</t>
</section>
<section title="Selection of the backup feasible successor">
<t>When selecting a backup feasible successor, the OF performs in order the
following checks:
<list style="numbers">
<t>When multiple interfaces are available,
a router on a higher order interface is preferable.</t>
<t>The backup feasible successor MUST NOT be the preferred parent.</t>
<t>
The backup feasible successor MUST be either in the same DODAG version as the
preferred parent or in an subsequent version. Note that if the backup feasible successor
is not from the current version then it can not be used as parent.</t>
<t>Along with RPL rules, a Router with a Rank that is higher than the Rank
computed for this node MUST NOT be selected as a feasible successor.
Further specifications might allow a node of a same Rank as a feasible successor.
<!--
Further specifications might extend the set of feasible successors,
for instance to nodes of a same Rank, aka siblings.It MAY still be selected as sibling if no better Back-up next hop is found. -->
</t>
<t>A router with a lesser Rank SHOULD be preferred.</t>
<t>A router that has been validated as usable by an implementation dependant
validation process SHOULD be preferred.</t>
<t>The backup feasible successor that was in use already SHOULD be preferred.</t>
</list>
</t>
</section>
<section anchor="API" title="Abstract Interface with RPL core">
<t>Objective Function 0 interacts with the core RPL in the following ways:
<list
hangIndent="11" style="hanging">
<t hangText="Processing DIO:">This core RPL triggers the OF when a new
DIO was received. OF0 analyses the information in the DIO and may select
the source as a parent or sibling.</t>
<t hangText="Providing DAG information">The OF0 support can be required to
provide the DAG information for a given instance to the RPL core. This includes
the material that is contained in a DIO base header.</t>
<!-- t>
hangText="Generating Options">The OF0 support can be required to
provide the material that is necessary to build a DIO option.</t -->
<t hangText="Providing a Parent List">The OF0 support can be required to
provide the ordered list of the parents and feasible successors
for a given instance to the RPL core.
This includes the material that is contained in the transit option for
each entry.</t>
<t hangText="Trigger">The OF0 support may trigger the RPL core
to inform it that a change occurred. This can be used to indicate whether the change
requires a new DIO to be fired or whether trickle timers need to be reset.</t>
</list></t>
</section>
<section anchor="const" title="OF0 Constants and Variables">
<t>OF0 uses the following constants:
<list style="hanging">
<t hangText="MinHopRankIncrease:">256</t>
<t hangText="DEFAULT_STEP_OF_RANK:">3 * MinHopRankIncrease</t>
<t hangText="MINIMUM_STEP_OF_RANK:">1 * MinHopRankIncrease</t>
<t hangText="MAXIMUM_STEP_OF_RANK:">9 * MinHopRankIncrease</t>
<t hangText="MAXIMUM_RANK_STRETCH:">5 * MinHopRankIncrease</t>
<t hangText="DEFAULT_RANK_FACTOR:">1</t>
<t hangText="MINIMUM_RANK_FACTOR:">1</t>
<t hangText="MAXIMUM_RANK_FACTOR:">4</t>
</list></t>
</section>
<section anchor="IANA" title="IANA Considerations">
<t>IThis specification requires the assignment of an OCP for OF0. The value of 0 is suggested.</t>
</section>
<section anchor="Sec" title="Security Considerations">
<t>Security Considerations for OCP/OF 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="Acknowledgements" title="Acknowledgements">
<t>Most specific thanks to Philip Levis for his help in finalizing this document, in
particular WRT wireless links, to Tim Winter, JP Vasseur, Julien Abeille, Mathilde
Durvy, Teco Boot, Navneet Agarwal and Henning Rogge for in-depth review and first
hand implementer's feedback.</t>
</section>
</middle>
<back>
<references title="Normative References">
<?rfc include="reference.RFC.2119"?>
</references>
<references title="Informative References">
<!-- 5867?rfc include='reference.I-D.draft-ietf-roll-building-routing-reqs-07.xml'? -->
<!-- 5826?rfc include='reference.I-D.draft-ietf-roll-home-routing-reqs-08.xml'? -->
<?rfc include='reference.I-D.ietf-roll-rpl.xml'?>
<?rfc include="reference.RFC.5548"?>
<?rfc include="reference.RFC.5826"?>
<?rfc include="reference.RFC.5867"?>
<?rfc include='reference.I-D.ietf-roll-terminology.xml'?>
<?rfc include="reference.RFC.5673"?>
<?rfc include='reference.I-D.ietf-roll-routing-metrics.xml'?>
<?rfc include='reference.I-D.tsao-roll-security-framework.xml'?>
<?rfc include="reference.I-D.ietf-roll-minrank-hysteresis-of"?>
<reference anchor="DeCouto03"
target="http://pdos.csail.mit.edu/papers/grid:mobicom03/paper.pdf">
<front>
<title abbrev="DeCouto03">
A High-Throughput Path Metric for Multi-Hop Wireless Routing
</title>
<author fullname="Douglas S. J. De Couto" surname="De Couto"> </author>
<author fullname="Daniel Aguayo" surname="Aguayo"> </author>
<author fullname="John Bicket" surname="Bicket"> </author>
<author fullname="Robert Morris" surname="Morris"> </author>
<date year="2003" />
</front>
<seriesInfo name="MobiCom '03"
value="The 9th ACM International Conference on Mobile
Computing and Networking, San Diego, California," />
<format target="http://pdos.csail.mit.edu/papers/grid:mobicom03/paper.pdf"
type="HTML" />
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<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" />
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