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ROLL                                                     P. Thubert, Ed.
Internet-Draft                                             Cisco Systems
Intended status: Standards Track                        October 17, 2011
Expires: April 19, 2012


                 RPL adaptation for asymmetrical links
                     draft-thubert-roll-asymlink-00

Abstract

   The Routing Protocol for Low Power and Lossy Networks defines a
   generic Distance Vector protocol for Low Power and Lossy Networks,
   many of which exhibit strongly asymmetrical characteristics.  This
   draft proposes an extension for that optimizes RPL operations whereby
   upwards and downwards direction-optimized RPL instances are
   associated.

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 April 19, 2012.

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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   (http://trustee.ietf.org/license-info) in effect on the date of
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . 3
   3.  The asymmetrical link problem . . . . . . . . . . . . . . . . . 4
   4.  Solution Overview . . . . . . . . . . . . . . . . . . . . . . . 4
   5.  Modified DODAG Information Object (DIO) . . . . . . . . . . . . 5
   6.  Operations  . . . . . . . . . . . . . . . . . . . . . . . . . . 5
   7.  Backward compatibility  . . . . . . . . . . . . . . . . . . . . 6
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
   9.  Security Considerations . . . . . . . . . . . . . . . . . . . . 7
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 7
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . . . 8
     11.1.  Normative References . . . . . . . . . . . . . . . . . . . 8
     11.2.  Informative References . . . . . . . . . . . . . . . . . . 8
   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 [RFC5548], [RFC5673], [RFC5826], and
   [RFC5867], many of which explicitly or implicitly refer to links with
   asymmetrical properties.

   Upon those requirements, the Routing Protocol for Low Power and Lossy
   Network [I-D.ietf-roll-rpl] was designed as a platform that can be
   extended by further specifications or guidances, by adding new
   metrics, Objective Functions, or additional options.

   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.

   In one hand, RPL requires bidirectional links for the control, but on
   the other, there is no requirement that the properties of a link are
   the same in both directions.  In fact, such a symmetry is rarely
   present in LLNs, whether links are based on radios or power-line.

   Some initial implementations require that the quality of both
   directions of a link is evaluated as very good so that the link can
   be used for control and data in both directions.  This eliminates
   asymmetrical links that are very good in one direction, but only good
   enough for scarce activity in the other direction.

   In practice, a DAG that is built to optimize upwards traffic is
   generally not congruent with a DAG that is built to optimize
   downwards traffic.  This is why this specification is designed to
   enable asymmetrical routing DAGs that are bound together to get the
   maximum benefits of all bidirectional links.


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 upwards qualifies a link, a DODAG or an instance that is
   optimal for sending traffic in the general direction of the root,
   though may be usable but suboptimal for traffic coming form the
   direction of the root.  The term downwards qualifies the same words
   for the opposite direction.




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   The term parenting applied to instances refers to the directional
   association of two instances.  The graph formed by parented instances
   must be a DAG.  Traffic may be transferred from an instance onto a
   parent instance under specified circumstances.


3.  The asymmetrical link problem


4.  Solution Overview

   With the core RPL specification, [I-D.ietf-roll-rpl] each instance is
   a separate routing topology, and packets must be forwarded within the
   same topology / same instance.  One direct consequence of that design
   choice is that a topology must be very good for both upwards and
   downwards traffic; otherwise, traffic between two nodes in the
   instance may suffer.

   A simple approach to address bidirectional but asymmetrical links
   with RPL is to construct two DAGs, one for upwards traffic and one
   for downwards traffic, each DAG a separate instance, and then bind
   the two together.  In order to benefit from both instances for a same
   packet, this solution extends RPL to allow traffic to be transferred
   from one instance to the next.

   It can be noted at this point that with [I-D.ietf-roll-rpl], traffic
   that goes down does not generally go back up again, whereas P2P
   traffic within a DODAG might go up to a common parent and then down
   to the destination.  In terms of instance relationship, this means
   that when an upwards and a downwards instance are bound together,
   traffic from the former may be transferred to the latter, but not the
   other way around.  In other words, there is an order, a parent-child
   relationship, between the two instances.

   Additionally, if there is no next-hop for a packet going down within
   the instance, then with [I-D.ietf-roll-rpl] the packet must be
   dropped.  In order to limit that risk, it is tempting though
   inefficient to lower the constraints that are applied to build the
   topology.  It can be more efficient to actually keep the constraints
   as they should be, but, instead, enable a less constrained, more
   spanning, fall-back topology into which traffic can be transferred.

   For that reason, this solution allows for more complex instance
   relationships than plain child-parent associations.  In order to
   avoids loops which could be created when transferring packets from
   one instance to the next, this solution requires that the instances
   be themselves organized as a superior Directed Acyclic Graph, and
   enforce that inter-DAG transfers occur only within that superior



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   super-DAG of DAG instances.


5.  Modified DODAG Information Object (DIO)

   The DODAG Information Object [I-D.ietf-roll-rpl] carries information
   that allows a node to discover a RPL Instance, learn its
   configuration parameters, select a DODAG parent set, and maintain the
   DODAG.  This specification defines a new flag bit to indicate that
   the DAG is directional.


        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       | RPLInstanceID |Version Number |             Rank              |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |G|D| MOP | Prf |     DTSN      |     Flags     |   Reserved    |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                                                               |
       +                                                               +
       |                                                               |
       +                            DODAGID                            +
       |                                                               |
       +                                                               +
       |                                                               |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Option(s)...
       +-+-+-+-+-+-+-+-+

                       Figure 1: The DIO Base Object

   Directional (D):  The Directional (D) flag is set to indicate that
         the instance is intended for directional operation, and reset
         otherwise.  When it is set, a MOP of 0 indicates the upwards
         direction whereas any other value specified in
         [I-D.ietf-roll-rpl] indicates downwards.  All other values of
         MOP will be considered downwards unless explicitly specified
         otherwise.


6.  Operations

   This specification allows an organization of Instances as follows:

      Instances MUST be organized as a Directed Acyclic Graph.  This
      information MUST be commissioned into the devices so they know
      both which instances they should participate in, and which



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      direction of transfer is allowed between instances.

      A spanning instance using OF0 [I-D.ietf-roll-of0] MAY be used as
      root in that instance DAG.

   This specification defines a new bit in the RPL [I-D.ietf-roll-rpl]
   DODAG Information Object (DIO) with the Directional (D) flag that
   indicates a directional operation for a given instance.  An
   implementation that does not support that new bit will not be able to
   propagate it.

   In case of a directional operation,

      The direction is indicated by the MOP field, a MOP of 0 means
      upwards and otherwise is downwards.

      Links are still REQUIRED to allow bidirectional operations

      Only the metrics that correspond to the DAG direction are used for
      the parent selection.

      An upward instance SHOULD install routes that lead to the root and
      beyond - typically the default route.

      A downwards instance MAY ONLY install more specific routes that
      are injected by nodes in the DODAG through the DAO process.

      P2P operations are achieved by associating a child upwards
      instance with a parent downwards instance.

      A packet MUST NOT be transferred from a parent instance to a child
      instance.

      A packet MAY be transferred from a child instance to its parent
      instance if and only if the child instance does not provide a
      route to the destination, or the parent instance provides a more
      specific route (longer match) to the destination.

      Transferring from an upwards instance to a downwards instance if
      generally desirable.  Other forms of transfers are generally not
      desirable.  Policies MAY be put in place to ovreride that general
      guidance.


7.  Backward compatibility

   An OF is generally designed to compute a Rank of a directional link
   in a fashion that is diffent from a bidirectional link, and in



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   particular will not use the same metrics and thusobtain different
   ranks for a same situation.  For that reason, it is important that
   the OF is aware that an instance is supposed to define a directional
   DODAG, and it is RECOMMENDED that only devices that support
   directional DODAGs are allowed in a directional instance.

   It might happen that for some purposes like higher availability, an
   implementation that does not support directional links is
   administratively allowed to join a directional DODAG.  In that case,
   the extension of the DODAG that starts at that device will not be
   directional, but the instance will still be functional.

   In that case, it might also happen that a device that supports
   directional DODAGs per this specification sees candidate neighbors
   that expose the Directional flag and some others that do not.  An OF
   that supports directional links SHOULD favor directional links over
   non directional links, in a fashion that is to be specified with the
   OF.  In the case of OF0 [I-D.ietf-roll-of0], the 'D' flag should be
   accounted for before the computation of item 8 in the "Selection Of
   The Preferred Parent" section 4.2.1., that is before Ranks and be
   calculated and compared.


8.  IANA Considerations

   This specification requires that a bit in DIO be assigned to indicate
   directional link operations as specified in section


9.  Security Considerations

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


10.  Acknowledgements

   The author wishes to recognize Richard Kelsey, JP Vasseur, Tom
   Phinney, Robert Assimiti, Don Sturek and Yoav Ben-Yehezkel for their
   various contributions.


11.  References







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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.

   [I-D.ietf-roll-terminology]
              Vasseur, J., "Terminology in Low power And Lossy
              Networks", draft-ietf-roll-terminology-06 (work in
              progress), September 2011.

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

11.2.  Informative References

   [I-D.ietf-roll-of0]
              Thubert, P., "RPL Objective Function Zero",
              draft-ietf-roll-of0-20 (work in progress), September 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,
              "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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