# Congestion Control and Traffic Differentiation for Heterogeneous 6 TiSCH Networks in IIoT
## 1 Introduction  
The Industrial Internet of Things (IIoT) is the sub-category of the IoT that targets the industrial sector to improve productivity and efficiency [1]. Unlike consumer IoT, IIoT applications are characterized by stringent communication requirements in terms of reliability and delay [2]. The IPv6 over Time-Slotted Channel Hopping (6 TiSCH) working group was established in 2013 with the aim to enable industrial-grade IPv6 networks to foster IIoT [3]. The 6 TiSCH network is a key component to enable the adoption of IPv6 in industrial standards and the convergence of Operational Technology (OT) with Information Technology (IT) where industrial devices (e.g., sensors, actuators, robots, etc.) are enabled to connect to the cloud. The collected information from the industrial site is integrated via a control and management platform to improve the operational efficiency and productivity of the manufacturing process [4]. In this context, the 6 TiSCH working group is developing solutions for missing components, such as schedule management, deterministic IPv6 flows, link management, and routing [5]. 6 TiSCH networks are Low-power and Lossy Networks (LLNs) that are constructed using low-cost and resource-constrained devices, namely LLN devices [6; 7]. In 6 TiSCH networks, communication routes are constructed and maintained through the Routing Protocol for LLN (RPL) [6]. RPL organizesthe 6 TiSCH network as a Destination-Oriented Acyclic Graph (DODAG) rooted at the sink node, namely the DODAG root. Each node is attached to a parent node that relays all its sensory information to the DODAG root. RPL employs the concept of RANKto define the relative position of a node with respect to the DODAG root. In RPL, the Objective Function (OF) describes the rules to compute the RANK and how it should be used to select the preferred parent [7]. Currently, there are two OFs defined by the RPL standard, OF zero (OF0) [8] and Minimum Rank with Hysteresis OF (MRHOF) [9]; however, there is no obligation to use a specific OF. Information about the RANK and OF is advertised through DODAG Information Object (DIO) messages, whose transmission interval is controlled by the Trickle timer algorithm [10]. Based on the information received by the DIO message and the adopted OF, the node selects a preferred parent from its candidate parent set. In heavy traffic load circumstances, parent nodes are likely to be congested due to the increased forwarding rate and the limited buffer size of LLN nodes. In the RPL standard, a parent node is selected either based on hop-counts or the link quality [6] without considering the congestion level causing an imbalanced network. Congestion has a profound impact on the network performance in terms of packet loss, delay, and energy consumption, and experimental measurements revealed that in high traffic scenarios, the main cause of packet loss is due to congestion [11]. The current RPL specifications do not specify how to detect and control congestion in 6 TiSCH networks. Moreover, in many industrial applications, such as monitoring and control scenarios, sensor nodes generate different traffic types with varying real-time requirements [12]. In certain scenarios, e.g., emergencies, critical traffic has higher importance than other types of traffic and must be delivered within a limited time to maintain stability, functionality, and to avoid dangerous situations. For data transmission scheduling, the current version of RPL considers the nodes to utilize either the First-In First-Out (FIFO) policy [13] or Last-In First-Out (LIFO) policy [14] in their output buffer. RPL has no explicit mechanism to efficiently handle the transmission of heterogeneous traffic based on the corresponding criticality and performance requirements. Accordingly, critical packets may be blocked and transmitted after non-critical ones, hence violating their timing limits. Therefore, traffic priority and network congestion are key research challenges in heterogeneous 6 TiSCH networks in IIoT applications.