## 2 Routing Challenges in TN-NTN Mobile Networks  
Integration of terrestrial, aerial, and satellite network segments will enable an omnipresent network. Fast mobility of the satellite part poses several challenges and issues regarding the organisation of routing and its optimisation. The key challenges to be tackled to enable unified 3D networks include the following [3,5,17]:  
* **Routing convergence--**applying traditional Internet Protocol (IP) routing schemes, e.g., Open Shortest Path First (OSPF), is problematic in NTNs due to mobility of network nodes. Frequent reconfigurations of connections between nodes require continuous updates of routing tables and link costs to maintain up-to-date information on the network state within the nodes. The changes occur repeatedly, causing almost constant updates, which impacts the convergence and leads to unstable routing and large signalling overhead [18]. Adoption of SDN to provide dynamic and flexible user traffic steering is a common solution [18]. However, the original SDN concept lacks CP scalability, so its wide-scale deployments are problematic. Distributed SDN architectures allow for mitigating SDNC overload at the cost of complexity--E2E functionality requires the development of coordination mechanisms across multiple SDNCs. Also, in wide-scale deployments, SDNCs placement for optimal network control needs careful consideration (in terms of both intra- and cross-layer CP latency, network observability (obtaining reliable network monitoring information), and recovery). * **Temporal and predictive routing--**topology changes caused by the mobile NTN nodes can lead to broken links or changes of link properties (e.g., bandwidth drops due to partial occlusion). Addressing these issues will require frequent rerouting of flows and path updates on a topology change, leading to extensive signalling traffic. To calculate viable routes with increased durability and mitigate the above issues, link availability prediction and mechanisms for fast updates of paths (via low-latency monitoring, TE metrics adjustments, etc.) are vital. These are required to support time-scheduled routing schemes and enhancements by contextual information (e.g., satellite orbits, air interfaces alignment, object occlusion, etc.). * **Resilience--**the disappearing links and broken network paths can lead to severe Service Level Agreement (SLA) violations. Hence, increasing resilience to minimise the impact of communication cutoffs is a core requirement to enable QoS-driven services. Potential solutions include multi-path and/or node/edge-disjoint routing [19], in-switch buffering mechanisms in case of access node isolation (i.e., store-and-forward) [20,21], or predictive routing schemes. * **Optimisation--**conventional TE algorithms--e.g., Multi-Protocol Label Switching-Traffic Engineering (MPLS-TE)--cannot be used effectively in NTNs due to long convergence time. The emerging routing and TE methods will need to consider both QoS constraints and effective traffic distribution to handle limited ISL capacity. Moreover, the synchronisation of network state information across TE databases and its unified representation in NTNs becomes crucial to facilitate E2E control and Artificial Intelligence (AI)-driven optimisation. To achieve the latter, the TE algorithms need to be able to extract the information from the nodes and link relationships rather than the fixed graph structure to avoid overly complex and monolithic models. * **Asset heterogeneity--**future mobile networks are expected to combine heterogeneous systems with diversified service capabilities and properties (radio interfaces, protocol stack, etc. ), i.e., the _Network of Networks_[22]. To enable E2E services in 3D networks, it is essential to develop dynamic QoS management and coordination mechanisms to provide paths satisfying E2E QoS regimes.