## 2 Related Work
### Decentralized Routing Protocols  
Decentralized algorithms can use the exchange of beacons to make efficient decisions for content delivery. The Greedy Perimeter Stateless Routing (GPSR) protocol [6] was proposed for wireless datagram networks that uses the positions of routers and a packer's destination to make packet forwarding decisions. By keeping the state only about the local topology, GPSR scales better in per-router state than shortest-path and ad hoc routing protocols as the number of network destinations increases. [PERSON] et al. [30] demonstrated that the performance of a video-based overtaking assistant can be significantly improved by exploiting information from the beacons about any forthcoming increase in the load of the multiple access channel used. [PERSON] et al. [31] proposed two data forwarding schemes particularly for choosing the best packet forwarder in a bi-directional multiple lane highway. [PERSON] et al. [32] proposed an improved flooding scheme to cope with variable vehicle density situations by exchanging beacons with each other. [PERSON] et al. [33] introduced a proactive content discovery scheme to tackle the mobility, large population and rich content challenges of VANETs.  
[PERSON] et al. [34] proposed a new routing protocol for vehicular sensor networks that uses four different metrics, which are the distance to the destination, the traffic density, the vehicular trajectory and the available bandwidth, to make forwarding decisions for minimizing packet losses and packet delay. [PERSON] et al. [35] proposed a multi-player game theory algorithm for intra-cluster data dissemination in vehicular sensor networks by analyzing the competitive and cooperative relationships among vehicles. [PERSON] et al. [36] proposed a utility-based sensing task dissemination algorithm, where the sensing tasks are decomposed and offloaded to neighboring vehicles according to the utilities of the neighboring vehicles to the decomposed sensing tasks. [PERSON] et al. [37] proposed an infrastructure-less traffic adaptive data dissemination protocol that takes into account road traffic and network traffic status for both highway and urban scenarios. Specifically, the proposed double broadcast suppression techniques in [37] can adapt efficiently to the irregular road topology.  
Based on the above-mentioned work, some cross-layer strategies were proposed to synchronize the resources in different communication layers. [PERSON] et al. [38; 39] provided a fully-distributed cross-layer solution, which leverages the properties of video coding to design a collision-resolution mechanism and the characteristics of variable bit rate traffic to efficiently exploit radio resources. [PERSON] et al. [40; 41] proposed an earliest deadline first scheme, which dynamically adjusts the priority of real-time streaming to avoid collision and introduces an admission control policy according to time constraints to provide guaranteed QoS in multi-channel environments. [PERSON] et al. [42] presented an analytical approach that takes cross-layer factors into account and propose a new routing metric based on optimizing a queueing model that considers local loads, interferences and packet droppings. [PERSON] et al. [43] addressed the problem of the stability of any-path communications in vehicular networks in the presence of inter-vehicle link failures. However, the dynamically-changing network topology will generate high packet transmission overhead, which motivates the adoption of beacon-less schemes.