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Versions: 00 01                                                         
Internet Engineering Task Force                                 M. Goyal
Internet-Draft                                   University of Wisconsin
Intended status: Experimental                                  Milwaukee
Expires: April 15, 2013                                       D. Barthel
                                                   France Telecom Orange
                                                             E. Baccelli
                                                        October 12, 2012

                           DIS Modifications


   This document specifies the DIS flags and options that allow an RPL
   node to control how neighbor RPL routers respond to its solicitiation
   for DIOs.

Status of this Memo

   This Internet-Draft is submitted to IETF 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-
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   Internet-Drafts are draft documents valid for a maximum of six months
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   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 15, 2013.

Copyright Notice

   Copyright (c) 2012 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
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   include Simplified BSD License text as described in Section 4.e of

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   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.  New Flags in the DIS Base Object . . . . . . . . . . . . . . .  4
   4.  DIS Options  . . . . . . . . . . . . . . . . . . . . . . . . .  4
     4.1.  Metric Container . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Response Spreading Option  . . . . . . . . . . . . . . . .  5
   5.  Applications . . . . . . . . . . . . . . . . . . . . . . . . .  6
     5.1.  A Leaf Node Joining a DAG  . . . . . . . . . . . . . . . .  6
     5.2.  Identifying A Defunct DAG  . . . . . . . . . . . . . . . .  6
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
     6.1.  DIS Flags  . . . . . . . . . . . . . . . . . . . . . . . .  8
     6.2.  RPL Control Message Options  . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

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1.  Introduction

   An RPL node can use a DODAG Information Solicitation (DIS) message to
   solicit DODAG Information Object (DIO) messages from its neighbor RPL
   routers.  A DIS may carry a Solicited Information option that
   specifies the predicates of the DAG(s) the node is interested in.  In
   the absence of a Solicited Information option, it is assumed that the
   node generating the DIS is interested in receiving DIOs for all the
   DAGs.  A DIS can be multicast to all the in-range routers or it can
   be unicast to a specific neighbor router.  RPL requires a router to
   consider the receipt of a multicast DIS as an inconsistency and hence
   reset its Trickle timers [RFC6206] for the matching DAGs.  The
   receipt of a unicast DIS causes an RPL router to generate the DIOs
   for all the matching DAGs without resetting the Trickle timers.

   Consider an RPL leaf node that desires to join a certain DAG.  This
   node can either wait for its neighbor RPL routers to voluntarily
   transmit DIOs or it can proactively solicit DIOs using a DIS message.
   Voluntary DIO transmissions may happen after a very long time if the
   network is stable and the Trickle timer intervals have reached large
   values.  Thus, proactively seeking DIOs using a DIS may be the only
   reasonable option.  Since the node does not know which neighbor
   routers belong to the DAG, it must solicit the DIOs using a multicast
   DIS (with predicates of the desired DAG specified inside a Solicited
   Information option).  On receiving this DIS, the neighbor routers
   that belong to the desired DAG will reset their Trickle timers and
   quickly transmit their DIOs.  The downside of resetting Trickle
   timers is that the routers would continue to transmit the DIOs
   frequently for a considerable time interval.  These DIO transmissions
   are unnecessary, consume precious energy and may contribute to
   congestion in the network.

   There are other scenarios where resetting of Trickle timer following
   the receipt of a multicast DIS is not appropriate.  For example,
   consider an RPL router that desires to free up memory by deleting
   state for the defunct DAGs it belongs to.  Identifying a defunct DAG
   may require the node to solicit DIOs from its DAG parents using a
   multicast DIS.

   To deal with the situations described above, this document specifies
   the DIS flags and options that allow an RPL node to control how
   neighbor RPL routers respond to its solicitiation for DIOs:

   o  Using routing constraints to limit the number of responding

   o  Whether the responding routers should reset their Trickle timers;

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   o  Whether the responding routers should send a unicast DIO or a
      multicast one;

   o  The time interval over which the responding routers must schedule
      their DIO transmissions.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in

   Additionally, this document uses terminology from [RFC6550].
   Specifically, the term RPL node refers to an RPL router or an RPL
   host as defined in [RFC6550].

3.  New Flags in the DIS Base Object

       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
       |    Flags  |N|T|    Reserved   | Option(s)...

                    Figure 1: Modified DIS Base Object

   This document defines two new flags inside the DIS base object:

   o  "No Inconsistency" (N) flag: On receiving a unicast/multicast DIS
      with N flag set, an RPL router MUST NOT reset the Trickle timers
      for the matching DAGs.  Also, a DIO generated in response to a DIS
      with N flag set MUST always contain a Configuration option.

   o  "DIO Type" (T) flag: This flag specifies whether the responding
      routers should transmit a multicast DIO or a unicast one.  The
      responding router MUST transmit a multicast DIO if this flag is

   The modified DIS base object is shown in Figure 1.

4.  DIS Options

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4.1.  Metric Container

   In order to limit the number of routers that will respond to a
   multicast DIS, this document allows the specification of routing
   constraints inside a DIS that a router must satisfy in order to
   respond to the DIS.  These routing constraints are specified inside a
   Metric Container option contained in the DIS.  Thus, this document
   allows the inclusion of a Metric Container option inside a DIS.  An
   RPL router that receives a DIS with a Metric Container option MUST
   ignore any Metric object in it, and MUST evaluate the "mandatory"
   Constraint objects in it by comparing the constraint value to the
   aggregated value of the corresponding routing metric that the router
   maintains for the matching DAG(s).  The aggregated routing metric
   values MUST satisfy all the mandatory constraints in order for the
   router to generate DIOs for the matching DAG(s).

4.2.  Response Spreading Option

             0                   1                   2
             0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
            |  Type = 0x0A  |    Length     | Spread. Inter.|

                  Figure 2: The Response Spreading Option

   A multicast DIS may lead to a large number of RPL routers responding
   with DIOs.  Concurrent transmissions by multiple routers are not
   desirable.  Such transmissions may end up in collisions.  Unicast
   DIOs may be able to avail of link-level retransmissions.  However,
   multicast DIOs have no such protection.  These transmissions and
   retransmissions may also cause congestion in the network.  To avoid
   such problems, this document specifies an optional DIO response
   spreading mechanism.

   This document defines a new RPL control message option called
   "Response Spreading", shown in Figure 2, with a recommended Type
   value 0x0A (to be confirmed by IANA).  A Response Spreading option
   may be included only inside a multicast DIS message.  An RPL router
   that responds to a multicast DIS, that includes a Response Spreading
   option, MUST wait for a time uniformly chosen in the interval
   [O..2^SpreadingInterval], expressed in ms, before attempting to
   transmit its DIO.  If the DIS does not include a Response Spreading
   option, the node is free to transmit the DIO as it otherwise would.

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5.  Applications

   This section details two example mechanisms that use the DIS flags
   and options defined in this document.  The first mechanism describes
   how a leaf node may join a desired DAG.  The second mechanism details
   how a node may identify defunct DAGs for which it still maintains

5.1.  A Leaf Node Joining a DAG

   A new leaf node that joins an established LLN runs an iterative
   algorithm in which it requests (using multicast DIS) DIOs from
   routers belonging to the desired DAG.  The DIS message has the "No
   Inconsistency" flag set (to prevent resetting of Trickle timer in
   responding routers) and the "DIO Type" flag reset (to make responding
   routers send unicast DIOs back).  The DIS message can include a
   Response Spreading option listing a suitable spreading interval and a
   Metric Container listing the routing constraints that the responding
   routers must satisfy.  In each iteration, the node multicasts such a
   DIS and waits for the DIOs.  Once the spreading interval has expired,
   the node considers the current iteration to be unsuccessful.  Now the
   node relaxes the routing constraints somewhat and proceeds to the
   next iteration.  The cycle repeats until the node receives one or
   more DIOs in a particular iteration or if maximum number of
   iterations have been reached.

5.2.  Identifying A Defunct DAG

   An RPL node may remove a neighbor from its parent set for a DAG for a
   number of reasons:

   o  The neighbor is no longer reachable, as determined using a
      mechanism such as Neighbor Unreachanility Detection (NUD)
      [RFC4861], Bidirectional Forwarding Detection (BFD) [RFC5881] or
      L2 triggers [RFC5184]; or

   o  The neighbor advertises an infinite rank in the DAG; or

   o  Keeping the neighbor as a parent would required the node to
      increase its rank beyond L + DAGMaxRankIncrease, where L is the
      minimum rank the node has had in this DAG; or

   o  The neighbor advertises membership in a different DAG within the
      same RPL Instance, where a different DAG is recognised by a
      different DODAGID or a different DODAGVersionNumber.

   Even if the conditions listed above exist, an RPL node may fail to
   remove a neighbor from its parent set because:

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   o  The node may fail to receive the neighbor's DIOs advertising an
      increased rank or the neighbor's membership in a different DAG;

   o  The node may not check, and hence may not detect, the neighbor's
      unreachability for a long time.  For example, the node may not
      have any data to send to this neighbor and hence may not encounter
      any event (such as failure to send data to this neighbor) that
      would trigger a check for the neighbor's reachability.

   In such cases, a node would continue to consider itself attached to a
   DAG even if all its parents in the DAG are unreachable or have moved
   to different DAGs.  Such a DAG can be characterized as being defunct
   from the node's perspective.  If the node maintains state about a
   large number of defunct DAGs, such state may prevent a considerable
   portion of the total memory in the node from being available for more
   useful purposes.

   To alleviate the problem described above, an RPL node may invoke the
   following procedure to identify a defunct DAG and delete the state it
   maintains for this DAG.  Note that, given the proactive nature of RPL
   protocol, the lack of data traffic using a DAG can not be considered
   a reliable indication of the DAG's defunction.  Further, the Trickle
   timer based control of DIO transmissions means the possibility of an
   indefinite delay in the receipt of a new DIO from a functional DAG
   parent.  Hence, the mechanism described next is based on the use of a
   DIS message to solicit DIOs about a DAG suspected of defunction.
   Further, a multicast DIS is used so as to avoid the need to query
   each parent individually and also to discover other neighbor routers
   that may serve as the node's new parents in the DAG.

   When an RPL node has not received a DIO from any of its parents in a
   DAG for more than a locally configured time duration:

   o  The node generates a multicast DIS message with:

      *  "No Inconsistency" flag set so that the responding routers do
         not reset their Trickle timers.

      *  "DIO Type" flag set so that the responding routers send
         multicast DIOs and other nodes in the vicinity do not need to
         invoke this procedure.

      *  A Solicited Information option to identify the DAG in question.
         This option must have the I and D flags set and the
         RPLInstanceID/DODAGID fields must be set to values identifying
         the DAG.  The V flag inside the Solicited Information option
         should not be set so as to allow the neighbors to send DIOs
         advertising the latest version of the DAG.

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      *  A Response Spreading option specifying a suitable time interval
         over which the DIO responses may arrive.

   o  After sending the DIS, the node waits for the duration specified
      inside the Response Spreading option to receive the DIOs generated
      by its neighbors.  At the conclusion of the wait duration:

      *  If the node has received one or more DIOs advertising newer
         version(s) of the DAG, it joins the latest version of the DAG,
         selects a new parent set among the neighbors advertising the
         latest DAG version and marks the DAG status as functional.

      *  Otherwise, if the node has not received a DIO advertising the
         current version of the DAG from a neighbor in the parent set,
         it removes that neighbor from the parent set.  As a result, if
         the node has no parent left in the DAG, it marks the DAG as
         defunct and schedule the deletion of the state it has
         maintained for the DAG after a locally configured "hold"
         duration.  (This is because, as per RPL specification, when a
         node no longer has any parents left in a DAG, it is still
         required to remember the DAG's identity (RPLInstanceID,
         DODAGID, DODAGVersionNumber), the lowest rank (L) it has had in
         this DAG and the DAGMaxRankIncrease value for the DAG for a
         certain time interval to ensure that the node does not join an
         earlier version of the DAG and does not rejoin the current
         version of the DAG at a rank higher than L +

6.  IANA Considerations

6.1.  DIS Flags

   IANA is requested to allocate bits 6 and 7 of the DIS Flag Field to
   become the "No Inconsistency" and "DIO Type" bits, the functionality
   of which is described in Section 3 of this document.

               | Value |      Meaning     |   Reference   |
               |   6   | No Inconsistency | This document |
               |   7   |     DIO Type     | This document |

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6.2.  RPL Control Message Options

   IANA is requested to allocate a new code point in the "RPL Control
   Message Options" registry for the "Response Spreading" option, the
   behavior of which is described in Section 4.2.

              | Value |       Meaning      |   Reference   |
              |  0x0A | Response Spreading | This document |

                        RPL Control Message Options

7.  Security Considerations


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.

   [RFC6550]  Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
              Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
              Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
              Lossy Networks", RFC 6550, March 2012.

8.2.  Informative References

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              September 2007.

   [RFC5184]  Teraoka, F., Gogo, K., Mitsuya, K., Shibui, R., and K.
              Mitani, "Unified Layer 2 (L2) Abstractions for Layer 3
              (L3)-Driven Fast Handover", RFC 5184, May 2008.

   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
              June 2010.

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, March 2011.

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Authors' Addresses

   Mukul Goyal
   University of Wisconsin Milwaukee
   3200 N Cramer St
   Milwaukee, WI  53201

   Phone: +1 414 2295001
   Email: mukul@uwm.edu

   Dominique Barthel
   France Telecom Orange
   28 Chemin Du Vieux Chene, BP 98
   Meylan,   38243

   Email: dominique.barthel@orange-ftgroup.com

   Emmanuel Baccelli

   Phone: +33-169-335-511
   Email: Emmanuel.Baccelli@inria.fr
   URI:   http://www.emmanuelbaccelli.org/

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