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Versions: (draft-zhong-roll-dis-modifications) 00 draft-ietf-roll-dis-modifications

Internet Engineering Task Force                         C. Gundogan, Ed.
Internet-Draft                                                 FU Berlin
Intended status: Standards Track                              D. Barthel
Expires: January 9, 2017                                          Orange
                                                             E. Baccelli
                                                                   INRIA
                                                            July 8, 2016


                           DIS Modifications
                draft-gundogan-roll-dis-modifications-00

Abstract

   This document augments [RFC6550] with DIS flags and options that
   allow a RPL node to better control how neighbor RPL routers respond
   to its solicitation for DIOs.

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 January 9, 2017.

Copyright Notice

   Copyright (c) 2016 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




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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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  RFC6550 refresher . . . . . . . . . . . . . . . . . . . .   2
     1.2.  Undesirable effects . . . . . . . . . . . . . . . . . . .   3
     1.3.  Desired improvments . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  DIS Base Object flags . . . . . . . . . . . . . . . . . . . .   5
   4.  DIS Options . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.1.  Metric Container  . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Response Spreading  . . . . . . . . . . . . . . . . . . .   6
     4.3.  DIO Option Request  . . . . . . . . . . . . . . . . . . .   7
   5.  Full behavior illustration  . . . . . . . . . . . . . . . . .   8
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
     6.1.  DIS Flags . . . . . . . . . . . . . . . . . . . . . . . .   9
     6.2.  RPL Control Message Options . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  11
   Appendix A.  Applications . . . . . . . . . . . . . . . . . . . .  11
     A.1.  A Leaf Node Joining a DAG . . . . . . . . . . . . . . . .  11
     A.2.  Identifying A Defunct DAG . . . . . . . . . . . . . . . .  12
     A.3.  Explicit and Implicit DIO Option Requests . . . . . . . .  14
   Appendix B.  Experimental data  . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   This document augments [RFC6550], the RPL routing protocol
   specification.

1.1.  RFC6550 refresher

   Per [RFC6550], a RPL node can send a DODAG Information Solicitation
   (DIS) message to solicit DODAG Information Object (DIO) messages from
   neighbor RPL routers.

   A DIS can be multicast to all the routers in range or it can be
   unicast to a specific neighbor router.

   A DIS may carry a Solicited Information option that specifies the
   predicates of the DAG(s) the soliciting node is interested in.  In
   the absence of such Solicited Information option, the soliciting node



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   is deemed interested in receiving DIOs for all the DAGs known by the
   solicited router(s).

   [RFC6550] requires a router to treat the receipt of a multicast DIS
   as an inconsistency and hence reset its Trickle timers for the
   matching DAGs.  As a result of the general Trickle timer mechanism,
   future DIOs will be sent at a higher rate.  See [RFC6206] for the
   specification of Trickle timers and the definition of
   "inconsistency".

   [RFC6550] requires a router that receives a unicast DIS to respond by
   unicasting a DIO for each matching DAG and to not reset the
   associated Trickle timer.  Such a DIO generated in response to a
   unicast DIS must contain a Configuration option.

   This description is summarized in Table 1.

   +----------------------------+----------------------+---------------+
   |                            |     Unicast DIS      | Multicast DIS |
   +----------------------------+----------------------+---------------+
   |     no option present      |  unicast DIO, don't  |    do reset   |
   |                            | reset Trickle timer  | Trickle timer |
   | -------------------------- | -------------------- | ------------- |
   |   Solicited Information    |      do nothing      |   do nothing  |
   |    option present, not     |                      |               |
   |          matching          |                      |               |
   | -------------------------- | -------------------- | ------------- |
   |   Solicited Information    |  unicast DIO, don't  |    do reset   |
   |  option present, matching  | reset Trickle timer  | Trickle timer |
   +----------------------------+----------------------+---------------+

        Table 1: Router behavior on receiving a DIS, as per RFC6550

   More precisely, Table 1 describes the behavior of routers for each
   DAG they belong to.  In the general case where multiple RPL instances
   co-exist in a network, routers will maintain a Trickle timer for the
   one DAG of each RPL instance they belong to, and nodes may send a DIS
   with multiple Solicited Information options pertaining to different
   DAGs or instances.  In this more general case, routers will respond
   for each individual DAG/instance they belong to as per Table 1.

1.2.  Undesirable effects

   Now, consider a 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



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   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 will keep transmitting frequent DIOs for a
   considerable duration until the Trickle timers again reach long
   intervals.  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 a 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.

   Certain scenarios may require a RPL router to solicit a DIO from a
   parent by using a unicast DIS.  The parent is forced to include a
   Configuration option within the unicast DIO, although the requesting
   node might still have this information locally available.  Since the
   information within the Configuration option is described as generally
   static and unchanging throughout the DODAG, it inflates the unicast
   DIO unnecessarily by 16 bytes for each request.

1.3.  Desired improvments

   To deal with the situations described above, there is a need in the
   industry for DIS flags and options that allow a RPL node to control
   how neighbor RPL routers respond to its solicitation for DIOs, for
   example by expressing:

   o  the routing constraints that routers should meet to be allowed to
      respond, thereby lowering the number of responders

   o  whether the responding routers should reset their Trickle timers
      or not, thereby limiting the cumulated number of transmitted DIOs

   o  whether the responding routers should respond with a unicast DIO
      instead of a multicast one, thereby lowering the overhearing cost
      in the network

   o  whether the responding routers should omit DIO options that were
      not requested explicitely and thus reducing the amount of traffic
      and giving full control over the options of the solicited DIO




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   o  the time interval over which the responding routers should
      schedule their DIO transmissions, thereby lowering the occurence
      of collisions.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

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

3.  DIS Base Object flags

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

   o  the "No Inconsistency" (N) flag: On receiving a multicast DIS with
      the N flag set, a RPL router MUST NOT reset the Trickle timers for
      the matching DAGs.  In addition, it MUST take specific action,
      which is to respond by explicitely sending a DIO.  This DIO MUST
      include a Configuration option.  This behavior augments [RFC6550],
      which had provision for such flag.  Since this specific, one-shot
      DIO is not a consequence of the general Trickle timer mechanism,
      it will be sent right away if no Response Spreading option is
      present or it will be scheduled according to the Response
      Spreading option if one is present in the DIS (see Section 4.2).

   o  the "DIO Type" (T) flag: In case the N flag is set, this T flag
      specifies what type of DIO is sent in response.  It MUST be a
      unicast DIO if this flag is set and it MUST be a multicast DIO if
      this flag is reset.

   o  the "DIO Option Request" (R) flag: On receiving a DIS with the R
      flag set, the receiver MUST include all options that were
      requested by the DIS containing one or multiple DIO Option Request
      options.  A responding RPL router MUST NOT include DIO options
      that were not explicitely requested.  Note that this behaviour
      contradicts with [RFC6550] for the case of including a
      Configuration option in all DIOs requested by a unicast DIS.

   When a unicast DIS is transmitted, both its N and T flags SHOULD be
   0, which are the default values per [RFC6550].  On receiving a
   unicast DIS, the N and T flags MUST be ignored and treated as 00.
   When the R flag is unset, then a RPL router may include or omit DIO
   options like specified in [RFC6550].  A rpl router responding to a



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   DIS with the R flag set MUST only include all requested DIO options
   in the solicited DIO.

   The modified DIS base object is shown in Figure 1.

       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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |N|T|R|  Flags  |    Reserved   | Option(s)...
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


                    Figure 1: Modified DIS Base Object

4.  DIS Options

4.1.  Metric Container

   In order to lower the number of routers that will respond to a DIS,
   this document allows routing constraints to be carried by a DIS.
   Only the router(s) that satisfy these constraints is (are) allowed to
   respond to the DIS.

   These routing constraints are described using a Metric Container
   option contained in the DIS.  Metric Containers are defined in
   [RFC6550] and [RFC6551].  Metric Containers options were previously
   only allowed in DIOs.  This document augments [RFC6550] by allowing
   the inclusion of a Metric Container option inside a DIS as well.

   A 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
   value of the corresponding routing metric that the router maintains
   for the matching DAG(s).  These routing metric values MUST satisfy
   all the mandatory constraints in order for the router to consider the
   solicitation successful for the matching DAG(s).  This augments the
   behavior already present in [RFC6550] with the Solicited Information
   option.

   This option can be used in both unicast and multicast DIS.

4.2.  Response Spreading









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             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 = 0x0B  |    Length     | Spread. Inter.|
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 2: The Response Spreading option

   Even with the use of the Solicited Information and the Section 4.1
   options, a multicast DIS may still lead to a large number of RPL
   routers taking immediate action and responding with DIOs.  Concurrent
   transmissions by multiple routers are not desirable since they may
   lead to poor channel utilization or even to packet loss.  Unicast
   DIOs may be able to avail of link-level retransmissions.  However,
   multicast DIOs usually have no such protection, since they commonly
   make use of link layer broadcast.  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 option, shown in Figure 2, with a recommended Type
   value 0x0B (to be confirmed by IANA).  A RPL router that explicitely
   responds with a specific, one-shot DIO to a 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.

   A Response Spreading option MAY be included inside a unicast DIS
   message, but there is no benefit in doing so.

   Multiple Response Spreading options SHOULD NOT be used inside a same
   DIS message.

   This mechanism MUST NOT affect the Trickle timer mechanism.

4.3.  DIO Option Request

             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 = 0x0C  |    Length     | Req. Opt. Type|
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                  Figure 3: The DIO Option Request option

   If a unicast DIS is used to request a DIO, then [RFC6550] mandates
   that a Configuration option MUST be included in this DIO.  The



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   Configuration option contains generally static information that stays
   unmodified throughout the DAG.  For scenarios where a RPL node is
   already part of a DAG and hence is holding the information that is
   propagated with the Configuration option, an inclusion of such leads
   to an unnecesary inflation of 16 bytes for each solicited DIO.

   As per [RFC6550], no process is defined to trigger the inclusion of
   other DIO options in a solicited DIO.

   This document defines a new RPL control message option called DIO
   Option Request option, shown in Figure 3, with a recommended Type
   value of 0x0C (to be confirmed by IANA).  This new option allows full
   control over the options of the solicited DIO.  The target of a
   unicast or multicast DIS with the R flag set and with one or more DIO
   Option Request options included, MUST include these requested options
   in the solicited DIO.  For a DIS with the R flag unset, a RPL router
   behaves like described in [RFC6550] with regard to DIO options.

5.  Full behavior illustration

   Figure 4 and Figure 5 illustrate the normative behavior described in
   Section 3 and Section 4.1.

                            +--------------------+----------------------
                            |    Unicast DIS     |         Multicast DIS
                            +--------------------+--------------------+-
                            |                    |         N=0        |
       +--------------------+--------------------+--------------------+-
       |                    | unicast DIO,       |                    |
       | no option present  | don't              | do                 |
       |                    | reset Trickle timer| reset Trickle timer|
       +--------------------+--------------------+--------------------+-
       | Solicited Informa- |                    |                    |
       | tion/Metric Contai-| do nothing         | do nothing         |
       | ner option present,|                    |                    |
       | not matching.      |                    |                    |
       +--------------------+--------------------+--------------------+-
       | Solicited Informa- | unicast DIO,       |                    |
       | tion/Metric Contai-| don't              | do                 |
       | ner option present,| reset Trickle timer| reset Trickle timer|
       | matching.          |                    |                    |
       +--------------------+--------------------+--------------------+-

                  Figure 4: Overall DIS behavior, part 1

   Notice that Figure 4 is indeed identical to Table 1 when Metric
   Container options are not used in DIS.




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         -------------------------------------------+
          Multicast DIS                             |
         ----------------------+--------------------+
          |      N=1, T=0      |      N=1, T=1      |
         -+--------------------+--------------------+
          | multicast DIO,     | unicast DIO,       |
          | don't              | don't              |
          | reset Trickle timer| reset Trickle timer|
         -+--------------------+--------------------+
          |                    |                    |
          | do nothing         | do nothing         |
          |                    |                    |
          |                    |                    |
         -+--------------------+--------------------+
          | multicast DIO,     | unicast DIO,       |
          | don't              | don't              |
          | reset Trickle timer| reset Trickle timer|
          |                    |                    |
         -+--------------------+--------------------+

                  Figure 5: Overall DIS behavior, part 2

   For the sake of completeness, let's remind here that a specific, one-
   shot DIO generated in response to a DIS with the R flag unset MUST
   contain a Configuration option.  If the R flag is set, then this DIO
   contains only explicitely requested DIO options.  This DIO's
   transmission is delayed according to the Delay Spreading option of
   the DIS, if one such option is present.

6.  IANA Considerations

6.1.  DIS Flags

   IANA is requested to allocate bits 0, 1 and 2 of the DIS Flag Field
   to become the "No Inconsistency", "DIO Type", and "DIO Option
   Request" bits, the functionality of which is described in Section 3
   of this document.

              +-------+--------------------+---------------+
              | Value |      Meaning       |   Reference   |
              +-------+--------------------+---------------+
              |   0   |  No Inconsistency  | This document |
              |   1   |      DIO Type      | This document |
              |   2   | DIO Option Request | 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 and the
   "Dio Option Request" option, the behavior of which are described in
   Section 4.2 and Section 4.3, respectively.

              +-------+--------------------+---------------+
              | Value |      Meaning       |   Reference   |
              +-------+--------------------+---------------+
              |  0x0B | Response Spreading | This document |
              |  0x0C | DIO Option Request | This document |
              +-------+--------------------+---------------+

                        RPL Control Message Options

7.  Security Considerations

   TBA

8.  Acknowledgements

   A lot of text in this document originates from now-expired [I-
   D.goyal-roll-dis-modifications] co-authored with M.  Goyal.  The
   requirements and solutions also draw from now-expired [I-D.dejean-
   roll-selective-dis] co-authored with N.  Dejean.  Their contribution
   is deeply acknowledged.

   We also thank (TBA) for their useful feedback and discussion.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [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,
              <http://www.rfc-editor.org/info/rfc6550>.






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   [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,
              <http://www.rfc-editor.org/info/rfc6551>.

9.2.  Informative References

   [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,
              <http://www.rfc-editor.org/info/rfc4861>.

   [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,
              <http://www.rfc-editor.org/info/rfc5184>.

   [RFC5881]  Katz, D. and D. Ward, "Bidirectional Forwarding Detection
              (BFD) for IPv4 and IPv6 (Single Hop)", RFC 5881,
              DOI 10.17487/RFC5881, June 2010,
              <http://www.rfc-editor.org/info/rfc5881>.

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, DOI 10.17487/RFC6206,
              March 2011, <http://www.rfc-editor.org/info/rfc6206>.

Appendix A.  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 in an energy efficient manner.
   The second mechanism details how a node may identify defunct DAGs for
   which it still maintains state.

A.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, 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.



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   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.

   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.

A.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

   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



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      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:

      *  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.





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   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 +
         DAGMaxRankIncrease.)

A.3.  Explicit and Implicit DIO Option Requests

   Certain information from a DIO is only needed occasionally or for
   specific events:

   o  A Configuration option contains information that is static and
      stays mostly unchanged during the lifetime of the DODAG.  Thus,
      the Configuration option is important when joining the DODAG, but
      inflates the DIO unnecessarily thereafter.

   o  A Prefix Information option is also useful when joining a DODAG to
      perform address autoconfiguration and propagating the prefixes.
      After that, it is only relevant when changes occure that would
      affect the prefix information (new prefixes, lifetime updates,
      ...).

   o  More DIO options may be added in the future that have similar
      properties as mentioned above.

   The Configuration and Prefix Information options may be omitted from
   the trickle timer based DIOs, leading to less bytes for each DIO.
   Once a RPL router decides to join a DODAG, it may solicit a DIO by
   sending a DIS with the R flag set and two "DIO Option Request"



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   options included.  One for the Configuration option, the other for
   the Prefix Information option.  Upon receiving these options in the
   next DIO, the RPL router can successfully finish the joining process.

Appendix B.  Experimental data

   The effectiveness of these flags and options has been measured on
   real industrial hardware.

   Data to be added

Authors' Addresses

   Cenk Gundogan (editor)
   FU Berlin

   Email: cenk.guendogan@fu-berlin.de


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

   Email: dominique.barthel@orange.com


   Emmanuel Baccelli
   INRIA

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

















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