ROLL                                                      R. Jadhav, Ed.
Intended status: Standards Track                                R. Sahoo
Expires: December 5, 2021                                        Juniper
                                                                   Y. Wu
                                                            June 3, 2021

                            RPL Observations


   This document describes RPL protocol design issues, various
   observations and possible consequences of the design and
   implementation choices.

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   This Internet-Draft will expire on December 5, 2021.

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   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Motivation  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Requirements Language and Terminology . . . . . . . . . .   3
   3.  DTSN increment in storing MOP . . . . . . . . . . . . . . . .   4
     3.1.  Deliberations . . . . . . . . . . . . . . . . . . . . . .   5
   4.  DAO retransmission and use of DAO-ACK in storing MOP  . . . .   5
     4.1.  Significance of bidirectional Path establishment
           indication and relevance of DAO-ACK . . . . . . . . . . .   6
     4.2.  Problems with hop-by-hop DAO-ACK  . . . . . . . . . . . .   6
     4.3.  Problems with end-to-end DAO-ACK  . . . . . . . . . . . .   6
     4.4.  Deliberations . . . . . . . . . . . . . . . . . . . . . .   6
     4.5.  Implementation Notes  . . . . . . . . . . . . . . . . . .   7
   5.  Interpreting Trickle Timer  . . . . . . . . . . . . . . . . .   7
   6.  Handling resource unavailability  . . . . . . . . . . . . . .   8
     6.1.  Deliberations . . . . . . . . . . . . . . . . . . . . . .   8
   7.  Handling aggregated targets . . . . . . . . . . . . . . . . .   9
     7.1.  Deliberations . . . . . . . . . . . . . . . . . . . . . .   9
   8.  RPL Transit Information in DAO  . . . . . . . . . . . . . . .   9
     8.1.  Deliberations . . . . . . . . . . . . . . . . . . . . . .  10
   9.  Upgrades or Extensions to RPL protocol  . . . . . . . . . . .  10
   10. Path Control bits handling  . . . . . . . . . . . . . . . . .  10
   11. Asymmetric Links and RPL  . . . . . . . . . . . . . . . . . .  11
   12. Adjacencies probing with RPL  . . . . . . . . . . . . . . . .  11
     12.1.  Deliberations  . . . . . . . . . . . . . . . . . . . . .  12
   13. Control Options eliding mechanism in RPL  . . . . . . . . . .  12
   14. Managing persistent variables across node reboots . . . . . .  12
     14.1.  Persistent storage and RPL state information . . . . . .  12
     14.2.  Lollipop Counters  . . . . . . . . . . . . . . . . . . .  13
     14.3.  RPL State variables  . . . . . . . . . . . . . . . . . .  14
       14.3.1.  DODAG Version  . . . . . . . . . . . . . . . . . . .  14
       14.3.2.  DTSN field in DIO  . . . . . . . . . . . . . . . . .  14
       14.3.3.  PathSequence . . . . . . . . . . . . . . . . . . . .  15
     14.4.  State variables update frequency . . . . . . . . . . . .  15
     14.5.  Deliberations  . . . . . . . . . . . . . . . . . . . . .  15
     14.6.  Implementation Notes . . . . . . . . . . . . . . . . . .  16
   15. Capabilities and its role in RPL  . . . . . . . . . . . . . .  16
     15.1.  Handshaking node capabilities  . . . . . . . . . . . . .  16
     15.2.  How do Capabilities differ from MOP and Configuration
            Option?  . . . . . . . . . . . . . . . . . . . . . . . .  17
     15.3.  Deliberations  . . . . . . . . . . . . . . . . . . . . .  17
   16. Backward Compatibility issues with RPL Options  . . . . . . .  17
   17. RPL under-specification . . . . . . . . . . . . . . . . . . .  17
   18. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18

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   19. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   20. Security Considerations . . . . . . . . . . . . . . . . . . .  18
   21. References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     21.1.  Normative References . . . . . . . . . . . . . . . . . .  18
     21.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Appendix A.  Additional Stuff . . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

1.  Motivation

   The primary motivation for this draft is to enlist different issues
   with RPL operation and invoke a discussion within the working group.
   This draft by itself is not intended for RFC tracks but as a WG
   discussion track.  This draft may in turn result in other work items
   taken up by the WG which may improvise on the issues mentioned

2.  Introduction

   RPL [RFC6550] specifies a proactive distance-vector routing scheme
   designed for LLNs (Low Power and Lossy Networks).  RPL enables the
   network to be formed as a DODAG and supports storing mode and non-
   storing mode of operations.  Non-storing mode allows reduced memory
   resource usage on the nodes by allowing non-BR nodes to operate
   without managing a routing table and involves use of source routing
   by the Root to direct the traffic along a specific path.  In storing
   mode of operation intermediate routers maintain routing tables.

   This work aims to highlight various issues with RPL which makes it
   difficult to handle certain scenarios.  This work will highlight such
   issues in context to RPL's mode of operations (storing versus non-
   storing).  There are cases where RPL does not provide clear rules and
   implementations have to make their choices hindering interoperability
   and performance.

   [I-D.clausen-lln-rpl-experiences] provides some interesting points.
   Some sections in this draft may overlap with some observations in
   [clausen], but this is been done to further extend some scenarios or
   observations.  It is highly encouraged that readers should also visit
   [I-D.clausen-lln-rpl-experiences] for other insights.  Regardless,
   this draft is self-sufficient in a way that it does not expect to
   have read [clausen-draft].

2.1.  Requirements Language and Terminology

   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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   NS-MOP = RPL Non-storing Mode of Operation

   S-MOP = RPL Storing Mode of Operation

   This document uses terminology described in [RFC6550] and [RFC6775].

3.  DTSN increment in storing MOP

   DTSN increment has major impact on the overall RPL control traffic
   and on the efficiency of downstream route update.  DTSN is sent as
   part of DIO message and signals the downstream nodes to trigger the
   target advertisement.  The 6LR needs to decide when to update the
   DTSN and usually it should do it in a conservative way.  The DTSN
   update mechanism determines how soon the downward routes are
   established along the new path.  RPL specifications does not provide
   any clear mechanism on how the DTSN update should happen in case of
   storing mode.

                                     / \
                                    /   \
                                   /     \
                                 (B)    -(C)
                                  |    /  |
                                  |   /   |
                                  |  /    |
                                 (D)-    (E)
                                   \      ;
                                    \    ;
                                     \  ;
                                      / \
                                     /   \
                                    /     \
                                  (G)     (H)

                         Figure 1: Sample topology

   Consider example topology shown in Figure 1, assume that node D
   switches the parent from node B to C.  Ideally the downstream nodes D
   and its sub-childs should send their target advertisement to the new
   path via node C.  To achieve this result in a efficient way is a
   challenge.  Incrementing DTSN is the only way to trigger the DAO on
   downstream nodes.  But this trigger should be sent not only on the

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   first hop but to all the grand-child nodes.  Thus DTSN has to be
   incremented in the complete sub-DODAG rooted at node D thus resulting
   in DIO/DAO storm along the sub-DODAG.  This is specifically a big
   issue in high density networks where the metric deteoration might
   happen transiently even though the signal strength is good.

   The primary implementation issue is whether a child node increment
   its own DTSN when it receives DTSN update from its parent node?  This
   would result in DAO-updates in the sub-DODAG, thus the cost could be
   very high.  If not incremented it may result in serious loss of
   connectivity for nodes in the sub-DODAG.

3.1.  Deliberations

   (1)  In S-MOP, should the child node increment its DTSN on seeing
        that its preferred parent has updated its DTSN?

   (2)  What are rules for DTSN increment for S-MOP, which multiple
        implementations can follow thus allowing consistent performance
        across different implementations?

4.  DAO retransmission and use of DAO-ACK in storing MOP

   [RFC6550] has an optional DAO-ACK mechanism using which an upstream
   parent confirms the reception of a DAO from the downstream child.  In
   case of storing mode, the DAO is addressed to the immediate hop
   upstream parent resulting in DAO-ACK from the parent.  There are two
   implementations possible:

   (1)  Hop-by-hop ACK: A parent responds with a DAO-ACK immedetialy
        after receiving the DAO.

   (2)  End-to-End ACK: A node waits for the upstream parent to send
        DAO-ACK to respond with a DAO-ACK downstream.  The upstream
        parent may do as many attempts to successfully send this DAO
        upstream.  In other words, the parent node accepts the
        responsibilty of sending the DAO upstream till the point it is
        ACKed the moment it responds back with its own ACK to the child.

                               1->          3->
                               DAO          DAO
                               ACK          ACK
                               <-2          <-4

                       Figure 2: Hop-by-hop DAO-ACK

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                               1->          2->
                               DAO          DAO
                               ACK          ACK
                               <-4          <-3

                       Figure 3: End-to-End DAO-ACK

4.1.  Significance of bidirectional Path establishment indication and
      relevance of DAO-ACK

   Lot of application traffic patterns requires that the bidirectional
   path be established between the target node and the root.  A typical
   example is that COAP request with ACK bit set would require an
   acknowledgement from the end receiver and thus warrants bidirectional
   path establishment.  It is imperative that the target node first
   ascertains whether such a bidirectional path is established before
   initiating such application traffic.  In case of non-storing MOP, the
   DAO-ACK works perfectly fine to ascertain such bidirectional
   connectivity since it is an indication that the root which usually is
   the direct destination of the DAO has received the DAO.  But in case
   of storing MOP, things are more complicated since DAO is sent hop-by-
   hop and the DAO-ACK semantics are not clear enough as per the current
   specification.  As mentioned in above section, an implementation can
   choose to implement hop-by-hop ACK or end-to-end ACK.

4.2.  Problems with hop-by-hop DAO-ACK

   The primary issue with this mode is that target node cannot ascertain
   bidirection path connectivity on the reception of the DAO-ACK.

4.3.  Problems with end-to-end DAO-ACK

   In this case, it is possible for the target node to ascertain if the
   DAO has indeed reached the root since the reception of DAO-ACK on
   target node confirms this.  However there is extra state information
   that needs to be maintained on the 6LRs on behalf of all the child
   nodes.  Also it is very difficult for the target node to ascertain a
   timer value to decide whether the DAO transmission has failed to
   reach the root.

4.4.  Deliberations

   (1)  How should an implementation interpret the DAO-ACK semantics?

   (2)  What is the best way for the target node to know that the end to
        end bidirectional path is successfully installed or updated?  In

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        NS-MOP, the DAO-ACK provides a clear way to do this.  Can the
        same be achieved for storing-MOP?

   (3)  What happens if the DAO-ACK with Status!=0 is responded by
        ancestor node?

   (4)  How to selectively NACK subset of targets in case target options
        are aggregated?

4.5.  Implementation Notes

   Current RPL open source implementations have both types of DAO-ACK
   implementations.  For e.g.  RIOT supports hop-by-hop DAO-ACK.
   Contiki older versions supported hop-by-hop ACK but the recent
   version have changed to end-to-end ACK implementation.

   The sequence of sending no-path DAO and DAO matters when updating the
   routing adjacencies on a parent switch.  If an implementation chooses
   to send no-path DAO before DAO then it results in significantly more
   overhead for route invalidation.  This is because no-path DAO would
   traverse all the way up to the BR clearing the routes on the way.  In
   case there is a common ancestor post which the old and new path
   remains same then it is better to send regular DAO first thus
   limiting the propagation of subsequent no-path DAO till this common

5.  Interpreting Trickle Timer

   Trickle algorithm defines a mechanism to reset the timer.  Trickle
   timer reset is unlike regular periodic timers wherein the timer is
   simply reset to start again.  Reset of trickle timer implies
   resetting the trickle back to Imin and starting with a new interval
   as mentioned in Section 4.2 of [RFC6206].

 |----|--------|----------------|------------------------------| . . . .
  Imin   I2             I3                     I4                    I5

                     Figure 4: Trickle Timer Operation

   The above figure shows an example of trickle intervals.  An interval
   is double that of the previous interval size.  Section 4.2. of
   [RFC6206] states that,

   "If Trickle hears a transmission that is "inconsistent" and I is
   greater than Imin, it resets the Trickle timer.  To reset the timer,
   Trickle sets I to Imin and starts a new interval as in step 2.  If I
   is equal to Imin when Trickle hears an "inconsistent" transmission,

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   Trickle does nothing.  Trickle can also reset its timer in response
   to external "events"."

   Thus if the trickle timer has advanced to subsequent intervals i.e.,
   >= I2, then a reset of trickle timer implies going back to Imin.
   However, if the trickle timer is currently in Imin and if it hears an
   inconsistent transmission then it does nothing.

   In context to multicast DIS/DIO operation, this implies that if the
   DIO trickle timer is already at Imin and if the node hears a
   multicast DIS, then the timer does nothing.  It MUST NOT reset the
   timer again in this case.

   An implementation MUST never restart the timer within an interval.
   For e.g., in the above figure, if the timer is in interval I2, the
   implementation MUST never restart the timer to the beginning of the
   current interval i.e., I2.  If the timer is in interval T2 and if the
   reset is to be done then the interval is set back to Imin.  If the
   timer is already in Imin, then the reset should do nothing.

6.  Handling resource unavailability

   The nodes in the constrained networks have to maintain various
   records such as neighbor cache entries and routing entries on behalf
   of other targets to facilitate packet forwarding.  Because of the
   constrained nature of the devices the memory available may be very
   limited and thus the path selection algorithm may have to take into
   consideration such resource constraints as well.

   RPL currently does not have any mechanism to advertise such resource
   indicator metrics.  The primary tables associated with RPL are
   routing table and the neighbor cache.  Even though neighbor cache is
   not directly linked with RPL protocol, the maintenance of routing
   adjacencies results in updates to neigbor cache.

6.1.  Deliberations

      Is it possible to know that an upstream parent/ancestor cannot
      hold enough routing entries and thus this path should not be used?

      Is it possible to know that an upstream parent cannot hold any
      more neighbor cache entry and thus this upstream parent should not
      be used?

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7.  Handling aggregated targets

   RPL allows and defines specific procedures so as to aid target
   aggregation in DAO.  Having said that, the specification does not
   mandate use of aggregated targets nor does it make any comment on
   whether a receiving node needs to handle it.  Target aggregation is
   an useful tool and especially helps with link layer technologies that
   does not suffer from low MTUs such as PLC.  Even if the
   implementation does not support aggregating targets, it should
   atleast mandate reception of aggregated targets in DAO.

   RPL has a mechanism currently to ACK the DAO but it does not have a
   mechanism to ACK the target option.  Thus in case of aggregated
   targets in the DAO, if the subset of the targets fail then it is
   impossible for the DAO-ACK to signal this to the DAO sender.

7.1.  Deliberations

      Even if the implementation does not support aggregating targets,
      should it atleast mandate reception and handling of aggregated
      targets in DAO?

      There is a good scope for compressing aggregated targets which can
      significantly reduce the RPL control overhead.

      How to selectively NACK subset of targets in case target options
      are aggregated?

      The DEFAULT_DAO_DELAY of 1sec does not help much with aggregation.
      The upstream parent nodes should wait for more time then the child
      nodes so as to effectively aggregate.  Can we have
      DEFAULT_DAO_DELAY a function of the level/rank the node is at?

8.  RPL Transit Information in DAO

   RPL allows associating a target or set of targets with a Transit
   Information Option which contains attributes for a path to one or
   more destinations identified by the set of targets.  In case of NS-
   MOP, the transit Information will contain the all critical Parent
   Address which allows the common ancestor usually the root to identify
   the source route header for the target node.  The Transit Information
   also contains other information such as Path Sequence and Path
   Lifetime which are critical for maintaining route adjacencies.

   RPL however does not mandate the use of Transit Information Option
   for targets.

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8.1.  Deliberations

      Is it ok to let implementations decide on the inclusion of Transit
      Information Option?

      Is it possible to achieve interop without mandating use of Transit
      Information Option?

      If the Transit Information Option is sent, should the handling of
      PathSequence be mandated?

9.  Upgrades or Extensions to RPL protocol

   RPL extensibility is highly desirable and is controlled by protocol
   elements within the messaging framework.  In the pursuit to keep the
   signalling overhead less, RPL specification has been restricting in
   its approach to extend its field ranges, thus in some cases putting
   extensibility at stakes.  Consider for example, the mode of operation
   bits which is three bits in the RPL specification.  These bits are
   already saturated and it may be difficult to add major upgrades
   without extending these bits.

   Addition of new Control Options or new RPL Codes almost certainly
   results in backward compatibility issues.  RFC6550 clearly mentions
   that a message with an unknown RPL Code MUST be silently discarded.
   However, no explicit handling is suggested for unknown RPL control
   option types.  In some cases, implementations simply copy-forward an
   unknown option as it is while in other cases the unknown option is
   stripped off before forwarding the message.


   (1)  What are the extensibility options RPL could implement?  How
        much overhead would it incur?

   (2)  Most of the extensions are in the form of new control options.
        Should RPL have a mechanism to only handle such extensions in a
        backward compatible but in a generic manner?

10.  Path Control bits handling

   RPL uses Path Control bits in the DAO's Transit Information Option
   for installing multiple downward routes to the nodes.  These multiple
   routes could be used for reliability, latency or traffic load-
   balancing within a DAG.  The path control bits are usable both in
   storing and non-storing mode of operation.

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   RFC6550 Section 9.9 bullet point 9 requires a mandatory setting of
   Path Control bits in all the unicast DAOs sent by the Target node.
   However, no existing implementation of RPL supports this.  There is
   no reason for a network which only requires a single path to the root
   to mandatorily support path control bits.


   (1)  Should the mandatory clause for supporting Path Control Bits in
        RFC6550 Section 9.9 point 9 be removed?

   (2)  Handling Path Control Bits may be complex.  An implementation
        guideline explaining the use-cases and resource (memory
        requirements) assumptions would help implementors decide the
        utility of this technique.

11.  Asymmetric Links and RPL

   Section 3.1 of [I-D.ietf-intarea-adhoc-wireless-com] explains
   asymmetric link characteristics and what it takes for a protocol to
   support asymmetric links.  RPL depends on bi-directional links for
   control even though near-perfect symmetry is not expected.  The
   implication of this is that the upstream and downstream path remains
   same within a given RPL instance for any pair of nodes.  There are
   following questions sprouting of this design:

   (1)  Is it possible to detect asymmetric links?

   (2)  In the presence of asymmetric links what is the impact on the
        control overhead and is there a way to possibly mitigate or
        alleviate any negative impact?

   [I-D.ietf-roll-aodv-rpl] defines a mechanism to use a pair of
   instances which are coupled.  This allows disjoint upstream and
   downstream paths between pair of nodes assuming that the link
   asymmetricity is detected using some outside techniques.  The link
   assumes that the link asymmetricity is already known to the nodes in
   the form of static configuration.  In case of 6tisch networks, the
   availability of transmission slots information can be used to
   identify link asymmetricity.  The challenge with regards to detecting
   link asymmetricity arises from scenarios where, for example, the
   nodes transmit with unequal power levels.

12.  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

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   overhead.  One of the fallout of this design choice is that, in the
   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

   (1)  There is no data traffic based on which the links could be

   (2)  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.

12.1.  Deliberations

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

   (2)  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.

13.  Control Options eliding mechanism in RPL

   RPL configuration changes are rare and thus various configuration
   options may not change over a long period of time.  RPL provides a
   way for the configuration options to be elided but there are no clear
   guidelines on how the eliding should be handled.  In the absence of
   such guidelines, it is possible that certain nodes may end up using
   stale configuration in the event of transient link failures.

14.  Managing persistent variables across node reboots

14.1.  Persistent storage and RPL state information

   Devices are required to be functional for several years without
   manual maintanence.  Usually battery power consumption is considered
   key for operating the devices for several (tens of) years.  But apart
   from battery, flash memory endurance may prove to be a lifetime
   bottleneck in constrained networks.  Endurance is defined as maximum
   number of erase-write cycles that a NAND/NOR cell can undergo before

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   losing its 'gauranteed' write operation.  In some cases (cheaper
   NAND-MLC/TLC), the endurance can be as less as 2K cycles.  Thus for
   e.g.  if a given cell is written 5 times a day, that NAND-flash cell
   assuming an endurance of 10K cycles may last for less than 6 years.

   Wear leveling is a popular technique used in flash memory to minimize
   the impact of limited cell endurance.  Wear leveling works by
   arranging data so that erasures and re-writes are distributed evenly
   across the medium.  The memory sectors are over-provisioned so that
   the writes are distributed across multiple sectors.  Many IoT
   platforms do not necessarily consider this over-provisioning and
   usually provision the memory only to what is required.  Some
   scenarios such as street-lighting may not require the application
   layer to write any information to the persistent storage and thus the
   over-provisioning is often ignored.  In such cases if the network
   stack ends up using persistent storage for maintaining its state
   information then it becomes counter-productive.

   In a star topology, the amount of persistent data write done by
   network protocols is very limited.  But ad-hoc networks employing
   routing protocols such as RPL assume certain state information to be
   retained across node reboots.  In case of IoT devices this storage is
   mostly floating gate based NAND/NOR based flash memory.  The impact
   of loss of this state information differs depending upon the type
   (6LN/6LR/6LBR) of the node.

14.2.  Lollipop Counters

   [RFC6550] Section 7.2. explains sequence counter operation defining
   lollipop [Perlman83] style counters.  Lollipop counters specify
   mechanism in which even if the counter value wraps, the algorithm
   would be able to tell whether the received value is the latest or
   not.  This mechanism also helps in "some cases" to recover from node
   reboot, but is not foolproof.

   Consider an e.g. where Node A boots up and initialises the seqcnt to
   240 as recommended in [RFC6550].  Node A communicates to Node B using
   this seqcnt and node B uses this seqcnt to determine whether the
   information node A sent in the packet is latest.  Now lets assume,
   the counter value reaches 250 after some operations on Node A, and
   node B keeps receiving updated seqcnt from node A.  Now consider that
   node A reboots, and since it reinitializes the seqcnt value to 240
   and sends the information to node B (who has seqcnt of 250 stored on
   behalf of node A).  As per section 7.2. of [RFC6550], when node B
   receives this packet it will consider the information to be old
   (since 240 < 250).

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                         |  A  |  B  |  Output  |
                         | 240 | 240 | A<B, old |
                         | 240 | 241 | A<B, old |
                         | 240 |  :: | A<B, old |
                         | 240 | 256 | A<B, old |
                         | 240 |  0  | A<B, new |
                         | 240 |  1  | A>B, new |
                         | 240 |  :: | A>B, new |
                         | 240 | 127 | A>B, new |

      Default values for lollipop counters considered from [RFC6550]
                               Section 7.2.

                Table 1: Example lollipop counter operation

   Based on this figure, there is dead zone (240 to 0) in which if A
   operates after reboot then the seqcnt will always be considered
   smaller.  Thus node A needs to maintain the seqcnt in persistent
   storage and reuse this on reboot.

14.3.  RPL State variables

   The impact of loss of RPL state information differs depending upon
   the node type (6LN/6LR/6LBR).  Following sections explain different
   state variables and the impact in case this information is lost on

14.3.1.  DODAG Version

   The tuple (RPLInstanceID, DODAGID, DODAGVersionNumber) uniquely
   identifies a DODAG Version.  DODAGVersionNumber is incremented
   everytime a global repair is initiated for the instance (global or
   local).  A node receiving an older DODAGVersionNumber will ignore the
   DIO message assuming it to be from old DODAG version.  Thus a 6LBR
   node (and 6LR node in case of local DODAG) needs to maintain the
   DODAGVersionNumber in the persistent storage, so as to be available
   on reboot.  In case the 6LBR could not use the latest
   DODAGVersionNumber the implication are that it won't be able to
   recover/re-establish the routing table.

14.3.2.  DTSN field in DIO

   DTSN (Destination advertisement Trigger Sequence Number) is a DIO
   message field used as part of procedure to maintain Downward routes.
   A 6LBR/6LR node may increment a DTSN in case it requires the

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   downstream nodes to send DAO and thus update downward routes on the
   6LBR/6LR node.  In case of RPL NS-MOP, only the 6LBR maintains the
   downward routes and thus controls this field update.  In case of
   S-MOP, 6LRs additionally keep downward routes and thus control this
   field update.

   In S-MOP, when a 6LR node switches parent it may have to issue a DIO
   with incremented DTSN to trigger downstream child nodes to send DAO
   so that the downward routes are established in all parent/ancestor
   set.  Thus in S-MOP, the frequency of DTSN update might be relatively
   high (given the node density and hysteresis set by objective function
   to switch parent).

14.3.3.  PathSequence

   PathSequence is part of RPL Transit Option, and associated with RPL
   Target option.  A node whichs owns a target address can associate a
   PathSequence in the DAO message to denote freshness of the target
   information.  This is especially useful when a node uses multiple
   paths or multiple parents to advertise its reachability.

   Loss of PathSequence information maintained on the target node can
   result in routing adjacencies been lost on 6LRs/6LBR/6BBR.

14.4.  State variables update frequency

    |   State variable   |  Update frequency |   Impacts node type    |
    | DODAGVersionNumber |        Low        | 6LBR, 6LR(local DODAG) |
    |        DTSN        | High(SM),Low(NSM) |       6LBR, 6LR        |
    |    PathSequence    | High(SM),Low(NSM) |        6LR, 6LN        |

   Low=<5 per day, High=>5 per day; SM=Storing MOP, NSM=Non-Storing MOP

                       Table 2: RPL State variables

14.5.  Deliberations

   (1)  Is it possible that RPL removes the use of persistent storage
        for maintaining state information?

   (2)  In most cases, the node reboots will happen very rarely.  Thus
        doing a persistent storage book-keeping for handling node reboot
        might not make sense.  Is it possible to consider signaling
        (especially after the node reboots) so as to avoid maintaining

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        this persistent state?  Is it possible to use one-time on-reboot
        signalling to recover some state information?

   (3)  It is necessary that RPL avoids using persistent storage as far
        as possible.  Ideally, extensions to RPL should consider this as
        a design requirement especially for 6LR and 6LN nodes.  DTSN and
        PathSequence are the primary state variables which have major

14.6.  Implementation Notes

   An implementation should use a random DAOSequence number on reboot so
   as to avoid a risk of reusing the same DAOSequence on reboot.
   Regardless the sequence counter size of 8bits does not provide much
   gurantees towards choosing a good random number.  A parent node will
   not respond with a DAO-ACK in case it sees a DAO with the same
   previous DAOSequence.

   Write-Before-Use: The state information should be written to the
   flash before using it in the messaging.  If it is done the other way,
   then the chances are that the node power downs before writing to the
   persistent storage.

15.  Capabilities and its role in RPL

   RPL is a distributed protocol and it requires that the participating
   nodes agree on basic set of primitives to follow.  RPL currently
   handles this using MOP (Mode of Operation) bits in the DIO.  MOP bits
   inform the nodes the basic mode of operation a node MUST support to
   join the Instance as a 6LR.  The MOP is decided and advertised by the
   root of the RPL Instance.  A node not supporting the given MOP may
   still join the Instance as a leaf node or 6LN.

   RPL further uses DIO Configuration Option to advertise the
   configuration each node needs to use (for e.g., for trickle timer).

15.1.  Handshaking node capabilities

   Currently there exist no mechanism to handshake capabilities of the
   root or 6LRs or 6LNs.  If a feature is optional and is supported by
   6LRs/6LNs then currently there exists no mechanism to signal it.
   There are several RPL extension proposals which are possibly optional
   features.  Root needs to know if the 6LR/6LN supports these optional
   features to enable the extension in that path context.  Similarly
   6LRs and 6LNs need to know whether the root supports certain
   extensions that it can make use of.

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15.2.  How do Capabilities differ from MOP and Configuration Option?

   Unlike MOP and Configuration Option which are issued by the root of
   the Instance, Capabilities can be issued by any node.  A 6LN/6LR node
   can advertise its capabilities such that those can be seen by
   intermediate 6LRs and the root of the Instance.

15.3.  Deliberations

   (1)  Is it possible for leaf nodes to advertise their set of
        capabilities, which can be used by root and/or intermediate 6LRs
        to make run time decisions?

   (2)  How should these capabilities be carried?  Should it be carried
        in DAO/DIO/DAO-ACK?

   (3)  Should the definition of capabilities be same in both directions

16.  Backward Compatibility issues with RPL Options

   Most of the new work in ROLL requires addition of new control
   options.  Everytime a new control option is added, it is required
   that all the nodes upgrade to support this option.  In many cases,
   the new specification declares using a Flag day to switch to the new

   New control options may not require mandatory handling on every node
   but it requires at-least some processing.  For e.g., assume that a
   new control option is added to DIO message.  The option does not
   require any handling on the nodes not supporting it but it requires
   at-least for these nodes to forward this new control option
   downstream.  Currently the new control option may be stripped off.

   It should be possible for the unknown control options to be copied
   as-is to the downstream/upstream node(s).  The specification defining
   the new control option will decide whether a node should strip-off or
   copy the unknown control option.

17.  RPL under-specification

   (a)  PathSequence: Is it mandatory to use PathSequence in DAO Transit
        Information Option?  RPL mentions that a 6LR/6LBR hosting the
        routing entry on behalf of target node should refresh the
        lifetime on reception of a new Path Sequence.  But RPL does not
        necessarily mandate use of Path Sequence.  Most of the open
        source implementation [RIOT] [CONTIKI] currently do not issue
        Path Sequence in the DAO message.

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   (b)  Target Option aggregation in DAO: RPL allows multiple targets to
        be aggregated in a single DAO message and has introduced a
        notion of DelayDAO using which a 6LR node could delay its DAO to
        enable such aggregation.  But RPL does not have clear text on
        handling of aggregated DAOs and thus it hinders

   (c)  DTSN Update: RPL does not clearly define in which cases DTSN
        should be updated in case of storing mode of operation.  More
        details for this are presented in Section 3.

18.  Acknowledgements

   Many thanks to Pascal Thubert for hallway chats and for helping
   understand the existing design rationales.  Thanks to Michael
   Richardson for Unstrung RPL implementation rationale.  Thanks to ML
   discussions, in particular (

19.  IANA Considerations

   This memo includes no request to IANA.

20.  Security Considerations

   This is an information draft and does add any changes to the existing

21.  References

21.1.  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,

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, DOI 10.17487/RFC6206,
              March 2011, <>.

   [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,

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   [RFC6775]  Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
              Bormann, "Neighbor Discovery Optimization for IPv6 over
              Low-Power Wireless Personal Area Networks (6LoWPANs)",
              RFC 6775, DOI 10.17487/RFC6775, November 2012,

21.2.  Informative References

              Clausen, T., Verdiere, A. C. D., Yi, J., Herberg, U., and
              Y. Igarashi, "Observations on RPL: IPv6 Routing Protocol
              for Low power and Lossy Networks", draft-clausen-lln-rpl-
              experiences-11 (work in progress), March 2018.

              Baccelli, E. and C. E. Perkins, "Multi-hop Ad Hoc Wireless
              Communication", draft-ietf-intarea-adhoc-wireless-com-02
              (work in progress), July 2016.

              Anamalamudi, S., Zhang, M., Perkins, C. E., Anand, S., and
              B. Liu, "Supporting Asymmetric Links in Low Power
              Networks: AODV-RPL", draft-ietf-roll-aodv-rpl-10 (work in
              progress), April 2021.

              Perlman, R., "Fault-Tolerant Broadcast of Routing
              Information", North-Holland Computer Networks, Vol.7,
              December 1983.

Appendix A.  Additional Stuff

Authors' Addresses

   Rahul Arvind Jadhav (editor)
   Bangalore, Karnataka  560037


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   Rabi Narayan Sahoo
   Bangalore, Karnataka  560037


   Yuefeng Wu
   No.101, Software Avenue, Yuhuatai District,
   Nanjing, Jiangsu  210012

   Phone: +86-15251896569

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