TOC 
6LoWPAN Working GroupE. Kim
Internet-DraftETRI
Expires: May 7, 2009D. Kaspar
 Simula Research Laboratory
 C. Gomez
 Tech. Univ. of Catalonia/i2CAT
 C. Bormann
 Universität Bremen TZI
 November 03, 2008


Problem Statement and Requirements for 6LoWPAN Routing
draft-dokaspar-6lowpan-routreq-07

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Abstract

This document provides the problem statement for 6LoWPAN routing. It also defines the requirements for 6LoWPAN routing considering the low-power characteristics of the network and its devices.


Table of Contents

1.  Problem Statement
2.  Design Space
3.  Scenario Considerations and Parameters for 6LoWPAN Routing
4.  6LoWPAN Routing Requirements
    4.1.  Support of 6LoWPAN Device Properties
    4.2.  Support of 6LoWPAN Link Properties
    4.3.  Support of 6LoWPAN Network Characteristics
    4.4.  Support of Security
    4.5.  Support of Mesh-under Forwarding
5.  Security Considerations
6.  Acknowledgements
7.  References
    7.1.  Normative References
    7.2.  Informative References
§  Authors' Addresses
§  Intellectual Property and Copyright Statements



 TOC 

1.  Problem Statement

Low-power wireless personal area networks (LoWPANs) are formed by devices complying to the IEEE 802.15.4 standard [5] (IEEE Computer Society, “IEEE Std. 802.15.4-2006 (as amended),” 2007.). 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] (Kushalnagar, N., Montenegro, G., and C. Schumacher, “IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and Goals,” August 2007.).

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] (Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, “Transmission of IPv6 Packets over IEEE 802.15.4 Networks,” September 2007.)) 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:

This creates new challenges on obtaining robust and reliable routing within LoWPANs.

The 6LoWPAN problem statement document ("6LoWPAN Problems and Goals" [3] (Kushalnagar, N., Montenegro, G., and C. Schumacher, “IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs): Overview, Assumptions, Problem Statement, and Goals,” August 2007.)) 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 (Mesh-under (left) and route-over routing (right))). Most statements in this document consider both the mesh-under and route-over cases.
[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.

Considering the problems above, The 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 detail requirements differ. (e.g., a few dozens nodes for home lighting system needs appropriate 'scalability' for the applications, while billions of nodes for highway infrastructure system also needs appropriate 'scalability'.) This document states the routing requirements to consider the features of 6LoWPAN applications in general, while trying to give example cases for different cases of routing. It is noted that this one routing requirement document does not mean that having a single routing solution may be the best one for all 6LoWPAN applications.


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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] (Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, “Transmission of IPv6 Packets over IEEE 802.15.4 Networks,” September 2007.)), 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. One radio hop would be seen as a single IP link [7] (Shelby, Z., Thubert, P., Hui, J., Chakrabarti, S., and E. Nordmark, “LoWPAN Neighbor Discovery Extensions, draft-shelby-6lowpan-nd-00 (work in progress),” October 2008.).


Figure 1 (Mesh-under (left) and route-over routing (right)) 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.

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. In a specific use-case can use mesh-header in [4] (Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, “Transmission of IPv6 Packets over IEEE 802.15.4 Networks,” September 2007.)for pre-defined L2 forwarding. When a route-over protocol is 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 (An example of a 6LoWPAN) , the PnC is the only IPv6 router in the LoWPAN in the assumption of [4] (Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, “Transmission of IPv6 Packets over IEEE 802.15.4 Networks,” September 2007.). 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 intermediate nodes become LoWPAN router and set up IPv6 paths for multi-hop transmission. 


    O   X
   /    |                      PnC: PAN Coordinator
PnC --- O --- O --- X          O: Intermediate node (FFD)
       / \                     X: End host (FFD or RFD)
      X   O --- X
          |
         / \
        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 at the edge of the LoWPAN) will be always the default router for the outgoing packet of the 6LoWPAN.


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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 [6] (Bulusu, N. and S. Jha, “Wireless Sensor Networks,” July 2005.). 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:

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:
b.
Node Parameters:
c.
Link Parameters:

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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 have to support multiple device types and roles, for example:

Due to these unique device types and roles 6LoWPANs need to consider the following two primary features:

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 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 following requirements:


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4.1.  Support of 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. The following requirements are based on battery-powered LoWPAN devices.

[R01] 6LoWPAN routing protocols SHOULD have small code size of routing protocol stack 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 LoWPAN routing protocol solution should consider the limited memory size typically starting at 4KB. RAM size of 6LoWPAN 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 [14] (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.). Small networks, can be designed to support a smaller number of hops. It is highly dependent on 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 protocol control messages SHOULD not create fragmentation of physical layer (PHY) frames.

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] (Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, “Transmission of IPv6 Packets over IEEE 802.15.4 Networks,” September 2007.), the maximum size of a 6LoWPAN frame, in order not to cause fragmentation on the PHY layer, is 81 octets.

[R03] 6LoWPAN routing protocols SHOULD cause minimal power consumption by the efficient use of control packets (e.g., minimize expensive multicast which cause broadcast to the entire LoWPAN).

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:

while maintaining 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 in many devices, computational operations or taking sensor samples, radio communications is by far the dominant factor of power consumption [8] (Pister, K. and B. Boser, “Smart Dust: Wireless Networks of Millimeter-Scale Sensor Nodes,” .). 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 [11] (Shih, E., “Physical Layer Driven Protocols and Algorithm Design for Energy-Efficient Wireless Sensor Networks,” July 2001.).

In [9] (Hill, J., “System Architecture for Wireless Sensor Networks,” .) the energy consumption of two example RF controllers for low-power nodes is shown. The TR1000 radio consumes 21mW on transmitting at 0.75mW, and 15mW on reception (with a receiver sensitivity of -85dBm). The CC1000 consumes 31.6mW on transmitting 0.75mW, and 20mW for receiving (with a receiver sensitivity of -105dBm). [9] (Hill, J., “System Architecture for Wireless Sensor Networks,” .) explains the power continuation under the concept of an idealized power source: based on the energy of idealized AA battery, the CC1000 can transmit for approximately 4 days straight or receive for 9 days straight.

One multicast packet causes reception of the entire nodes in the LoWPAN, while only the nodes in the path use the reception energy at unicast. Thus, 6LoWPAN routing protocol SHOULD minimize the control cost by the routing packets.


 TOC 

4.2.  Support of 6LoWPAN Link Properties

6LoWPAN links have the characteristics of low bandwidth and possibliy high loss rates. The routing requirements described in this subsection are from the link properties.

[R04] The design of routing protocols for 6LoWPANs must consider the fact that packets are to be delivered with reasonable probability.

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 ingress and egress, while forced entry securty alerts must be routed to one or more fixed or mobile user devices within 5 seconds [16] (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-01 (work in progress),” .). 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 [14] (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.). 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 or minutes [15] (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-02 (work in progress),” October 2008.).

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 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, different from address types and reliable/unreliable mode.

Considering the 6LoWPAN link latency, routing protocols can calculate the probable delivering time of the control packets in a normal case not more than a few hundred ms between two nodes.

[R05] 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).

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 [13] (Brandt, A., Buron, J., and G. Porcu, “Home Automation Routing Requirement in Low Power and Lossy Networks, draft-ietf-roll-home-routing-reqs-04 (work in progress),” October 2008.). The tolerance of the recovery time can vary dependent 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.


 TOC 

4.3.  Support of 6LoWPAN Network Characteristics

6LoWPAN can be deployed with different tolerance levels, network scales, topologies, levels of mobility, etc. In any case, 6LoWPAN must be maintain low energy consumption. The requirements described in the following subsection are derived from the network feature of 6LoWPANs.

[R06] 6LoWPAN routing protocols SHOULD be reliable despite unresponsive nodes due to periodic hibernation. (e.g., management with the duty cycle)

Many nodes in 6LoWPAN 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, 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.

In [9] (Hill, J., “System Architecture for Wireless Sensor Networks,” .) it is explained that CC1000-based nodes must operate at a duty cycle of approximately 2% to survive for one year from idealized AA battery power source. For home automation purposes, it is suggested that that the devices have to maximize the sleep phase with a duty cycle lower than 1% [13] (Brandt, A., Buron, J., and G. Porcu, “Home Automation Routing Requirement in Low Power and Lossy Networks, draft-ietf-roll-home-routing-reqs-04 (work in progress),” October 2008.), while in building automation, batteries must be operational for at least 5 years when the sensing devices are transmitting data (e.g. 64 bytes) once per minute [16] (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-01 (work in progress),” .).

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 pathes.

[R07] The metric used by 6LoWPAN routing protocols MAY utilize a combination of the inputs provided by the MAC layer and other measures to obtain optimal path considering energy balance and link quality.

Simple hop-count-only mechanisms may be inefficient in 6LoWPANs. In home, buildings, or infrastructure, some nodes will be installed with mains-powered. Such power-installed node MUST be considered as a relay point for more roles in packet delivery. 6LoWPAN routing protocols MUST know the power source of the nodes.

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 (An example network) represent the Link Delivery Ratio (LDR) between each pair of nodes. There are studies that show a piecewise linear dependence between LQI and LDR [12] (Chen, B., Muniswamy-Reddy, K., and M. Welsh, “Ad-Hoc Multicast Routing on Resource-Limited Sensor Nodes,” 2006.). 


                                  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
  • 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.

[R08] 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 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). For home automation applications it is envisioned that the routing protocol must support 250 devices in the network [13] (Brandt, A., Buron, J., and G. Porcu, “Home Automation Routing Requirement in Low Power and Lossy Networks, draft-ietf-roll-home-routing-reqs-04 (work in progress),” October 2008.), while routing protocols for metropolitan-scale sensor networks must be capable of clustering a large number of sensing nodes into regions containing on the order of 10^2 to 10^4 sensing nodes each [15] (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-02 (work in progress),” October 2008.). 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.

[R09] 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 [10] (Lee, S., Belding-Royer, E., and C. Perkins, “Scalability Study of the Ad Hoc On-Demand Distance-Vector Routing Protocol,” March 2003.).

[R10] 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.

Building monitoring applications, for instance, require that the mobile devices SHOULD be capable of unjoining (handing-off) from an old network joining onto a new network within 15 seconds [16] (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-01 (work in progress),” .). 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 [13] (Brandt, A., Buron, J., and G. Porcu, “Home Automation Routing Requirement in Low Power and Lossy Networks, draft-ietf-roll-home-routing-reqs-04 (work in progress),” October 2008.). In industrial environments, where mobile equipment such as cranes move around, vehicular speeds of up to 35 kmph are required to be supported by the routing protocol [14] (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.). Currently, 6LoWPANs are not being used for such a fast mobility cases, but dynamic association and deassociation 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:

[R11] 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.


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4.4.  Support of Security

The routing requirement described in this subsection allow secure transmission of routing messages. Solutions may take into account the specific features of IEEE 802.15.4 MAC layers.

[R12] 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] (Nikander, P., Kempf, J., and E. Nordmark, “IPv6 Neighbor Discovery (ND) Trust Models and Threats,” May 2004.). 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.


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4.5.  Support of Mesh-under Forwarding

One 6LoWPAN may be built as one IPv6 link. In this case, mesh-under forwarding/routing must be supported. The routing requirements described in this subsection allow optimization and correct operation of routing solutions taking into account the specific features mesh-under routing.

[R13] 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.

[R14] 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.

[R15] 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.

[R16] 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.


 TOC 

5.  Security Considerations

Security issues are described in Section 4.4 (Support of Security). More security considerations will follow the 6LoWPAN security analysis work.


 TOC 

6.  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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7.  References


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7.1. Normative References

[1] Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML).
[2] Nikander, P., Kempf, J., and E. Nordmark, “IPv6 Neighbor Discovery (ND) Trust Models and Threats,” RFC 3756, May 2004 (TXT).
[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 (TXT).
[4] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler, “Transmission of IPv6 Packets over IEEE 802.15.4 Networks,” RFC 4944, September 2007 (TXT).
[5] IEEE Computer Society, “IEEE Std. 802.15.4-2006 (as amended),” 2007.

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7.2. Informative References

[6] Bulusu, N. and S. Jha, “Wireless Sensor Networks,” July 2005.
[7] Shelby, Z., Thubert, P., Hui, J., Chakrabarti, S., and E. Nordmark, “LoWPAN Neighbor Discovery Extensions, draft-shelby-6lowpan-nd-00 (work in progress),” October 2008.
[8] Pister, K. and B. Boser, “Smart Dust: Wireless Networks of Millimeter-Scale Sensor Nodes.”
[9] Hill, J., “System Architecture for Wireless Sensor Networks.”
[10] Lee, S., Belding-Royer, E., and C. Perkins, “Scalability Study of the Ad Hoc On-Demand Distance-Vector Routing Protocol,” March 2003.
[11] Shih, E., “Physical Layer Driven Protocols and Algorithm Design for Energy-Efficient Wireless Sensor Networks,” July 2001.
[12] Chen, B., Muniswamy-Reddy, K., and M. Welsh, “Ad-Hoc Multicast Routing on Resource-Limited Sensor Nodes,” 2006.
[13] Brandt, A., Buron, J., and G. Porcu, “Home Automation Routing Requirement in Low Power and Lossy Networks, draft-ietf-roll-home-routing-reqs-04 (work in progress),” October 2008.
[14] 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.
[15] 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-02 (work in progress),” October 2008.
[16] 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-01 (work in progress).”
[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.

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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
  Universität 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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