## 2 Literature Review  
There are numerous routing protocols proposed by researchers in this area, to improve performance demands of different applications through the network layer of wireless sensor networks protocol stack [7; 8]. Most of the existing routing protocols in wireless sensor networks are designed based on the single-path routing strategy without considering the effects of various traffic load intensities [9; 10]. In this approach, each source node selects a single path which can satisfy performance requirements of the intended applications for transmitting its traffic towards the sink node. Although route discovery through single-path routing approach can be performed with minimum computational complexity and resource utilization, the limited capacity of a single path significantly reduces the achievable network throughput [11; 12]. Furthermore, the low flexibility of this approach against node or link failures may significantly reduce the network performance in critical situations. For instance, whenever the active path fails to transmit data packets (as a result of limited power supply of the sensor nodes, high dynamics of wireless links and physical damages), finding an alternative path to continue data transmission process may cause extra overhead and delay in data delivery. Therefore, due to the resource constraints of sensor nodes and the unreliability of wireless links, single-path routing approaches cannot be considered effective to meet the performance demands of various applications. In order to cope with the limitations of single-path routing techniques, another type of routing strategy, known as multipath routing, has become a promising technique in wireless sensor and ad hoc networks. Dense deployment of the sensor nodes enables a multipath routing approach to construct several paths from individual sensor nodes towards the destination [13]. Discovered paths can be utilized concurrently to provide adequate network resources in intensive traffic conditions. Alternatively, each source node can use only one path for data transmission and switch to another path upon node or link failures. The latter one is mainly used for fault-tolerance purposes, and this is known as alternative path routing. In the past decade, the multipath routing approach has been widely utilized for different network management purposes such as improving data transmission reliability, providing fault-tolerance routing, congestion control and Quality of Service (QoS) support in traditional wired and wireless networks. However, the unique features of wireless sensor networks (e.g., constrained power supply, limited computational capability, and low-memory capacity) and the characteristics of short-range radio communications (e.g., fading and interference [14; 15]) introduce new challenges that should be addressed in the design of multipath routing protocols. Accordingly, existing multipath routing protocols for traditional wireless networks (such as ad hoc networks) cannot be used directly in low-power sensor networks. During the past years, this issue has motivated the research community of wireless sensor networks to develop multipath routing protocols which are suitable for sensor networks. In [7], routing challenges and design issues in wireless sensor networks were discussed. They classified all the existing routing strategies based on the network structure and protocol operation. In [16] the researchers provided a brief overview on the existing fault-tolerant routing protocols in wireless sensor networks and categorized these protocols into retransmission-based and replication-based protocols. The researchers in [17] and [18], classified the existing multipath routing protocols in ad hoc networks based on the primary criterion used in their design. Many routing protocols have been proposed for Wireless Sensor and Ad Hoc Networks.