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Network Working Group                                         T. Clausen
Internet-Draft                                      A. Colin de Verdiere
Intended status: Informational                                     J. Yi
Expires: August 2, 2014                         LIX, Ecole Polytechnique
                                                              U. Herberg
                                         Fujitsu Laboratories of America
                                                             Y. Igarashi
                                        Hitachi, Ltd., Yokohama Research
                                                              Laboratory
                                                        January 29, 2014


   Observations of RPL: IPv6 Routing Protocol for Low power and Lossy
                                Networks
                  draft-clausen-lln-rpl-experiences-07

Abstract

   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 limits of RPL, notably the possible
   directions that further protocol developments should explore, in
   order to address these.

Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 2, 2014.




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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
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   described in the Simplified BSD License.





































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  RPL Overview . . . . . . . . . . . . . . . . . . . . . . . . .  5
     3.1.  RPL Message Emission Timing - Trickle Timers . . . . . . .  7
   4.  Requirement Of DODAG Root  . . . . . . . . . . . . . . . . . .  8
     4.1.  Observations . . . . . . . . . . . . . . . . . . . . . . .  8
   5.  RPL Data Traffic Flows . . . . . . . . . . . . . . . . . . . .  9
     5.1.  Observations . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Fragmentation Of RPL Control Messages And Data Packet  . . . . 12
     6.1.  Observations . . . . . . . . . . . . . . . . . . . . . . . 12
   7.  The DAO Mechanism: Downward and Point-to-Point Routes  . . . . 14
     7.1.  Observations . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  Address Aggregation and Summarization  . . . . . . . . . . . . 16
     8.1.  Observations . . . . . . . . . . . . . . . . . . . . . . . 17
   9.  Link Bidirectionality Verification . . . . . . . . . . . . . . 18
     9.1.  Observations . . . . . . . . . . . . . . . . . . . . . . . 18
   10. Neighbor Unreachability Detection For Unidirectional Links . . 19
     10.1. Observations . . . . . . . . . . . . . . . . . . . . . . . 20
   11. RPL Implementability and Complexity  . . . . . . . . . . . . . 21
     11.1. Observations . . . . . . . . . . . . . . . . . . . . . . . 22
   12. Underspecification . . . . . . . . . . . . . . . . . . . . . . 22
     12.1. Observations . . . . . . . . . . . . . . . . . . . . . . . 22
   13. Protocol Convergence . . . . . . . . . . . . . . . . . . . . . 23
     13.1. Observations . . . . . . . . . . . . . . . . . . . . . . . 24
     13.2. Caveat . . . . . . . . . . . . . . . . . . . . . . . . . . 24
   14. Loops  . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     14.1. Observations . . . . . . . . . . . . . . . . . . . . . . . 25
   15. Security Considerations  . . . . . . . . . . . . . . . . . . . 26
   16. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 27
   17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
   18. Informative References . . . . . . . . . . . . . . . . . . . . 27
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 30

















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1.  Introduction

   RPL - the "Routing Protocol for Low Power and Lossy Networks"
   [RFC6550] - 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 LLNs.

   The objective of RPL and ROLL is to provide routing in networks which
   "comprise up to thousands of nodes" [roll-charter], where the
   majority of the nodes have very constrained resources
   [I-D.ietf-roll-terminology], and where handling mobility is not an
   explicit design criteria [RFC5867], [RFC5826], [RFC5673], [RFC5548].

   [roll-charter] states that "Typical traffic patterns are not simply
   unicast flows (e.g. in some cases most if not all traffic can be
   point to multipoint)", and [I-D.ietf-roll-terminology] further
   categorizes the supported traffic types into "upward" traffic from
   sensors to a collection sink or LBR (LLN Border Router) (denoted
   multipoint-to-point), "downward" traffic from the collection sink or
   LBR to the sensors (denoted point-to-multipoint) and traffic from
   "sensor to sensor" (denoted point-to-point traffic), and establishes
   this terminology for these traffic types.  Thus, while the target for
   RPL and ROLL is to support all of these traffic types, the emphasis
   among these, according to [roll-charter], appears to be to optimize
   for multipoint-to-point traffic, while also supporting point-to-
   multipoint and point-to-point traffic.

   With experiences obtained since the publication of RPL as [RFC6550],
   it is opportune to document observations of the protocol, in order to
   understand which aspects of it work well and which necessitate
   further investigations.  Understanding possible limitations is
   important to identify issues which may restrict the deployment scope
   of the protocol and which may need further protocol work or
   enhancements.

   The observations made in this document, except for when explicitly
   noted otherwise, do not depend on any specific implementation or
   deployment, but can be understood from simply analyzing the protocol
   specification [RFC6550].  That said, all observations made have been
   confirmed to also be present in, at least, some deployments or test
   platforms with RPL, i.e., have been experimentally confirmed.

   This document is explicitly not an implementation guidebook for RPL.
   It has as objective to document observations of behaviors of
   [RFC6550], in the spirit of better understanding the characteristics
   and limits of the protocol.



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2.  Terminology

   This document uses the terminology and notation defined in [RFC6550].

   Additionally, this document uses terminology from
   [I-D.ietf-roll-terminology], specifically the terms defined for the
   traffic types "MP2P" (Multipoint-to-Point), "P2P" (Point To Point)
   and "P2MP" (Point-to-Multipoint).

   Finally, this document introduces the following terminology:

   RPL Router -  A device, running the RPL protocol, as specified by
      [RFC6550].


3.  RPL Overview

   The basic construct in RPL is a "Destination Oriented Directed
   Acyclic Graph" (DODAG), depicted in Figure 1, with a single RPL
   Router acting as DODAG Root.  The DODAG Root has responsabilities in
   addition to those of other RPL Routers, including for initiating,
   configuring, and managing the DODAG, and (in some cases) acting as a
   central relay for traffic through and between RPL Routers in the LLN.

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

                            Figure 1: RPL DODAG

   In an LLN, in which RPL has converged to a stable state, each RPL
   Router has identified a stable set of parents, each of which is a
   potential next-hop on a route towards the DODAG Root.  One of the
   parents is selected as preferred parent.  Each RPL Router, which is
   part of a DODAG (i.e., which has selected parents and a preferred
   parent) 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 messages.



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   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
   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
   messages.

   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, and 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 (and, possibly, to destinations
   reachable through the DODAG Root).

   "Downward routes" are enabled by having sensors issue Destination
   Advertisement Object (DAO) messages, propagating as unicast via
   preferred 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.  Those
   two modes are non-interoperable, i.e., a mixture of RPL Routers
   running in different modes is impossible in the same routing domain.
   Depending on the MOP, DAO messages are forwarded differently towards
   the DODAG Root:

   o  In "non-storing mode", an RPL Router originates a DAO messages,
      advertising one or more of its parents, and unicasts these to the



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      DODAG Root.  Once the DODAG Root has received DAOs from an RPL
      Router, and from all RPL Routers on the route 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 route 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 a
   data packet, 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 packet
   for reaching the destination sensor (downward routes in non-storing
   mode), or simply use hop-by-hop routing (downward routes in storing
   mode) for forwarding the data packet.  In the case of storing mode,
   if the source and the destination for a point-to-point data packet
   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 reaches 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
   DIO.

   With Trickle, an RPL Router will schedule emission of a DIO at some



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   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.

   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.

   When operating in non-storing mode, 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 routes
   from itself and to all destinations and to calculate routes.

   When operating in storing mode, this entails that the DODAG Root
   needs enough memory to keep a list of all RPL Routers in the RPL
   instance, and a next hop for each of those RPL Routers.  If
   aggregation is used, the memory requirements can be reduced in
   storing mode (see Section 8 for observations about aggregation in
   RPL).

   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 data packets transit
   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



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   DODAG Roots, and be administratively configured as such - with the
   remainder of the RPL Routers in the network being of typically lesser
   capacity.  In storing mode, the DODAG root needs to keep a routing
   entry for all RPL Routers in the RPL instance.  In non-storing mode,
   the resource requirements on the DODAG Root are likely much higher
   than in storing mode, as the DODAG Root needs to store a network
   graph containing complete routes to all destinations in the RPL
   instance, in order to calculate the routing table (whereas in storing
   mode, only the next hop for each destination in the RPL instance
   needs to be stored, and aggregation may be used to further reduce the
   resource requirements).

   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 the DODAG
   Root and for 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 RPL Router may not have the necessary
   resources to satisfy this role as (temporary) 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, all (or at least a large number) of the RPL
   routers need to have resources to act as roots to support floating
   DODAG, especially in non-storing mode.

   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.  In those cases, it would be
   hard to specify such "super-device" as DODAG root, and can result in
   non-optimal routes.


5.  RPL Data Traffic Flows

   RPL makes a-priori assumptions of data traffic types, and explicitly
   defines three such [I-D.ietf-roll-terminology] traffic types: 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 [RFC6550], the resulting protocol
   design, however, reflects these assumptions in that the mechanism
   constructing multipoint-to-point routes is efficient in terms of



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   control traffic generated and state required, point-to-multipoint
   route construction much less so - and point-to-point routes 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) (see
   Section 7) - or, in case of storing mode, considerable memory
   requirements in all LLN routers inside the network (see 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 route 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.

5.1.  Observations

   RPL is suited for networks where sensor-to-root traffic is dominant,
   by distribution of DIO messages and building of a collection tree.
   The one way traffic from the sensor to the root can be forwarded
   through the preferred parent.

   However, the data traffic characteristics, assumed by RPL, do not
   represent a universal distribution of traffic types in LLNs:

   o  There are scenarios where sensor-to-sensor traffic is a more
      common occurrence, documented, e.g., in [RFC5867] ("Building
      Automation Routing Requirements in Low Power and Lossy Networks").

   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 DODAG Root to a specific sensor, and



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      the sensor value is expected returned.  In fact, unless all
      traffic in the LLN is unidirectional, without acknowledgements
      (e.g., as in UDP), and no control messages (e.g., for service
      discovery) or other data packets are sent from the DODAG Root to
      the RPL Routers, traffic will be bi-directional.  The IETF
      protocol for use in constrained environments, CoAP
      [I-D.ietf-core-coap], makes use of acknowledgements to control
      packet loss and ensure that packets are received by the packet
      destination.  In the four message types defined for CoAP:
      confirmable, acknowledgement, reset and non-confirmable, the first
      three are dedicate for sending/acknowledgement cycle.  Another
      example is that the ZigBee Alliance SEP 2.0 specification [SEP2.0]
      describes the use of HTTP over TCP over ZigBeeIP, between RPL
      Routers and the DODAG Root - and with the use of TCP inherently
      causing bidirectional traffic by way of data-packets and their
      corresponding acknowledgements.  In fact, current Internet
      protocols generally require some form of acknowledgment, and
      foregoing an acknowledgment probably means a trade-off in the area
      of reliable transmission or repeated retransmissions or both.

   For the former, all sensor-to-sensor routes include the DODAG Root,
   possibly causing congestions on the communication medium near the
   DODAG Root, and draining energy from the intermediate RPL Routers on
   an unnecessarily long route.  If sensor-to-sensor traffic is common,
   RPL Routers near the DODAG Root will be particularly solicited as
   relays, especially in non-storing mode.

   For the latter, as there is no provision for on-demand generation of
   routing information from the DODAG Root to a proper subset of all RPL
   Routers, each RPL Router (besides the Root) is required to generate
   DAOs.  In particular in non-storing mode, each RPL Router will
   unicast a DAO to the DODAG Root (whereas in storing mode, the DAOs
   propagate upwards towards the Root).  The effects of the requirement
   to establish downward routes to all RPL Routers are:

   o  Increased memory and processing requirements at the DODAG Root (in
      particular in non-storing mode) and in RPL Routers near the DODAG
      Root (in storing mode).

   o  A considerable control traffic overhead [bidir], in particular at
      and near the DODAG Root, therefore:

   o  Potentially congested channels, and:

   o  Energy drain from the RPL Routers.






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6.  Fragmentation Of RPL Control Messages And Data Packet

   Link layers, used in LLNs, are often unable to provide an MTU of, at
   least, 1280 octets - as otherwise required for IPv6 [RFC2460].  In
   such LLNs, link-specific fragmentation and reassembly of IP packets
   at a layer below IPv6 is used to transport larger IP packets,
   providing the required minimum 1280 octet MTU [RFC4919].

   When such below-the-IP-layer fragmentation is used, the IP packet has
   to be reassembled at every hop.  Every fragment must be received
   successfully by the receiving device, or the entire IP packet is
   lost.  Moreover, the additional link-layer frame overhead (and IPv6
   Fragment header overhead in case of IP fragmentation) 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 [ieee802154], with a maximum frame size of 127
   bytes - a much smaller value than the specified minimum MTU of 1280
   bytes for IPv6 [RFC2460].  Reducing the need of fragmentation of IP
   datagrams on such a link layer, 6LoWPAN provides an adaptation layer
   [RFC4944], [RFC6282], providing "Layer 2.5 fragmentation" in order to
   accommodate IPv6 packet transmissions over the maximum IEEE 802.15.4
   frame size of 127 octets, as well as compressing the IPv6 header,
   reducing the overhead of the IPv6 header from at least 40 octets to a
   minimum of 2 octets.  Given the IEEE 802.15.4 frame 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 if link-layer fragmentation is to be avoided.

   The second L in LLN indicating Lossy [roll-charter], higher loss
   rates than typically seen in IP networks are expected, rendering
   fragmentation important to avoid.  This, in particular because, as
   mentioned above, the whole IP packet is dropped if only a single
   fragment is lost.

6.1.  Observations

   [RFC4919] makes the following observation regarding using IP in
   LoWPAN networks based on IEEE 802.15.4 frames:

      Applications within LoWPANs are expected to originate small
      packets.  Adding all layers for IP connectivity should still allow
      transmission in one frame, without incurring excessive
      fragmentation and reassembly.  Furthermore, protocols must be
      designed or chosen so that the individual "control/protocol



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      packets" fit within a single 802.15.4 frame.  Along these lines,
      IPv6's requirement of sub-IP reassembly [...] may pose challenges
      for low-end LoWPAN devices that do not have enough RAM or storage
      for a 1280-octet packet.

   In order to avoid the link-layer fragmentation and thus to adhere to
   the recommendation in [RFC4919], each control packet of RPL must fit
   into the remaining 79 octets of the 802.15.4 frame.  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 when using a simple 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 link-layer 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
   fragmentation.

   RPL may further increase the probability of link-layer fragmentation
   of data traffic: for non-storing mode, RPL employs source-routing for
   all downward traffic.  [RFC6554] specifies the RPL Source Routing
   header, which imposes a fixed overhead of 8 octets per IP packet
   leaving 71 octets remaining from the link-layer MTU in order to
   contain the whole IP packet into a single frame - 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 link-layer fragmentation of also data
   packets.  This, in particular, for longer routes, e.g., for point-to-
   point data traffic between sensors inside the LLN, where data traffic
   transit through the DODAG Root and is then source-routed to the
   destination.  The overhead of source routing is further detailed in
   Section 7.

   Given the minimal packet size of LLNs, the routing protocol must
   impose low (or no) overhead on data packets, hopefully independently
   of the number of hops [RFC4919].  However, source-routing not only
   causes increased overhead in the IP header, it also leads to a
   variable available payload for data (depending on how long the source
   route is).  In point-to-point communication and when non-storing mode
   is used for downward traffic, the source of a data packet will be
   unaware of how many octets will be available for payload (without



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   incurring L2.5 fragmentation) when the DODAG Root relays the data
   packet and adds the source routing header.  Thus, the source may
   choose an inefficient size for the data payload: if the data payload
   is large, it may exceed the link-layer MTU at the DODAG Root after
   adding the source-routing header; on the other hand, if the data
   payload is low, the network resources are not used efficiently, which
   introduces more overhead and more frame transmissions.

   Unless the DODAG Root is the source of an IPv6 packet to be forwarded
   through an RPL LLN, the IPv6 packet must be encapsulated in IPv6-in-
   IPv6 tunneling, with the RPL extension added to the outer IPv6
   header.  Similarly, in non-storing mode, the original IPv6 packet
   must be carried in IPv6-in-IPv6 tunneling, with the RPL routing
   header added to the outer IPv6 header.  Both of these mechanisms add
   additional overhead, increasing the likelihood that link-layer
   fragmentation will be required to deliver the IPv6 packet.  In
   addition, even IPv6 packets that are the minimum MTU size of 1280
   octets will require IPv6 fragmentation to accommodate the RPL tunnel
   and headers on a deployment using the [RFC4944] specification to
   carry IPv6 over IEEE 802.15.4, because RFC4944 defines the MTU for
   such deployments to be 1280 octets.  The ZigBee Alliance is
   considering relaxing [RFC4944] to use an MTU of 1360 octets in its
   specification for IPv6 over IEEE 802.15.4 to accommodate 1280 octet
   IPv6 packets with the required tunnel overhead without fragmentation.


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
   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 [RFC6554] 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 route 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.



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   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
   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 need to be the same as DODAG root -
   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 of devices, as the network evolves, is needed.

   In an experimental testbed, [rpl-eval-UCB] argues that practical
   experiences suggest that RPL in storing mode, with RPL Routers having
   10kB of RAM (TELOSB mote with TinyOS, 16-bit RISC, 48 kB program
   flash memory, 16 kB configuration EEPROM), should be limited to
   networks of less than ~30 RPL Routers.  Note that observation of less
   than 30 RPL Routers only presents the results obtained from specified
   testbed and implementation in [rpl-eval-UCB].  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 apply.  Moreover, if the LoWPAN adaption
   layer [RFC4944] is used in the LLN, route aggregation is not possible
   since the same /64 is applied across the entire network.

   In short, the mechanisms in RPL 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 Root (non-storing
   mode).

   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
   particular, in order to calculate routes 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 increase DTSN (Destination
   Advertisement Trigger Sequence Number) to trigger DAO message
   transmission, or eventually increase the DODAG version number in case
   the destination is still unreachable, which possibly provokes a
   broadcast-storm-like situation.  This, in particular, as [RFC6550]
   does not specify DAO message transmission constraints, nor any
   mechanism for adapting DAO emission to the network capacity.



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   In storing mode, a DTSN increment by the DODAG Root works only if all
   RPL Routers, on the path from the DODAG Root to the "lost" target RPL
   Router, have kept their routing table up-to-date by triggering DAO
   updates, and thus have a route to the target RPL Router.  In non-
   storing mode, the DODAG Root incrementing its DTSN will trigger
   global DAO updates, and thus extra overhead in the network and delay
   in the recalculation of the missing route.

   Furthermore, DTSN increments are carried by way of DIO messages.  In
   case the "lost" target RPL Router has lost all of its parents, it
   will not be able to receive DIO messages from them, and thus will
   have to wait until it has poisoned its sub-DODAG and joined the DODAG
   through another parent.  The only way the DODAG Root can speed up
   this process is by incrementing the DODAG version number, thus
   triggering global recalculation of the DODAG.

   Even in case the DTSN increment is carried to the "lost" target RPL
   Router through another parent, the triggered DAO will need to go up
   the DODAG to the DODAG Root via another route, which might itself be
   broken.  This would necessitate the use of local repair mechanisms,
   potentially causing loops in the network (see Section 14) and
   eventually global DODAG recalculation.


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 route 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
   available.










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                                   .---.
                                   |   |
                                   '---'
                                     |
                                     |
                                    (a)
                                     |
                                     |1.x.x.x/8
                                     |
                                    (b)
                                    / \
                         1.1.x.x/16/   \ 1.2.x.x/16
                                  /     \
                              .---.     .---.
                              | c |     | d |
                              '---'     '---'


                       Figure 2: Address Hierarchies

   Any aggregated routes require the use of a prefix shorter than /64,
   and subsequent hierarchical assignment of prefixes down to a /64 (as
   any RPL Router itself provides a /64 subnet to any hosts connected to
   the RPL Router).

   Moreover, if the 6lowpan adaption layer [RFC4944] is used in the LLN,
   route aggregation is not possible since the same /64 is applied
   across the entire network.

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 route 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, while [RFC6550]
   specifically allows for aggregation (e.g., in Section 18.2.6.: "[...]
   it is recommended to delay the sending of DAO message to DAO parents
   in order to maximize the chances to perform route aggregation").



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   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, losing redundancy in the network
   connectivity.  RPL does 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 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 routes.  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 also have to 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.  Link Bidirectionality Verification

   Parents (and the preferred parent) are selected based on receipt of
   DIOs.  This, alone, does not guarantee the ability of an RPL Router
   to successfully communicate with the parent.  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.  Preliminary results from a test-bed of
   AMI (Automated Metering Infrastructure) devices using 950MHz radio
   interfaces, and with a total of 22 links, show that 36% of these
   links are unidirectional.  If an RPL Router receives a DIO on such a
   unidirectional link, and selects the originator of the DIO as parent,



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   which 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.

   [RFC6550] discusses some mechanisms which can (if deemed needed) be
   used to verify that a link is bidirectional before choosing an RPL
   Router as a parent.  While requiring one mechanism for bidirectional
   verification to be used, the document does not specify which method
   to be used, and how to be used.  The mechanisms discussed include NUD
   [RFC4861], BFD [RFC5881] and [RFC5184].  BFD is explicitly called out
   as "often not desirable" as it uses a proactive approach (exchange of
   periodic HELLO messages), and thus would "lead to excessive control
   traffic".  Furthermore, not all L2 protocols provide L2
   acknowledgements; even less so for multicast packets - and so, not on
   RPL DIOs, the multicast transmission of which is a requirement for
   the Trickle timer flooding reduction to be effective (see
   Section 3.1).  This has as consequence that such L2 acknowledgements
   can only be used to determine if a given link is bidirectional or
   unidirectional once the RPL Router already has selected parents AND
   actually has data traffic to forward by way of these parents - in
   contradiction with RPL's stated design principle that require that
   the reachability of an RPL Router be verified before choosing it as a
   parent ([RFC6550], Section 1.1).  Absent any mechanism specified by
   RPL to verify the bidirectionality of links, RPL Routers thus have to
   rely on NUD to choose their parent correctly (see Section 10).


10.  Neighbor Unreachability Detection For Unidirectional Links

   [RFC6550] 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 tries (and fails) to actually use another RPL
   Router 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.






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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 L2 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 seemingly 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" over the network lifetime.

   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.

   An RPL Router may detect that its preferred parent is lost by way of
   NUD, when trying to communicate to the DODAG Root.  If that RPL
   Router has no other parents in its parent set, all it can do is wait:
   RPL does not provide other mechanisms for an RPL Router to react to
   such an event.  In the case where there is no downward traffic (i.e.,
   no data or acknowledgements are sent from the DODAG Root), neither
   the DODAG Root nor the preferred parent, to which upward connectivity
   was lost, will be able to detect and react to the event of
   connectivity loss.

   In other words, for upward traffic, the RPL Routers that by way of
   NUD detect connectivity loss, will be unable to act in order to
   restore connectivity (e.g., by way of a signaling mechanism to the
   DODAG Root, to request DODAG reconstruction by way of version number
   increase).  The RPL Routers, which could react (the "preferred
   parents") will for upward traffic not generate any traffic "downward"
   allowing NUD to engage and detect connectivity loss.




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   It is worth noting that RPL is optimized for upward traffic
   (multipoint-to-point traffic), and that this is exactly the type of
   traffic where NUD is not applicable as a mechanism for detecting and
   reacting to connectivity loss.

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

   Finally, upon having been notified by NUD that the "next hop" is
   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 further ahead, possibly
   outside the LLN, thus changing preferred parent will not alleviate
   the situation.  Moreover, using information from an upper-layer
   protocol, e.g., to return TCP ACKs back to the source, requires
   established downward routes in the DODAG (i.e., each RPL Router needs
   to send DAO messages to the DODAG Root, as described in Section 7).

   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)" [RFC6550].
   However, the 163 pages long specification of RPL, plus additional
   specifications for routing headers [RFC6554], Trickle timer
   [RFC6206], routing metrics [RFC6551] and objective function
   [RFC6552], 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
   (http://www.sics.se/contiki), 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



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   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 likely that (i) due to limited
   memory capacity of the RPL Routers, and (ii) due to expensive
   development cost of the routing protocol implementation, 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
   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 [RFC6550] provides various options and extensions in many
   parts, which makes a complex protocol, 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:





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   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 route.  If not all of these DAOs from intermediate RPL
       Routers have been received, route calculation is not possible,
       and DAO-ACKs or data traffic cannot be sent to that destination.

   Other examples of underspecification include detection of
   connectivity loss, as described in Section 10, as well as 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 of minimizing 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
   emissions.






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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
   theory.

   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 within 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].

   In another experimental testbed with 17 RPL routers (Tmote Sky,
   Contiki platform [powertrace]), the authors also showed that the DIO
   emission continues with constant rate.  Even with a relatively high
   data rate for sensor networks (every router sends 1 packet to the
   root per minute), the energy used for routing control packets is
   higher than the data traffic transmission.

   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
   [trickle-multicast].

13.2.  Caveat

   Note that these observations do not claim that it is impossible to
   parametrize Trickle timers so that a given deployment exhibits the
   theoretical characteristics (or, characetristics sufficiently close
   thereto) of the Trickle mechanism.  These observations suggest that
   the default parameter values, provided for Trickle timers in
   [RFC6550], did not apply to the small network tested.  These
   observations also suggest that special care is required when
   selecting the values for the parameters for Trickle timers, and that
   these values likely are to be determined experimentally, and
   individually for each deployment.




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14.  Loops

   [RFC6550] states that it "guarantees neither loop free route
   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 RPL Router
   identifier (i.e., IP address) appears at least twice in the route
   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.

   As for (ii), RPL is supposed to run on LLN routers with "constraints
   on [...] memory" [RFC6550]; 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,
   frequently.  During the same experiments described in Section 13, a
   snapshot of the DODAG was made every ten seconds.  In 74.14% of the



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   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 routes are not built correctly.  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
   channel.

   Finally, loop detection for every data packet increases the
   processing overhead.  RPL is targeted for deployments on very
   constrained devices with little CPU power, therefore a loop detection
   for every packet reduces available resources of the LLN router for
   other tasks (such as sensing).  Moreover, each IPv6 packet needs to
   contain the "RPL Option for Carrying RPL Information in Data-Plane
   Datagrams" [RFC6553] in order to use loop detection (as well as
   determining the RPL instance), which in turn implies an extra IPv6
   header (and thus overhead) for IPv6-in-IPv6 tunneling.  As this RPL
   option is a hop-by-hop option, it needs to be in an encapsulating
   IPv6-in-IPv6 tunnel and then regenerated at each hop.


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.

   Moreover, the authors would like to express their gratitude to Ralph
   Droms (Cisco) for his careful review of various versions of this
   document, for many long discussions, and for his considerable
   contributions to both the content and the quality of this document.


18.  Informative References

   [I-D.ietf-core-coap]
              Shelby, Z., Hartke, K., and C. Bormann, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-18
              (work in progress), June 2013.

   [I-D.ietf-roll-terminology]
              Vasseur, J., "Terms used in Routing for Low power And
              Lossy Networks", draft-ietf-roll-terminology-13 (work in
              progress), October 2013.

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

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

   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals",
              RFC 4919, August 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.



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   [RFC5184]  Aggarwal, R., Kompella, K., Nadeau, T., and G. Swallow,
              "Bidirectional Forwarding Detection (BFD) for IPv4 and
              IPv6 (Single Hop)", RFC 5184, June 2010.

   [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.

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

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

   [RFC6551]  Vasseur, J., Pister, K., Dejan, N., and D. Barthel,
              "Routing Metrics Used for Path Calculation in Low-Power
              and Lossy Networks", RFC 6551, March 2012.

   [RFC6552]  Thubert, P., "Objective Function Zero for the Routing
              Protocol for Low-Power and Lossy Networks (RPL)",
              RFC 6552, March 2012.

   [RFC6553]  Hui, J. and J. Vasseur, "The Routing Protocol for Low-
              Power and Lossy Networks (RPL) Option     for Carrying RPL
              Information in Data-Plane Datagrams", RFC 6553,
              March 2012.



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   [RFC6554]  Hui, J., Vasseur, J., Culler, D., and V. Manral, "An IPv6
              Routing Header for Source Routes with the Routing Protocol
              for Low-Power and Lossy Networks (RPL)", RFC 6554,
              March 2012.

   [SEP2.0]   Alliance, Zigbee., "ZigSmart Energy version 2.0 (SEP 2.0)
              draft 0.7", July 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.

   [ieee802154]
              Computer Society, IEEE., "IEEE Std. 802.15.4-2003",
              October 2003.

   [powertrace]
              Dunkels, A., Eriksson, J., Finne, N., and N. Tsiftes,
              "Powertrace: Network-level Power Profiling for Low-power
              Wireless Networks", Technical Report SICS T2011:05.

   [roll-charter]
              "ROLL Charter",
              web http://datatracker.ietf.org/wg/roll/charter/,
              February 2012.

   [rpl-contiki]
              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.

   [rpl-eval]
              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.

   [rpl-eval-UCB]
              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.



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   [trickle-multicast]
              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,
              2011.


Authors' Addresses

   Thomas Clausen
   LIX, Ecole Polytechnique
   91128 Palaiseau Cedex,
   France

   Phone: +33 6 6058 9349
   Email: T.Clausen@computer.org
   URI:   http://www.thomasclausen.org


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

   Phone: +33 6 1264 7119
   Email: axel@axelcdv.com
   URI:   http://www.axelcdv.com/


   Jiazi Yi
   LIX, Ecole Polytechnique
   91128 Palaiseau Cedex,
   France

   Phone: +33 1 6933 4031
   Email: jiazi@jiaziyi.com
   URI:   http://www.jiaziyi.com/













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

   Email: ulrich@herberg.name
   URI:   http://www.herberg.name/


   Yuichi Igarashi
   Hitachi, Ltd., Yokohama Research Laboratory

   Phone: +81 45 860 3083
   Email: yuichi.igarashi.hb@hitachi.com
   URI:   http://www.hitachi.com/



































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