Research Article RPL Mobility Support for Point-to-Point Traffic Flows towards Mobile Nodes David Carels, Eli De Poorter, Ingrid Moerman, and Piet Demeester Department of Information Technology (INTEC), Ghent University and iMinds, Gaston Crommenlaan 8 Bus 201, 9050 Ghent, Belgium Correspondence should be addressed to David Carels; david.carels@intec.ugent.be Received 13 February 2015; Revised 4 June 2015; Accepted 10 June 2015 Academic Editor: Christos Anagnostopoulos Copyright © 2015 David Carels et al. his is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. he “routing protocol for low-power and lossy networks” (RPL) from the IETF ROLL working group is a widely used standard to support routing in wireless sensor networks (WSNs). Although the RPL protocol was originally designed with static topologies in mind, recently a number of extensions have been proposed to support traic lows from mobile nodes towards a static gateway. However, this paper demonstrates that these solutions do not support traic lows going the other direction, for example, from the gateway towards mobile devices. To remedy this, the paper irst analyses the problems that prevent reliable traic lows towards mobile devices when using RPL. Aterwards, a new mechanism to improve downward route updating is proposed. Our new approach minimizes the probability of connectivity loss by ensuring that the internal state of the static network remains consistent. Our solution is implemented and evaluated using both simulation tools and experimental facilities and it is shown that it improves the end-to-end packet delivery ratio to mobile nodes from 20–30% up to 80% while reducing the overall RPL signalling overhead without the use of location information. 1. Introduction he IETF IPv6 Routing Protocol for Low-Power and Lossy Networks (RPL) [1] is widely used to support routing between sensor nodes. In most scenarios, a backbone network of intermediary nodes is installed, which is assumed to be static. Although handling mobility is not an explicit design cri- teria for RPL [2], a number of use cases have been proposed in which also mobile nodes could be added to this static network [3–6]. Examples include mobile nodes in intelli- gent transportation systems (ITS) for the monitoring of air quality, mobile robots collecting information, and location tracking of occupants and assets. For most of these use cases, communication lows go from the mobile node to the sink and vice versa. In addition, more advanced use cases, such as continuous monitoring of patients where patients (data sources) and doctors (data collectors) are mobile [3], also require direct communication lows between mobile nodes. RPL is optimised for data traic in which the sink is colo- cated with the root of the RPL tree, but also other commu- nication patterns are supported [1, 2]. he protocol is based on the exchange of DIO (DODAG information object) mes- sages to construct a DODAG (destination-oriented directed acyclic graph) for routing of data towards a DODAG root. RPL uses DAO (destination advertisement object) messages to establish downward routes towards the network nodes. hese downward routes also enable the point-to-point com- munication between network nodes. he establishment of downwards routes leads to higher control traic overhead and increased memory and processing requirements for (nodes near) the DODAG root [2]. RPL uses a tree-based routing approach for the ixed backbone network. However, the main characteristic of mo- bility is a highly dynamic topology which results in frequent disconnections with neighbouring nodes. Due to these dis- connections, packets that are routed towards a mobile device can be routed towards edges (parents) even when the mobile device is already out of reach of these parents. he main contributions of this paper are as follows: (i) he paper thoroughly analyses the conditions during which packet loss can occur when routing towards mobile nodes in a RPL network. (ii) In contrast to previous papers about mobility in RPL, that focused mainly on eicient parent selection by mobile nodes, this paper proposes a solution to keep Hindawi Publishing Corporation International Journal of Distributed Sensor Networks Volume 2015, Article ID 470349, 13 pages http://dx.doi.org/10.1155/2015/470349