Protocol for Efficient Opportunistic Communication Ranjana Pathak *† , Peizhao Hu † , Jadwiga Indulska *† and Marius Portmann *† * The University of Queensland, School of Information Technology and Electrical Engineering † National ICT Australia (NICTA) Email: {Firstname.Lastname}@nicta.com.au Abstract—In typical wireless networks, end-to-end routing is a usual way to deliver data packets from source nodes to destination nodes. In the case of link failures when no alternative route is found, the routing protocols will drop these packets. As a way to improve the packet delivery ratio, an integration of the store-carry-forward features with the traditional end-to-end communication has been already proposed. The existing solutions propose one-time only switching from one communication mode to another should the link failures occur. In this paper, we propose a hybrid protocol to support the dynamic switch between the two modes of communication should the link conditions changed. That is, the protocol utilises the ability to buffer packets when end-to-end routes are not possible, and leverages the end-to-end routes whenever they become available to ensure performance. We evaluate the proposed protocol using a set of comprehensive simulation scenarios to systematically demonstrate its significant improvement in packet delivery over one of the best representative routing protocol for end-to-end routing. I. INTRODUCTION Due to various reasons (including mobility and interference) wireless networks are vulnerable to changes of link conditions and therefore can experience link failures. Typical routing protocols (e.g., AODV, OLSR) are designed to discover and maintain the route between a source and a destination. They are responsible for evaluating and acting upon the changes of link conditions. When link failures occur, these routing protocols often attempt to reroute (or repair the route) and drop the subsequent data packets if no alternative route exists. Another way of communication in wireless networks is opportunistic networking which shares the communication con- cept with delay-tolerant networking. In these networks, data packets are delivered to neighbouring nodes on encounter between mobile devices and packets travel in this manner hop- by-hop until they reach the destination. The performance of opportunistic protocols is much lower than routing protocols which find an end-to-end route if such a path from the source to the destination exists. There already exists research on combining the opportunis- tic and end-to-end protocols [1], [2], [3], [4]. These ap- proaches tend to switch over to the opportunistic communi- cation paradigm for the lifetime of the packet flow when the packets are dropped due to link failures. For example, SF- BATMAN [3] is an attempt to extend BATMAN (a reactive protocol similar to AODV) with the store-and-forward function- ality. However, only a preliminary design is presented and the evaluations are very preliminary. In [1], Ott et al. proposed an approach to extend AODV to support DTN routing when path to the destination breaks and cannot be repaired. The switching from AODV to DTN is always at source nodes and the switch- ing back from DTN to AODV is not supported. In contrast, we propose a truly hybrid protocol in which the packets that would be dropped due to route failure are delivered opportunistically through the network until they reach a node that is able to create an end-to-end path to the destination. Therefore the approach leverages the potential partial end-to-end routes that can be created in wireless networks. By doing so, this hybrid approach not only improves the packet delivery ratio compared to end-to- end routing protocols, but also shows efficiency improvement when compared to opportunistic protocols. In our previous paper [5], we demonstrated the initial concept of the hybrid protocol by extending AODV 1 . We named the extended protocol — AODV-OPP. When a link failed and no alternative route existed, AODV would drop all subsequent packets. However, AODV-OPP buffers these packets and deliv- ers them at a later time to all its neighbours. Whenever packets arrive at a node that has end-to-end route to destination, these packets will be sent using the route. The switching between mode of communications is managed dynamically. As the protocol forwards packets to all one-hop neighbours, overhead is certainly a concern. In this paper, we propose a new AODV- OPP+ protocol that balances the trade-off between delivery ratio and overhead and retains the ability to dynamically switch between communication modes if necessary. The main contri- butions of this paper are: (i) a metric to evaluate neighbours of a node and to identify the neighbours that have the best chance of forwarding the packets toward the destination; (ii) a low- overhead algorithm to support the dynamic switching between communication modes; (iii) extensive simulation evaluations (including synthetic) of the proposed hybrid protocol; and (iv) performance comparison of the proposed protocol with the AODV [6] protocol. The remainder of the paper is organized as follows. Section II describes the overall design of the hybrid protocol — AODV- OPP+, including the new reachability metric and algorithm. This is followed by the evaluation results and discussions on the performance of the proposed protocol. Finally, we conclude the paper in Section IV. 1 The idea can be easily applied to other routing protocols, such as OLSR. 38th Annual IEEE Conference on Local Computer Networks 978-1-4799-0537-9/13/$31.00 ©2013 IEEE 244