Networking Working Group D. Culler
Internet-Draft Arch Rock
Intended status: Informational JP. Vasseur
Expires: January 8, 2008 Cisco Systems, Inc
July 7, 2007
Routing Requirements for Low Power And Lossy Networks
draft-culler-rl2n-routing-reqs-01
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Abstract
The need for high quality routing for Low power and Lossy Network
(L2N) such as sensor networks comprised of highly constrained devices
(CPU, memory, ...) in sometimes unstable wireless environments is
critical now and will continue to increase. Interest in this class
of applications has grown dramatically in recent years and a routing
solution addressing the specific environments of such networks is
highly required considering the numerous, incompatible open-source
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and proprietary routing protocols as well as several industrial
forums. The aim of this document is to define the routing
requirements for Sensor Networks at the IP layer. Such routing
protocol(s) would need to address several unique aspects of this
class of embedded devices and would operate in networks comprising
links of various nature.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. "Route over" versus "Mesh under" . . . . . . . . . . . . . . . 3
4. Unique Routing Requirements of Low Power Wireless
Networkson . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4.1. Spatially-Driven Multihop . . . . . . . . . . . . . . . . 4
4.2. Light footprint . . . . . . . . . . . . . . . . . . . . . 5
4.3. Small MTU . . . . . . . . . . . . . . . . . . . . . . . . 6
4.4. Deep Power Management . . . . . . . . . . . . . . . . . . 6
4.5. Heterogeneous Capabilities . . . . . . . . . . . . . . . . 7
4.6. Highly Variable Connectivity . . . . . . . . . . . . . . . 7
4.7. Structured Workload and Traffic Pattern . . . . . . . . . 8
4.8. Partial Information . . . . . . . . . . . . . . . . . . . 8
4.9. Multi-topology Routing . . . . . . . . . . . . . . . . . . 9
4.10. Data Aware routing . . . . . . . . . . . . . . . . . . . . 9
5. Relationship with other Routing Protocols . . . . . . . . . . 9
6. Security Issues . . . . . . . . . . . . . . . . . . . . . . . 10
7. Manageability Issues . . . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. Security Considerations . . . . . . . . . . . . . . . . . . . 10
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
11.1. Normative References . . . . . . . . . . . . . . . . . . . 10
11.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
Intellectual Property and Copyright Statements . . . . . . . . . . 12
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1. Terminology
CMOS: Complementary metal-oxide-semiconductor. DRAM: Dynamic Random
Access Memory. L2N: Low power and Lossy Network.
MAC: Medium Access control. PAN: Personal Area Network. RAM: Random
Access Memory. RF Links: Radio Frequency Links. RL2N: Routing in
Low power and Lossy Networks.
SNR: Signal Noise Ratio. SRAM: Static Random Access Memory.
2. Introduction
The need for high quality routing for wireless networks comprised of
highly constrained (memory, power, bandwidth, CPU ...) and typically
embedded devices in a potentially variable environment (thus the
therm "Lossy") is critical now and will continue to increase.
Interest in this class of applications, including sensor networks,
device networks, environmental monitoring, home automation, building
automation, process control, automated meter readings, condition-
based maintenanace, security, and others, has grown dramatically in
recent years; a routing solution addressing the specific environments
of such networks is highly required considering the numerous,
incompatible open-source and proprietary routing protocols that have
emerged, as well as several industrial forums that have emerged over
the IEEE 802.15.4 link and various proprietary links.
Such routing protocol(s) would need to address several unique
requirements of this class of embedded devices and would operate in
networks comprising links of various nature. Considering the variety
of Sensor and Controller-based applications, there may not be a
single routing protocol satisfying the entire list of requirements,
in which case it may be decided to define a limited set of routing
protocols that could be combined to satify the overall objective. It
is also envisioned that the designed solution will not address very
specific requirements of some more "exotic" networks.
3. "Route over" versus "Mesh under"
Within the IETF, working groups are attending to aspects of this
issue with, for example, 6LoWPAN considering layer 2 "mesh-under" for
IEEE 802.15.4 links and MANET considering layer 3 and higher layer
routing in mobile environments with relatively high powered nodes and
links. Meanwhile, industry forums, including Zigbee, Zwave, Wireless
HART, and ISA SP100.11a, and numerous proprietary offerings address
the combination of low-power and wireless, but only within the
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equivalent of a single IP link and only within the context of stacks
vertically integrated from the physical layer to the application with
no provisions for routing to other kinds of links.
It is clearly envisioned that Low Power and Lossy Networks (L2Ns)
will comprise a variety of nodes interconnected by links of different
nature including IEEE 802.15.4, IEEE 802.11, IEEE 802.16 and so on.
The IETF 6LoWPAN working group has defined a format for IPv6 over
802.15.4 with extensive header compression, fragmentation for very
small link frames, and support for mesh routing under the IP link
(see [I-D.ietf-6lowpan-format]).
Clearly routing techniques can be defined at the link layer (also
referred as to the "Mesh Under" approach). By contrast, the "Route
over" approach exclusively relies on IP routing over a network made
of a variety of nodes interconnected by links of various nature. The
aim of this document is to define the routing requirements L2Ns at
the IP layer. As such, it pertains to collections of IEEE 802.15.4
devices, but is not limited to communication within a single IP link.
It pertains to IP level routing among multiple such PANs, routing
among IEEE 802.15.4 PANs and other links, and routing in other low
power (wireless) networks.
4. Unique Routing Requirements of Low Power Wireless Networkson
L2Ns present a variety of unique routing requirements driven partly
by implementation technology constraints, partly by the domain of
usage, and partly by application characteristics. These issues are
listed roughly in order of criticality.
[I-D.levis-rl2n-overview-protocols]provides an overview of existing
protocols in light of L2Ns' specific requirements. Of course, none
of these protocols were designed with all these considerations in
mind, and so it is not surprising that some of the issues remain
unsolved.
Whereas this document lists the set of generic requirement for RL2N
other documents lists application specific routing requirements. The
routing requirements for home control and automation are discussed in
[I-D.brandt-rl2n-home-routing-reqs].
4.1. Spatially-Driven Multihop
The low transmission power of PAN (Personal Area Network) radios
(e.g. collection of IEEE 802.15 links, implies that the range is
relatively short; multiple hops are required to achieve communication
over greater distances. Variously referred to as mesh or multihop
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routing, such multihop routing communication is important from a
basic energy viewpoint. The energy cost to traverse a given distance
with multiple fixed-power hops grows only linearly with distance,
whereas the energy of a single RF "hop" grows as a cubic or higher
power of the distance, depending on elevation and other factors.
It is also essential from a reliability viewpoint. Lower
transmission power generally means lower SNR, relatively high per-hop
loss rates and greater sensitivity to fading, interference,
attenuation, and occlusion. Mutihop communication permits routing
around obstacles and provides temporal diversity through
retransmission as well as spatial diversity through multiple
receivers, i.e., multipath routing. In addition, with multihop
routing use to cover distance, route formation and reliability are
intimately linked. Taking a longer hop will typically incur a larger
loss rate, while a more reliable hop incurs more transmissions to
reach the destination. These issues occur potentially both at layer
2, with IP routing over mesh-routed links, and, of course, at layer
3, with IP routing over similar or dissimilar links.
Furthermore, with multiple points of egress between low-power
wireless networks and conventional powered networks, route selection
over on type of link may be influenced by factors in the low-power
links.
4.2. Light footprint
Integrated CMOS radios typically have sophisticated physical layer
and MAC support integrated with the transceiver. However, the
network layer over this MAC (or sub-MAC) is generally implemented on
a microcontroller device with the capabilities and resources
historically associated with serial links (e.g., RS-232 and RS-485).
In particular, as of today, these devices have only a few kilobytes
of RAM and a few to several tens of kilobytes of program ROM. The
memory capacity of these device has been growing, but at much slower
rate than the SRAM and DRAM storage found in microprocessor-based
systems. The marginal cost of memory in embedded devices is much
greater than in conventional computers and standby power consumption
increases with RAM capacity due to leakage, so memory capacity
impacts the lifetime of battery powered, low-duty cycle devices.
Thus, the small memory capacity of these units is fundamental and
constrains routing table size, buffer capacity, and all routing
states, including neighbor tables, link estimators, sequence number
and other caches. For example, link state algorithms, distance
vector algorithms, and various intermediates and hybrids may have
quite different relative merits when footprint is at premium, as
compared to convergence rate, information exchange rate, and so on.
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Existing routing protocols generally attend to constraints imposed by
the links more than to constraints imposed by the nodes that connect
those links. The prime exception to this is scalability concerns of
very large networks given fixed, albeit powerful, routers. Here we
are concerned with how routing protocols scale down to less capable
nodes, even a fixed network scale. We are also concerned with how
routing protocols can allow more capable nodes to relieve less
capable ones, even with common link characteristics. Compression
techniques, such as that in 6LoWPAN, enable the opportunity to
perform routing on low-power devices (and permit the use of small
MTUs and modest forwarding buffers), but do not address the resource
requirements of the routing protocols that guide the exchange of such
compressed packets.
4.3. Small MTU
Potentially high bit error rates, limited buffer capacity, limited
channel capacity shared among numerous devices, and pervasive hidden-
terminal occurrences due to the presence of many devices spread over
physical regions all lead to the use of relatively small frames.
Thus, per packet routing and header information comes at a premium.
These issues, as well as limited energy, storage and bandwidth
resources, imply that routing needs to be more aware of underlying
physical factors than in traditional, even wireless, networks. For
example, protocols involving the exchange of lists of all 1-hop or
all 2-hop neighbors may be forced to reckon with longs lists (if the
physical density is high compared to the communication range).
Alternatively, efforts to limit the degree of the network by
adjusting transmission range bring additional physical factors into
the purvue of routing. Moreover, such measures to optimize route
formation may be at odds with optimizing forwarding cost.
4.4. Deep Power Management
In most L2Ns, average transmission rates are very low, relative to
channel capacity and powering on the radio to be ready receive costs
power consumption that is roughly equal to that of actual
transmission or reception.
Thus, power budgets tend to be dominated by idle listening costs,
unless the receivers are heavily duty cycled. Thus, routing
protocols MUST permit deep power management in the underlying link
layers. Currently, these link level techniques fall into three
general categories: variants of TDMA either local or global, variants
of cluster-based beaconing, and variants of preamble sample. While
power management is typically viewed as a layer 2 responsibility, few
routing protocols anticipate that the devices responsible for
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forwarding (and for route maintenance) have their network link off
most (often over 99%) of the time. Alternatively, certain link-level
power management strategies may introduce extreme constraints on
routing protocols.
4.5. Heterogeneous Capabilities
While the majority of devices are highly constrained, in many
settings they operate in conjunction with more capable devices,
including microprocessors hosting the same RF link but with greater
RAM capacity, devices on mains power with either large or small
storage, devices with directional to high-gain antennas, and devices
that bridge or route to higher bandwidth links. The existence of
such a wide scope of device types within L2N (e.g Sensor Networks
must be taken into account by the routing protocol to increase the
lifetime and robustness of the most constrained devices. In some
cases, it may be advantageous to decrease the routing optimality at
the benefit of energy saving for the most constrained (set of)
devices. Thus the routing protocol must not only be capable of
supporting such a wide variety a devices but should consider the
device capability as a key element of the routing decision, domain
scope for the exchange of routing control plane messages.
The routing protocol MUST support the ability to perform constrained
based routing taking into account a variety of static or dynamic node
metrics.
4.6. Highly Variable Connectivity
In many use cases for low power wireless devices, mobility is a
central element. However, even where all the devices are stationary,
changes in environmental conditions gives rise to substantial changes
in the connectivity relationships. Moving objects, opening and
closing of doors, background interference due to machinery,
electronic equipment, radios, or other wireless networks, even
atmospheric changes which increase or decrease absorption all alter
the connection topology over which routing takes place. Thus,
routing protocols MUST be adaptive to a changing underlying topology
and able to utilize connectivity and related information, such as
link quality or signal strength, to maintain viable paths.
The routing protocol MUST be particularly robust to topology changes
in the network due to frequent change of link states. For many
embedded networks with substantial, often the mobility is structured,
rather than ad hoc, such as items moving through a manufacturing
process, shipping exchanges, mobile devices moving through a
stationary network of similar devices, or collections of devices
moving together as a network. The most extreme variations in
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connectivity, including mobility over large distances and enclosure
into RF-opaque settings, give rise to intermittent connectivity (DTN:
Delay Tolerant Networks). Many use cases involve logging over long
periods of disconnected operation and dispersion of logged data upon
arrival and detection of a point of connectivity. Such topology
changing environments are usually challenging for routing protocols
and may lead to frequent rerouting decisions: careful consideration
must be given to bound the number of rerouting decisions for the most
constrained devices so as to save energy.
4.7. Structured Workload and Traffic Pattern
The above characteristics suggest that effective general-purpose RL2N
can be very hard - multiple hops are required over spontaneously
varying connections where bandwidth is precious, packets are small
and little state can be expended at each router. However, the same
observations suggests that routing protocols can take advantage of
the constrained setting to simplify the general problem. The
workload or traffic pattern of use cases for these networks tend to
be highly structured (Point-to-Multipoint or Multipoint-to-point due
to the specific role of one or more sinks), unlike the any-to-any
data transfers and interactive key-strokes that dominate typical
client and server workloads. Instrumentation and monitoring
typically involve regular, periodic, or alarm-based collection from a
large collection of devices. Configuration, tasking, and management
typically involve dissemination of commands to an aggregate of
devices. Automation, such as lighting control, involve numerous
long-lived aggregates of actuation points and control points. Uses
in transportation and shipping involve opportunistic communication
bursts upon arrival at suitable way points. General-purpose any-to-
any connectivity arises in situations such as management, diagnosis,
and field access. In many cases, exploiting such structure may
simplify difficult problems arising from resource constraints or
variation in connectivity. Thus the routing protocols MUST support
Point to Point, Point to Multipoint and Multipoint to Point routing.
However, the highly correlated, repetitive use of particular traffic
patterns will typically allow routing protocols to optimize for very
common simple cases.
4.8. Partial Information
The density of connectivity varies dramatically from long nearly-
linear structures (e.g., over a transect of land, a bridge or a road)
to extremely dense collections in a single RF 'cell' (e.g., parcels
on a dock or containers in transport). Thus, routing protocols and
addressing SHOULD avoid placing arbitrary limits on the underlying
connection topology. Conversely, routing with partial information is
an important property in L2Ns as it facilitates scaling down of the
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node or scaling up of the network to points where algorithmic
concepts such "all 1-hop neighbors", "all 2-hop neighbors", all
nodes, or all pairs may not be representable with the resources
available per node.
4.9. Multi-topology Routing
Multi-topology Routing (MTR) is also important to consider both with
the goal of improving service where it is desirable, but in reducing
effort where service requirements are lax. Although many L2Ns use
initially provide fairly latency in-sensitive monitoring, many
applications have emerged that require timely delivery of the vast
majority of the readings, eventual delivery of the remainder, time-
sensitive delivery of alarms, and/or increasing predictability for
soft and moderately real-time.
These issues impact path selection and path quality optimization, as
well as the impact of protocol and route maintenance traffic on data
traffic, especially during times of critical physical change. Thus,
the mix of applications with a wide range of requirements in term of
path quality leads to the requirements for MTR.
4.10. Data Aware routing
Ultimately, scalability may benefit from the ability to perform
computations for data reduction or fusion within the network, not
just at the data processing sink level. The most common case being
aggregation along a dynamically computed path to a sink.
Thus the routing protocol MUST take points of aggregation (another
node capability) into account when calculating routes.
5. Relationship with other Routing Protocols
This family of unique characteristics pose unique routing challenges.
At the same time, these challenges have deep similarities (and
substantial points of difference) with several other IETF routing
protocols. Like MANET, the interconnection topology over which
routing is performed must, in general, be deduced from observed
communication events, in addition to physical wiring or explicit
configuration. This topology may be static or dynamic, depending on
physical conditions. However, the routing state, neighbor table
size, and cache state per node will in many cases be highly
constrained. Devices themselves have important structure and
characteristics, as many are stationary and some are unconstrained.
In general, the average bandwidth and power demand per node should
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stay bounded and not grow unreasonably with the size of a network.
Thus, it may be unacceptable to generate unscoped floods, unless the
frequency of floods per node diminishes with the size of the network.
In these respects, light footprint routing has much in common with
IGP. Effective routing must be carried out in the presence of
partial (space limited) and somewhat imperfect information. Note
that mixed routing protocol may be considered (Distance Vector and
Link state). That said, none of the currently available routing
protocol fulfills the requirement of L2Ns network listed above.
[I-D.levis-rl2n-overview-protocols]aims at providing an overview
survey of existing routing protocols. The aforementioned
requirements may be conflicting and defining a new routing protocol
fully satisfying those requirements might be challenging. The
objective of this work would be to define a routing protocol that
will satisfy those requirements as much as possible and that would
potentially adapt itself to the particular deployment context.
6. Security Issues
TBD
7. Manageability Issues
TBD
8. IANA Considerations
This document includes no request to IANA.
9. Security Considerations
TBD
10. Acknowledgements
11. References
11.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
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11.2. Informative References
[I-D.brandt-rl2n-home-routing-reqs]
Vasseur, J., "Routing Requirement in Low Power and Lossy
Networks in Home automation requirements for RL2N-
routing", draft-brandt-rl2n-home-routing-reqs-00 (work in
progress), July 2007.
[I-D.ietf-6lowpan-format]
Montenegro, G., "Transmission of IPv6 Packets over IEEE
802.15.4 Networks", draft-ietf-6lowpan-format-13 (work in
progress), April 2007.
[I-D.levis-rl2n-overview-protocols]
Vasseur, J., "Overview of Existing Wireless Mesh Routing
Protocols for Low Power and Lossy Networks",
draft-levis-rl2n-overview-protocols-00 (work in progress),
July 2007.
Authors' Addresses
D Culler
Arch Rock
657 Mission St. Suite 600
San Francisco, CA 94105
USA
Email: dculler@archrock.com
JP Vasseur
Cisco Systems, Inc
1414 Massachusetts Avenue
Boxborough, MA 01719
USA
Email: jpv@cisco.com
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