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ROLL                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                           June 20, 2011
Expires: December 22, 2011

                        RPL Objective Function 0


   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",
   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
   working documents as Internet-Drafts.  The list of current Internet-
   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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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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

   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

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

   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

   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

   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

   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

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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

   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

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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:









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

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

              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

              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

              Gnawali, O. and P. Levis, "The Minimum Rank Objective
              Function with Hysteresis",
              draft-ietf-roll-minrank-hysteresis-of-04 (work in
              progress), May 2011.

              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.

              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.

              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

   Phone: +33 497 23 26 34
   Email: pthubert@cisco.com

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