# Vortex: Network-Driven Opportunistic Routing for Ad Hoc Networks
## 2 Related Works
### Local information-based routing protocols
ExOR [18; 19] uses ETX (estimated transmission count), which is calculated from the inverse of the transmission success rate of each link collected from entire networks, to minimize the number of end-to-end forwarding with prioritized forwarders. In ExOR, the highest priority will be given to the terminal with the smallest ETX to forward packets to a destination terminal and ETX-based priority will be assigned to other forwarders in the same manner based on comprehensive ETX information. The forwarding procedure of ExOR is based on the assigned priority and ExOR appends the set of eligible forwarder lists to a packet header to be able to make a forwarding decision on each forwarder between a source and destination terminal. GeRaF [12] is a location-based OR that selects a terminal closer to a destination terminal as the next forwarder. In GeRaF, terminals determine their geographic location using GPS and calculate the relative geographic relationships between terminals within the entire network prior to actual forwarding. The terminals periodically broadcast HELLO messages to exchange the relative geographic location among the terminals in the network, that is, a source terminal knows the destination terminal location prior to actual forwarding. The source terminal initiates packet forwarding by exchanging RTS and CTS packets which are different from the ones in the MAC layer. In the RTS/CTS exchange, source terminals add their own location and destination terminals to the RTS packet header. The receiver of the RTS packet checks its location and replies CTS packet in response to the received RTS packet if the receiver location is closer to the destination terminal compared to the source terminal. The CTS packet reply is performed sequentially from the terminal closer to the destination terminal to the further terminals. Then, the source terminal forwards the packet to the terminal that is the closest to the destination with higher priority. GeRaF repeats the same procedure recursively until the packet reaches the destination terminal. For more improvement of routing performance, routing protocols enhancing the legacy ORs have been proposed [22; 23; 24; 25; 26; 27]. Wide-ranging strategies such as network coding, geographic location-based, and cross-layered approaches, have been taken to address the drawbacks of conventional routing protocols. Although scrupulous routes or topology management could improve performance in specific environments, complex routing procedures are vulnerable to the fluctuations of a MANET communication environment. Therefore, we would explain routing protocols with a tolerance to the fluctuation and layer-independent ones to improve their applicability to various environments. In [16], the authors proposed a routing protocol using a bloom filter called HRAN (heat routing for ad hoc networks). HRAN uses the concept of \"Heat gradients\" that indicate the distance to a specific destination terminal, that is, the terminals closer to the destination terminal feel hotter than the terminals further to the destination terminal. The bloom filters in HRAN are used to create a heat map in each terminal which classifies the neighboring terminals around the destination terminal into several heat levels and creates a heat zone according to the level. With the zone assignment, HRAN starts its route discovery procedure and the route request is forwarded along the assigned heat zone, namely heat gradients. Once the request reaches the destination terminal, the destination terminal sends back a route reply using the backward route that had already been created during the request forwarding procedure. Then, the source terminal starts data packet forwarding and HRAN also creates a heat tunnel which assigns a \"hotter\" zone along the discovered route for further route discovery.