6LoWPAN Working Group D. Kaspar
Internet-Draft E. Kim
Expires: September 29, 2007 M. Shin
H. Kim
ETRI / PEC
March 28, 2007
Design Goals and Requirements for 6LoWPAN Mesh Routing
draft-dokaspar-6lowpan-routreq-01
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Abstract
This document defines the problem statement, design goals, and
requirements for mesh routing in low-power wireless personal area
networks (LoWPANs).
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Scenario Considerations . . . . . . . . . . . . . . . . . 3
2. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 5
3. LoWPAN Mesh Routing Design Goals . . . . . . . . . . . . . . . 6
3.1. Power Conservation . . . . . . . . . . . . . . . . . . . . 6
3.2. Low Protocol Complexity . . . . . . . . . . . . . . . . . 7
3.3. Reliability . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Mobility . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.5. Security . . . . . . . . . . . . . . . . . . . . . . . . . 8
4. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Analysis of Existing Mesh Routing Candidates . . . . . . . . . 11
6. Security Considerations . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
Intellectual Property and Copyright Statements . . . . . . . . . . 15
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1. Introduction
Low-power wireless personal area networks (LoWPANs) are formed by
devices complying to the IEEE 802.15.4 standard [1]. LoWPAN devices
are distinguished by their low bandwidth, short range, scarce memory
capacity, limited processing capability and other attributes of
inexpensive hardware. In this document, the characteristics of nodes
participating in LoWPANs are assumed to be those described in [2].
IEEE 802.15.4 networks support star and mesh topologies and consist
of two different device types: reduced-function devices (RFDs) and
full-function devices (FFDs). RFDs have the most limited
capabilities and are intended to perform only simple and basic tasks.
RFDs may only associate with a single FFD at a time, but FFDs may
form arbitrary topologies among each other and accomplish more
advanced functions, such as multi-hop routing.
Neither the IEEE 802.15.4 standard nor the "IPv6 over IEEE 802.15.4"
[3] specification provide any information as to how mesh topologies
could be obtained and maintained. However, routing in mesh networks
has been the subject of much research in the IETF. A number of
experimental protocols have been developed in the Mobile Ad-hoc
Networks (MANET) working group, such as AODV [6], OLSR [7], or DYMO
[8].
This document defines the problem statement of mesh routing in LoWPAN
environments and outlines the relevant design goals and requirements.
1.1. Scenario Considerations
Low-power WPAN technology is still in its early stage of development,
but the range of conceivable usage scenarios is tremendous. The
numerous possible applications of sensor networks make it obvious
that mesh topologies will be prevalent in LoWPAN environments and
routing will be a necessity for expedient communication.
In general, the requirements on a routing protocol are depending on
the network architecture - the hardware parameters of the
participating nodes and the properties of the network;
1. Network properties:
* Device number and density
* Network scale and topology
* Mobility issues
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* ... etc.
2. Node parameters:
* Physical dimensions
* Hardware Cost
* Processing speed and memory size
* Power consumption and source
* Transmission range, rate and bandwidth
* ... etc.
Network properties may vary arbitrarily for application scenarios in
both LoWPAN or MANET contexts. For instance, even though LoWPANs are
often regarded as more dense and consisting of more nodes than MANET
networks, it might as well occur that a MANET consists of more
devices than a LoWPAN.
On the other hand, the node parameters themselves define whether a
certain node is a LoWPAN device, a MANET device or neither of both.
The classification boundaries are only vaguely defined and heavily
depend on the LoWPAN architecture. However, it is perceivable that
LoWPAN devices enable services that MANET devices can not provide
because of their nature of being more expensive, occupying greater
physical space or requiring too much battery power. We might think
that the characteristics of LoWPANs are device features only, but
they enable a whole new segment of services that MANET devices are
not necessarily able to satisfy.
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2. Problem Statement
If possible, the easiest way to achieve LoWPAN mesh routing is by
reusing existing MANET protocols without making any modifications.
However, the modification or simplification of existing MANET routing
protocols may be required for mesh routing to be feasible in a LoWPAN
domain, because other requirements apply to LoWPAN devices. Unlike
MANET devices, LoWPAN nodes are characterized by much lower power
supplies, smaller memory sizes, and lower processing power, which
creates new challenges on obtaining robust and reliable mesh routing
within LoWPANs.
There exists a trade-off relationship between routing effectiveness
and the requirements posed upon the devices participating in a mesh
network. The challenge is to create a balance between protocol
simplicity and routing performance. But stripping down existing
protocols to power-aware, low-overhead protocols decreases the
efficacy and functionality of their sophisticated routing techniques,
or possibly even endangers the goals they were designed for.
The document "6LoWPAN Problems and Goals" [2] briefly mentions four
requirements on the routing protocol in the section 4.2; low overhead
on data packets, low routing overhead, minimal memory computation,
and support for sleeping nodes considering battery saving. These
high-level requirements only describe low overhead and power saving.
Based on these fundamental features of LoWPAN routing, more detailed
requirements need to be considered, which can lead to protocol design
and analysis.
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3. LoWPAN Mesh Routing Design Goals
This section outlines the fundamental design goals, which serve to
define the issues and to impose a list of requirements for
forthcoming LoWPAN routing solutions. The general design goals for
routing are to provide a loop-free, transparent to lower and upper
layers, and robust and scalable solution. Based on these fundamental
routing goals, this section describes special features and goals
concerning LoWPAN routing design. If the application of MANET
routing protocols is feasible in LoWPAN environments, the reuse of
existing solutions must be considered. However, LoWPANs have two
unique goals to consider;
o Power conservation: some devices are mains-powered, but most are
battery-operated and needed to last several months to a few years
with single AA battery Section 3.1.
o Hardware miniaturization: tiny devices, memory sizes, and/or
processors Section 3.2.
These fundamental features of LoWPANs can affect the design of
routing solutions, so that existing specifications from MANET should
be simplified and modified to the smallest extent possible, in order
to fit the low-power requirements of LoWPANs.
3.1. Power Conservation
Saving energy is crucially important to LoWPAN devices that are not
mains-powered but have to rely on a depleting source, such as a
battery. The lifetime of a LoWPAN node depends on the energy it can
store and harvest.
Compared to functions such as computational operations or taking
sensor samples, radio communications is by far the dominant factor of
power consumption [9]. Therefore, optimization of mesh routing
protocols in terms of achieving a minimal number of control messages
is essential for the longevity of battery-powered nodes. Routing
overhead must be minimized in order to prevent fragmentation of
frames on the physical layer (PHY), reducing the energy required for
transmission and preventing the need for packet reassembly.
In order to find energy-optimal routing paths, LoWPAN mesh routing
protocols should minimize power consumption by utilizing a
combination of the link quality indication (LQI) provided by the MAC
layer and other measures, such as hop count. Route repair and route
error messages should be avoided for minimizing the overall number of
control messages and the required energy for sending them.
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Many nodes in LoWPAN environments might periodically hibernate (i.e.
disable their transceiver activity) to save energy. Therefore, mesh
routing protocols must ensure robust packet delivery despite of nodes
frequently shutting off their radio transmission interface.
Neighbor discovery is a major precondition to allow routing in a
network. Especially in a low-power environment, where nodes might be
in periodic sleeping states, it is difficult to define whether a node
is a neighbor of another or not. In addition to that, LoWPAN devices
must be very economical in terms of power consumption and should
avoid sending "Hello" messages. Instead, link layer mechanisms (such
as acknowledgments or beacon responses) could be utilized to keep
track of active neighbors. Neighbor discovery for LoWPANs is
described more detailed in [5].
Transparent IP routing between LoWPAN domains and higher layer
networks must be provided bidirectionally. A LoWPAN mesh routing
protocol must allow for gateways to forward packets out of the local
domain and it must be possible to query a LoWPAN device from outside
of the local domain. Strategies must be considered to avoid battery
depletion of nodes by too many queries from higher level networks.
End-to-end communication is not a design goal of LoWPAN.
3.2. Low Protocol Complexity
A routing protocol of low complexity certainly assists to achieve the
previously described goal of reducing power consumption. However,
this section's focus is on designing a LoWPAN protocol that is
robust, easy to analyze and implicitly less prone to security
attacks. A LoWPAN routing protocol solution MUST consider the
limited memory size and computation capabilities of participating
devices. And even though implementation details are out of IETF
scope, due to the hardware restrictions of LoWPAN devices, code
length SHOULD be considered to fit within their small memory size.
Because network layer routing imposes too much overhead for LoWPANs
and link layer techniques are out of scope of IETF, LoWPAN mesh
routing should be performed within the adaptation layer defined in
[3]. Both addressing modes provided by the IEEE 802.15.4 standard,
16 bit short addresses and 64 bit extended addresses, must be
considered by LoWPAN mesh routing protocols. It is also assumed that
nodes participating in LoWPAN mesh routing are assigned only a single
address/identifier and do not support multiple interfaces.
A LoWPAN routing protocol should be designed to minimize the required
memory size, computational and algorithmical complexity. Possible
techniques might include the omission of routing tables or local
repair procedures, but the actual details are left to the protocol
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designers.
3.3. Reliability
Another important characteristic of LoWPAN devices is their
unreliability due to limited system capabilities, and also because
they might be closely coupled to the physical world with all its
unpredictable variation, noise, and asynchrony. It is predicted that
LoWPANs will be comprised of significantly higher numbers of devices
than counted in current networks, causing user interaction and
maintenance to become impractical and therefore requiring robustness
and self-healing capabilities in their fundamental network structure.
The design of mesh routing protocols must consider the fact that
packets are to be reliably delivered over a much higher number of
hops, too. Mesh routing must be aware of unreliable nodes. One
method to estimate the reliability of a node is to use the link
quality indication (LQI) provided by the MAC layer in combination
with other indicators.
The required reliability is heavily dependent on the individual
service scenario and the LoWPAN architecture.
3.4. Mobility
Routing must be allowed both for fixed and dynamically adaptive
topologies. Mesh networks are likely to consist of nodes with a
certain degree of mobility. Due to the low performance
characteristics of LoWPAN devices, mobility support should be
provided for unmodified nodes. Fast mobility detection will be a
huge challenge and LoWPAN nodes might even change their location
while being in state of hibernation. Routing must be robust and
remain energy- efficient in all such cases. The detailed mobility
requirements and goals are described in [4]. Also, as recently seen
in discussions related to MANEMO (Network mobility for MANET), a
similar point was stated regarding network mobility in LoWPAN
environments.
The required mobility is heavily dependent on the individual service
scenario and the LoWPAN architecture.
3.5. Security
Security threats within LoWPANs may be different from existing threat
models in MANET environments. Neighbor Discovery in IEEE 802.15.4
links may be susceptible to threats as listed in RFC3756 [10].
Bootstrapping may also impose additional threats. Nevertheless,
security should not cause significant transmission overhead.
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4. Requirements
This section lists the requirements on mesh routing protocols for
LoWPANs. The first few of these requirements are rather unique to
LoWPANs and might not be required in other networks;
R01: Existing MANET protocols (e.g., AODV, OLSR, DYMO) SHOULD be
reused.
R02: 6LoWPAN routing MUST be performed in the adaptation layer, as
defined in [3].
R03: Protocol control messages MUST not create fragmentation of
physical layer (PHY) layer frames.
R04: 6LoWPAN routing protocols MUST keep power consumption at a
minimum. The LQI (Link Quality Indicator) provided by the MAC
layer MUST be matched with other indicators to make good decisions
on route calculation.
R05: The solution MUST be simple and of low complexity.
R05.1: Operating with low routing state SHOULD be maintained
(e.g., devices may have only 32 forwarding entries available).
R06: Code length SHOULD be considered to fit within LoWPAN
devices' small memory size.
R07: Both addressing modes, 16 bit short addresses and 64 bit
extended addresses MUST be supported.
R08: For neighbor discovery, 6LoWPAN devices MUST avoid sending
"Hello" messages to check on periodic sleeping states. Instead,
link layer mechanisms (such as acknowledgments or beacon
responses) SHOULD be utilized to keep track of active neighbors.
More general requirements, important for LoWPANs, are listed below.
Many of these requirements are dependent on the 6LoWPAN architecture;
R09: The procedure of local repair and related control messages
(e.g., Route Error Messages) MAY be omitted.
R10: A 6loWPAN routing protocol MUST allow for gateways to forward
packets out of the local domain and it MUST be possible to query a
6LoWPAN device from outside of the local domain.
R11: 6LoWPAN routing protocols MUST be robust and SHOULD work
independent of the sleep schedule (e.g., flooding) of hibernating
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nodes.
R12: Bootstrapping mechanisms MAY be a prerequisite for 6LowPAN
routing.
R13: The solution MUST allow for dynamically adaptive topologies.
It may support local mobility and global mobility.
R14: The solution MUST be as highly scalable as defined by the
LoWPAN architecture (e.g. a large geography).
R15: Protocol control messages MUST be secured. L2 (WEP-like)
security might be enough.
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5. Analysis of Existing Mesh Routing Candidates
This section is still to be defined.
An objective analysis on existing MANET routing protocols should be
carried out to evaluate their suitability within LoWPANs.
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6. Security Considerations
Security issues are handled as described in Section 3.5
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7. References
[1] IEEE Computer Society, "IEEE Std. 802.15.4-2003", October 2003.
[2] Kushalnagar, N., Montenegro, G., and C. Schumacher, "6LoWPAN:
Overview, Assumptions, Problem Statement and Goals,
draft-ietf-6lowpan-problem-07 (work in progress)",
February 2007.
[3] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4 Networks,
draft-ietf-6lowpan-format-10 (work in progress)", October 2005.
[4] Chakrabarti, S. and S. Daniel Park, "LoWPAN Mobility
Requirements and Goals, draft-chakrabarti-mobopts-lowpan-req-01
(work in progress)", March 2007.
[5] Chakrabarti, S. and E. Nordmark, "LoWPAN Neighbor Discovery
Extensions, draft-chakrabarti-6lowpan-ipv6-nd-03 (work in
progress)", March 2007.
[6] Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On-Demand
Distance Vector (AODV) Routing", RFC 3561, July 2003.
[7] Clausen, T. and P. Jacquet, "Optimized Link State Routing
Protocol (OLSR)", RFC 3626, October 2003.
[8] Chakeres, I., "Dynamic MANET On-demand (DYMO) Routing,
draft-ietf-manet-dymo-06 (work in progress)", June 2005.
[9] Pfister, K. and B. Boser, "Smart Dust: Wireless Networks of
Millimeter-Scale Sensor Nodes".
[10] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.
[11] Bradner, S., "Intellectual Property Rights in IETF Technology",
BCP 79, RFC 3979, March 2005.
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Authors' Addresses
Dominik Kaspar
ETRI / PEC
161 Gajeong-dong
Yuseong-gu
Daejeon 305-700
Korea
Phone: +82-42-860-1072
Email: dominik@etri.re.kr
Eunsook Kim
ETRI / PEC
161 Gajeong-dong
Yuseong-gu
Daejeon 305-700
Korea
Phone: +82-42-860-6124
Email: eunah@etri.re.kr
Myungki Shin
ETRI / PEC
161 Gajeong-dong
Yuseong-gu
Daejeon 305-700
Korea
Phone: +82-42-860-4847
Email: mkshin@etri.re.kr
Hyoungjun Kim
ETRI / PEC
161 Gajeong-dong
Yuseong-gu
Daejeon 305-700
Korea
Phone: +82-42-860-6576
Email: hjk@etri.re.kr
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