Network Working Group T. Clausen
Internet-Draft A. Colin de Verdiere
Intended status: Informational J. Yi
Expires: July 18, 2013 LIX, Ecole Polytechnique
U. Herberg
Fujitsu Laboratories of America
Y. Igarashi
Hitachi, Ltd., Yokohama Research
Laboratory
January 14, 2013
Observations of RPL: IPv6 Routing Protocol for Low power and Lossy
Networks
draft-clausen-lln-rpl-experiences-05
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 weaknesses and 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
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on July 18, 2013.
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Copyright Notice
Copyright (c) 2013 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 . . . . 11
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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
14.1. Observations . . . . . . . . . . . . . . . . . . . . . . . 25
15. Security Considerations . . . . . . . . . . . . . . . . . . . 26
16. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 26
17. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 26
18. Informative References . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 29
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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 approximately one year past since publication of RPL as
[RFC6550], it is opportune to document observations of 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.
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. [rpl-eval-UCB] indicates that, in storing mode, a TelosB
mote with 10KB of RAM has sufficient memory to support up to about 30
RPL Routers in the LLN - in a larger network (in storing or non-
storing mode, both) the DODAG Root would require at least that much,
likely much more, memory. 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 - 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.
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-device" as DODAG Root is both
unnecessary and undesirable.
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
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design, however, reflects these assumptions in that the mechanism
constructing multipoint-to-point routes is efficient in terms of
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
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
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
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discovery) or other data packets are sent from the DODAG Root to
the RPL Routers, traffic will be bi-directional. As an example,
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.
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.
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
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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 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.
In RPL, 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
[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
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.
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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.
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
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
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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.
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
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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
of devices, as the network evolves, 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 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.
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
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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 - but does not specify nor recommend one of these
for use. 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, 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] 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
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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.
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
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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].
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.
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.
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"
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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
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
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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
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.
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.
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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-roll-terminology]
Vasseur, JP., "Terminology in Low power And Lossy
Networks", work in
progress draft-ietf-roll-terminology-08, December 2012.
[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.
[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.
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[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.
[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.
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[ieee802154]
Computer Society, IEEE., "IEEE Std. 802.15.4-2003",
October 2003.
[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.
[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
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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/
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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