## 3 NDN Enabled Communication and Transport Research in V2X
### Dynamic Forwarding  
In recent literature, a plethora of forwarding strategies for NDN were proposed. Examples include, but are not limited to (ideal) Nearest Replica Routing ([i]NRR) [9], adaptive hop-by-hop forwarding strategy using reinforcement learning inspired by the Q-routing framework (INFORM) [10], Request Forwarding Algorithm (RFA) [11], Stochastic Adaptive Forwarding (SAF) [12]. [PERSON] et al. [13] give a comprehensive overview on existing forwarding strategies for NDN in a general context. However, this survey does not cover forwarding strategies designed for vehicular networks. [PERSON] et al. [14] provided an implementation of NDN for vehicular networks (V-NDN). Their forwarding implementation forwards each Interest to all available interfaces. Following work describes Navigo [15], a forwarding strategy using the geolocation of vehicles. Another forwarding strategy leveraging trajectories of vehicles in addition to geolocation is proposed by [PERSON] et al. [16]. Controlled data propagation in vehicular named data networking (CODIE), a forwarding strategy is presented in [17]. CODIE features a data dissemination limit which allows to limit the hops a Data packet travels back. A forwarding strategy based on the link stability (LISIC) for V-NDN is provided in [18]. A density-aware and delay tolerant forwarding strategy is introduced by [PERSON] et al. [19]. Their approach proposes a two phase forwarding strategy (rebroadcast and retransmissions). Density-awareness is achieved by utilization of a neighborhood list which is updated whenever the vehicle observes a packet in-the-air. A multi-hop and multi-path routing approach for fast content retrieval in VANETs is proposed in [20]. Figure 3: The architecture of a NDN Face. A Face consists of two mechanisms: link service and transport. The advantage of NDN over IP for vehicular ad-hoc networks is that content itself is addressed. This resolves a lot of issues of routing and forwarding. The principles of dynamic forwarding and a stateful data plane have tremendous advantages for V2X communication. For instance, forwarding strategies can exploit their neighborhood in order to find new _routes_ to specific content. This somehow mimics protocols we know quite well from ad-hoc routing in IP networks (e.g., Ad-Hoc On-demand Vector routing [21]). However, the request response principle of NDN also introduces a problem that has not yet been extensively investigated in current literature. The Data packet (whether at the origin or in a cache) that matches an Interest, is sent along the reverse path of the matching Interest. When an intermediate node leaves communication range, the Data packet will be discarded. Figure 4 illustrates the problem of discarded Data packet(s) in case that intermediate nodes become out of range after Interests have been forwarded to the origin or hit a cached copy. As vehicle E _replaces_ vehicle D (position wise) the Data packet will be discarded by D because it has no corresponding PIT entries or by B due to the missing Face. This example sketches a problem that has a tremendous impact on the performance of NDN (throughput and delay wise) in medium to high velocity V2X scenarios. Even if the Interest was successfully forwarded to the origin, the corresponding Data packet cannot be delivered which would trigger retransmissions of Interests and consequentially increase delay. The implementation of the forwarding core of NDN (part of the NFD) does support dynamic hand over of Data packets which match a specific PIT entry but unsolicited data may only be cached or discarded. Therefore, we suggest to delegate the decision what happens with unsolicited Data to the forwarding strategy registered for the corresponding prefix. This would allow that Data may be even handed over to nodes without a corresponding PIT entry (or PIT entries are created on-demand) in such scenarios.