## 1 Introduction
### Related Work
#### 1.1.1 Mobile Data Offloading  
Multiple approaches have been proposed to support mobile data offloading in cellular networks, depending on the types of constraints that should be satisfied for content delivery and on the techniques used to transmit the content. The reader is referred to [1; 13] for extensive surveys of existing solutions. D2D-based data offloading is a specific class of mobile data offloading solutions in which the content must be delivered within a deadline, and D2D communications are used to disseminate the content to intended destinations. Most research studies on D2D-based data offloading focus on designing efficient strategies to select the best subset of users (often called seed users) who should initially receive a copy of the requested content and bootstrap the dissemination of that content in the opportunistic network. The authors of [14] formulate a utility maximisation problem to model the subset selection, considering the mobility of users, traffic heterogeneity, and the cache size on mobile nodes. The authors of [6] propose injecting a small number of initial copies of the content and defining control loops that keep track of the delivered content and assess the opportunity of re-injecting copies. An influence maximisation problem is adopted in [15] to select suitable seed nodes based on topological information, such as node centrality. A community-based algorithm is developed in [16] to select seed users based on the characteristics of the contact patterns between end-users. A similar approach based on the contact graph of mobile nodes is proposed in [17]. The importance of energy efficiency and resource optimisation in D2D communications has also been studied. For instance, the authors in [18] address power optimisation in D2D-enabled 5G cellular networks, demonstrating how energy-efficient beam-forming and power control strategies can reduce network congestion while satisfying user quality constraints. Similarly, an energy-efficient power allocation scheme for D2D underlay communication in distributed antenna systems (DASs) is proposed in [19]. While these studies focus on resource and power management, they implicitly support traffic offloading by enabling direct communication between devices and reducing the need for base station involvement. Recent works have also explored user-side optimization strategies for D2D-assisted offloading in cache-enabled networks. For instance, the study in [20] proposes a joint user-side recommendation and D2D-based offloading approach to reshape content requests dynamically. The authors of [21] propose leveraging the predicted mobility of users to select as seed nodes those users with the highest probability of encountering users requesting that content. An encounter prediction model coupled with a game-theoretic approach for seed selection is also developed in [22]. A learning framework is proposed in [4] to determine how many content copies should be injected into the seed nodes over time, not only to bootstrap the content dissemination. Unlike the aforementioned solutions, we analyse an offloading scheme called OORS, which does not rely on seed nodes for content dissemination. Additionally, the content is not propagated among nodes in an epidemic-like manner but is restricted to those nodes that have explicitly requested it. We highlight that by involving only nodes that express interest in the specific content, OORS optimises resource usage by reducing unnecessary transmissions and buffer occupancy. However, a caching and routing policy that is too restrictive may negatively impact offloading efficiency if not carefully configured.