Multi-hop broadcast is a key technique to disseminate time-sensitive event-driven safety warning messages (WMs) in Vehicular Ad hoc Networks (VANETs). Due to the lossy nature of the vehicular wireless environment and the fact that the implementation of broadcast at the link layer uses unreliable transmissions (i.e., lack of positive ACKs), highly reliable, scalable, and fast multi-hop broadcast protocol is particularly difficult to design in VANETs with lossy links. Schemes that use redundant network layer broadcasts have been proposed. However, the tradeoff between reception reliability and transmission count in such schemes needs to be carefully considered.

In this paper (The preliminary version of this paper appeared in [1], IEEE MASS 2009.,) we propose an opportunistic broadcast protocol (OppCast) that aims at simultaneously achieving high WM packet reception ratio (PRR) and fast multi-hop message propagation while minimizing the number of transmissions. A double-phase broadcast strategy is proposed to achieve fast message propagation in one phase and to ensure high PRR in the other. OppCast exploits opportunistic forwarding in each transmission to enhance the WM reception reliability and to reduce the hop delay, and to carry out reliable and efficient broadcast coordination, we propose the use of explicit broadcast acknowledgements (BACKs) which effectively reduces the number of redundant transmissions. OppCast is also extended to handle sparse and disconnected VANETs, where the protocol adaptively switches between fast opportunistic forwarding and the store-carry-and-forward paradigm. Extensive simulation results show that, compared with existing competing protocols, OppCast achieves close to 100% PRR and faster dissemination rate under a wide range of vehicular traffic densities, while using significantly smaller number of transmissions.