6LoWPAN Working Group E. Kim
Internet-Draft ETRI
Expires: September 9, 2009 D. Kaspar
Simula Research Laboratory
C. Gomez
Tech. Univ. of Catalonia/i2CAT
C. Bormann
Universitaet Bremen TZI
March 8, 2009
Problem Statement and Requirements for 6LoWPAN Routing
draft-ietf-6lowpan-routing-requirements-01
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Abstract
This document provides the problem statement for 6LoWPAN routing. It
also defines the requirements for 6LoWPAN routing considering IEEE
802.15.4 specificities and the low-power characteristics of the
network and its devices.
Table of Contents
1. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Design Space . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. 6LoWPAN Headers for Routing . . . . . . . . . . . . . . . 9
3.2. Reference Network Model . . . . . . . . . . . . . . . . . 11
4. Scenario Considerations and Parameters for 6LoWPAN Routing . . 13
5. 6LoWPAN Routing Requirements . . . . . . . . . . . . . . . . . 17
5.1. Support of 6LoWPAN Device Properties . . . . . . . . . . . 17
5.2. Support of 6LoWPAN Link Properties . . . . . . . . . . . . 19
5.3. Support of 6LoWPAN Network Characteristics . . . . . . . . 21
5.4. Support of Security . . . . . . . . . . . . . . . . . . . 25
5.5. Support of Mesh-under Forwarding . . . . . . . . . . . . . 26
6. Security Considerations . . . . . . . . . . . . . . . . . . . 27
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
8.1. Normative References . . . . . . . . . . . . . . . . . . . 29
8.2. Informative References . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31
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1. Problem Statement
In the context of this document, low-power wireless personal area
networks (LoWPANs) are formed by devices that are compatible with the
IEEE 802.15.4 standard [6]. Most of the LoWPAN devices are
distinguished by their low bandwidth, short range, scarce memory
capacity, limited processing capability and other attributes of
inexpensive hardware. The characteristics of nodes participating in
LoWPANs are assumed to be those described in RFC 4919 [3].
IEEE 802.15.4 networks support star and mesh topologies. However,
neither the IEEE 802.15.4 standard nor the 6LoWPAN format
specification ("IPv6 over IEEE 802.15.4" [4]) define how mesh
topologies could be obtained and maintained. Thus, the 6LoWPAN
formation and multi-hop routing should be supported by higher layers,
either the 6LoWPAN adaptation layer or the IP layer. A number of IP
routing protocols have been developed in various IETF working groups.
However, these existing routing protocols may not satisfy the
requirements of multi-hop routing in 6LoWPANs, for the following
reasons:
o 6LoWPAN nodes have special types and roles, such as primary
battery-operated nodes, power-affluent nodes, mains-powered and
high-performance gateways, data aggregators, etc. 6LoWPAN routing
protocols should support multiple device types and roles.
o The more stringent requirements apply to LoWPANs, as opposed to
higher performance or non-battery-operated networks. 6LoWPAN nodes
are characterized by small memory sizes, low processing power, and
are running on very limited power supplied by primary non-
rechargeable batteries (a few KBytes of RAM, a few dozens of
KBytes of ROM/flash memory, and a few MHz of CPU is typical). A
node's lifetime is usually defined by the lifetime of its battery.
o Handling sleeping nodes is very critical in LoWPANs, more than in
traditional ad-hoc networks. LoWPAN nodes might stay in sleep-
mode for most of the time. Time synchronization is important for
efficient forwarding of packets.
o Routing in 6LoWPANs might possibly translate to a simpler problem
than routing in higher-performance networks. LoWPANs might be
either transit networks or stub networks. Under the assumption
that LoWPANs are never transit networks (as implied by [4]),
routing protocols may be drastically simplified. This document
will primarily focus on stub networks. Based on the necessity,
this document may be extended with 6LoWPAN network configurations
that include transit networks.
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o Routing in LoWPANs might possibly translate to a harder problem
than routing in higher-performance networks. Routing in LoWPANs
requires power-optimization, stable operation in harsh
environments, data-aware routing, etc. These requirements are not
easily satisfiable all at once.
This creates new challenges on obtaining robust and reliable routing
within LoWPANs.
The 6LoWPAN problem statement document ("6LoWPAN Problems and Goals"
[3]) briefly mentions four requirements on routing protocols;
(a) low overhead on data packets
(b) low routing overhead
(c) minimal memory and computation requirements
(d) support for sleeping nodes considering battery saving
These four high-level requirements describe the basic need for
6LoWPAN routing. Based on the fundamental features of 6LoWPAN, more
detailed routing requirements are presented in this document, which
can lead to further analysis and protocol design.
Using the 6LoWPAN header format [4], there are two layers routing
protocols can be defined at, commonly referred to as "Mesh Under" and
"Route Over". The Mesh Under approach supports routing under the IP
link and is directly based on the link-layer IEEE 802.15.4 standard
in 6LoWPAN, therefore using (64-bit or 16-bit short) MAC addresses.
On the other hand, the Route Over approach relies on IP routing and
therefore supports routing over possibly various types of
interconnected links (see also Figure 1). Most statements in this
document consider both the Mesh Under and Route Over cases.
Note: The ROLL WG is now working on Route Over approaches for Low
power and Lossy Networks (LLNs), not specifically for 6LoWPAN. This
document is focused on 6LoWPAN-specific requirements, in alignment
with the ROLL WG.
Considering the problems above, detailed 6LoWPAN routing requirements
must be defined. Application-specific features affect the design of
6LoWPAN routing requirements and the corresponding solutions.
However, various applications can be profiled by similar technical
characteristics, although the related detailed requirements might
differ (e.g., a few dozens of nodes for home lighting system need
appropriate scalability for the applications, while billions of nodes
for a highway infrastructure system also need appropriate
scalability). This document states the routing requirements of
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6LoWPAN applications in general, while trying to give examples for
different cases of routing. This routing requirements document does
not imply that a single routing solution may be the best one for all
6LoWPAN applications.
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2. Terminology
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 [1].
Readers are expected to be familiar with all the terms and concepts
that are discussed in "IPv6 over Low-Power Wireless Personal Area
Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and
Goals" [3], and " Transmission of IPv6 Packets over IEEE 802.15.4
Networks" [4].
This document defines additional terms:
LoWPAN Coordinator Node
A logical functional entity that performs the special role of
coordinating its child nodes for local data aggregation, status
management of local nodes, etc. Thus, the Coordinator Node does
not need to coincide with a link-layer PAN coordinator and there
may be multiple instance in a LoWPAN.
LoWPAN Mesh Node
A LoWPAN node that forwards data between arbitary source-
destination pairs in 6LoWPAN adaptation layer using link address
(and thus only exist in Mesh Under LoWPANs). A Mesh Node may also
serve as a LoWPAN Host.
Additionally, in alignment with all other 6LoWPAN drafts, this
document uses the same terms and definitions as provided by the
6LoWPAN ND draft [8]:
LoWPAN Host
A node that only sources or sinks IPv6 datagrams. Referred to as
a host in this document. The term node (see LoWPAN Node) is used
when the the differentiation between host and router is not
important.
LoWPAN Edge Router
An IPv6 router that interconnects the LoWPAN to another network.
Referred to as an edge router in this document.
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LoWPAN Router
A node that forwards datagrams between arbitrary source-
destination pairs using a single 6LoWPAN interface performing IP
routing (and thus only exist in route over LoWPANs). A LoWPAN
Router may also serve as a LoWPAN Host - both sourcing and sinking
IPv6 datagrams. Refered to as a router in 6LoWPAN documents. All
LoWPAN Routers perform ND message relay on behalf of other nodes.
LoWPAN Node
A node that composes a LoWPAN. In mesh under, each intermidiate
node performs multi-hop forwarding at L2. In route over, each
intermidiate node serves as a LoWPAN router performing IP routing.
Mesh Under
A LoWPAN configuration where the link-local scope is defined by
the boundaries of the LoWPAN and includes all nodes within.
Forwarding and multihop routing functions are achieved at L2
between mesh nodes.
Route Over
A LoWPAN configuration where the link-local scope is defined by
those nodes reachable over a single radio transmission. Due to
the time-varying characteristics of wireless communication, the
neighbor set may change over time even when nodes maintain the
same physical locations. Multihop is achieved using IP routing.
Backbone Link
This is an IPv6 link that interconnects two or more edge routers.
It is expected to be deployed as a high speed backbone in order to
federate a potentially large set of LoWPANs.
Extended LoWPAN
This is the aggregation of multiple LoWPANs as defined in [3]
interconnected by a backbone link via Edge Routers and forming a
single subnet.
LoWPAN Link
A low-power wireless link which is shared by a link-local scope in
a LoWPAN. In a LoWPAN, a link can be a very instable set of
nodes, for instance the set of nodes that can receive a packet
that is broadcast over the air in a route over LoWPAN, or the set
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of nodes currently reachable in an L2 mesh in a mesh under LoWPAN.
Such a set may vary from one packet to the next as the nodes move
or as the radio propagation conditions change.
LoWPAN Subnet
A subnet including a LoWPAN or an Extended LoWPAN, together with
the backbone link with the same subnet prefix and prefix length.
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3. Design Space
Apart from a wide variety of routing algorithms possible for 6LoWPAN,
the question remains as to whether routing should be performed Mesh
Under (in the adaptation layer defined by the 6lowpan format document
[4]), or by the IP-layer using a Route Over approach. The most
significant consequence of Mesh Under routing is that the inherited
stringent characteristics of IEEE 802.15.4 would directly affect the
6LoWPAN routing mechanisms, therefore using (64-bit or 16-bit short)
MAC addresses instead of IP addresses, and a 6LoWPAN would be seen as
a single IP link. In case a Route Over mechanism is to be applied to
a 6LoWPAN it must also support 6LoWPAN's unique properties using
global IPv6 addressing.
Figure 1 shows the place of 6LoWPAN routing in the entire network
stack.
+-----------------------------+ +-----------------------------+
| Application Layer | | Application Layer |
+-----------------------------+ +-----------------------------+
| Transport Layer (TCP/UDP) | | Transport Layer (TCP/UDP) |
+-----------------------------+ +-----------------------------+
| Network Layer (IPv6) | | Network +---------+ |
+-----------------------------+ | Layer | Routing | |
| 6LoWPAN +---------+ | | (IPv6) +---------+ |
| Adaptation | Routing | | +-----------------------------+
| Layer +---------+ | | 6LoWPAN Adaptation Layer |
+-----------------------------+ +-----------------------------+
| IEEE 802.15.4 (MAC) | | IEEE 802.15.4 (MAC) |
+-----------------------------+ +-----------------------------+
| IEEE 802.15.4 (PHY) | | IEEE 802.15.4 (PHY) |
+-----------------------------+ +-----------------------------+
Figure 1: Mesh-under (left) and route-over routing (right)
In order to avoid packet fragmentation and the overhead for
reassembly, routing packets should fit into a single IEEE 802.15.4
physical frame and application data should not be expanded to an
extent that they no longer fit.
3.1. 6LoWPAN Headers for Routing
In the simplest case for a Mesh Under where predefined layer two
forwarding is appropriate, the mesh-header defined in RFC 4944 [4] is
sufficient. Frame Delivery in a Link-Layer Mesh is described in the
Section 11 in RFC 4944. The mesh type and header defined in RFC 4944
are as follows:
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A 6LoWPAN Mesh Header:
1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 0|V|F|HopsLft| originator address, final address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: 6LoWPAN Mesh Header
However, the mesh header is not sufficient when it needs full routing
functionalities applying more routing metrics and functions. If a
Mesh Under routing protocol is built for operation in 6LoWPAN's
adaptation layer, routing control packets with MAC addresses are
placed after the 6LoWPAN Dispatch. A new Dispatch value is REQUIRED
to be assigned for Mesh Under routing. As shown in Figure 3,
multiple routing protocols can be supported by the usage of different
Dispatch bit sequences.
A 6LoWPAN encapsulated Mesh Under Routing packet:
+---------------------+----------------+---------+----
| Dispatch (new val.) | Routing header | ...
+---------------------+----------------+---------+----
Figure 3: 6LoWPAN packet format and Mesh Under routing
When a Route Over protocol is built over the IPv6 layer, the Dispatch
value can be chosen as one of the Dispatch patterns for 6LoWPAN,
followed by a compressed or uncompressed IPv6 header, and Route Over
routing header will be included in the payload of IPv6 packet.
Figure 4 depicts an example of 6LoWPAN encapsulated Route Over
routing packets for HC1 defined in RFC 4944:
+----------------+-------------+------------------------+---
| Dispatch + HC1 | IPv6 Header | Payload(Routing packet)| ...
+----------------+-------------+------------------------+---
Figure 4: 6LoWPAN HC1 packet format and Route Over routing
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Figure 4 depicts an example of 6LoWPAN encapsulated Route Over
routing packets for IPHC defined in the ongoing packet format work in
6LoWPAN [9]:
+----------------------+-------------+------------------------+--
|Dispatch + LOWPAN_IPHC| IPv6 Header | Payload(Routing packet)|...
+----------------------+-------------+------------------------+--
Figure 5: 6LoWPAN IPHC packet format and Route Over routing
3.2. Reference Network Model
When a 6LoWPAN follows the Mesh Under configuration, the LoWPAN Edge
Router (ER) is the only IPv6 router in the 6LoWPAN (see Figure 6).
This means that the IPv6 link-local scope includes all nodes in the
LoWPAN. A Mesh Under routing mechanism MUST be provided to support
multi-hop transmission.
If a Route Over routing is used in the stub-network, not only the ER
but also other intermediate nodes become LoWPAN Routers and perform
standard layer 3 routing (see Figure 7). The link-local scope is
defined by one radio hop.
h h
/ | ER: Edge Router
ER --- m --- m --- h m: Mesh Node
/ \ h: LoWPAN Host
h m --- h
|
/ \
m - m -- h
Figure 6: An example of a Mesh Under LoWPAN
h h
/ | ER: Edge Router
ER --- r --- r --- h r: LoWPAN Router
/ \ h: LoWPAN Host
h r --- h
|
/ \
r - r -- h
Figure 7: An example of a Route Over LoWPAN
When multiple 6LoWPANs are formed with globally unique IPv6 addresses
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in the 6LoWPANs, and node (a) of 6LoWPAN [A] wants to communicate
with node (b) of 6LoWPAN [B], the normal IPv6 mechanisms can be
employed. For Mesh Under, there is one IP hop from a node (a) to ER
of [A], no matter how many radio hops stay apart from each other.
This, of course, assumes the existence of a Mesh Under routing
protocol in order to reach the ER. For Route Over, the IPv6 address
of (b) is set as the destination of the packets, and the nodes
perform IP routing to the ER for these outgoing packets. In this
case, one radio hop is one IPv6 link. Additionally, a default route
to the ER could be inserted into the 6LoWPAN routing system.
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4. Scenario Considerations and Parameters for 6LoWPAN Routing
IP-based LoWPAN 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 robust routing will be a necessity for expedient
communication. Research efforts in the area of sensor networking
have put forth a large variety of multi-hop routing algorithms [7]
and [21]. Most related work focuses on optimizing routing for
specific application scenarios, which can largely be categorized into
several models of communication, including the following ones:
o Flooding (in very small networks)
o Hierarchical routing
o Geographic routing
o Self-organizing coordinate routing
Depending on the topology of a 6LoWPAN and the application(s) running
over it, different types of routing may be used. However, this
document abstracts from application-specific communication and
describes general routing requirements valid for overall routing in
6LoWPANs.
The following parameters can be used to describe specific scenarios
in which the candidate routing protocols could be evaluated.
a. Network Properties:
* Number of Devices, Density and Network Diameter:
These parameters usually affect the routing state directly
(e.g. the number of entries in a routing table or neighbor
list). Especially in large and dense networks, policies must
be applied for discarding "low-quality" and stale routing
entries in order to prevent memory overflow.
* Connectivity:
Due to external factors or programmed disconnections, a
6LoWPAN can be in several states of connectivity; anything in
the range from "always connected" to "rarely connected". This
poses great challenges to the dynamic discovery of routes
across a 6LoWPAN.
* Dynamicity (including mobility):
Location changes can be induced by unpredictable external
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factors or by controlled motion, which may in turn cause route
changes. Also, nodes may dynamically be introduced into a
6LoWPAN and removed from it later. The routing state and the
volume of control messages may heavily dependent on the number
of moving nodes in a LoWPAN and their speed.
* Deployment:
In a 6LoWPAN, it is possible for nodes to be scattered
randomly or to be deployed in an organized manner. The
deployment can occur at once, or as an iterative process,
which may also affect the routing state.
* Spatial Distribution of Nodes and Gateways:
Network connectivity depends on the spatial distribution of
the nodes, and on other factors like device number, density
and transmission range. For instance, nodes can be placed on
a grid, or can be randomly placed in an area (bidimensional
Poisson distribution), etc. In addition, if the 6LoWPAN is
connected to other networks through infrastructure nodes
called gateways, the number and spatial distribution of
gateways affects network congestion and available data rate,
among others.
* Traffic Patterns, Topology and Applications:
The design of a LoWPAN and the requirements on its application
have a big impact on the network topology and the most
efficient routing type to be used. For different traffic
patterns (point-to-point, multipoint-to-point, point-to-
multipoint) and network architectures, various routing
mechanisms have been introduced, such as data-aware, event-
driven, address-centric, and geographic routing.
* Classes of Service:
For mission-critical applications, support of multiple classes
of service may be required in resource-constrained LoWPANs and
may require a certain degree of routing protocol overhead.
* Security:
LoWPANs may carry sensitive information and require a high
level of security support where the availability, integrity,
and confidentiality of data are primordial. Secured messages
cause overhead and affect the power consumption of LoWPAN
routing protocols.
b. Node Parameters:
* Processing Speed and Memory Size:
These basic parameters define the maximum size of the routing
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state. LoWPAN nodes may have different performance
characteristics.
* Power Consumption and Power Source:
The number and topology of battery- and mains-powered nodes in
a LoWPAN affect routing protocols in their selection of
optimal paths for network lifetime optimization.
* Transmission Range:
This parameter affects routing. For example, a high
transmission range may cause a dense network, which in turn
results in more direct neighbors of a node, higher
connectivity and a larger routing state.
* Traffic Pattern: This parameter affects routing since high-
loaded nodes (either because they are the source of packets to
be transmitted or due to forwarding) may incur a greater
contribution to delivery delays and may consume more energy
than lightly loaded nodes. This applies to both data packets
and routing control messages.
c. Link Parameters:
This section discusses link parameters that apply to IEEE
802.15.4 legacy mode (i.e. not making use of improved schemes).
* Throughput:
The maximum user data throughput of a bulk data transmission
between a single sender and a single receiver through an
unslotted IEEE 802.15.4 2.4 GHz channel in ideal conditions is
as follows [20]:
+ 16-bit MAC addresses, unreliable mode: 151.6 kbps
+ 16-bit MAC addresses, reliable mode: 139.0 kbps
+ 64-bit MAC addresses, unreliable mode: 135.6 kbps
+ 64-bit MAC addresses, reliable mode: 124.4 kbps
In the case of 915 MHz band:
+ 16-bit MAC addresses, unreliable mode: 31.1 kbps
+ 16-bit MAC addresses, reliable mode: 28.6 kbps
+ 64-bit MAC addresses, unreliable mode: 27.8 kbps
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+ 64-bit MAC addresses, reliable mode: 25.6 kbps
In the case of 868 MHz band:
+ 16-bit MAC addresses, unreliable mode: 15.5 kbps
+ 16-bit MAC addresses, reliable mode: 14.3 kbps
+ 64-bit MAC addresses, unreliable mode: 13.9 kbps
+ 64-bit MAC addresses, reliable mode: 12.8 kbps
* Latency:
The range of latencies of a frame transmission between a
single sender and a single receiver through an unslotted IEEE
802.15.4 2.4 GHz channel in ideal conditions are as shown next
[20]. For unreliable mode, the actual latency is provided.
For reliable mode, the round-trip-time including transmission
of a layer two acknowledgment is provided:
+ 16-bit MAC addresses, unreliable mode: [1.92 ms, 6.02 ms]
+ 16-bit MAC addresses, reliable mode: [2.46 ms, 6.56 ms]
+ 64-bit MAC addresses, unreliable mode: [2.75 ms, 6.02 ms]
+ 64-bit MAC addresses, reliable mode: [3.30 ms, 6.56 ms]
In the case of 915 MHz band:
+ 16-bit MAC addresses, unreliable mode: [5.85 ms, 29.35 ms]
+ 16-bit MAC addresses, reliable mode: [8.35 ms, 31.85 ms]
+ 64-bit MAC addresses, unreliable mode: [8.95 ms, 29.35 ms]
+ 64-bit MAC addresses, reliable mode: [11.45 ms, 31.85 ms]
In the case of 868 MHz band:
+ 16-bit MAC addresses, unreliable mode: [11.7 ms, 58.7 ms]
+ 16-bit MAC addresses, reliable mode: [16.7 ms, 63.7 ms]
+ 64-bit MAC addresses, unreliable mode: [17.9 ms, 58.7 ms]
+ 64-bit MAC addresses, reliable mode: [22.9 ms, 63.7 ms]
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5. 6LoWPAN Routing Requirements
This section defines a list of requirements for 6LoWPAN routing. The
most important design property unique to low-power networks is that
6LoWPANs have to support multiple device types and roles, for
example:
o primarily battery-operated host nodes (called "power-constrained
nodes" in the following)
o mains-powered host nodes (an example for what we call "power-
affluent nodes")
o power-affluent (but not necessarily mains-powered) high-
performance gateway(s)
o nodes with various functionality (data aggregators, relays, local
manager/coordinators, etc.)
Due to these unique device types and roles LoWPANs need to consider
the following two primary attributes:
o Power conservation: some devices are mains-powered, but most are
battery-operated and need to last several months to a few years
with a single AA battery. Many devices are mains-powered most of
the time, but still need to function for possibly extended periods
from batteries (e.g. on a construction site before building power
is switched on for the first time).
o Low performance: tiny devices, small memory sizes, low-performance
processors, low bandwidth, high loss rates, etc.
These fundamental attributes of LoWPANs affect the design of routing
solutions, so that existing routing specifications should be
simplified and modified to the smallest extent possible when there
are appropriate solutions to adapt, otherwise, new solutions should
be introduced in order to fit the low-power requirements of LoWPANs,
meeting the requirements described in the following.
5.1. Support of 6LoWPAN Device Properties
The general objectives listed in this section should be followed by
6LoWPAN routing protocols. The importance of each requirement is
dependent on what node type the protocol is running on and what the
role of the node is. The following requirements are based on
battery-powered LoWPAN nodes.
[R01] 6LoWPAN routing protocols SHOULD allow to be implemented with
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small code size and require low routing state to fit the typical
6LoWPAN node capacity (e.g., code size considering its typical flash
memory size, and routing table less than 32 entries).
A 6LoWPAN routing protocol solution should consider the limited
memory size typically starting at 4KB. RAM size of LoWPAN nodes
often ranges between 2KB and 10KB, and program flash memory
normally consists of 48KB to 128KB. (e.g., in the current market,
MICAz has 128KB program flash, 4KB EEPROM, 512KB external flash
ROM; TIP700CM has 48KB program flash, 10KB RAM, 1MB external flash
ROM).
Due to these hardware restrictions, code length should be
considered to fit within a small memory size; no more than 48KB to
128KB of flash memory including at least a few tens of KB of
application code size. A routing protocol of low complexity helps
to achieve the goal of reducing power consumption, improves
robustness, requires lower routing state, is easier to analyze,
and may be implicitly less prone to security attacks.
In addition, operation with low routing state (such as routing
tables and neighbor lists) SHOULD be maintained since some typical
memory sizes preclude to store state of a large number of nodes.
For instance, industrial monitoring applications need to support
at maximum 20 hops [16]. Small networks can be designed to
support a smaller number of hops. It is highly dependent on the
network architecture, but considering the 6LoWPAN device
properties, there should be at least one mode of operation that
can function with 32 forwarding entries or less.
[R02] 6LoWPAN routing protocols SHOULD cause minimal power
consumption by the efficient use of control packets (e.g., minimize
expensive IP multicast which causes link broadcast to the entire
LoWPAN) and by the efficient routing of data packets.
One way of battery lifetime optimization is by achieving a minimal
control message overhead. Compared to functions such as
computational operations or taking sensor samples, radio
communications is by far the dominant factor of power consumption
[10]. Power consumption of transmission and/or reception depends
linearly on the length of data units and on the frequency of
transmission and reception of the data units [13].
The energy consumption of two example RF controllers for low-power
nodes is shown in [11]. The TR1000 radio consumes 21mW when
transmitting at 0.75mW, and 15mW on reception (with a receiver
sensitivity of -85dBm). The CC1000 consumes 31.6mW when
transmitting 0.75mW, and 20mW for receiving (with a receiver
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sensitivity of -105dBm). The power continuation under the concept
of an idealized power source is explained in [11]. Based on the
energy of an idealized AA battery, the CC1000 can transmit for
approximately 4 days straight or receive for 9 consecutive days.
Note that availability for reception consumes power as well.
Multicast may causes flooding in the LoWPAN. On consideration
this, 6LoWPAN routing protocol SHOULD minimize the control cost by
the routing packets. Another document discusses control cost of
routing protocols in low power and lossy networks [19].
5.2. Support of 6LoWPAN Link Properties
6LoWPAN links have the characteristics of low data rate and possibly
high loss rates. The routing requirements described in this section
are derived from the link properties.
[R03] 6LoWPAN routing protocol control messages SHOULD NOT exceed a
single IEEE 802.15.4 frame size in order to avoid packet
fragmentation and the overhead for reassembly.
In order to save energy, routing overhead should be minimized to
prevent fragmentation of frames. Therefore, 6LoWPAN routing
should not cause packets to exceed the IEEE 802.15.4 frame size.
This reduces the energy required for transmission, avoids
unnecessary waste of bandwidth, and prevents the need for packet
reassembly. As calculated in RFC4944 [4], the maximum size of a
6LoWPAN frame, in order not to cause fragmentation, is 81 octets.
This may imply the use of semantic fragmentation and/or algorithms
that can work on small increments of routing information.
[R04] The design of routing protocols for LoWPANs must consider the
fact that packets are to be delivered with sufficient probability
according to application requirements.
Requirements on successful end-to-end packet delivery ratio (where
delivery may be bounded within certain latency) vary depending on
applications. In industrial applications, some non-critical
monitoring applications may tolerate successful delivery ratio of
less than 90% with hours of latency; in some other cases, a
delivery ratio of 99.9% is required [16]. In building automation
applications, application layer errors must be below 0.01% [18].
Successful end-to-end delivery of packets in a IEEE 802.15.4 mesh
depends on the quality of the path selected by the routing
protocol and on the ability of the routing protocol to cope with
short-term and long-term quality variation. The metric of the
routing protocol strongly influences performance of the routing
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protocol in terms of delivery ratio.
The quality of a given path depends on the individual qualities of
the links (including the devices) that compose that path. IEEE
802.15.4 settings affect the quality perceived at upper layers.
In particular, in IEEE 802.15.4 reliable mode, if an
acknowledgment frame is not received after a given period, the
originator retries frame transmission up to a maximum number of
times. If an acknowledgment frame is still not received by the
sender after performing the maximum number of transmission
attempts, the MAC layer assumes the transmission has failed and
notifies the next higher layer of the failure. Note that
excessive retransmission may be detrimental, see RFC 3819 [5].
[R05] The design of routing protocols for LoWPANs must consider the
latency requirements of applications and IEEE 802.15.4 link latency
characteristics.
Latency requirements may differ from a few hundreds milliseconds
to minutes, depending on the type of application. Real-time
building automation applications usually need response times below
500 ms between egress and ingress, while forced entry security
alerts must be routed to one or more fixed or mobile user devices
within 5 seconds [18]. Non-critical closed loop applications for
industrial automation have latency requirements that can be as low
as 100 ms but many control loops are tolerant of latencies above
1s [16]. In contrast to this, urban monitoring applications allow
latencies smaller than the typical intervals used for reporting
sensed information; for instance, in the order of seconds to
minutes [17].
The range of latencies of a frame transmission between a single
sender and a single receiver through an ideal unslotted IEEE
802.15.4 2.4 GHz channel is between 2.46ms and 6.02ms in 64 bit
MAC address unreliable mode and 2.20 ms to 6.56ms in 64 bit
address reliable mode. The range of latencies of 868 MHz band is
from 11.7 ms to 63.7 ms, depending on the address type and
reliable/unreliable mode used. Note that the latencies may be
larger than that depending on channel load, MAC layer settings ,
and reliable/unreliable mode choice. Note that other MAC
approaches than the legacy 802.15.4 may be used (e.g. TDMA).
Duty cycling may further affect latency (see [R08 ]).
Note that a tradeoff exists between [R05] and [R04].
[R06] 6LoWPAN routing protocols SHOULD be robust to dynamic loss
caused by link failure or device unavailability either in short-term
(e.g. due to RSSI variation, interference variation, noise and
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asynchrony) or in long-term (e.g. due to a depleted power source,
hardware breakdown, operating system misbehavior, etc).
An important trait of 6LoWPAN 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. In harsh environments, LoWPANs easily suffer from link
failure. Collision or link failure easily increases Send Queue/
Receive Queue (SQ/RQ) and it can lead to queue overflow and packet
losses.
For home applications, where users expect feedback after carrying
out actions (such as handling a remote control while moving
around), routing protocols must converge within 2 seconds if the
destination node of the packet has moved and must converge within
0.5 seconds if only the sender has moved [15]. The tolerance of
the recovery time can vary depending on the application, however,
the routing protocol must provide the detection of short-term
unavailability and long-term disappearance. The routing protocol
has to exploit network resources (e.g. path redundancy) to offer
good network behavior despite of node failure.
[R07] 6LoWPAN routing protocols SHOULD be designed to correctly
operate in the presence of link asymmetry.
Link asymmetry occurs when the probability of successful
transmission between two nodes is significantly higher in one
direction than in the other one. This phenomenon has been
reported in a large number of experimental studies and it is
expected that 6LoWPANs will exhibit link asymmetry.
5.3. Support of 6LoWPAN Network Characteristics
6LoWPANs can be deployed in different sizes and topologies, adhere to
various models of mobility, tolerate various levels of interference,
etc. In any case, LoWPANs must maintain low energy consumption. The
requirements described in the following subsection are derived from
the network attributes of 6LoWPANs.
[R08] 6LoWPAN routing protocols SHOULD be reliable despite
unresponsive nodes due to periodic hibernation.
Many nodes in LoWPAN environments might periodically hibernate
(i.e. disable their transceiver activity) in order to save energy.
Therefore, routing protocols must ensure robust packet delivery
despite nodes frequently shutting off their radio transmission
interface. Feedback from the lower IEEE 802.15.4 layer may be
considered to enhance the power-awareness of 6LoWPAN routing
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protocols.
CC1000-based nodes must operate at a duty cycle of approximately
2% to survive for one year from idealized AA battery power source
[11]. For home automation purposes, it is suggested that that the
devices have to maximize the sleep phase with a duty cycle lower
than 1% [15], while in building automation applications, batteries
must be operational for at least 5 years when the sensing devices
are transmitting data (e.g. 64 bytes) once per minute [18].
Dependent on the application in use, packet rates differ from
1/sec to 1/day. Routing protocols need to know the cycle of the
packet transmission and utilize the information to calculate
routing paths.
[R09] The metric used by 6LoWPAN routing protocols MAY utilize a
combination of the inputs provided by the lower layers and other
measures to obtain the optimal path considering energy balance and
link qualities.
In homes, buildings, or infrastructure, some nodes will be
installed with mains power. Such power-installed nodes MUST be
considered as a relay points for more roles in packet delivery.
6LoWPAN routing protocols MUST know the power constraints of the
nodes.
Simple hop-count-only mechanisms may be inefficient in 6LoWPANs.
There is a Link Quality Indication (LQI), or/and RSSI from IEEE
802.15.4 that may be taken into account for better metrics. The
metric to be used (and its goal) may depend on applications and
requirements.
The numbers in Figure 8 represent the Link Delivery Ratio (LDR) of
each pair of nodes. There are studies that show a piecewise
linear dependence between LQI and LDR [14].
0.6
A-------C
\ /
0.9 \ / 0.9
\ /
B
Figure 8: An example network
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In this simple example, there are two options in routing from node
A to node C, with the following features:
A. Path AC:
+ (1/0.6) = 1.67 avg. transmissions needed for each packet
+ one-hop path
+ good in energy consumption and end-to-end latency of data
packets, bad in delivery ratio (0.6)
+ bad in probability of route reconfigurations
B. Path ABC
+ 2*(1/0.81) = 2.47 avg. transmissions needed for each packet
+ two-hop path
+ bad in energy consumption and end-to-end latency of data
packets, good in delivery ratio (0.81)
If energy consumption of the network must be minimized, path AC is
the best (this path would be chosen based on a hop count metric).
However, if the delivery ratio in that case is not sufficient, the
best path is ABC (it would be chosen by an LQI based metric).
Combinations of both metrics can be used.
The metric also affects the probability of route reconfiguration.
Route reconfiguration, which may be triggered by packet losses,
may require transmission of routing protocol messages. It is
possible to use a metric aimed at selecting the path with low
route reconfiguration rate by using LQI as an input to the metric.
Such a path has good properties, including stability and low
control message overhead.
[R10] 6LoWPAN routing protocols SHOULD be designed to achieve both
scalability from a few nodes to millions of nodes and minimality in
terms of used system resources.
A LoWPAN may consist of just a couple of nodes (for instance in a
body-area network), but may expand to much higher numbers of
devices (e.g. monitoring of a city infrastructure or a highway).
For home automation applications it is envisioned that the routing
protocol must support 250 devices in the network [15], while
routing protocols for metropolitan-scale sensor networks must be
capable of clustering a large number of sensing nodes into regions
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containing on the order of 10^2 to 10^4 sensing nodes each [17].
It is therefore necessary that routing mechanisms are designed to
be scalable for operation in various network sizes. However, due
to a lack of memory size and computational power, 6LoWPAN routing
might limit forwarding entries to a small number, such as at
maximum 32 routing table entries.
[R11] The procedure of route repair and related control messages
should not harm overall energy consumption from the routing
protocols.
Local repair improves throughput and end-to-end latency,
especially in large networks. Since routes are repaired quickly,
fewer data packets are dropped, and a smaller number of routing
protocol packet transmissions are needed since routes can be
repaired without source initiated Route Discovery [12]. One
important consideration here may be to avoid premature depletion,
even in case that impairs other requirements.
[R12] 6LoWPAN routing protocols SHOULD allow for dynamically adaptive
topologies and mobile nodes. When supporting dynamic topologies and
mobile nodes, route maintenance should keep in mind the goal of a
minimal routing state and routing protocol message overhead.
Building monitoring applications, for instance, require that the
mobile devices SHOULD be capable of leaving (handing-off) from an
old network joining onto a new network within 15 seconds [18].
More interactive applications such as used in home automation
systems, where users are giving input and expect instant feedback,
mobility requirements are also stricter and a convergence time
below 0.5 seconds is commonly required [15]. In industrial
environments, where mobile equipment such as cranes move around,
the support of vehicular speeds of up to 35 km/h are required to
be supported by the routing protocol [16]. Currently, 6LoWPANs
are not being used for such a fast mobility, but dynamic
association and disassociation MUST be supported in 6LoWPAN.
There are several challenges that should be addressed by a 6LoWPAN
routing protocol in order to create robust routing in dynamic
environments:
* Mobile nodes changing their location inside a LoWPAN:
If the nodes' movement pattern is unknown, mobility cannot
easily be detected or distinguished by the routing protocols.
Mobile nodes can be treated as nodes that disappear and re-
appear in another place. Movement pattern tracking increases
complexity and can be avoided by handling moving nodes using
reactive route updates.
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* Movement of a LoWPAN with respect to other (inter)connected
LoWPANs:
Within stub networks, more powerful gateway nodes need to be
configured to handle moving LoWPANs.
* Nodes permanently joining or leaving the LoWPAN:
In order to ease routing table updates, reduce their size, and
minimize error control messages, nodes leaving the network may
announce their disassociation to the closest edge router or if
any, to a specific node which takes charge of local association
and disassociation.
[R13] 6LoWPAN routing protocol SHOULD support various traffic
patterns; point-to-point, point-to-multipoint, and multipoint-to-
point, while avoid excessive multicast traffic in a LoWPAN.
6LoWPANs often have point-to-multipoint or multipoint-to-point
traffic patterns. Many emerging applications include point-to-
point communication as well. 6LoWPAN routing protocols should be
designed with the consideration of forwarding packets from/to
multiple sources/destinations. Current WG drafts in the ROLL
working group explain that the workload or traffic pattern of use
cases for LoWPANs tend to be highly structured, unlike the any-to-
any data transfers that dominate typical client and server
workloads. In many cases, exploiting such structure may simplify
difficult problems arising from resource constraints or variation
in connectivity.
5.4. Support of Security
The routing requirement described in this subsection allows secure
transmission of routing messages. Solutions may take into account
the specific features of IEEE 802.15.4 MAC layers.
[R14] 6LoWPAN protocols SHOULD support secure delivery of control
messages. A minimal security level can be achieved by utilizing AES-
based mechanism provided by IEEE 802.15.4.
Security threats within LoWPANs may be different from existing
threat models in ad-hoc network environments. Neighbor Discovery
in IEEE 802.15.4 links may be susceptible to threats as listed in
RFC3756 [2]. Bootstrapping may also impose additional threats.
Security is also very important for designing robust routing
protocols, but it should not cause significant transmission
overhead. While there are applications which require very high
security, such as in traffic control, other applications are less
easily harmed by wrong node behavior, such as a home entertainment
system.
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The IEEE 802.15.4 MAC provides an AES-based security mechanism.
Routing protocols need to define how this mechanism can be used to
obtain the intended security. Byte overhead of the mechanism,
which depends on the security services selected, must be
considered. In the worst case in terms of overhead, the mechanism
consumes 21 bytes of MAC payload.
IEEE 802.15.4 does not specify protection for acknowledgement
frames. Since the sequence numbers of data frames are sent in the
clear, an adversary can forge an acknowledgement for each data
frame. This weakness can be combined with targeted jamming to
prevent delivery of selected packets. In consequence, IEEE
802.15.4 acknowledgements cannot be relied upon. In applications
that require high security, the routing protocol must not exploit
feedback from acknowledgements (e.g. to keep track of neighbor
connectivity, see [R16]).
5.5. Support of Mesh-under Forwarding
One LoWPAN may be built as one IPv6 link. In this case, Mesh Under
forwarding/routing mechanisms must be supported. The routing
requirements described in this subsection allow optimization and
correct operation of routing solutions taking into account the
specific features of the mesh-under configuration.
[R15] In case a routing protocol operates in 6LoWPAN's adaptation
layer, routing tables and neighbor lists MUST support 16-bit short
and 64-bit extended addresses.
[R16] In order to perform discovery and maintenance of neighbors,
LoWPAN Nodes SHOULD avoid sending "Hello" messages of NS, NA, RS or
RA messages. Instead, link-layer mechanisms (such as
acknowledgments) MAY be utilized to keep track of active neighbors.
Reception of an acknowledgement after a frame transmission may
render unnecessary the transmission of explicit Hello messages,
for example. In a more general view, any frame received by a node
may be used as an input to evaluate the connectivity between the
sender and receiver of that frame.
[R17] In case there are one or more nodes allocated for the specific
role of local management, the nodes MAY take the role of keeping
track of node association and de-association within the charging area
of the LoWPAN.
[R18] If the routing protocol functionality includes enabling IP
multicast, then it may want to employ relay points of group-targeting
messages instead of using link-layer multicast (broadcast).
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6. Security Considerations
Security issues are described in Section 4.4. Security
considerations of RFC 4919 [3] and RFC 4944 [4] apply as well. More
security considerations will result from the 6LoWPAN security
analysis work.
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7. Acknowledgements
The authors thank Myung-Ki Shin for giving the idea of writing this
draft. The authors also thank to S. Chakrabarti who gave valuable
comments for mesh-under requirements.
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8. References
8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756, May 2004.
[3] 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.
[4] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4 Networks",
RFC 4944, September 2007.
[5] Karn, P., Bormann, C., Fairhurst, G., Grossman, D., Ludwig, R.,
Mahdavi, J., Montenegro, G., Touch, J., and L. Wood, "Advice
for Internet Subnetwork Designers", BCP 89, RFC 3819,
July 2004.
[6] IEEE Computer Society, "IEEE Std. 802.15.4-2006 (as amended)",
2007.
8.2. Informative References
[7] Bulusu, N. and S. Jha, "Wireless Sensor Networks", July 2005.
[8] Shelby, Z., Thubert, P., Hui, J., Chakrabarti, S., and E.
Nordmark, "LoWPAN Neighbor Discovery Extensions,
draft-ietf-6lowpan-nd-01 (work in progress)", February 2009.
[9] Hui, J. and P. Thubert, "Compression Format for IPv6 Datagrams
in 6LoWPAN Networks, draft-ietf-6lowpan-hc-04 (work in
progress)", December 2008.
[10] Pister, K. and B. Boser, "Smart Dust: Wireless Networks of
Millimeter-Scale Sensor Nodes".
[11] Hill, J., "System Architecture for Wireless Sensor Networks".
[12] Lee, S., Belding-Royer, E., and C. Perkins, "Scalability Study
of the Ad Hoc On-Demand Distance-Vector Routing Protocol",
March 2003.
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[13] Shih, E., "Physical Layer Driven Protocols and Algorithm Design
for Energy-Efficient Wireless Sensor Networks", July 2001.
[14] Chen, B., Muniswamy-Reddy, K., and M. Welsh, "Ad-Hoc Multicast
Routing on Resource-Limited Sensor Nodes", 2006.
[15] Brandt, A., Buron, J., and G. Porcu, "Home Automation Routing
Requirement in Low Power and Lossy Networks,
draft-ietf-roll-home-routing-reqs-06 (work in progress)",
November 2008.
[16] Pister, K., Thubert, P., Dwars, S., and T. Phinney, "Industrial
Routing Requirements in Low Power and Lossy Networks,
draft-ietf-roll-indus-routing-reqs-04 (work in progress)",
January 2009.
[17] Dohler, M., Watteyne, T., Winter, T., Barthel, D., and C.
Jacquenet, "Urban WSNs Routing Requirements in Low Power and
Lossy Networks, draft-ietf-roll-urban-routing-reqs-03 (work in
progress)", January 2009.
[18] Martocci, J., De Mil, P., Vermeylen, W., and N. Riou, "Building
Automation Routing Requirements in Low Power and Lossy
Networks, draft-ietf-roll-building-routing-reqs-05 (work in
progress)".
[19] Levis, P., Tavakoli, A., and S. Dawson-Haggerty, "Overview of
Existing Routing Protocols for Low Power and Lossy Networks ,
draft-ietf-roll-protocols-survey-06 (work in progress)".
[20] Latre, M., De Mil, P., Moerman, I., Dhoedt, B., and P.
Demeester, "Throughput and Delay Analysis of Unslotted IEEE
802.15.4", May 2006.
[21] Lu, J., Valois, F., Dohler, M., and D. Barthel, "Quantifying
Organization by Means of Entropy", 2008.
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Authors' Addresses
Eunsook Eunah Kim
ETRI
161 Gajeong-dong
Yuseong-gu
Daejeon 305-700
Korea
Phone: +82-42-860-6124
Email: eunah.ietf@gmail.com
Dominik Kaspar
Simula Research Laboratory
Martin Linges v 17
Snaroya 1367
Norway
Phone: +47-6782-8223
Email: dokaspar.ietf@gmail.com
Carles Gomez
Tech. Univ. of Catalonia/i2CAT
Escola Politecnica Superior de Castelldefels
Avda. del Canal Olimpic, 15
Castelldefels 08860
Spain
Phone: +34-93-413-7206
Email: carlesgo@entel.upc.edu
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Fax: +49-421-218-7000
Email: cabo@tzi.org
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