Routing Protocol for Low Power and Lossy Networks (RPL) is the de-facto routing standard for Internet of Things (IoT) applications. It has been designed to work with resource-constrained devices in constrained environment. In RPL, objective functions (OFs) play the role of selecting optimal paths. However, the existing OFs are not suitable for Mission Critical IoT (MC-IoT) applications. MC-IoT applications expect that the data should reach the destination within a strict deadline. So, reliability and delay are the two significant requirements in such type of applications. An existing work RMP-RPL achieves the hard reliability requirement for MC-IoT applications. However, it did not consider the delay requirement. Also, it was not tested for different traffic rates. Therefore, this paper proposes a reliable and delay-efficient multi-path RPL (RDMP-RPL) that optimizes both reliability and delay. It considers buffer and channel loss probabilities while calculating reliability. For delay calculation, it considers queueing and link delays. In addition, the protocol presents a method to select backup parents to meet end-to-end delay requirements. The results demonstrate that RDMP-RPL maintains a packet delivery ratio (PDR) of at least 99% and at most one second end-to-end delay (EED) considering a maximum of 5 hops for a packet to reach the destination from any of the source nodes for grid topology. But, for random topology, the number of hops can exceed 5. RDMP-RPL cannot achieve the strict requirements in situations where traffic rate, mobility and number of nodes cross a specific threshold. However, it outperforms other benchmark protocols by a significant margin.

Mission-critical Internet of Things (MC-IoT) applications span from the industrial field to the battlefield and from smart homes to healthcare. Reliability is one of the stringent requirements of such applications. Routing in low-power and Lossy Networks (RPL) is a standard routing protocol traditionally used in IoT applications. Unlike the traditional single-path-based RPL, the reliable multi-path RPL (RMP-RPL) selectskparents for each transmitting node to achieve the hard reliability requirement in MC-IoT applications. However, the RMP-RPL fails to achieve the reliability requirement when the number of source nodes and traffic rate increases. In this paper, a multi-path game theoretic congestion control (GTMP-RPL) approach is proposed on top of the RMP-RPL to reduce congestion at the parent nodes of any child. In case of congestion at any one of the selectedkparents, its child node replaces the congested parent by a non-congested parent with minimum rank from the set of remaining nodes. The rank of a node is calculated based on the Data Packet Drop Ratio (DPDR), Expected Transmission Count (ETX), and Node Mobility (NM) of that node. If no non-congested parent is available for that child node, a non-cooperative game is played among siblings to adjust their data transmission rates based on congestion occurrence at the parent, energy spent by the child, and parent connectivity of the child node. The solution obtained for the proposed game, using the Lagrange multiplier and Karush-Kuhn-Tucker (KKT) conditions, is used to reduce congestion at the parent node. The proposed scheme is validated using the cooja simulator in Contiki OS. Simulation results show that GTMP-RPL achieves a 99% packet delivery ratio for at most 50 source nodes with a traffic rate of 30 pkts/min present in a random topology of 101 nodes. In addition, it outperforms the other benchmark schemes by a significant margin to achieve hard reliability.