# SDN Architecture for 6 LoWPAN Wireless Sensor Networks
## 2 Related Work  
The goal of SD6 WSN is to bring the SDN approach to 6 LoWPAN, supporting the implementation of complex networking applications not supported by RPL. The last is an IPv6 routing protocol that specifies how to construct a routing topology based on a destination oriented directed acyclic graph (DODAG), using an objective function that defines how a node in a DODAG selects its parents according to the routing metrics [3]. The objective function computes a rank value, which represents the distance from the node up to the root with respect to a given metric. RPL uses the trickle algorithm [10] to react to changes in topology that, at steady state, sends control messages at low speed, and speeding up to resolve inconsistencies [11; 12; 13]. DODAG is best suited for converge-cast traffic, and when direct communication between sensors occurs, WSN would be subject to congestion in the links near the root. The authors in [4] point out difficulties regarding the possible fragmentation of RPL control messages and issues regarding the hypothesis of bi-directionality assumed for the links since their quality is only evaluated in the routes towards the root. They also point out that RPL does not guarantee the absence of loops, and even if it can repair a loop when it is detected, this increases latency. The performance of RPL in AMI networks, with the emphasis on the reliability and stability, is evaluated in [14]. The conclusion is that the protocol often chooses sub-optimized paths due to inefficient routing decisions, which is the main factor in decreasing the packet delivery rate. In a subsequent paper [15], the same authors analyzed the role of the RPL protocol in a broader context of smart grid networks. Although the same restrictions were mentioned, the authors concluded that the application of RPL is viable. RPL uses the expected transmission count (ETX) [16] as one of the metrics to build the DODAG; however, it does not specify how to implement it. Several authors implemented measurement of ETX in the MAC layer [17; 18], but this approach has a limitation. The calculation of ETX in the MAC layer can only be done when messages are sent in unicast mode. Because RPL constructs DODAG using control messages sent in multicast, the topology is built using pre-defined default values, which leads to the use of poor quality links. This difficulty was pointed out by [PERSON] et al. [18], who studied the problem of neighborhood management in high-density RPL networks. Another known limitation of RPL is the size of the parent set of a node in the DODAG. If the size is small and the quality of the links is variable, the DODAG will be unstable. When the measurement engine identifies that the ETX has increased too much, the link is considered not usable, causing the use of an alternate parent. However, when there are no alternative parents, the node sends messages requesting a parent, which can cause the entire subtree to be rebuilt below it. A parent set with a size too large inhibits a child node from selecting the best parent, leading to poor routing. This is because the rank of a node must be greater than the rank of any member of its set of parents [19]. RPL has also been compared to other alternatives. [PERSON] et al. [20] presented a comparative study between RPL and AODV (ad-hoc on-demand distance vector protocol) in intelligent energy metering networks. The simulations concluded that the AODV presents inferior performance in termsof latency and packet delivery rate (PDR) with respect to RPL in networks with a large number of nodes. [PERSON] and [PERSON] [PERSON] and [PERSON] (2012) compared RPL with the LOAD (6 LoWPAN ad-Hoc on-demand distance vector routing protocol), in large LLN networks with a predominance of bidirectional traffic. The authors argued that RPL is optimized for multipoint-to-point networks, with rare point-to-multipoint and point-to-point traffic.