## 2 Literature Review
### Flat Routing Approaches  
The first category of routing approaches is multi-hop flat routing. In such routing approaches, each sensor node typically plays the same role, and sensor nodes collaborate to perform sensing tasks.  
Flooding and gossiping [11] are two classical approaches to relay packets in sensor networks without the need for any routing algorithms or topology maintenance. In flooding, each sensor that receives a packet broadcasts the packet to all of its neighbours, and this process continues until the packet arrives at the destination or the maximum number of hops for the packet is reached. On the other hand, gossiping is a slightly enhanced version of flooding where the receiving node sends the packet to a randomly-selected neighbour, which picks another random neighbour to forward the packet to, and so on, until the destination or the maximum hop is reached.  
[PERSON] _et al._[12; 13] proposed a family of adaptive protocols, called sensor protocols for information via negotiation (SPIN). The SPIN family is designed to address the deficiencies of classical routing approaches, such as flooding and gossiping, which waste much energy and bandwidth when sending extra and unnecessary copies of packets by sensors that cover overlapping areas. SPIN consists of three steps. First, when a sensor has a data packet to transfer, it broadcasts the metadata of the data packet to its neighbours. The size of the meta packets is much smaller than that of the data packet. Then, if any neighbours are interested in the data packet, they will send back a request message to the sensor. Once the sensor receives the request message, it will transfer the data packet to those neighbours that are interested in that packet.  
[PERSON] _et al._[14] devised an energy-aware self-organised routing algorithm for WSNs. Their algorithm enables individual sensors to follow locally selfish strategies, which, in turn, result in a framework of a routing network with desirable global properties. In addition, the algorithm can adaptively deal with the changing sensor numbers and network topology.  
[PERSON] _et al._[15] studied the lowest latency path problem and then devised a forwarding protocol based on biased random walks, where sensors use only local information about their neighbours and their next active period to make forwarding decisions. Specifically, their protocol, lukewarm potato, is a compromise between the shortest path forwarding and hot potato forwarding. Hot potato forwarding is a greedy approach to the fast propagation of a message toward the sink: a node, upon receiving a message, forwards the message to the neighbour that is the first to become active.  
[PERSON] and [PERSON] [16] devised a gradient routing algorithm that uses two-hop information for industrial WSNs to enhance real-time performance with energy efficiency. The routing algorithm is based on the number of hops to the sink instead of distance. Specifically, the routing algorithm applied two-hop velocity-based routing [17] to a gradient-based WSNs so as to reduce both energy consumption and end-to-end delay.  
[PERSON] _et al._[18] introduced a routing enhancement scheme. Their scheme first finds a robust guide path during the route discovery phase. Then, along this guide path, data packets are greedily forwarded toward the destination through nodes' cooperation without utilising location information. The forwarding procedure is based on the priorities of forwarding candidates, where nodes with the highest priorities will be picked up to forward data packets.