Networking Working Group                                 P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                       February 18, 2010
Expires: August 22, 2010

                        RPL Objective Function 0


   The Routing Protocol for Low Power and Lossy Networks (RPL) defines a
   generic Distance Vector protocol for Low Power and Lossy Networks
   (LLNs).  RPL is instantiated to honor a particular routing objective/
   constraint by the adding a specific Objective Function (OF) that is
   designed to solve that problem.  This specification defines a basic
   OF, OF0, that uses only the abstract properties exposed in RPL
   messages to maximize connectivity.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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Copyright Notice

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   described in the BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  Basic Objective Function  . . . . . . . . . . . . . . . . . . . 3
     3.1.  Goal  . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
     3.2.  Selection of the Preferred Parent . . . . . . . . . . . . . 5
     3.3.  Selection of the Backup next_hop  . . . . . . . . . . . . . 6
   4.  OF0 Constants and Variables . . . . . . . . . . . . . . . . . . 6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     8.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     8.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8

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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 [I-D.ietf-roll-building-routing-reqs],
   [I-D.ietf-roll-home-routing-reqs], [RFC5673], and [RFC5548].

   Considering the wide variety of use cases, link types and metrics,
   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 instantiated using Objective Functions.

   RPL forms Destination Oriented Directed Acyclic Graphs (DODAGs)
   within instances of the protocol, each instance being set up to honor
   a particular routing objective/constraint of a given deployment.
   This instantiation is achieved by plugging into the RPL core a
   specific Objective Function (OF) that is designed to solve that
   problem to be addressed by that instance.

   the Objective Function selects the DODAG iteration that a device
   joins, and a number of neighbor routers within that iteration as
   parents and siblings.  The OF is also responsible for computing the
   Rank of the device, that abstracts a relative position within the
   DODAG and is used by the RPL core to enable a degree of loop
   avoidance and verify forward progression towards a destination, as
   specified in [I-D.ietf-roll-rpl].

   This document defines the Objective Function 0 (OF0), which operates
   on information defined in the RPL specification [I-D.ietf-roll-rpl]
   and does not require any additional option.

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].

3.  Basic Objective Function

   This specification defines the basic Objective Function, that is also
   called OF0 since it corresponds to the Objective Code Point 0.  OF0
   does not leverage link layer metrics such as described in
   [I-D.ietf-roll-routing-metrics], but is only based on abstract
   information from the DIO base container, such as Rank and
   administrative preference.

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3.1.  Goal

   The Goal of the OF0 is to join a DODAG iteration 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.
   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.

   The metric used in OF0 is the RPL Rank, as defined in
   [I-D.ietf-roll-rpl].  Using a metric that in essence is similar to
   hop count implies that the quality of the connectivity should be
   asserted so that only neighbors with a good enough connectivity are
   presented to the OF.  How that connectivity is asserted and
   maintained is out of scope.

   The default step of Rank is DEFAULT_RANK_INCREMENT for each hop.  An
   implementation MAY allow a step between MINIMUM_RANK_INCREMENT and
   MAXIMUM_RANK_INCREMENT to reflect a large variation of link quality.
   It MAY stretch its step of Rank by up to MAXIMUM_RANK_STRETCH in
   order to enable the selection of a sibling when only one parent is
   available.  For instance, say that a node computes a step of Rank of
   4 units of MINIMUM_RANK_INCREMENT from a preferred parent with a Rank
   of 6 units resulting in a Rank of 10 units for this node.  Say that
   with that Rank of 10 units, this node would end up with only one
   parent and no sibling, though there is a neighbor with a Rank of 12
   units.  In that case, the node is entitled to stretch its step of
   Rank by a value of 2 units, thus using a step of Rank of 6 units so
   as to reach a Rank of 12 units and find a sibling.  But the node is
   not entitled to use a step of Rank larger than 6 units since that
   would be a greedy behavior that would deprive the neighbor of this
   snode as a successor.  Also, if the neighbor had exposed a Rank of 16
   units, the stretch of Rank from 10 to 16 units would have exceeded
   MAXIMUM_RANK_STRETCH of 4 units and thus the neighbor would not have
   been selectable even as a sibling.

   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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   OF0 selects a preferred parent and a backup next_hop if one is
   available.  The backup next_hop might be a parent or a sibling.  All
   the traffic is routed via the preferred parent.  When the link
   conditions do not let a packet through the preferred parent, the
   packet is passed to the backup next_hop.

3.2.  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 iteration.  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

   4.   In the absence of a Grounded DODAG iteration, 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 iteration
        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 sequence SHOULD be

   7.   When computing a resulting Rank for this node from a parent Rank
        and a Step of Rank from that parent, the parent that causes the
        lesser resulting Rank SHOULD be preferred.

   8.   A DODAG iteration for which there is an alternate parent SHOULD
        be preferred.  This check is optional.  It is performed by
        computing the backup next_hop while assuming that the router
        that is currently examined is finally selected as preferred

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   9.   The DODAG iteration that was in use already SHOULD be preferred.

   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.

3.3.  Selection of the Backup next_hop

   o  When multiple interfaces are available, a router on a higher order
      interface is preferable.

   o  The preferred parent MUST be ignored.

   o  A Router that is not in the same DODAG as the preferred parent,
      either in the current or a subsequent iteration, MUST be ignored.

   o  A Router with a Rank that is higher than the Rank computed for
      this node out of the preferred parent SHOULD NOT be selected as
      parent, to avoid greedy behaviors.  It MAY still be selected as
      sibling if no better Back-up next hop is found.

   o  A router with a lesser Rank SHOULD be preferred.

   o  A router that has been validated as usable by an implementation
      dependant validation process SHOULD be preferred.

   o  The backup next_hop that was in use already SHOULD be preferred.

4.  OF0 Constants and Variables

   OF0 uses the following constants:

   MinHopRankIncrease:  256

   DEFAULT_RANK_INCREMENT:  4 * MinHopRankIncrease

   MINIMUM_RANK_INCREMENT:  1 * MinHopRankIncrease

   MAXIMUM_RANK_INCREMENT:  16 * MinHopRankIncrease

   MAXIMUM_RANK_STRETCH:  4 * MinHopRankIncrease

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5.  IANA Considerations

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

6.  Security Considerations

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

7.  Acknowledgements

   Most specific thanks to Tim Winter, JP Vasseur, Julien Abeille and
   Mathilde Durvy for in-depth review and first hand implementer's

8.  References

8.1.  Normative References

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

8.2.  Informative References

              Martocci, J., Riou, N., Mil, P., and W. Vermeylen,
              "Building Automation Routing Requirements in Low Power and
              Lossy Networks", draft-ietf-roll-building-routing-reqs-07
              (work in progress), September 2009.

              Brandt, A., Buron, J., and G. Porcu, "Home Automation
              Routing Requirements in Low Power and Lossy Networks",
              draft-ietf-roll-home-routing-reqs-08 (work in progress),
              September 2009.

              Vasseur, J. and D. Networks, "Routing Metrics used for
              Path Calculation in Low Power and Lossy Networks",
              draft-ietf-roll-routing-metrics-04 (work in progress),
              December 2009.


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              Winter, T., Thubert, P., and R. Team, "RPL: IPv6 Routing
              Protocol for Low power and Lossy Networks",
              draft-ietf-roll-rpl-06 (work in progress), February 2010.

              Vasseur, J., "Terminology in Low power And Lossy
              Networks", draft-ietf-roll-terminology-02 (work in
              progress), October 2009.

              Tsao, T., Alexander, R., Dohler, M., Daza, V., and A.
              Lozano, "A Security Framework for Routing over Low Power
              and Lossy Networks", draft-tsao-roll-security-framework-01
              (work in progress), September 2009.

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

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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