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RAW                                                      G. Papadopoulos
Internet-Draft                                            IMT Atlantique
Intended status: Standards Track                           March 9, 2020
Expires: September 10, 2020

                    Use cases for DIS Modifications


   This document presents some of the use-cases which call for DIS flags
   and options modifications.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 10, 2020.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Applications  . . . . . . . . . . . . . . . . . . . . . . . .   2
     3.1.  A Leaf Node Joining a DAG . . . . . . . . . . . . . . . .   2
     3.2.  Identifying A Defunct DAG . . . . . . . . . . . . . . . .   3
     3.3.  Adjacencies probing with RPL  . . . . . . . . . . . . . .   5
       3.3.1.  Deliberations . . . . . . . . . . . . . . . . . . . .   6
   4.  Informative References  . . . . . . . . . . . . . . . . . . .   6
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .   7

1.  Introduction

2.  Terminology

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

3.  Applications

   This section details some use cases that require DIS modifications
   compared to the behaviour currently defined in [RFC6550].  The first
   use case is thatof a new leaf node joining an established DAG in an
   energy efficient manner.  The second use case describes why node
   might want to use DIS to identify defunct DAGs for which it still
   maintains state.  The third use case describes the need for adjacency
   probing and how DIS can used for that.

3.1.  A Leaf Node Joining a DAG

   This use case is typically of a smart meter being replaced in the
   field, while a RPL network is operating and stable.  The new smart
   meter must join the network quickly, without draining the energy of
   the surrounding nodes, be they battery-operated RPL routers or leaf
   nodes.  In this use case, the issues with the current RPL
   specification are

   o  Just waiting for a gratuitous DIO may take a long time if the
      Trickle timers have relaxed to the steady state.  A technician who
      has just installed the new meter needs to positively assess that
      the meter has joined the network before it leaves the premise.  It
      is not economically viable to ask the technician to standby the
      meter until a gratuitous DIO has arrived, which may take hours.

   o  If the meter sends a DIS, it needs to do so using multicast,
      because it has no knowledge of its surroundings.  Sending a

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      multicast DIS is considered an inconsistency by the nearby RPL
      routers.  They will reset their Trickle timer to the shortest
      period.  This will trigger sending a stream of DIOs until the
      Trickle timers relax again.  The DIOs will be sent in multicast,
      which will trigger energy expenditure at nearby nodes, which had
      no need for the DIOs.

   A proposed solution could be the following.  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, thereby keeping the
   aggregated number of transmissions low.  It also has the "DIO Type"
   flag set to make responding routers send unicast DIOs back, thereby
   not triggering full reception in nearby nodes that have state-of-the-
   art radio receivers with hardware-based address filtering.

   The DIS message can include a Response Spreading option prescribing a
   suitable spreading interval based on the expected density of nearby
   routers and on the expected Layer 2 technology.

   The DIS will likely include a Metric Container listing the routing
   constraints that the responding routers must satisfy in order to be
   allowed to respond [RFC6551].

   At each iteration, the node multicasts such a DIS and waits for
   forthcoming DIOs.  After a time equal to the spreading interval, the
   node considers the current iteration to be unsuccessful.  The node
   consequently relaxes the routing constraints somewhat and proceeds to
   the next iteration.

   The cycle repeats until the node receives one or more DIOs or until
   it has relaxed the constraints to the lowest acceptable values.

   This algorithm has been proven in the field to be extremely energy-
   efficient, especially when routers have a wide communication range.

3.2.  Identifying A Defunct DAG

   A 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

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   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, a RPL node may fail to
   remove a neighbor from its parent set because:

   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, a 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 here 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 a 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:

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

      *  the "DIO Type" flag not 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.

      *  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 +

3.3.  Adjacencies probing with RPL

   RPL avoids periodic hello messaging as compared to other distance
   vector protocols.  It uses trickle timer based mechanism to update
   configuration parameters.  This significantly reduces the RPL control
   overhead.  One of the fallout of this design choice is that, in the

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   absence of regular traffic, the adjacencies could not be tested and
   repaired if broken.

   RPL provides a mechanism in the form of unicast DIS to query a
   particular node for its DIO.  A node receiving a unicast DIS MUST
   respond with a unicast DIO with Configuration Option.  This mechanism
   could as well be made use of for probing adjacencies and certain
   implementations such as Contiki uses this.  The periodicity of the
   probing is implementation dependent, but the node is expected to
   invoke probing only when

   o  There is no data traffic based on which the links could be tested.

   o  There is no L2 feedback.  In some case, L2 might provide periodic
      beacons at link layer and the absence of beacons could be used for
      link tests.

3.3.1.  Deliberations

   o  Should the probing scheme be standardized?  In some cases using
      multicast based probing may prove advantageous.

   o  In some cases using multicast based probing may prove
      advantageous.  Currently RPL does not have multicast based
      probing.  Multicast DIS/DIO may not be suitable for probing
      because it could possibly lead to change of states.

4.  Informative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, 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,
              DOI 10.17487/RFC5184, May 2008,

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   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
              DOI 10.17487/RFC5881, June 2010,

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., 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,
              DOI 10.17487/RFC6550, March 2012,

   [RFC6551]  Vasseur, JP., Ed., Kim, M., Ed., Pister, K., Dejean, N.,
              and D. Barthel, "Routing Metrics Used for Path Calculation
              in Low-Power and Lossy Networks", RFC 6551,
              DOI 10.17487/RFC6551, March 2012,

Author's Address

   Georgios Z. Papadopoulos
   IMT Atlantique
   Office B00 - 102A
   2 Rue de la Chataigneraie
   Cesson-Sevigne - Rennes  35510

   Phone: +33 299 12 70 04
   Email: georgios.papadopoulos@imt-atlantique.fr

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