Network Working Group                                         T. Clausen
Internet-Draft                                      A. Colin de Verdiere
Intended status: Informational                                     J. Yi
Expires: August 2, 2012                         LIX, Ecole Polytechnique
                                                              U. Herberg
                                         Fujitsu Laboratories of America
                                                        January 30, 2012

  Experiences with RPL: IPv6 Routing Protocol for Low power and Lossy


   With RPL - the "IPv6 Routing Protocol for Low-power Lossy Networks" -
   having been published as a Proposed Standard after a ~2-year
   development cycle, this document presents an evaluation of the
   resulting protocol of its applicability and of its limits.  The
   documents presents a selection of observations of the protocol
   characteristics, exposes experiences acquired when producing various
   prototype implementations of RPL, and presents results obtained from
   testing this protocol - by way of network simulations, in network
   testbeds and in deployments.  The document aims at providing a better
   understanding of possible weaknesses and limits of RPL, notably the
   possible directions that further protocol developments should
   explore, in order to address these.

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on August 2, 2012.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  RPL Overview . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  RPL Message Emission Timing - Trickle Timers . . . . . . .  6
   4.  Requirement Of DODAG Root  . . . . . . . . . . . . . . . . . .  6
     4.1.  Observations . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  RPL Data Traffic Flows . . . . . . . . . . . . . . . . . . . .  8
     5.1.  Observations . . . . . . . . . . . . . . . . . . . . . . .  9
   6.  Fragmentation Of RPL Control Messages And Data Packet  . . . .  9
     6.1.  Observations . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  The DAO Mechanism: Downward and Point-to-Point Routes  . . . . 10
     7.1.  Observations . . . . . . . . . . . . . . . . . . . . . . . 11
   8.  Address Aggregation and Summarization  . . . . . . . . . . . . 12
     8.1.  Observations . . . . . . . . . . . . . . . . . . . . . . . 13
   9.  Links Assumed Bi-Directional . . . . . . . . . . . . . . . . . 14
     9.1.  Observations . . . . . . . . . . . . . . . . . . . . . . . 14
   10. Neighbor Unreachability Detection For Unidirectional Links . . 14
     10.1. Observations . . . . . . . . . . . . . . . . . . . . . . . 15
   11. RPL Implementability and Complexity  . . . . . . . . . . . . . 16
     11.1. Observations . . . . . . . . . . . . . . . . . . . . . . . 16
   12. Underspecification . . . . . . . . . . . . . . . . . . . . . . 17
     12.1. Observations . . . . . . . . . . . . . . . . . . . . . . . 17
   13. Protocol Convergence . . . . . . . . . . . . . . . . . . . . . 18
     13.1. Observations . . . . . . . . . . . . . . . . . . . . . . . 19
   14. Loops  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     14.1. Observations . . . . . . . . . . . . . . . . . . . . . . . 19
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 20
   16. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 21
   18. Informative References . . . . . . . . . . . . . . . . . . . . 21
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23

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1.  Introduction

   RPL - the "Routing Protocol for Low Power and Lossy Networks"
   [I-D.ietf-roll-rpl] - is a proposal for an IPv6 routing protocol for
   Low-power Lossy Networks (LLNs), by the ROLL Working Group in the
   Internet Engineering Task Force (IETF).  This routing protocol is
   intended to be the IPv6 routing protocol for LLNs and sensor
   networks, applicable in all kinds of deployments and applications of

   The objective of RPL and ROLL is to target networks which "comprise
   up to thousands of nodes" [roll-charter], where the majority of the
   nodes have very constrained resources, where the network to a large
   degree is "managed" by a (single or few) central "supernodes"
   [I-D.ietf-roll-rpl-terminology], and where handling mobility is not
   an explicit design criteria ([RFC5867], [RFC5826], [RFC5673],
   [RFC5548]).  Supported traffic patterns include multipoint-to-point,
   point-to-multipoint and point-to-point traffic.  The emphasis among
   these traffic patterns is to optimize for multipoint-to-point
   traffic, to reasonably support point-to-multipoint traffic and to
   provide basic features for point-to-point traffic, in that order.

   As of early 2011, RPL has been approved by the IESG, for publication
   as a "Proposed Standard" RFC (Request for Comments).  The implication
   of a protocol being labeled "Proposed Standard" is that it is
   considered generally stable: well-understood and community reviewed,
   no known design issues pending, and with some community support

   "Proposed Standard" is, however, only the first step on the Standards
   Track, and it is thus opportune to consider the protocol in order to
   understand which aspects of it necessitate further investigations,
   and in order to identify possibly weak points which may restrict the
   deployment scope of the protocol.  This document has as objective to
   provide observations of RPL, in the spirit of better understanding
   its characteristics and limits.

2.  Terminology

   This document uses the terminology and notation defined in

   Additionally, it uses the following terminology:

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   RPL Router -  A device, running the RPL protocol, as specified by

3.  RPL Overview

   The basic construct in RPL is a "Destination Oriented Directed
   Acyclic Graph" (DODAG), depicted in Figure 1.  In a converged LLN,
   each RPL Router has identified a stable set of parents, each of which
   is a potential next-hop on a path towards the "root" of the DODAG, as
   well as a preferred parent.  Each RPL Router, which is part of a
   DODAG (i.e. has selected parents) will emit DODAG Information Object
   (DIO) messages, using link-local multicast, indicating its respective
   rank in the DODAG (i.e. distance to the DODAG Root according to some
   metric(s), in the simplest form hop-count).  Upon having received a
   (number of such) DIO messages, an RPL Router will calculate its own
   rank such that it is greater than the rank of each of its parents,
   select a preferred parent and then itself start emitting DIO

                                 ^ ^ ^
                                /  |  \
                              (a)  |   (b)
                              ^   (c)    ^
                             /     ^     (d)
                            (f)    |    ^  ^
                                  (e)--/    \

                            Figure 1: RPL DODAG

   The DODAG formation thus starts at the DODAG Root (initially, the
   only RPL Router which is part of a DODAG), and spreads gradually to
   cover the whole LLN as DIOs are received, parents and preferred
   parents are selected and further RPL Routers participate in the
   DODAG.  The DODAG Root also includes, in DIO messages, a DODAG
   Configuration Object, describing common configuration attributes for
   all RPL Routers in that network - including their mode of operation,
   timer characteristics etc.  RPL Routers in a DODAG include a verbatim
   copy of the last received DODAG Configuration Object in their DIO
   messages, permitting also such configuration parameters propagating
   through the network.

   As a Distance Vector protocol, RPL restricts the ability for an RPL
   Router to change rank.  An RPL Router can freely assume a smaller
   rank than previously advertised (i.e. logically move closer to the
   DODAG Root) if it discovers a parent advertising a lower rank, and

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   must then disregard all previous parents of ranks higher than the
   router's new rank.  The ability for an RPL Router to assume a greater
   rank (i.e. logically move farther from the DODAG Root) than
   previously advertised is restricted in order to avoid count-to-
   infinity problems.  The DODAG Root can trigger "global recalculation"
   of the DODAG by increasing a sequence number, DODAG version, in DIO

   The DODAG so constructed is used for installing routes: the
   "preferred parent" of an RPL Router can serve as a default route
   towards the DODAG Root, or the DODAG Root can embed in its DIO
   messages the destination prefixes, included by DIOs generated by RPL
   Routers through the LLN, to which connectivity is provided by the
   DODAG Root.  Thus, RPL by way of DIO generation provides "upward
   routes" or "multipoint-to-point routes" from the sensors inside the
   LLN and towards the DODAG Root.

   "Downward routes" are enabled by having sensors issue Destination
   Advertisement Object (DAO) messages, propagating as unicast via
   parents towards the DODAG Root.  These describe which prefixes belong
   to, and can be reached via, which RPL Router.  In a network, all RPL
   Routers must operate in either of storing-mode or non-storing-mode,
   specified by way of a "Mode of Operation" (MOP) flag in the DODAG
   Configuration Object from the DODAG Root.  Depending on the MOP, DAO
   messages are forwarded differently towards the DODAG Root:

   o  In "non-storing-mode", an RPL Router originates DAO messages,
      advertising one or more of its parents, and unicast it to the
      DODAG Root.  Once the DODAG Root has received DAOs from an RPL
      Router, and from all RPL Routers on the path between it and the
      DODAG Root, it can use source routing for reaching advertised
      destinations inside the LLN.

   o  In "storing-mode", each RPL Router on the path between the
      originator of a DAO and the DODAG Root records a route to the
      prefixes advertised in the DAO, as well as the next-hop towards
      these (the RPL Router, from which the DAO was received), then
      forwards the DAO to its preferred parent.

   "Point-to-point routes", for communication between devices inside the
   LLN and where neither of the communicating devices are the DODAG
   Root, are as default supported by having the source sensor transmit
   via its default route to the DODAG Root (i.e., using the upward
   routes) which will then, depending on the "Mode of Operation" for the
   DODAG, either add a source-route to the received data for reaching
   the destination sensor (downward routes in non-storing-mode) or
   simply use hop-by-hop routing (downward routes in storing-mode).  In
   the case of storing-mode, if the source and the destination for a

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   point-to-point communication share a common ancestor other than the
   DODAG Root, a downward route may be available in an RPL Router (and,
   thus, used) before the data packet reaching the DODAG Root.

3.1.  RPL Message Emission Timing - Trickle Timers

   RPL message generation is timer-based, with the DODAG Root being able
   to configure back-off of message emission intervals using Trickle
   [RFC6206].  Trickle, as used in RPL, stipulates that an RPL Router
   transmits a DIO "every so often" - except if receiving a number of
   DIOs from neighbor RPL routers, enabling the RPL Router to determine
   if its DIO transmission is redundant.

   When an RPL Router transmits a DIO, there are two possible outcomes:
   either every neighbor RPL Router that hears the message finds that
   the information contained is consistent with its own state (i.e., the
   received DODAG version number corresponds with that which the RPL
   Router has recorded and no better rank is advertised than that which
   is recorded in the parent set) - or, a recipient RPL Router detects
   that either the sender of the DIO or itself has out-of-date
   information.  If the sender has out-of-date information, then the
   recipient RPL Router schedules transmission of a DIO to update this
   information.  If the recipient RPL Router has out-of-date
   information, then it updates based on the information received in the

   With Trickle, an RPL Router will schedule emission of a DIO at some
   time, t, in the future.  When receiving a DIO containing information
   consistent with its own information, the RPL Router will record that
   "redundant information has been received" by incrementing a
   redundancy counter, c.  At the time t, if c is below some "redundancy
   threshold", then it transmits its DIO.  Otherwise, transmission of a
   DIO at this time is suppressed, c is reset and a new t is selected to
   twice as long time in the future - bounded by a pre-configured
   maximum value for t.  If, on the other hand, the RPL Router has
   received an out-of-date DIO from one of its neighbors, t is reset to
   a pre-configured minimum value and c is set to zero.  In both cases,
   at the expiration of t, the RPL Router will verify if c is below the
   "redundancy threshold" and if so transmit - otherwise, increase t and
   stay quiet.

4.  Requirement Of DODAG Root

   As indicated in Section 3, the DODAG Root has both a special
   responsibility and is subject to special requirements.  The DODAG
   Root is responsible for determining and maintaining the configuration
   parameters for the DODAG, and for initiating DIO emissions.

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   The DODAG Root is also responsible (in both storing and non-storing
   mode) for being able to, when downward routes are supported, maintain
   sufficient topological information to be able to construct routes to
   all destinations in the network.  This entails that the DODAG Root is
   required to have sufficient memory and sufficient computational
   resources to be able to record a network graph containing all paths
   from itself and to all destinations and calculate paths - this,
   regardless of if the network is operating in storing or non-storing

   The DODAG Root is also required to have sufficient energy available
   so as to be able to ensure the relay functions required (this,
   especially for non-storing mode, where all traffic transits through
   the DODAG Root).

4.1.  Observations

   In a given deployment, select RPL Routers can be provisioned with the
   required energy, memory and computational resources so as to serve as
   DODAG Roots, and be administratively configured as such - with the
   remainder of the RPL routers in the network being of typically lesser
   capacity. [rpl-eval-UCB] indicates that, in storing mode, a device
   with 10KB of memory scales up to about 30 RPL Routers - in a larger
   network (in storing or non-storing mode, both) the DODAG Root would
   require at least that much, most likely much more, memory.  In non-
   storing mode, only the DODAG Root will require that much, or likely
   much more, memory with the other RPL Routers being able to make do
   with considerably less.

   RPL Routers provisioned with resources to act as DODAG Roots, and
   administratively configured to act as such, represent a single point
   of failure in the network.  As the memory requirements for DODAG Root
   and other RPL Routers are substantially different, unless all RPL
   Routers are provisioned with resources (memory, energy, ...) to act
   as DODAG Roots, effectively if the designated DODAG Root fails, the
   network fails and RPL is unable to operate.  Even if electing another
   RPL Router as temporary DODAG root (e.g., for forming a "Floating"
   DODAG) for providing internal connectivity between RPL Routers, this
   router may not - likely, will not - have the necessary resources to
   satisfy the role as DODAG Root.

   Thus, although in principle RPL provides, by way of "Floating
   DODAGs", protocol mechanisms for establishing a DODAG for providing
   internal connectivity even in case of failure of the administratively
   provisioned DODAG Root - especially in non-storing mode - it is
   unlikely that any RPL Routers not explicitly provisioned as DODAG
   Roots will have sufficient resources to undertake this task.

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   Another possible LLN scenario is that only internal point-to-point
   connectivity is sought, and no RPL Router has a more "central" role
   than any other - a self-organizing LLN.  Requiring special
   provisioning of a specific "super-node" as DODAG Root is both
   unnecessary and undesirable.

5.  RPL Data Traffic Flows

   RPL makes a-priori assumptions of data traffic patterns: sensor-to-
   root data traffic (multipoint-to-point) is predominant, root-to-
   sensor data traffic (point-to-multipoint) is rare and sensor-to-
   sensor (point-to-point) data traffic is extremely rare.  While not
   specifically called out thus in [I-D.ietf-roll-rpl], the resulting
   protocol design reflects these assumptions in that the mechanism
   constructing multipoint-to-point paths is efficient in terms of
   control traffic generated and state required, point-to-multipoint
   path construction much less so - and point-to-point paths subject to
   potentially significant route stretch (routes going through the DODAG
   Root in non-storing mode) and over-the-wire overhead from using
   source routing (from the DODAG Root to the destination) (cf
   Section 7) - or, in case of storing mode, considerable memory
   requirements in all LLN routers inside the network (cf Section 7).

   An RPL Router selects from among its parents a "preferred parent", to
   serve as a default route towards the DODAG Root (and to prefixes
   advertised by the DODAG Root).  Thus, RPL provides "upward routes" or
   "multipoint-to-point routes" from the RPL Routers below the DODAG
   Root and towards the DODAG Root.

   An RPL Router which wishes to act as a destination for data traffic
   ("downward routes" or "point-to-multipoint") issues DAOs upwards in
   the DODAG towards the DODAG Root, describing which prefixes belong
   to, and can be reached via, that RPL Router.

   Point-to-Point routes between RPL Routers below the DODAG Root are
   supported by having the source RPL Router transmit, via its default
   route, data traffic towards the DODAG Root.  In non-storing mode, the
   data traffic will reach the DODAG Root, which will reflect the data
   traffic downward towards the destination RPL Router, adding a strict
   source routing header indicating the precise path for the data
   traffic to reach the intended destination RPL Router.  In storing
   mode, the source and the destination may possibly (although, may also
   not) have a common ancestor other than the DODAG Root, which may
   provide a downward route to the destination before data traffic
   reaching the DODAG Root.

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5.1.  Observations

   The data traffic characteristics assumed by RPL do not represent a
   universal distribution of traffic patterns in LLNs:

   o  There are scenarios where sensor-to-sensor traffic is a more
      common occurrence, documented e.g. in [RFC5867].

   o  There are scenarios, where all traffic is bi-directional, e.g. in
      case sensor devices in the LLN are, in majority, "actively read":
      a request is issued by the root to a specific sensor, and the
      sensor value is expected returned.

   For the former, all sensor-to-sensor paths include the root, possibly
   causing congestion on the communications medium near the root, and
   draining energy from the intermediate RPL Routers on an unnecessarily
   long path.  If sensor-to-sensor traffic is common, RPL Routers near
   the root will be particularly solicited as relays, especially in non-
   storing mode.  For the latter, all RPL Routers are required to
   generate DAOs, which generates a considerable control traffic
   overhead [bidir].

6.  Fragmentation Of RPL Control Messages And Data Packet

   Fragmentation of IP packets appears when the size of the IP datagram
   is larger than the Maximum Transmission Unit (MTU) supported by the
   link layer.  When an IP packet is fragmented, all fragments of that
   IP packet must be successfully received by a router, in order that
   the IP packet is successfully received - otherwise, the whole IP
   packet is lost.  Moreover, the additional link-layer frame overhead
   for each of the fragments increases the capacity required from the
   medium, and may consume more energy for transmitting a higher number
   of frames on the network interface.

   RPL is an IPv6 routing protocol, designed to operate on constrained
   link layers, such as IEEE 802.15.4 [ieee802154], with a maximum MTU
   of 127 bytes - a deviation from the otherwise specified minimum MTU
   of 1280 bytes for IPv6 [RFC2460].  Reducing the need of fragmentation
   of packets on such a link layer, compression adaptation layers exist
   [RFC4944], [RFC6282], reducing the overhead of the IPv6 header from
   at least 40 octets to a minimum of 2 octets.  With a physical layer
   packet size of 127 octets, a maximum frame overhead of 25 octets and
   21 octets for link layer security [RFC4944], 81 octets remain for L2
   payload.  Further subtracting 2 octets for the compressed IPv6 header
   leaves 79 octets for L3 data payload.

   The second L in LLN indicating Lossy [roll-charter], higher loss

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   rates than typically seen in IP networks are expected, rendering
   fragmentation important to avoid.

   DIO messages consist of a mandatory base object, facilitating DODAG
   formation, and additional options for e.g. autoconfiguration and
   network management.  The base object contains two unused octets,
   reserved for future use, resulting in two bytes of unnecessary zeros,
   sent with each DIO message.  The Prefix Information option, used for
   automatic configuration of address, carries even four unused octets
   in order to be compatible with IPv6 neighbor discovery.

6.1.  Observations

   While 79 octets may seem to be sufficient to carry RPL control
   messages, consider the following: RPL control messages are carried in
   ICMPv6, and the mandatory ICMPv6 header consumes 4 octets.  The DIO
   base another 24 octets.  If link metrics are used, that consumes at
   least another 8 octets - and this is using a hop count metric; other
   metrics may require more.  The DODAG Configuration Object consumes up
   to a further 16 octets, for a total of 52 octets.  Adding a Prefix
   Information Object for address configuration consumes another 32
   octets, for a total of 84 octets - thus exceeding the 79 octets
   available for L3 data payload and causing fragmentation of such a
   DIO.  As a point of reference, the ContikiRPL [rpl-contiki]
   implementation includes both the DODAG Configuration option and the
   Prefix Information option in all DIO messages.  Any other options,
   e.g.  Route Information options indicating prefixes reachable through
   the DODAG Root, increase the overhead and thus the probability of

   RPL may further increase the probability of fragmentation of also
   data traffic: for non-storing-mode, RPL employs source-routing for
   all downward traffic.  [I-D.ietf-6man-header] specifies the RPL
   Source Routing header, which imposes a fixed overhead of 8 octets per
   IP packet leaving 71 octets remaining from the MTU - from which must
   be deducted a variable number of octets, depending on the length of
   the route.  With fewer octets available for data payload, RPL thus
   increases the probability for fragmentation of also data packets.
   This, in particular, for longer paths, e.g. in point-to-point data
   traffic between sensors inside the LLN, where data traffic transit
   through the DODAG Root and are then source-routed to the destination.

7.  The DAO Mechanism: Downward and Point-to-Point Routes

   RPL specifies two distinct and incompatible "modes of operation" for
   downward traffic: storing mode, where each RPL Router is assumed to
   maintain routes to all destinations in its sub-DODAG, i.e.  RPL

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   Routers that are "deeper down" in the DODAG, and non-storing mode,
   where only the DODAG Root stores routes to destinations inside the
   LLN, and where the DODAG Root employs strict source routing in order
   to route data traffic to the destination RPL Router.

7.1.  Observations

   In addition to possible fragmentation, as occurs when using
   potentially long source routing headers over a medium with a small
   MTU - similar to what is discussed in Section 6 - the maximum length
   of the source routing header [I-D.ietf-6man-header] is limited to 136
   octets, including an 8 octet long header.  As each IPv6 address has a
   length of 16 octets, not more than 8 hops from the source to the
   destination are possible for "raw IPv6".  Using address compression
   (e.g. as specified in [RFC4944]), the maximum path length may not
   exceed 64 hops.  This excludes deployment of RPL for scenarios with
   long "chain-like" topologies, such as traffic lights along a street.

   In storing mode, each RPL Router has to store routes for destinations
   in its sub-DODAG.  This implies that, for RPL Routers near the DODAG
   Root, the required storage is only bounded by the number of paths to
   all other destinations in the network.  As RPL targets constrained
   devices with little memory, but also has as ambition to be operating
   networks consisting of thousands of routers [roll-charter], the
   storing capacity on these RPL Routers may not be sufficient - or, at
   least, the storage requirements in RPL Routers "near the DODAG Root"
   and "far from the DODAG Root" is not homogenous, thus some sort of
   administrative deployment, and continued administrative maintenance
   as the network evolves, of devices is needed.  Indeed, [rpl-eval-UCB]
   argues that practical experiences suggest that RPL in storing mode,
   with RPL routers having 10kB of RAM, should be limited to networks of
   less than ~30 RPL Routers.  Aggregation / summarization of addresses
   may be advanced as a possible argument that this issue is of little
   significance - Section 8 discusses why such an argument does not

   In short, the mechanisms in RPL, when using storing mode, force the
   choice between requiring all RPL Routers to have sufficient memory to
   store route entries for all destinations (storing-mode) - or, suffer
   increased risk of fragmentation, and thus loss of data packets, while
   consuming network capacity by way of source routing through the DODAG

   In RPL, the "mode of operation" stipulates that either downward
   routes are not supported (MOP=0), or that they are supported by way
   of either storing or non-storing mode.  In case downward routes are
   supported, RPL does not provide any mechanism for discriminating
   between which routes should or should not be maintained.  In

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   particular, in order to calculate paths to a given destination, all
   intermediaries between the DODAG Root and that destination must
   themselves be reachable - effectively rendering downward routes in
   RPL an "all-or-none" situation.  In case a destination is
   unreachable, all the DODAG Root may do is require all destinations to
   re-issue their DAOs, by way of issuing a DIO with an increased DODAG
   version number, possibly provoking a broadcast-storm-like situation.
   This, in particular, as [I-D.ietf-roll-rpl] does not specify DAO
   message transmission constraints, nor any mechanism for adapting DAO
   emission to the network capacity.

   A final point on the DAO mechanism: RPL supports point-to-point
   traffic only by way of relaying through the DODAG.  In networks where
   point-to-point traffic is no rare occasion, this causes unduly long
   paths (with possibly increased energy consumption, increased
   probability of packet losses) as well as possibly congestion around
   the DODAG Root.

8.  Address Aggregation and Summarization

   As indicated in Section 7, in storing mode, an RPL Router is expected
   to be able to store routing entries for all destinations in its "sub-
   DODAG", i.e., routing entries for all destinations in the network
   where the path to the DODAG Root includes that RPL Router.

   In the Internet, no single router stores explicit routing entries for
   all destinations.  Rather, IP addresses are assigned hierarchically,
   such that an IP address does not only uniquely identify a network
   interface, but also its topological location in the network, as
   illustrated in Figure 2.  All addresses with the same prefix are
   reachable by way of the same router - which can, therefore, advertise
   only that prefix.  Other routers need only record a single routing
   entry for that prefix, knowing that as the IP packet reaches the
   router advertising that prefix, more precise routing information is

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                                   |   |
                                    / \
                         1.1.x.x/16/   \ 1.2.x.x/16
                                  /     \
                              .---.     .---.
                              | c |     | d |
                              '---'     '---'

                       Figure 2: Address Hierarchies

8.1.  Observations

   In RPL, each RPL Router acquires a number of parents, as described in
   Section 3, from among which it selects one as its preferred parent
   and, thus, next-hop on the path to the DODAG Root.  RPL Routers
   maintain a parent set containing possibly more than a single parent
   so as to be able to rapidly select an alternative preferred parent,
   should the previously selected such become unavailable.  Thus
   expected behavior is for an RPL Router to be able to change its point
   of attachment towards the DODAG Root.  If IP addresses are assigned
   in a strictly hierarchical fashion, and if scalability of the routing
   state maintained in storing mode is based on this hierarchy, then
   this entails that each time an RPL Router changes its preferred
   parent, it must also change its own IP address - as well as cause RPL
   routers in its "sub-DODAG" to do the same.  RPL does not specify
   signaling for reconfiguring addresses in a sub-DODAG.

   A slightly less strict hierarchy can be envisioned, where an RPL
   Router can change its preferred parent without necessarily changing
   addresses of itself and of its sub-DODAG, provided that its former
   and new preferred parents both have the same preferred parent, and
   have addresses hierarchically assigned from that - from the
   "preferred grandparent".  With reference to Figure 1, this could be e
   changing its preferred parent from d to c, provided that both d and c
   have b as preferred parent.  Doing so would impose a restriction on
   the parent-set selection, admitting only parents which have
   themselves the same parent - thus, no longer having a DODAG but a
   simple tree, losing redundancy in the network connectivity.  RPL does

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   not specify rules for admitting only parents with identical grand-
   parents into the parent set - although such is not prohibited either,
   if the loss of redundancy from constructing a tree is acceptable.

   The DODAG Root incrementing the DODAG version number is the mechanism
   by which RPL enables global reconfiguration of the network,
   reconstructing the DODAG with (intended) more optimal paths.  In case
   of addressing hierarchies being enforced, so as to enable
   aggregation, this will either restrict the ability for an optimal
   DODAG construction, or will trigger global address autoconfiguration
   so as to ensure addressing hierarchies.

   Finally, with IP addresses serving a dual role of an identifier of
   both an end-point for communication and a topological location in the
   network, changing the IP address of a device, so as to reflect a
   change in network topology, also entails interrupting ongoing
   communication to or through that device.  Additional mechanisms (e.g.
   a DNS-like system) mapping "communications identifiers" and "IP
   addresses" are required.

9.  Links Assumed Bi-Directional

   Parents (and the preferred parent) are selected based on receipt of
   DIOs, without verification of the ability for an RPL Router to
   successfully communicate with the parent - i.e. without any
   bidirectionality check of links.  However, the basic use of links is
   for "upward" routes, i.e. for the RPL Router to use a parent (the
   preferred parent) as relay towards the DODAG Root - in the opposite
   direction of the one in which the DIO was received.

9.1.  Observations

   Unidirectional links are no rare occurrence, such as is known from
   wireless multi-hop networks.  If an RPL Router receives a DIO on such
   a unidirectional link, and selects the originator of the DIO as
   parent, that would be a bad choice: unicast traffic in the upward
   direction would be lost.  If the RPL Router had verified the
   bidirectionality of links, it might have selected a better parent, to
   which it has a bidirectional link.

10.  Neighbor Unreachability Detection For Unidirectional Links

   [I-D.ietf-roll-rpl] suggests using Neighbor Unreachability Detection
   (NUD) [RFC4861] to detect and recover from the situation of
   unidirectional links between an RPL Router and its (preferred)
   parent(s).  When, e.g., an RPL Router a tries (and fails) to actually

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   use RPL Router b for forwarding traffic, NUD is supposed engaged to
   detect and prompt corrective action, e.g. by way of selecting an
   alternative preferred parent.

   NUD is based upon observing if a data packet is making forward
   progress towards the destination, either by way of indicators from
   upper-layer protocols (such as TCP and, though not called out in
   [RFC4861], also from lower-layer protocols such as Link Layer ACKs )
   or - failing that - by unicast probing by way of transmitting a
   unicast Neighbor Solicitation message and expecting that a solicited
   Neighbor Advertisement message be returned.

10.1.  Observations

   An RPL Router may receive, transiently, a DIO from an RPL Router,
   closer (in terms of rank) to the DODAG Root than any other RPL Router
   from which a DIO has been received.  Some, especially wireless, link
   layers may exhibit different transmission characteristics between
   multicast and unicast transmissions (such is the case for some
   implementations of IEEE 802.11b, where multicast/broadcast
   transmissions are sent at much lower bit-rates than are unicast.
   IEEE 802.11b is, of course, not suggested as a viable interface for
   LLNs, but serves to illustrate that such asymmetric designs exist),
   leading to a (multicast) DIO being received from farther away than a
   unicast transmission can reach.  DIOs are sent (downward) using link-
   local multicast, whereas the traffic flowing in the opposite
   direction (upward) is unicast.  Thus, a received (multicast) DIO may
   not be indicative of useful unicast connectivity - yet, RPL might
   cause this RPL Router to select this attractive RPL Router as its
   preferred parent.  This may happen both at initialization or at any
   time during the LLN lifetime, as RPL allows attachment to a "better
   parent" at any time.

   A DODAG so constructed may appear stable and converged until such
   time that unicast traffic is to be sent and, thus, NUD invoked.
   Detecting only at that point that unicast connectivity is not
   maintained, and causing local (and possibly global) repairs exactly
   at that time, may lead to traffic not being deliverable.  As
   indicated in Section 8, if scalability is dependent on addresses
   being assigned hierarchically, changing point-of-attachment may
   entail more than switching preferred parent.

   Also, absent all RPL Routers consistently advertising their
   reachability through DAO messages, a protocol requiring bidirectional
   flows between the communicating devices, such as TCP, will be unable
   to operate.

   Finally, upon having been notified by NUD that the "next hop" is

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   unreachable, an RPL Router must discard the preferred parent and
   select another - hoping that this time, the preferred parent is
   actually reachable.  Also, if NUD indicates "no forward progress"
   based on an upper-layer protocol, there is no guarantee that the
   problem stems exclusively from the preferred parent being
   unreachable.  Indeed, it may be a problem farther ahead, possibly
   outside the LLN, thus changing preferred parent will not alleviate
   the situation.

   Incidentally, this stems from a fundamental difference between "fixed
   links in the Internet" and "wireless links": whereas the former, as a
   rule, are reliable, predictable and with losses being rare
   exceptions, the latter are characterized by frequent losses and
   general unpredictability.

11.  RPL Implementability and Complexity

   RPL is designed to operate on "RPL Routers [...] with constraints on
   processing power, memory, and energy (battery power)"
   [I-D.ietf-roll-rpl].  However, the 163 pages long specification of
   RPL, plus additional specifications for routing headers
   [I-D.ietf-6man-header], Trickle timer [RFC6206], routing metrics
   [I-D.ietf-roll-routing-metrics] and objective function
   [I-D.ietf-roll-rpl-of0], describes complex mechanisms (e.g. the
   upwards and downward data traffic, a security solution, manageability
   of RPL Routers, auxiliary functions for autoconfiguration of RPL
   Routers, etc.), and provides no less than 9 message types, and 10
   different message options.

   To give one example, the ContikiRPL implementation
   (, which provides only storing-mode and no
   security features, consumes about 50 KByte of memory.  Sensor
   hardware, such as MSP430 sensor platforms, does not contain much more
   memory than that, i.e. there may not be much space left to deploy any
   application on the RPL Router.

11.1.  Observations

   Since RPL is intended as the routing protocol for LLNs, which covers
   all the diverse applications requirements listed in [RFC5867],
   [RFC5673], [RFC5826], [RFC5548], it is possible that (i) due to
   limited memory capacity of the RPL Routers, and (ii) due to expensive
   development cost of the routing protocol implementation, many RPL
   implementations will only support a partial set of features from the
   specification, leading to non-interoperable implementations.

   In order to accommodate for the verbose exchange format, route

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   stretching and source routing for point-to-point traffic, several
   additional Internet-Drafts are being discussed for adoption in the
   ROLL Working Group - adding complexity to an already complex
   specification which, it is worth recalling, was intended to be of a
   protocol for low-capacity devices.

12.  Underspecification

   While [I-D.ietf-roll-rpl] is verbose in many parts, as described in
   Section 11, some mechanisms are underspecified.

   While for DIOs, the Trickle timer specifies a relatively efficient
   and easy-to-understand timing for message transmission, the timing of
   DAO transmission is not explicit.  As each DAO may have a limited
   lifetime, one "best guess" for implementers would be to send DAO
   periodically, just before the life-time of the previous DAO expires.
   Since DAOs may be lost, another "best guess" would be to send several
   DAOs shortly one after the other in order to increase probability
   that at least one DAO is successfully received.

   The same underspecification applies for DAO-ACK messages: optionally,
   on reception of a DAO, an RPL Router may acknowledge successful
   reception by returning a DAO-ACK.  Timing of DAO-ACK messages is
   unspecified by RPL.

12.1.  Observations

   By not specifying details about message transmission intervals and
   required actions when receiving DAO and DAO-ACKs, implementations may
   exhibit a bad performance if not carefully implemented.  Some
   examples are:

   1.  If DAO messages are not sent in due time before the previous DAO
       expires (or if the DAO is lost during transmission), the routing
       entry will expire before it is renewed, leading to a possible
       data traffic loss.

   2.  RPL does not specify to use jitter [RFC5148] (i.e. small random
       delay for message transmissions).  If DAOs are sent periodically,
       adjacent RPL Routers may transmit DAO messages at the same time,
       leading to link layer collisions.

   3.  In non-storing mode, the "piece-wise calculation" of routes to a
       destination from which a DAO has been received, relies on
       previous reception of DAOs from intermediate RPL Routers along
       the path.  If not all of these DAOs from intermediate RPL Routers
       have been received, route calculation is not possible, and DAO-

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       ACKs or data traffic cannot be sent to that destination.

   Other examples of underspecification include the local repair
   mechanism, which may lead to loops and thus data traffic loss, if not
   carefully implemented: an RPL Router discovering that all its parents
   are unreachable, may - according to the RPL specification - "detach"
   from the DODAG, i.e. increase its own rank to infinity.  It may then
   "poison" its sub-DODAG by advertising its infinite rank in its DIOs.
   If, however, the RPL Router receives a DIO before it transmits the
   "poisoned" DIO, it may attach to its own sub-DODAG, creating a loop.
   If, instead, it had waited some time before processing DIOs again,
   chances are it would have succeeded in poisoning its sub-DODAG and
   thus avoided the loop.

13.  Protocol Convergence

   Trickle [RFC6206] is used by RPL to schedule transmission of DIO
   messages, with the objective to minimize the amount of transmitted
   DIOs while ensuring a low convergence time of the network.  The
   theoretical behavior of Trickle is well understood, and the
   convergence properties are well studied.  Simulations of the
   mechanism, such as documented [trickle-multicast], confirm these
   theoretical studies.

   In real-world environments, however, varying link qualities may cause
   the algorithm to converge less well: frequent message losses entail
   resets of the Trickle timer and more frequent and unpredicted message

   This has been observed, e.g., in an experimental testbed: 69 RPL
   Routers (MSP430-based wireless sensor routers with IEEE 802.15.4,
   using [rpl-contiki] IPv6 stack and RPL without downward routes; the
   parameters of the Trickle timer were set to the implementation
   defaults (minimum DIO interval: 4 s, DIO interval doublings: 8,
   redundancy constant: 10)) were positioned in a fixed grid topology.
   This resulted in DODAGs being constructed with an average of 2.45
   children per RPL Router and an average rank of 3.58.

   In this small test network, the number of DIO messages emitted -
   expectedly spiked wihin the first ~10 seconds.  Alas, rather than
   taper off to become zero (as the simulation studies would suggest),
   the DIO emission rate remained constant at about 70 DIOs per second.
   Details on this experiment can be found in [rpl-eval].

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13.1.  Observations

   The varying link quality in real-world environments results in
   frequent changes of the best parent, which triggers a reset of the
   Trickle timer and thus the emission of DIOs.  Therefore Trickle does
   not converge as well for links that are fluctuating in quality as in

   The resulting higher control overhead due to frequent DIO emission,
   leads to higher bandwidth and energy consumption as well as possibly
   to an increased number of collisions of frames, as observed in

14.  Loops

   [I-D.ietf-roll-rpl] states that it "guarantees neither loop free path
   selection nor tight delay convergence times, but can detect and
   repair a loop as soon as it is used.  RPL uses this loop detection to
   ensure that packets make forward progress [...] and trigger repairs
   when necessary".  This implies that a loop may only then be detected
   and fixed when data traffic is sent through the network.

   In order to trigger a local repair, RPL relies on the "direction"
   information (with values "up" or "down"), contained in an IPv6 hop-
   by-hop option header from received a data packet.  If an "upward"
   data packet is received by an RPL Router, but the previous hop of the
   packet is listed with a lower rank in the neighbor set, the RPL
   Router concludes that there must be a routing loop and it may
   therefore trigger a local repair.  For downward traffic in non-
   storing mode, the DODAG Root can detect loops if the same router
   identifier (i.e.  IP address) appears at least twice in the path
   towards a destination.

14.1.  Observations

   The reason for RPL to repair loops only when detected by a data
   traffic transmission is to reduce control traffic overhead.  However,
   there are two problems in repairing loops only when so triggered: (i)
   the triggered local repair mechanism delays forward progress of data
   packets, increasing end-to-end delays, and (ii) the data packet has
   to be buffered during repair.

   (i) may seem as the lesser of the two problems, since in a number of
   applications, such as data acquisition in smart metering
   applications, an increased delay may be acceptable.  However, for
   applications such as alarm signals or in home automation (e.g. a
   light switch), increased delay may be undesirable.

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   As for (ii), RPL is supposed to run on LLN routers with "constraints
   on [...] memory" [I-D.ietf-roll-rpl]; buffering incoming packets
   during the route repair may not be possible for all incoming data
   packets, leading to dropped packets.  Depending on the transport
   protocol, these data packets must be retransmitted by the source or
   are definitely lost.

   If carefully implemented with respect to avoiding loops before they
   occur, the impact of the loop detection in RPL may be minimized.
   However, it can be observed that with current implementations of RPL,
   such as the ContikiRPL implementation, loops do occur - and that
   frequently.  During the same experiments described in Section 13, a
   snapshot of the DODAG was made every ten seconds.  In 74.14% of the
   4114 snapshots, at least one loop was observed.  Further
   investigation revealed that in all these cases the DODAG was
   partitioned, and the loop occurred in the sub-DODAG that no longer
   had a connection to the DODAG Root.  When the link to the only parent
   of an RPL Router breaks, the RPL Router may increase its rank and -
   when receiving a DIO from an RPL Router in its sub-DODAG - attach
   itself to its own sub-DODAG, thereby creating a loop - as detailed in
   Section 12.1.

   While it can be argued that the observed loops are harmless since
   they occur in a DODAG partition that has no connection to the DODAG
   Root, they show that the state of the network is inconsistent.  Even
   worse, when the broken link re-appears, it is possible that in
   certain situations, the loop is only repaired when data traffic is
   sent, possibly leading to data loss (as described above).  This can
   occur if the link to the previous parent is reestablished, but the
   rank of that previous parent has increased in the meantime.

   Another problem with the loop repair mechanism arises in non-storing
   mode when using only downward traffic: while the DODAG Root can
   easily detect loops (as described above), it has no direct means to
   trigger a local repair where the loop occurs.  Indeed, it can only
   trigger a global repair by increasing the DODAG version number,
   leading to a Trickle timer reset and increased control traffic
   overhead in the network caused by DIO messages, and therefore a
   possible energy drain of the RPL Routers and congestion of the

15.  Security Considerations

   This document does currently not specify any security considerations.
   This document also does not provide any evaluation of the security
   mechanisms of RPL.

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16.  IANA Considerations

   This document has no actions for IANA.

17.  Acknowledgements

   The authors would like to thank Matthias Philipp (INRIA) for his
   contributions to conducting many of the experiments, revealing or
   confirming the issues described in this document.

18.  Informative References

              Hui, J., Vasseur, J., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with RPL", work in
              progress draft-ietf-6man-rpl-routing-header-07.txt,
              December 2011.

              Vasseur, J., Pister, K., Dejan, N., and D. Barthel,
              "Routing Metrics used for Path Calculation in Low Power
              and Lossy Networks", work in
              progress draft-ietf-roll-routing-metrics-19, March 2011.

              Winther, T., Thubert, P., Hui, J., Vasseur, J., Brandt,
              A., Kelsey, R., Levis, P., Piester, K., and R. Struik,
              "RPL: IPv6 Routing Protocol for Low power and Lossy
              Networks", work in progress draft-ietf-roll-rpl-19.txt,
              March 2011.

              Thubert, P., "RPL Objective Function Zero", work in
              progress draft-ietf-roll-of0-20, September 2011.

              Vasseur, JP., "Terminology in Low power And Lossy
              Networks", work in
              progress draft-ietf-roll-terminology-06, September 2011.

   [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision
              3", BCP 9, RFC 2026, October 1996.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, Decemer 1998.

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

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, September 2007.

   [RFC5148]  Clausen, T., Dearlove, C., and B. Adamson, "Jitter
              Considerations in Mobile Ad Hoc Networks (MANETs)",
              RFC 5148, February 2008.

   [RFC5548]  Dohler, M., Watteyne, T., Winter, T., and D. Barthel,
              "Routing Requirements for Urban Low-Power and Lossy
              Networks", RFC 5548, May 2009.

   [RFC5673]  Pister, K., Thubert, P., Dwars, S., and T. Phinney,
              "Industrial Routing Requirements in Low-Power and Lossy
              Networks", RFC 5673, October 2009.

   [RFC5826]  Brandt, A., Buron, J., and G. Porcu, "Home Automation
              Routing Requirements in Low-Power and Lossy Networks",
              RFC 5826, April 2010.

   [RFC5867]  Martocci, J., Mi, P., Riou, N., and W. Vermeylen,
              "Building Automation Routing Requirements in Low Power and
              Lossy Networks", RFC 5867, June 2010.

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

   [RFC6282]  Hui, J. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              September 2011.

   [bidir]    Clausen, T. and U. Herberg, "A Comparative Performance
              Study of the Routing Protocols LOAD and RPL with Bi-
              Directional Traffic in Low-power and Lossy Networks
              (LLN)", Proceedings of the Eighth ACM International
              Symposium on Performance Evaluation of Wireless Ad Hoc,
              Sensor, and Ubiquitous Networks (PE-WASUN), 2011.

              Computer Society, IEEE., "IEEE Std. 802.15.4-2003",
              October 2003.

              "ROLL Charter",

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              Tsiftes, N., Eriksson, J., and A. Dunkels, "Low-Power
              Wireless IPv6 Routing with ContikiRPL",
              Proceedings Proceedings of the 9th ACM/IEEE International
              Conference on Information Processing in Sensor Networks
              (ISPN), 2011.

              Clausen, T., Herberg, U., and M. Philipp, "A Critical
              Evaluation of the IPv6 Routing Protocol for Low Power and
              Lossy Networks (RPL)", Proceedings of the 5th IEEE
              International Conference on Wireless & Mobile Computing,
              Networking & Communication (WiMob), 2011.

              Ko, J., Dawson-Haggerty, S., Culler, D., and A. Terzis,
              "Evaluating the Performance of RPL and 6LoWPAN in TinyOS",
              Proceedings of the Workshop on Extending the Internet to
              Low power and Lossy Networks (IP+SN), 2011.

              Clausen, T. and U. Herberg, "Study of Multipoint-to-Point
              and Broadcast Traffic Performance in the 'IPv6 Routing
              Protocol for Low Power and Lossy Networks' (RPL)",
              Journal of Ambient Intelligence and Humanized Computing,

Authors' Addresses

   Thomas Clausen
   LIX, Ecole Polytechnique
   91128 Palaiseau Cedex,

   Phone: +33 6 6058 9349

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   Axel Colin de Verdiere
   LIX, Ecole Polytechnique
   91128 Palaiseau Cedex,

   Phone: +33 6 1264 7119

   Jiazi Yi
   LIX, Ecole Polytechnique
   91128 Palaiseau Cedex,

   Phone: +33 1 6933 4031

   Ulrich Herberg
   Fujitsu Laboratories of America
   1240 E Arques Ave
   Sunnyvale, CA 94085


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