ROLL P. Thubert, Ed.
Internet-Draft Cisco Systems
Intended status: Standards Track June 20, 2011
Expires: December 22, 2011
RPL Objective Function 0
draft-ietf-roll-of0-14
Abstract
The Routing Protocol for Low Power and Lossy Networks (RPL)
specification defines a generic Distance Vector protocol that is
adapted to a variety of networks types by the application of specific
Objective Functions. An Objective Function defines how a RPL node
selects and optimizes routes within a RPL Instance based on the
information objects available. This document specifies a basic
Objective Function that relies only on the objects that are defined
in RPL and does not use any extension.
Requirements Language
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 RFC 2119 [RFC2119].
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on December 22, 2011.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Objective Function 0 Overview . . . . . . . . . . . . . . . . 4
4. OF0 Operations . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Computing Rank . . . . . . . . . . . . . . . . . . . . . . 5
4.2. Feasible Successors Selection . . . . . . . . . . . . . . 7
4.2.1. Selection Of The Preferred Parent . . . . . . . . . . 7
4.2.2. Selection Of The Backup Feasible Successor . . . . . . 8
5. Abstract Interface to OF0 . . . . . . . . . . . . . . . . . . 8
6. OF0 Operands . . . . . . . . . . . . . . . . . . . . . . . . . 9
6.1. Variables . . . . . . . . . . . . . . . . . . . . . . . . 9
6.2. Configurable Parameters . . . . . . . . . . . . . . . . . 9
6.3. Constants . . . . . . . . . . . . . . . . . . . . . . . . 10
7. Manageability Considerations . . . . . . . . . . . . . . . . . 10
7.1. Device Configuration . . . . . . . . . . . . . . . . . . . 10
7.2. Device Monitoring . . . . . . . . . . . . . . . . . . . . 11
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
11.1. Normative References . . . . . . . . . . . . . . . . . . . 12
11.2. Informative References . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
The Routing Protocol for LLN (RPL) [I-D.ietf-roll-rpl] specification
defines a generic Distance Vector protocol that is adapted to a
variety of Low Power and Lossy Networks (LLN) types by the
application of specific Objective Functions. An Objective Function
defines how a RPL node selects and optimizes routes within a RPL
Instance based on the information objects available. This separation
of Objective Functions from the core protocol specification allows
RPL to be adapted to meet the different optimization criteria
required by the wide range of deployments, applications and network
designs.
RPL forms Directed Acyclic Graphs (DAGs) as collections of
Destination Oriented DAGs (DODAGs) within instances of the protocol.
Each instance is associated with a specialized Objective Function. A
DODAG is periodically reconstructed as a new DODAG Version to enable
a global reoptimization of the graph.
An instance of RPL running on a device uses an Objective Function to
help it determine which DODAG Version it should join. The OF is also
used by the RPL instance to select a number of routers within the
DODAG Version to serve as parents or as feasible successors.
The RPL instance uses the OF to compute a Rank for the device. This
value represents an abstract distance to the root of the DODAG within
the DODAG Version. The Rank is exchanged between nodes using RPL and
allows other RPL nodes to avoid loops and verify forward progression
toward the destination, as specified in [I-D.ietf-roll-rpl].
The Objective Function Zero (OF0) operates on parameters that are
obtained from provisioning, the RPL DODAG Configuration option and
the RPL DIO base container [I-D.ietf-roll-rpl].
The Rank of a node is obtained by adding a normalized scalar,
rank_increase (Section 6.1), to the Rank of a selected preferred
parent. The rank_increase can vary with a ratio from 1 (excellent)
to 9 (worst acceptable) to represent the link properties. By
default, OF0 encodes the 2-octet Rank in units of 256, and the
default settings allow to encode a minimum of 28 (worst acceptable)
hops and a maximum of 255 (excellent) hops.
It is important that devices deployed in a particular network or
environment use the same OF to build and operate DODAGs. If they do
not, it is likely that sub-optimal paths will be selected). In
practice, without a common definition of an OF, RPL implementations
cannot guarantee to interoperate correctly. The RPL specification
[I-D.ietf-roll-rpl] does not include any OF definitions. This is
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left for other documents specific to different deployments and
application environments. Since there is no default OF or metric
container in the RPL main specification, it might happen that, unless
two given implementations follow the same guidance for a specific
problem or environment, those implementations will not support a
common OF with which they could interoperate.
OF0 is designed as a default OF that will allow interoperation
between implementations in a wide spectrum of use cases. This is why
it is not specific as to how the link properties are transformed into
a rank_increase and leaves that responsibility to the 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 its computation. This is also why OF0 ignores metric
containers.
2. Terminology
The Terminology used in this document is consistent with and
incorporates that described in `Terminology in Low power And Lossy
Networks' [I-D.ietf-roll-terminology] and [I-D.ietf-roll-rpl].
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 in the RPL specification [I-D.ietf-roll-rpl].
3. Objective Function 0 Overview
The RPL specification describes constraints on how nodes select
potential parents, called a parent set, from their neighbors. All
parents are feasible successors for upward traffic (towards the
root). Additionally, RPL allows the use of parents 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.
The Goal of the OF0 is for a node to join a DODAG Version that offers
good enough connectivity to a specific set of nodes or to a larger
routing infrastructure though there is no guarantee that the path
will be optimized according to a specific metric. Thus, for the
purpose of OF0, the term Grounded [I-D.ietf-roll-rpl] means that the
DODAG root provides such connectivity. How that connectivity is
asserted and maintained is out of scope.
Objective Function 0 is designed to find the nearest Grounded root.
This can be achieved if the Rank of a node is very close to an
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abstract function of its distance to the root. This need is balanced
with the other need of maintaining some path diversity, which may be
achieved by increasing the Rank. In the absence of a Grounded root,
inner connectivity within the Low Power and Lossy Network is still
desirable and floating DAGs will form, rooted at the nodes with the
highest administrative preference.
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 with no attempt to perform any load balancing. When
the link conditions do not let an upward packet through the preferred
parent, the packet is passed to the backup feasible successor.
A RPL node monitors links to a number of neighbor nodes, and can use
OF0 to assign a rank_increase to each link. Though the exact method
for computing the rank_increase is implementation-dependent, the
computation must follow the rules that are specified in Section 4.1.
4. OF0 Operations
4.1. Computing Rank
An OF0 implementation first computes a variable step_of_rank
(Section 6.1) associated with a given parent from relevant link
properties and metrics. The step_of_rank is used to compute the
amount by which to increase the rank along a particular link, as
explained later in this section.
Computing a step_of_rank based on a static metric such as an
administrative cost implies that the OF0 implementation only
considers parents with good enough connectivity, and results in a
Rank that is analogous to hop-count. In most LLNs, this favors paths
with fewer but longer hops of poorer connectivity; it is thus
RECOMMENDED to base the computation of the step_of_rank on dynamic
link properties such as the expected transmission count metric (ETX)
[I-D.ietf-roll-routing-metrics],[DeCouto03]. The Minimum Rank
Objective Function with Hysteresis
[I-D.ietf-roll-minrank-hysteresis-of] provides guidance on how link
cost can be computed and on how hysteresis can improve Rank
stability.
OF0 allows an implementation to stretch the step_of_rank in order to
enable the selection of at least one feasible successor and thus
maintain path diversity. Stretching the step_of_rank is NOT
RECOMMENDED, because it augments the apparent distance from the node
to the root, distorts the DODAG from the optimal shape and may cause
instabilities due to greedy behaviors whereby depending nodes augment
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their Ranks to use each other as parents in a loop. Still, an
implementation MAY stretch the step_of_rank with at most a
configurable stretch_of_rank (Section 6.2) of any value between 0 (no
stretch) and the fixed constant MAXIMUM_RANK_STRETCH (Section 6.3).
An implementation MUST maintain the stretched step_of_rank between
the fixed constants MINIMUM_STEP_OF_RANK and MAXIMUM_STEP_OF_RANK
(Section 6.3). This range allows to reflect a large variation of
link quality.
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 to configure a factor called rank_factor
(Section 6.2) and to apply the factor on all links and peers to
multiply the effect of the stretched step_of_rank in the
rank_increase computation as further detailed below.
Additionally, an implementation MAY recognize 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 MUST be set between the fixed constants
MINIMUM_RANK_FACTOR and MAXIMUM_RANK_FACTOR (Section 6.3) .
The variable rank_increase is represented in units expressed by the
variable MinHopRankIncrease which defaults to the fixed constant
DEFAULT_MIN_HOP_RANK_INCREASE ([I-D.ietf-roll-rpl]); with that
setting, the least significant octet in the RPL Rank field in the DIO
Base Object is not used.
The step_of_rank Sp that is computed for that link is multiplied by
the rank_factor Rf and then possibly stretched by a stretch_of_rank
Sr. The resulting rank_increase is added to the Rank of preferred
parent R(P) to obtain that of this node R(N):
R(N) = R(P) + rank_increase where:
rank_increase = (Rf*Sp + Sr) * MinHopRankIncrease
Optionally, the administrative preference of a root MAY be configured
to supersede the goal to join a Grounded DODAG. 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
the 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 vicinity.
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4.2. Feasible Successors Selection
4.2.1. Selection Of The Preferred Parent
As it scans all the candidate neighbors, OF0 keeps the parent that is
the best for the following criteria (in order):
1. [I-D.ietf-roll-rpl] section 8 spells out the generic rules for a
node to re-parent 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.
2. 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
succeeded that validation process is preferable.
3. When multiple interfaces are available, a policy might be
locally configured to order them and that policy applies first;
that is a router on a higher order interface in the policy is
preferable.
4. If the administrative preference of the root is configured to
supersede the goal to join a Grounded DODAG, a router that
offers connectivity to a more preferable root SHOULD be
preferred.
5. A router that offers connectivity to a grounded DODAG Version
SHOULD be preferred over one that does not.
6. A router that offers connectivity to a more preferable root
SHOULD be preferred.
7. When comparing 2 parents that belong to the same DODAG, a router
that offers connectivity to the most recent DODAG Version SHOULD
be preferred.
8. The parent that causes the lesser resulting Rank for this node,
as specified in Section 4.1, SHOULD be preferred.
9. 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.
10. The preferred parent that was in use already SHOULD be
preferred.
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11. A router that has announced a DIO message more recently SHOULD
be preferred.
These rules and their order MAY be varied by an implementation
according to configured policy.
4.2.2. Selection Of The Backup Feasible Successor
When selecting a backup feasible successor, the OF performs in order
the following checks:
1. When multiple interfaces are available, a router on a higher
order interface is preferable.
2. The backup feasible successor MUST NOT be the preferred parent.
3. The backup feasible successor MUST be either in the same DODAG
Version as this node or in an subsequent DODAG Version.
4. Along with RPL rules, a Router in the same DODAG Version as this
node and with a Rank that is higher than the Rank computed for
this node MUST NOT be selected as a feasible successor.
5. A router with a lesser Rank SHOULD be preferred.
6. A router that has been validated as usable by an implementation-
dependant validation process SHOULD be preferred.
7. The backup feasible successor that was in use already SHOULD be
preferred.
These rules and their order MAY be varied by an implementation
according to configured policy.
5. Abstract Interface to OF0
Objective Function 0 interacts for its management and operations in
the following ways:
Processing DIO: When a new DIO is received, the OF that corresponds
to the Objective Code Point (OCP) in the DIO is triggered with the
content of the DIO. OF0 is identified by OCP 0 (to be validated
by IANA Section 8).
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Providing DAG information: The OF0 support provides an interface
that returns information about a given instance. This includes
material from the DIO base header, the role (router, leaf), and
the Rank of this node.
Providing a Parent List: The OF0 support provide an interface that
returns 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.
Triggered Updates: the OF0 support provides events to inform it that
a change in DAG information or Parent List as occurred. This can
be caused by an interaction with another system component such as
configuration, timers, and device drivers, and the change may
cause the RPL core to fire a new DIO or reset trickle timers.
6. OF0 Operands
On top of variables and constants defined in [I-D.ietf-roll-rpl],
this specification introduces the following variables and constants:
6.1. Variables
OF0 uses the following variables:
step_of_rank (unsigned integer): an intermediate computation based
on the link properties with a certain neighbor.
rank_increase (unsigned integer): delta between the Rank of the
preferred parent and self
6.2. Configurable Parameters
OF0 can use the following optional configurable values that are used
as parameters to the rank_increase computation:
stretch_of_rank (unsigned integer): the maximum augmentation to the
step-of-rank of a preferred parent to allow the selection of an
additional feasible successor. If none is configured to the
device, then the step_of_rank is not stretched.
rank_factor (unsigned integer): A configurable factor that is used
to multiply the effect of the link properties in the rank_increase
computation. If none is configured, then a rank_factor of 1 is
used.
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6.3. Constants
Section 17 of [I-D.ietf-roll-rpl] defines RPL constants. OF0 fixes
the values of the following constants:
DEFAULT_STEP_OF_RANK: 3
MINIMUM_STEP_OF_RANK: 1
MAXIMUM_STEP_OF_RANK: 9
DEFAULT_RANK_STRETCH: 0
MAXIMUM_RANK_STRETCH: 5
DEFAULT_RANK_FACTOR: 1
MINIMUM_RANK_FACTOR: 1
MAXIMUM_RANK_FACTOR: 4
7. Manageability Considerations
Section 18 of [I-D.ietf-roll-rpl] depicts the management of the
protocol. This specification inherits from that section and its
subsections, with the exception that metrics as specified in
[I-D.ietf-roll-routing-metrics] are not used and do not require
management.
7.1. Device Configuration
An implementation SHOULD allow to configure at least a global
rank_factor that applies to all links. Additionally, the
implementation may allow to group interfaces, links and/or neighbors
and configure a more specific rank_factor to such groups.
An implementation MAY allow to configure a maximum stretch_of_rank as
discussed in Section 4.1. If none is configured, a value of 0 is
assumed and the step_of_rank is not stretched.
An OF0 implementation SHOULD support the DODAG Configuration option
as specified in section 6.7.6 of [I-D.ietf-roll-rpl] and apply the
parameters contained therein. When the option is used, the
parameters are configured to the nodes that may become DODAG roots,
and the nodes are configured to redistribute the information using
the DODAG Configuration option. In particular, the value of
MinHopRankIncrease can be distributed with that option and override
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the fixed constant of DEFAULT_MIN_HOP_RANK_INCREASE that is defined
section 17 of [I-D.ietf-roll-rpl] with a fixed value of 256.
At build-time, the default constant values should be used, that is:
the rank_factor is set to the fixed constant DEFAULT_RANK_FACTOR
(Section 6.3).
the maximum stretch_of_rank is set to the fixed constant
DEFAULT_RANK_STRETCH (Section 6.3).
the MinHopRankIncrease is set to the fixed constant
DEFAULT_MIN_HOP_RANK_INCREASE ([I-D.ietf-roll-rpl]).
The values can be overridden at anytime and apply at the next Version
of the DODAG.
7.2. Device Monitoring
As discussed in Section 5, the OF support must be able to provide
information about its operations, and trigger events when tthat
information changes. At a minimum, the information should include:
DAG information as specified in Section 6.3.1 of
[I-D.ietf-roll-rpl], and including the DODAGID, the RPLInstanceID,
the Mode of Operation, the Rank of this node, the current Version
Number, and the value of the Grounded flag.
A list of neighbors indicating the preferred parent and an
alternate feasible if available. For each neighbor, the Rank, he
current Version Number, and the value of the Grounded flag should
be indicated
8. IANA Considerations
This specification requires the assignment of an Objective Code Point
(OCP) for OF0 in the Objective Code Point Registry that is requested
in section 20.5. of [I-D.ietf-roll-rpl].
OCP code: The value of 0 is suggested.
Description: A basic Objective Function that relies only on the
objects that are defined in [I-D.ietf-roll-rpl].
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Defining RFC: This.
9. Security Considerations
This specification makes simple extensions to RPL and so is
vulnerable to and benefits from the security issues and mechanisms
described in [I-D.ietf-roll-rpl] and [I-D.ietf-roll-rpl]. This
document does not introduce new flows or new messages, thus requires
no specific mitigation for new threats.
OF0 depends on information exchanged in the Rank and OCP protocol
elements. If those elements were compromised, then an implementation
of OF0 might generate the wrong path for a packet resulting in it
being misrouted. Therefore, deployments are RECOMMENDED to use RPL
security mechanisms if there is a risk that routing information might
be modified or spoofed.
10. Acknowledgements
Most specific thanks to Philip Levis and Phoebus Chen for their help
in finalizing this document.
Many thanks also to Adrian Farrel, Tim Winter, JP Vasseur, Julien
Abeille, Mathilde Durvy, Teco Boot, Navneet Agarwal, Meral
Shirazipour and Henning Rogge for in-depth review and first hand
implementers' feedback.
11. References
11.1. Normative References
[I-D.ietf-roll-rpl]
Winter, T., Thubert, P., Brandt, A., Clausen, T., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., and J.
Vasseur, "RPL: IPv6 Routing Protocol for Low power and
Lossy Networks", draft-ietf-roll-rpl-19 (work in
progress), March 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
11.2. Informative References
[DeCouto03]
De Couto, Aguayo, Bicket, and Morris, "A High-Throughput
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Path Metric for Multi-Hop Wireless Routing", MobiCom
'03 The 9th ACM International Conference on Mobile
Computing and Networking, San Diego, California,, 2003, <h
ttp://pdos.csail.mit.edu/papers/grid:mobicom03/paper.pdf>.
[I-D.ietf-roll-minrank-hysteresis-of]
Gnawali, O. and P. Levis, "The Minimum Rank Objective
Function with Hysteresis",
draft-ietf-roll-minrank-hysteresis-of-04 (work in
progress), May 2011.
[I-D.ietf-roll-routing-metrics]
Vasseur, J., Kim, M., Pister, K., Dejean, N., and D.
Barthel, "Routing Metrics used for Path Calculation in Low
Power and Lossy Networks",
draft-ietf-roll-routing-metrics-19 (work in progress),
March 2011.
[I-D.ietf-roll-security-framework]
Tsao, T., Alexander, R., Dohler, M., Daza, V., and A.
Lozano, "A Security Framework for Routing over Low Power
and Lossy Networks", draft-ietf-roll-security-framework-06
(work in progress), June 2011.
[I-D.ietf-roll-terminology]
Vasseur, J., "Terminology in Low power And Lossy
Networks", draft-ietf-roll-terminology-05 (work in
progress), March 2011.
Author's Address
Pascal Thubert (editor)
Cisco Systems
Village d'Entreprises Green Side
400, Avenue de Roumanille
Batiment T3
Biot - Sophia Antipolis 06410
FRANCE
Phone: +33 497 23 26 34
Email: pthubert@cisco.com
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