6LoWPAN Working Group E. Kim
Internet-Draft ETRI
Expires: January 15, 2009 D. Kaspar
Simula Research Laboratory
C. Gomez
Technical University of
Catalonia/i2CAT
C. Bormann
Universitaet Bremen TZI
July 14, 2008
Problem Statement and Requirements for 6LoWPAN Mesh Routing
draft-dokaspar-6lowpan-routreq-06
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on January 15, 2009.
Kim, et al. Expires January 15, 2009 [Page 1]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
Abstract
This document provides the problem statement for 6LoWPAN mesh
routing. It also defines the requirements for 6LoWPAN mesh routing
considering the low-power characteristics of the network and its
devices.
Table of Contents
1. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 3
2. Design Space . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Scenario Considerations and Parameters for 6LoWPAN Routing . . 8
4. 6LoWPAN Routing Requirements . . . . . . . . . . . . . . . . . 12
4.1. Routing Requirements depending on the 6LoWPAN Device
Properties . . . . . . . . . . . . . . . . . . . . . . . . 12
4.2. Routing Requirements depending on Types of 6LoWPAN
Applications . . . . . . . . . . . . . . . . . . . . . . . 15
4.3. MAC-coupled Requirements . . . . . . . . . . . . . . . . . 17
4.4. Mesh-under specific Requirements . . . . . . . . . . . . . 19
4.5. Route-over specific Requirements . . . . . . . . . . . . . 19
5. Security Considerations . . . . . . . . . . . . . . . . . . . 21
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
7.1. Normative References . . . . . . . . . . . . . . . . . . . 23
7.2. Informative References . . . . . . . . . . . . . . . . . . 23
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
Intellectual Property and Copyright Statements . . . . . . . . . . 26
Kim, et al. Expires January 15, 2009 [Page 2]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
1. Problem Statement
Low-power wireless personal area networks (LoWPANs) are formed by
devices complying to the IEEE 802.15.4 standard [5][6]. 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
[3].
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 and accomplish more advanced functions,
such as multi-hop routing.
However, neither the IEEE 802.15.4 standard nor the 6LoWPAN format
specification ("IPv6 over IEEE 802.15.4" [4]) specify how mesh
topologies could be obtained and maintained. Thus, the 6LoWPAN
formation and multi-hop routing should be supported by higher layer,
either 6LoWPAN adaptation layer or IP layer. In the IETF, a number
of experimental protocols in IP layer have been developed in many
working groups. However, these existing routing protocols may not be
satisfying for mesh routing in a LoWPAN domain, for the following
reasons:
o 6LoWPAN nodes have special types and roles, such as primary
battery-operated RFDs, battery-operated and mains-powered FFDs,
possibly various levels of RFDs and FFDs, mains-powered and high-
performance gateways, data aggregators, etc. 6LoWPAN routing
protocols should support multiple device types and roles.
o The more stringent requirements that apply to 6LoWPANs 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 node's lifetime is usually defined
by the lifetime of its battery.
o Handling sleeping nodes is very critical in 6LoWPANs, more than in
traditional ad-hoc networks. 6LoWPAN nodes might stay in sleep-
mode for most of the time. Time synchronization is important for
efficient forwarding of packets.
o A possibly simpler routing problem; 6LoWPANs might be either
transit-networks or stub-networks. We can focus on stub networks
Kim, et al. Expires January 15, 2009 [Page 3]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
first as existing 6LoWPAN documents (implicitly or explicitly)
show the 6LoWPAN picture under the configuration of stub networks
at this moment [4], [8]. We can simplify networks by an
assumption of no transit networks. (based on the necessity, we may
extend the network configuration including transit network case)
o A possibly harder routing problem; routing in 6LoWPANs requires to
consider power-optimization, harsh environment, data-aware
routing, etc. These are not easy requirements to satisfy
together.
This creates new challenges on obtaining robust and reliable mesh
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 only describe the need for low
overhead and power saving. But, based on the fundamental features of
LoWPAN, more detailed routing requirements are presented in this
document, which can lead to further analysis and protocol design.
Using the 6LoWPAN header format[6], 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,
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 to both the mesh-under and
route-over cases.
In summary, the main issues of mesh routing in 6LoWPANs are:
1. The precise 6LoWPAN routing requirements must be defined.
2. If a routing protocol for 6LoWPAN is designed to run in the IP
layer, it should meet the 6LoWPAN-specific requirements. It
Kim, et al. Expires January 15, 2009 [Page 4]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
needs to be clarified how existing routing solutions can be
adapted to meet the low-power requirements presented in
Section 4.
[Note] ROLL WG is now working on the protocol survey for Low
power and Lossy Networks (LLNs), not specifically for 6LoWPAN.
It will decide whether new solution will be developed or not,
after that survey. This document is focused on 6LoWPAN specific
requirements, in alignment with ROLL WG.
3. If a routing procotol for 6LoWPAN within a subnet is designed to
run in adaptation layer, existing routing solutions do not
operate in 6LoWPAN's adaptation layer (and do not support the
addressing scheme defined by IEEE 802.15.4).
Considering the problems above, this draft addresses routing
requirements for 6LoWPANs for both mesh-under and route-over routing
protocol design.
Application-specific features affect the design of 6lowpan routing
requirements and the corresponding solutions. However, various
applications can be profiled by similar technical characteristics.
This document states the requirements to consider the features of
6LoWPAN applications in general. However, it does not mean that one
single routing solution may be the best one for all 6LoWPAN
applications.
Kim, et al. Expires January 15, 2009 [Page 5]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
2. 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 in the IP-layer using a route-over approach. The most
significant consequence of mesh-under routing is that routing would
be directly based on the IEEE 802.15.4 standard, therefore using (64-
bit or 16-bit short) MAC addresses instead of IP addresses, and a
LoWPAN would be seen as a single IP link. In case a route-over
mechanism is to be applied to a LoWPAN it must also support 6LoWPAN's
unique properties using global IPv6 addressing.
Additionally, because of the low-performance characteristics of
LoWPANs, a light-weight routing protocol must be provided that meets
the design goals and requirements presented in this document.
Figure 1 shows the place of 6LoWPAN mesh 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.
If a mesh-under routing protocol is built for operation in 6LoWPAN's
adaptation layer, routing control packets are placed after the
6LoWPAN Dispatch, unless a new code type is assigned for mesh-under
routing. Multiple routing protocols can be supported by the usage of
different Dispatch bit sequences. When a route-over protocol is
Kim, et al. Expires January 15, 2009 [Page 6]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
built in the IPv6 layer, the Dispatch value can be chosen as one of
the Dispatch patterns for 6LoWPAN compressed or uncompressed IPv6,
followed by the IPv6 header.
If a 6LoWPAN is formed like the Figure 2 , the PnC is the only IPv6
router in the LoWPAN in the assumption of [8]. The mesh-under
routing mechanism MUST be provided to forward packets which require
multi-hop forwarding. If route-over routing is used in the stub-
network, not only the PnC but also other FFDs need to set up IP paths
for multi-hop transmission.
O X
| | PnC: PAN Coordinator
PnC --- O --- O --- X C: Coordinator
/ \ O: FFD
X C--X X: RFD
|
/ \
O---O--X
Figure 2: An example of a 6LoWPAN
If multiple 6LoPWANs are formed with globally unique IPv6 addresses
in the 6LoWPANs, and node (a) of 6LoWPAN [A] wants to communicate
with node (b) of 6LoWPAN [B], the PnC (= IPv6 router) will be always
the default router for the outgoing packet of the 6LoWPAN.
Kim, et al. Expires January 15, 2009 [Page 7]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
3. Scenario Considerations and Parameters for 6LoWPAN Routing
IP-based 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. Research efforts in the area of sensor networking
have put forth a large variety of multi-hop routing algorithms [7].
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 Data-aware routing (dissemination vs. gathering)
o Event-driven vs. query-based routing
o Geographic routing
o Probabilistic routing
o Hierarchical 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 any type of 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:
* Device Number, 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
Kim, et al. Expires January 15, 2009 [Page 8]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
across a LoWPAN.
* Dynamicity (include mobility):
Location changes can be induced by unpredictable external
factors or by controlled motion, which may in turn cause route
changes. Also, nodes may dynamically be introduced into a
LoWPAN and removed from it later. The routing state and the
volume of control messages is heavily dependent on the number
of moving nodes in a LoWPAN and their speed.
* Deployment:
In a LoWPAN, 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 node spatial distribution
besides 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 LoWPAN is
connected to other networks through infrastructure nodes
called gateways, the number and spatial distribution of
gateways affects network congestion and available bandwidth,
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.
* Quality of Service (QoS):
For mission-critical applications, support of QoS is mandatory
in resource-constrained LoWPANs and cannot be achieved without
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.
Kim, et al. Expires January 15, 2009 [Page 9]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
b. Node Parameters:
* Processing Speed and Memory Size:
These basic parameters define the maximum size of the routing
state. LoWPAN nodes may have different performance
characteristics beyond the common RFD/FFD distinction.
* 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 maximization.
* 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 than low-loaded nodes. This
applies to both data packets and routing control messages
themselves.
c. Link Parameters:
* 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 [17]:
+ 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
Kim, et al. Expires January 15, 2009 [Page 10]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
+ 64-bit MAC addresses, unreliable mode: 27.8 kbps
+ 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 follows
[20]:
+ 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, 31.82 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]
Kim, et al. Expires January 15, 2009 [Page 11]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
4. 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 support multiple device types and roles, for example:
o primary battery-operated RFDs
o battery-operated and mains-powered FFDs
o possibly various levels of RFDs and FFDs
o mains-powered, high-performance gateway(s)
o data aggregators, etc.
Due to these unique device types and roles 6LoWPANs need to consider
the following two primary features:
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 possible 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 features of LoWPANs affect the design of routing
solutions, so that existing routing specifications should be
simplified and modified to the smallest extent possible, in order to
fit the low-power requirements of LoWPANs, meeting the following
requirements:
4.1. Routing Requirements depending on the 6LoWPAN Device Properties
The general objectives listed in this subsection should be followed
by 6LoWPAN routing protocols. The importance of each requirement is
dependent on what device type the protocol is running on and what the
role of the device is.
[R01] A 6LoWPAN routing protocol SHOULD be designed to minimize the
required computational and algorithmical complexity.
The lifetime of a wireless sensor node depends on the energy it
can store and harvest. Saving energy is crucially important to
6LoWPAN devices that are not mains-powered. However, one major
Kim, et al. Expires January 15, 2009 [Page 12]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
factor of power consumption in 6LoWPAN nodes is caused by the
microcontroller. Typical low power sensor nodes have 8 or 16 bit
microcontrollers. They normally consume between 0.250 mA and 2.5
mA per MHz [10]. Low power microcontrollers can have a
significant impact on system performance. A routing protocol of
low complexity helps to achieve the goal of reducing power
consumption, improves robustness, requires lower routing state, is
more easy to analyze, and is implicitly less prone to security
attacks.
See also Section 4.2 in [14] and Section 6.2 in [16].
[R02] 6LoWPAN routing protocols SHOULD have a low routing state to
fit the typical 6LoWPAN node capacity.
Typical RAM size of 6LoWPAN nodes 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). 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 example, devices may have only 32 forwarding entries
available. A LoWPAN routing protocol solution should consider the
limited memory size typically starting at 4KB, in which it is hard
to store neighbor state of hundreds of nodes) and computation
capabilities of participating devices; due to these hardware
restrictions, code length should be considered to fit within a
small memory size. In addition, it should consider that flash
writing/reading is energy expensive, while power consumption for
RAM is may be comparably negligible. For instance, in MICA motes,
writing and reading consumes 1.1 nAh/byte and 83.3 nAh/byte,
respectively.
See also Section 4.2 in [14], Section 5 in [15] and Section 6.2 in
[16].
[R03] 6LoWPAN routing protocols SHOULD cause minimal power
consumption, both in the efficient use of control packets and also in
the process of packet forwarding after route establishment.
Routing protocol design for 6LoWPAN should consider IEEE 802.15.4
link layer feedback on energy consumption. Power-aware routing is
a non-trivial task, because it is affected by many mutually
conflicting goals:
Kim, et al. Expires January 15, 2009 [Page 13]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
* Minimization of total energy consumed in the network
* Maximization of the time until a network partition occurs
* Minimizing the energy variance at each node
* Minimizing the cost per packet
while keeping packet delivery ratio, latency or other requirements
depending on each application.
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 [9]. 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 [12]
In [10] the energy consumption of two example RF controllers for
low-power nodes is shown. The TR1000 radio consumes: 21mW on
transmitting at 0.75mW; 15mW on reception (a receiver sensitivity
of -85dBm). The CC1000 consumes: 31.6mW on transmitting 0.75mW;
20mW on reception (a receiver sensitivity of -105dBm). [10]
explains the power continuation under the concept of an idealized
power source: the CC1000 can transmit for approximately 4 days
straight or receive for 9 days straight. The CC1000 must operate
at a duty cycle of approximately 2% to survive for one year.
See also Section 4.2 in [14].
[R04] 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 is needed since routes can be
repaired without source initiated Route Discovery [11].
[R05] Neighbor discovery for 6LoWPAN routing SHOULD be energy-
efficient.
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
Kim, et al. Expires January 15, 2009 [Page 14]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
a node is a neighbor of another or not. Mesh-under neighbor
discovery for 6LoWPANs is currently still in progress and
described more detailed in [8].
[R06] 6LoWPAN routing protocols SHOULD be reliable despite
unresponsive nodes due to periodic hibernation.
Many nodes in 6LoWPAN environments might periodically hibernate
(i.e. disable their transceiver activity) in order to save energy.
Therefore, mesh routing protocols must ensure robust packet
delivery despite nodes frequently shutting off their radio
transmission interface. Feedback, for instance from periodic
beacons, from the lower IEEE 802.15.4 layer may be considered to
enhance the power-awareness of 6LoWPAN routing protocols.
See also Section 4.2 in [14].
4.2. Routing Requirements depending on Types of 6LoWPAN Applications
The routing requirements described in this subsection are heavily
dependent on application needs.
[R07] 6LoWPAN routing protocol SHOULD support various traffic
patterns; point-to-point, point-to-multipoint, and multipoint-to-
point, while avoid excessive multicast traffic (broadcast in Link) in
6LoWPAN.
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 6LoWPANs 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.
See also Section 5 in [14], Section 2.1 in [15] and Section 5 in
[16].
[R08] 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
asynchrony) or in long-term (e.g. due to a depleted power source,
hardware breakdown, operating system misbehavior, etc).
Kim, et al. Expires January 15, 2009 [Page 15]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
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.
The routing protocol has to exploit network resources (e.g. path
redundancy) to offer good network behavior despite node failure.
For instance, in case of structural health monitoring, hundreds of
nodes could be placed on 20 bridge pillars spanning over a river.
Once the nodes are distributed, they are hardly accessible for
maintenance and in case of single node failure, the network should
not break down.
The design of routing protocols for 6LoWPANs must consider the
fact that packets are to be delivered with reasonable probability
despite unreliable and unresponsive nodes.
See also Section 4 in [15].
[R09] 6LoWPAN routing protocols SHOULD allow for dynamically adaptive
topologies and mobile nodes. When supporting dynamic topologies and
mobile nodes, route maintenance should be managed by keeping in mind
the goal of a minimal routing state.
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 6LoWPAN:
If the nodes' movement pattern is unknown, mobility cannot
easily be detected or distinguished from 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.
* Movement of a 6LoWPAN with respect to other (inter)connected
6LoWPANs:
Within stub networks, more powerful gateway nodes need to be
configured to handle moving 6LoWPANs.
* Nodes permanently joining or leaving the 6LoWPAN:
In order to ease routing table updates and reduce error control
messages, it would be helpful if nodes leaving the network
inform their coordinator about their intention to disassociate.
Kim, et al. Expires January 15, 2009 [Page 16]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
* See also Section 6.7 in [14], Section 8 in [15] and Section 4.3
in [16].
[R10] 6LoWPAN routing protocols SHOULD be designed to achieve both
scalability and minimality in terms of used system resources.
A 6LoWPAN 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).
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 32
routing table entries.
See also Section 4.5 in [14] and Section 6.1 in [16].
4.3. MAC-coupled Requirements
The routing requirements described in this subsection allow
optimization and correct operation of routing solutions taking into
account the specific features of IEEE 802.15.4 physical and MAC
layers.
[R11] 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.
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.
[R12] 6LoWPAN routing protocol control messages SHOULD not create
fragmentation of physical layer (PHY) frames.
Kim, et al. Expires January 15, 2009 [Page 17]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
In order to save energy, routing overhead should be minimized to
prevent fragmentation of frames on the physical layer (PHY).
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 on the PHY layer, is 81 octets.
[R13] The metric used by 6LoWPAN routing protocols MAY utilize a
combination of the inputs provided by the MAC layer and other
measures
Simple hop-count-only mechanisms may be inefficient in 6LoWPANs.
There is a Link Quality Indicator (LQI), Link Delivery Ratio
(LDR), 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 application and requirements.
The numbers in Figure 3 represent the Link Delivery Ratio (LDR)
between each pair of nodes. There are studies that show a
piecewise linear dependence between LQI and LDR [13].
0.6
A-------C
\ /
0.9 \ / 0.9
\ /
B
Figure 3: An example network
In this simple example, there are two options in routing from node
A to node C:
A. Path AC:
+ (1/0.6) = 1.67 avg. transmissions needed for each packet
+ one-hop path
+ good in energy consumption, bad in delivery ratio (0.6)
B. Path ABC
+ 2*(1/0.81) = 2.47 avg. transmissions needed for each packet
Kim, et al. Expires January 15, 2009 [Page 18]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
+ two-hop path
+ bad in energy consumption, 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 by hop count metric).
However, if delivery ratio in that case is not sufficient, best
path is ABC (it would be chosen by an LQI based metric).
Combinations of both of metrics can be used.
4.4. Mesh-under specific Requirements
The routing requirements described in this subsection allow
optimization and correct operation of routing solutions taking into
account the specific features mesh-under routing.
[R14] In case a routing protocol operates in 6LoWPAN's adaptation
layer, then routing tables and neighbor lists MUST support 16-bit
short and 64-bit extended addresses.
[R15] For neighbor discovery, 6LoWPAN devices SHOULD avoid sending
"Hello" messages. Instead, link-layer mechanisms (such as
acknowledgments or beacon responses) MAY be utilized to keep track of
active neighbors.
After an IEEE 802.15.4 PAN coordinator permits a device to join,
the new device adds the PAN coordinator to its neighbor list and
starts transmitting periodic beacons. These beacons can be used
as an indication of current neighbors.
[R16] In case there are one or more alternative PAN coordinators, the
coordinators MAY take the role of keeping track of node association
and de-association within the LoWPAN.
[R17] Alternative PAN coordinators, if any, MAY be a relay point of
group-targeting message instead of using multicast (broadcast in the
link layer).
For example, RS and RA can be only sent to the coordinators,
instead of being multicast. The coordinators take the role to
pass the packets to their own neighbors.
4.5. Route-over specific Requirements
The routing requirements described in this subsection allow
optimization and correct operation of routing solutions taking into
account the specific features of route-over routing.
Kim, et al. Expires January 15, 2009 [Page 19]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
[R18] In a mesh topology, 6LoWPAN network formation MUST support to
build IP routing connection.
[R19] IP multicast SHOULD be optimized.
In case route-over protocols is applied to 6LoWPAN, excessive IP
multicast should be avoided, as it causes not-necessary
broadcasting in Link of 6LoWPANs. For example, Multicast
(broadcast in Link layer) messages will consume a lot of energy if
IPv6 ND cannot be optimized well to suit for 6LoWPAN device.
Kim, et al. Expires January 15, 2009 [Page 20]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
5. Security Considerations
Security issues are described in Section 4.2
Kim, et al. Expires January 15, 2009 [Page 21]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
6. Acknowledgements
The authors would like to thank Myung-Ki Shin for giving the idea of
writing this draft.
Kim, et al. Expires January 15, 2009 [Page 22]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
7. References
7.1. 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] IEEE Computer Society, "IEEE Std. 802.15.4-2003", October 2003.
[6] IEEE Computer Society, "IEEE Std. 802.15.4-2006",
September 2006.
7.2. Informative References
[7] Bulusu, N. and S. Jha, "Wireless Sensor Networks", July 2005.
[8] Chakrabarti, S. and E. Nordmark, "LoWPAN Neighbor Discovery
Extensions, draft-chakrabarti-6lowpan-ipv6-nd-04 (work in
progress)", November 2007.
[9] Pister, K. and B. Boser, "Smart Dust: Wireless Networks of
Millimeter-Scale Sensor Nodes".
[10] Hill, J., "System Architecture for Wireless Sensor Networks".
[11] Lee, S., Belding-Royer, E., and C. Perkins, "Scalability Study
of the Ad Hoc On-Demand Distance-Vector Routing Protocol",
March 2003.
[12] Shih, E., "Physical Layer Driven Protocols and Algorithm Design
for Energy-Efficient Wireless Sensor Networks", July 2001.
[13] Chen, B., Muniswamy-Reddy, K., and M. Welsh, "Ad-Hoc Multicast
Routing on Resource-Limited Sensor Nodes", 2006.
[14] Brandit, A. and G. Porcu, "Home Automation Routing Requirement
Kim, et al. Expires January 15, 2009 [Page 23]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
in Low Power and Lossy Networks,
draft-ietf-roll-home-routing-reqs-01 (work in progress)",
July 2008.
[15] Pister, K., Thubert, P., Dwars, S., and T. Phinney, "Industrial
Routing Requirements in Low Power and Lossy Networks,
draft-ietf-roll-indus-routing-reqs-01 (work in progress)",
July 2008.
[16] Dohler, M., Watteyne, T., and T. Winter, "Urban WSNs Routing
Requirements in Low Power and Lossy Networks,
draft-ietf-roll-urban-routing-reqs-01 (work in progress)",
July 2008.
[17] Latre, M., De Mil, P., Moerman, I., Dhoedt, B., and P.
Demeester, "Throughput and Delay Analysis of Unslotted IEEE
802.15.4", May 2006.
Kim, et al. Expires January 15, 2009 [Page 24]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
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
Technical University 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
Kim, et al. Expires January 15, 2009 [Page 25]
Internet-Draft 6LoWPAN Mesh Routing Requirements July 2008
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Kim, et al. Expires January 15, 2009 [Page 26]