## 2 Previous Works  
There have been many routing schemes proposed to achieve efficient routing in the underwater acoustic networks[21; 22; 23]. In literature, there are many schemes that avoid the duplicate packet forwarding using the backoff timers in the flooding-based underwater acoustic networks, as in [24]. When a node receives the data packet, it initiates the backoff timer and holds the communication of that packet. If it receives a certain number of copies of the same data packet, which is \(floor(2.5)\), the node simply discards the packet. The timer is selected randomly between the predefined minimum and maximum backoff duration that is 5 s and 65 s. This scheme does not use any additional network parameter in the forwarding decision. The above work was extended by authors in [25], where they used hop count information, additional to the backoff, to better decide which packet should be considered duplicate and discarded. There are many similar works in the literature that involve the backoff timers, however, in this section, we discuss the depth based opportunistic void avoidance schemes, which are closely related to our proposed EDOVE scheme. As discussed in the previous text, the depth information is the prime metric that is used by the routing protocols. It is estimated locally from the reading of the pressure sensor. Data packets are forwarded from the higher depth nodes to the lower depth towards the surface where sink(s) that are floating on the water surface. In addition to the depth information, several other metrics have been adapted by the routing schemes to efficiently forward the data packets to sink(s). Depth based routing schemes are further categorized into two categories; _sender initiated_ (The next hop forwarder of the data packet is selected by the _Sender_ node) [17; 19] and the _receiver initiated_ (where the data packet receiver decides whether it forwards the packet or not) [15; 16; 18; 20]. In the _sender initiated_ protocols, the data packet _Sender_ node selects the next hop relay node, and the information of the next forwarder is added in the data packet. Once the node receives that data packet, it easily determines from the header information of the data packet that node has to further relay the packet or not. On the other hand in the _receiver initiated_ routing protocols, the data packet receiving nodes distributively contend to be the next forwarder. These nodes compute their respective holding times, and the potential forwarding candidate(s) has/have very small holding time compared to the other nodes. The node with smallest holding time forwards the data packet, and when the other nodes receive a copy of the same data packet while their holding time is still on, the simply discard the transmission [15]. In this manner, the _receiver initiated_ forwarding schemes suppress the data packet broadcast. Hence, the data packet broadcast suppression depends upon the variance of the holding times computed by the nodes. In this section, we merely focus on the _receiver initiated_ depth based routing schemes because the proposed EDOVE belongs to that routing category. In [15], authors proposed a holding time based packet suppression scheme where packet receiving nodes take the forwarding decision using their depth information, called Depth Based Routing (DBR). All the nodes that are in transmission range of the sender and have smaller depths are candidate forwarders. Rest of the nodes that receive data packet and are placed at the larger depth will simply discard the data packet. Consider the DBR scenario in Figure 2, where _Sender_ node \(S\) either sends or forwards the data packet. All nodes within the transmission range of \(S\), \(t_{r}^{S}\), e.g., nodes 1 to 5, receive the data message. The receiving nodes 2, 3 and 4 with depths \(d_{2}\), \(d_{3}\) and \(d_{4}\), respectively, are the potential forwarding candidates because their depths are less than the depth of \(S\). According to DBR, node 3 is a preferable forwarder due to the lowest depth among all the other nodes.