## 2 Related Work
### Localization-Free Routing Protocols  
Depth based routing (DBR) [9], energy-efficient DBR (EEDBR) [10], adaptive mobility of courier nodes in threshold-optimized depth based routing (AMCTD) [11], improved AMCTD (iAMCTD) [12] and delay-sensitive routing schemes [13] are popular localization-free routing protocols. In DBR, the neighbor with least depth is selected as forwarder. The holding time is computed based on depth of the forwarder node to avoid redundant transmissions at the destination. The node having smaller depth with respect to destination is always given the highest priority to proceed with the data delivery. In EEDBR, two control parameters are used to ensure cyclic selection of forwarder nodes. The parameters are: depth and energy. Based on these parameters, source node initiates a data packet within the communication range. Every node receives the packet and computes its holding time, then node with lowest holding time proceed with the data communication. AMCTD introduces the approach of courier nodes to achieve higher delivery ratio, better network stability and optimized lifetime. It uses weight function for selecting next forwarder of data packet on the basis of network density. Optimal weight computation not only balances load among the network nodes but also optimizes the holding time. These protocols do not find next hop eligible forwarder when void hole occurs, which results in low packet delivery ratio. [PERSON] et al. extended AMCTD protocol to iAMCTD protocol by considering three routing metrics for calculating weight function: signal quality index (SQL), energy cost function (ECF), and depth-dependent function (DDF). Moreover, it implements on-demand data routing and maximizes the lifetime of UWSNs by optimized mobility pattern of courier nodes. [PERSON] et al. [13] proposed three routing schemes: delay-sensitive DBR (DSDBR), delay-sensitive EEDBR(DSEEDBR), and delay-sensitive AMCTD (DSAMCTD). These protocols formulate delay-efficient priority factors (PF) and delay-sensitive holding time (DSH\({}_{T}\)) to minimize end-to-end delay. [PERSON] et al. proposed hop-by-hop dynamic addressing based (H2-DAB) [16] beacon-based routing protocol, which does not require any location information. In H2-DAB, each sensor node is assigned a unique hop-id by using beacon message. A surface sink broadcasts a beacon message, while the receiving nodes are assigned hop-ids. The receiving nodes increment the hop-id and rebroadcast it. In this way, each node is assigned a unique hop-id. This hop-id is used as routing metric where sensor node selects next forwarder with the small hop-id. [PERSON] et al. proposed reliable energy efficient routing protocol (R-ERP\({}^{2}\)R) [18] to improve the reliability of the delivered data and efficiently utilize the node battery. The R-ERP\({}^{2}\)R uses multiple metrics: link quality, physical distance and residual energy to nominate next forwarder. In [19], [PERSON] et al. proposed channel aware routing protocol (CARP), which combines link quality and hop count as routing metrics, to avoid void hole effectively. It uses cross layer approach for channel access and relay selection. In CARP, node is selected as relay if it has a history of successful delivery to its neighbor node with high residual energy. E-CARP [20] is the enhanced version of CARP. It reduces energy consumption of the network by avoiding control packets during relay selection and reduces sensory data to sink, when the underwater environment is relatively steady. In [15], hydraulic pressure based anycast (HydroCast) routing scheme is proposed to avoid void hole using backward transmissions. This process continues until the neighbor node finds lower pressure node. The occurrence of void hole can be avoided through efficient energy consumption among the network nodes. In this regard, [PERSON] et al. proposed cluster based algorithm to cater the problem of high data at the nodes deployed nearby the sink [21].