HyS-R: A Hybrid Subscription-Recovery Method for Downlink Connectivity in 6TiSCH Networks

The Routing Protocol for Low power and lossy network (RPL) is designed to support communication requirements in 6TiSCH networks in Industrial Internet of Things (IIoT) applications. RPL is mostly optimized for uplink communication, however, less attention is given to maintain connectivity for downlink communications. Supporting downlink communications is non-trivial task in process automation and control scenarios within the IIoT. RPL in its current definition is inefficient to support reliable downlink communications in terms of scalability and memory requirements leading to significant degradation in network performance. This paper introduces HyS-R, a Hybrid Subscription-Recovery method to maintain downlink connectivity and mitigate memory limitations in large-scale 6TiSCH networks. The proposed method is based on a relief group that is used as alternative route to unreachable destinations in the network. An intermediate node subscribes to the relief group when it fails to advertise a destination to its next-hop node. In addition, members of the relief group keep searching for alternative forwarders to keep the communication traffic to a minimum. Performance evaluations are carried out and the results demonstrate that the proposed HyS-R attains significant improvements in downlink communications compared to RPL storing and non-storing modes with a margin of energy cost.