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ROLL                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                          March 29, 2011
Expires: September 30, 2011


                        RPL Objective Function 0
                         draft-ietf-roll-of0-08

Abstract

   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.


































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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
   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 September 30, 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
   (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.














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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Objective Function 0 Overview  . . . . . . . . . . . . . . . .  6
   4.  Selection of the Preferred Parent  . . . . . . . . . . . . . .  8
   5.  Selection of the backup feasible successor . . . . . . . . . .  9
   6.  Abstract Interface with RPL core . . . . . . . . . . . . . . . 10
   7.  OF0 Constants and Variables  . . . . . . . . . . . . . . . . . 11
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 12
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 14
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     11.1.  Normative References  . . . . . . . . . . . . . . . . . . 15
     11.2.  Informative References  . . . . . . . . . . . . . . . . . 15
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17



































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1.  Introduction

   The IETF ROLL Working Group has defined application-specific routing
   requirements for a Low Power and Lossy Network (LLN) routing
   protocol, specified in [RFC5548], [RFC5673], [RFC5826], and
   [RFC5867].

   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 the wide range of use cases requires.

   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.

   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 [I-D.ietf-roll-rpl].

   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 [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.

   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.



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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 outside of this specification, in particular in the RPL
   specification.








































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3.  Objective Function 0 Overview

   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.

   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.

   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.

   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.  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 Step-of-Rank to each link to another node that it
   monitors.  The exact method for computing the Step-of-Rank is
   implementation-dependent.

   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.

   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) [DeCouto03] should be used instead to compute the
   Step-of-Rank.  For instance, the Minimum Rank Objective Function with



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   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.

   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.

   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.

   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 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):

   R(N) = R(P) + Ri where Ri = Rf*Sp + Sr.

   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.






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4.  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] 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 has
        been validated is preferrable.

   3.   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.

   4.   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.

   5.   A router that offers connectivity to a grounded DODAG version
        SHOULD be preferred over one that does not.

   6.   When comparing 2 routers that belong to the same DODAG, a router
        that offers connectivity to the freshest version SHOULD be
        preferred.

   7.   The parent that causes the lesser resulting Rank for this node
        SHOULD be preferred.

   8.   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.

   9.   The preferred parent that was in use already SHOULD be
        preferred.

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





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5.  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 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.

   4.  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.

   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.
























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6.  Abstract Interface with RPL core

   Objective Function 0 interacts with the core RPL in the following
   ways:

   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.

   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.

   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.

   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.



























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7.  OF0 Constants and Variables

   OF0 uses the following constants:

   MinHopRankIncrease:  256

   DEFAULT_STEP_OF_RANK:  3 * MinHopRankIncrease

   MINIMUM_STEP_OF_RANK:  1 * MinHopRankIncrease

   MAXIMUM_STEP_OF_RANK:  9 * MinHopRankIncrease

   MAXIMUM_RANK_STRETCH:  5 * MinHopRankIncrease

   DEFAULT_RANK_FACTOR:  1

   MINIMUM_RANK_FACTOR:  1

   MAXIMUM_RANK_FACTOR:  4
































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8.  IANA Considerations

   IThis specification requires the assignment of an OCP for OF0.  The
   value of 0 is suggested.















































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9.  Security Considerations

   Security Considerations for OCP/OF are to be developed in accordance
   with recommendations laid out in, for example,
   [I-D.tsao-roll-security-framework].














































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10.  Acknowledgements

   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.












































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11.  References

11.1.  Normative References

   [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
              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-01 (work in
              progress), February 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-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.

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

   [I-D.tsao-roll-security-framework]
              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.

   [RFC5548]  Dohler, M., Watteyne, T., Winter, T., and D. Barthel,



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              "Routing Requirements for Urban Low-Power and Lossy
              Networks", RFC 5548, May 2009.

   [RFC5673]  Pister, K., Thubert, P., Dwars, S., and T. Phinney,
              "Industrial Routing Requirements in Low-Power and Lossy
              Networks", RFC 5673, October 2009.

   [RFC5826]  Brandt, A., Buron, J., and G. Porcu, "Home Automation
              Routing Requirements in Low-Power and Lossy Networks",
              RFC 5826, April 2010.

   [RFC5867]  Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
              "Building Automation Routing Requirements in Low-Power and
              Lossy Networks", RFC 5867, June 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
   FRANCE

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







































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