## 3 Related Work  
The impact of interference on wireless networks is a well-studied problem, and interference mitigation strategies have been proposed for every IETF protocol layer. For example, the PHY configuration of IEEE 802.15.4 [20] links can be adapted based on link quality [21]. At the Medium Access Control (MAC) layer, TSCH (one of several IEEE 802.15.4 MAC protocols) may use, e.g., interference-aware scheduling algorithms [22] or channel blacklisting schemes [23]. At the routing layer, large amounts of (non-intentional) cross-technology interference (e.g., in the 2.4 GHz range) can cause RPL end-to-end reliability to drop below 10% [24], whilst periodic jamming may cause severe network instability [5]. Despite the numerous scientific papers focusing on RPL optimizations (for, e.g., mobility support, authentication and security, Quality of Service (QoS), etc. ), few RPL optimizations focus on improving interference robustness in general, let alone robustness against jamming. One example proposes the use of back-up RPL parents that are least likely to be impacted by the same jammer [25]. However, the main drawback of the aforementioned interference mitigation strategies (\(\
eq\)routing solutions) is that they employ single-interface routing solutions, and hence do not cope well with wide-band jamming/cross-technology interference and/or unfavorable electromagnetic properties of the environment (e.g., multi-path fading), which may render an entire frequency band unusable. Since our primary goal is to fully exploit modern multi-radio platforms, this section gives an overview of routing-centered solutions for multi-modal (i.e., multi-interface/multi-PHY) RPL-based communication approaches (\(\
eq\)interference mitigation strategies). [PERSON] et al. [9; 10] identified three approaches to multi-interface management: Multiple RPL Instances (MI), Parent Oriented (PO), and Interface Oriented (IO):  
1. The MI solution requires one RPL Instance per interface type. Since a node can only belong to one DODAG per instance, belonging to multiple DODAGs means joining multiple instances (each governed by an OF). The OFs can then be tailored to interface types. As such, the main advantage of the MI approach is that it can reuse a generic RPL implementation with a per-technology OF. However, to prevent loops, packets may only switch once between instances to a higher RPLInstanceID. Hence, the MI approach can handle only one link failure along a path and packet switching is unidirectional. In contrast, the proposed PO and IO solutions leverage multi-interfaced nodes in a single DODAG. 2. The PO solution is based on OF0 (see Section 2.2). It defines \(\Delta R_{c}\) as the average link metric to a neighbor calculated over all interfaces. If an interface is unavailable, its link metric defaults to a fixed maximum value during averaging. Thus, a neighbor incurs a penalty for each interface through which it is unavailable. Unlike OF0, the parent set size is unlimited. The parent yielding the lowest computed rank still becomes the preferred parent, except when there already is one, and one of its interfaces becomes unavailable. Then, metric averaging for the preferred parent is deferred by one OF call. This delay aims to prevent DODAG instability. Note that the interface providing the best link metric towards a neighbor becomes the default interface for (unicast) communication with said neighbor, i.e., it becomes the preferred interface. Preferred interface selection is transparent to RPL. [PERSON] et al. 's [9; 10] addressingarchitecture is rather vague. The authors simply stated that \"the PO solution combines multiple links into a single virtual link\" ([PERSON], 2018) (p. 2). To the best of our understanding (no code was available for further examination), this means that each node owns a single link-layer address and a single routable and link-local IPv6 address, regardless of how many interfaces it possesses.