## 2 In-Network Computing: What and Why
### Representative Use Cases
#### 2.2.2 DTNs  
DTNs extend the traditional concept of Digital Twins (DTs) by introducing inter-twin communications, enabling a networked ecosystem of digital entities that collaboratively manage complex systems or environments [43]. Traditionally, DTs rely on intra-twin communications, focusing on interactions with the physical twin only, with the targets of synchronizing its state or analyzing its behavior. However, DTNs expand this paradigm by creating communities of DTs that interact with each other in the virtual world, thus enabling new services like cooperative decision making, distributed simulation, and collective optimization [44]. DTNs are particularly relevant in 6G for addressing real-world challenges in key domains. For example, in smart cities, DTNs can enable real-time monitoring and optimization of urban infrastructure, such as traffic flow, energy distribution, and publicsafety systems, by facilitating seamless communication and coordination among the digital representations of these components [45]. In healthcare, DTNs can enable innovative solutions like remote surgery simulations or collaborative diagnostics by synchronizing DTs of medical devices, patients, and healthcare professionals [46]. Therefore, in the context of 6G, DTNs are envisioned as foundational components for achieving comprehensive network intelligence [42] and advanced user-centric services [47]. DTs can be hosted in various parts of the network, depending on their application and performance requirements [47]. For instance, they can reside in centralized cloud servers, leveraging the vast computational and storage resources of the cloud for tasks requiring significant processing power or long-term data analysis. Alternatively, DTs can be deployed closer to data sources, such as at the edge of the network, where latency-sensitive tasks and real-time interactions demand minimal delays. The choice of hosting location introduces additional challenges in managing communication and synchronization between DTs, particularly when they span multiple levels of the network infrastructure. Therefore, inter-twin communications in DTNs can vary significantly in their nature and underlying requirements. For instance, DTs hosted on the same physical device, such as an edge server, can interact with minimal latency and high reliability. Conversely, when DTs are distributed across different network devices--such as two distinct edge servers or a remote cloud server and an edge server--latency and reliability can fluctuate depending on the quality of the underlying communication networks. These variations in latency, bandwidth, and reliability, exacerbated by the additional need of maintaining consistent and synchronized interactions between DTs, make the design of robust and efficient inter-twin communication mechanisms a critical challenge for realizing DTNs. Scalability is another significant challenge, as 6G is expected to support millions of interconnected DTs. Despite their significant potential, the practical deployment of DTNs remains largely unexplored in the literature, though emerging technologies suggest they could become a reality in the near future. 6G advancements, such as sub-terahertz communication, IRS and semantic communication, can empower DTNs by enabling ultra-reliable low-latency communications and dynamic resource allocation [42], seamlessly supporting inter-twin interactions even in highly heterogeneous and geographically dispersed environments. Additionally, pervasive AI in 6G enables DTNs to process and act on contextual information, improving their ability to dynamically adapt to network conditions and application demands. In this context, INC can also play a transformative role. By embedding computational capabilities directly into the network, it can facilitate the dynamic instantiation and management of DTs closer to their physical counterparts or other points of interaction.