ROLL                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                            May 17, 2011
Expires: November 18, 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 a
   specific Objective Function.  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
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   This Internet-Draft will expire on November 18, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents

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   ( 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
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   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  . . . . . . . . . . . . . .  6
       4.2.1.  Selection of the Preferred Parent  . . . . . . . . . .  6
       4.2.2.  Selection of the backup feasible successor . . . . . .  7
   5.  Abstract Interface with RPL core . . . . . . . . . . . . . . .  8
   6.  OF0 Operands . . . . . . . . . . . . . . . . . . . . . . . . .  8
     6.1.  Variables  . . . . . . . . . . . . . . . . . . . . . . . .  8
     6.2.  Configurable parameters  . . . . . . . . . . . . . . . . .  8
     6.3.  Constants  . . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  9
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 10
     10.2. Informative References . . . . . . . . . . . . . . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   The Routing Protocol for Low Power and Lossy Networks
   [I-D.ietf-roll-rpl] 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 required by the wide range of use cases.

   RPL forms Destination Oriented Directed Acyclic Graphs (DODAGs)
   within instances of the protocol.  Each instance is associated with a
   specialized Objective Function.  A DODAG is periodically
   reconstructed in a new Version to enable a global reoptimization of
   the graph.

   An Objective Function selects the DODAG Version that a device joins
   within an instance, and a number of neighbor routers within that
   DODAG 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 avoid loops and verify forward progression towards a
   destination, as specified in [I-D.ietf-roll-rpl].

   The Objective Function 0 (OF0) operates on parameters that are
   obtained from provisionning, 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, 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.  As a result, OF0 with
   default settings allows to encode a minimum of 28 (worst acceptable)
   hops and a maximum of 255 (excellent) hops.

   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

   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

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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 upgoing 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.  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.

   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, which may be achieved by increasing
   the Rank.  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.

   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.

   OF0 assigns a rank_increase to each link to another node that it
   monitors.  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.

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4.  OF0 operations

4.1.  Computing Rank

   An OF0 implementation first computes a step_of_rank associated with a
   given parent from relevant link properties and metrics.

   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 analoguous to hop-count.  In most LLNs, this favors
   paths with less 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)
   [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

   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 at least one feasible successor and thus
   maintain path diversity.  The use of a stretch_of_rank is not
   recommended as 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 behaviours whereby depending nodes
   augment their Ranks to use each other as parents in a loop.  An
   implementation MUST maintain the stretched step_of_rank between
   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 a configurable factor called
   rank_factor and to apply the factor on all links and peers.

   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

   The rank_increase is expressed in units of MinHopRankIncrease, which
   defaults to DEFAULT_MIN_HOP_RANK_INCREASE; with that setting, the
   least significant octet in the RPL Rank is not used.

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   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 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 vicinity.

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 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.

   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.

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   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

   11.  A router that has announced a DIO message more recently SHOULD
        be preferred.

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

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5.  Abstract Interface with RPL core

   Objective Function 0 interacts with the RPL core in the following

   Processing DIO:  When a new DIO is received, the RPL core calls the
      OF that corresponds to the Objective Code Point OCP) in the DIO .
      OF0 corresponds to the OCP 0 (to be validated by IANA).

   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.

   Calling back:  The RPL core provides a call back interface for the
      OF0 support 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

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 parameters:

   stretch_of_rank (unsigned integer):  an optional augmentation to the
      step-of-rank of the preferred parent to allow the selection of
      additional parents.

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   rank_factor (unsigned integer):  A configurable factor that is used
      to multiply the effect of the link properties in the rank_increase

6.3.  Constants

   OF0 fixes the following constants:










7.  IANA Considerations

   This specification requires the assignment of a RPL Objective Code
   Point (OCP) for OF0.  The value of 0 is suggested.

8.  Security Considerations

   Security Considerations for OCP/OF are to be developed in accordance
   with recommendations laid out in, for example,

9.  Acknowledgements

   Most specific thanks to Philip Levis and Phoebus Chen for their help
   in finalizing this document.

   Many thanks also 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.

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10.  References

10.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

10.2.  Informative References

              De Couto, Aguayo, Bicket, and Morris, "A High-Throughput
              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-03 (work in
              progress), May 2011.

              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.

              Vasseur, J., "Terminology in Low power And Lossy
              Networks", draft-ietf-roll-terminology-05 (work in
              progress), March 2011.

              Tsao, T., Alexander, R., Daza, V., and A. Lozano, "A
              Security Framework for Routing over Low Power and Lossy
              Networks", draft-tsao-roll-security-framework-02 (work in
              progress), March 2010.

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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

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