# Vortex: Network-Driven Opportunistic Routing for Ad Hoc Networks
## 1 Introduction  
Recently, wireless multi-hop networks such as ad hoc networks have been considered to be a promising architecture since the autonomous, self-organized, and distributed network management would play an important role in the communications of IoT (Internet of Things) or in situations where infrastructure support is expected such as disaster sites. Such a self-organized nature of ad hoc networks enables us to provide communication opportunities, and this can be realized only with terminals composing the networks. Thus, there are some advantages in terms of deployment cost, environment adaptability, responsiveness, and so forth. Against this background, wireless multi-hop communication is thought to be a promising technology for deploying networks where infrastructure support is not available. However, ad hoc networks generally use wireless communication for data transmission, which causes reliability degradation. Moreover, the mobility of terminals in ad hoc networks results in a change in network topology. In other words, the connectivity of terminals changes momentarily, and this makes it difficult to establish stable end-to-end routes in the networks. There are many routing protocols in ad hoc networks to address the unique communication environment. The conventional routing protocols for ad hoc networks can be broadly classified into three categories: proactive (table-driven), reactive (on-demand), and hybrid. The proactive routing protocols such as OLSR (optimized link state routing) [1] continuously manage forwarding routes in the advance of the actual forwarding procedure, whereas the reactive routing protocols such as DSR (dynamic source routing) [2] and AODV (ad hoc on-demand distance Vector) [3] establish end-to-end routes after communication demands arise at source terminals. Hybrid routing protocols generally take advantage of these protocols to achieve better routing performance. However, these conventional unicast routing protocols generally have a limitation in routing flexibility, which is mainly derived from the single communication pair-based routing procedure and end pair-based route management. A novel routing paradigm named opportunistic routing (OR) has been proposed to address the former issues by exploiting the broadcast nature of wireless communication [4; 5]. Unlike the conventional routing protocols, OR does not rely on a specific end-to-end route to forward packets from a source terminal to a destination terminal, but selects optimal forwarders on a hop-by-hop or packet-by-packet basis. This forwarding principle also allows it to forward packets with better link quality and results in simultaneous multi-path forwarding, which greatly improves end-to-end reliability. However, these conventional OR strategies have drawbacks in gathering routing information and adapting to dynamic network environments. This is because the eligible forwarder selection for ORs requires extensive information such as ETX (expected transmission count), and scalability limitations occur as the network size increases. Moreover, a routing protocol with a fixed route or guide performs reasonably well in stable network environments while its performance degrades in dynamic environments due to its rigid routing procedure. Therefore, the problems that should be addressed in ad hoc network routing protocol are summarized as the problem of adaptation to terminal mobility. For more details, difficulties in the following points should be addressed to overcome the problem. * Route discovery/Route maintenance;
* Information collection for routing procedure;
* Handle terminal participation and leave. In addition, conventional routing protocols for ad hoc networks are generally designed for a specific end-to-end paired terminal, which also degrades the efficiency of information gathering, route discovery, and route maintenance.