# Caching Transient Contents in Vehicular Named Data Networking: A Performance Analysis
## 1 Introduction  
Recent advancements in the fields of sensing, computing, communication, and networking technologies are contributing in making vehicles multi-faceted elements (equipped with cameras, sensors, radars, storage, processing, and positioning capabilities) of smart and connected cities. Vehicular on-board units (OBUs) are able to interact among each other through vehicle-to-vehicle (V2V) communications, and with nearby road-side units (RSUs), traffic lights, pedestrians, and edge nodes through vehicle-to-infrastructure (V2I) communications. Thanks to vehicle-to-everything (V2X) connectivity, interactions with remote entities, such as cloud servers and Internet facilities, are also available. V2X connectivity would overall increase the driving and traveling experience, by enabling a rich set of applications, ranging from safety and traffic efficiency to infotainment and, more recently, cooperative and automated driving. Data exchanged by such revolutionary vehicular application will exhibit different features (e.g., size, lifetime, generation frequency, dissemination scope, popularity) and requirements (e.g., latency, throughput, reliability). Named Data Networking (NDN) has natural advantages to greatly overcome the challenges of vehicular ad-hoc networks (VANETs), such as rapidly changing topology, harsh propagation environments and short-lived and intermittent connectivity, thanks to its name-based routing and native in-network caching capabilities [1]. Moreover, being focused on _what_ content to retrieve, instead of _where_ the content is located, NDN well matches vehicular applications where typically,(_i_) communicating entities are interested in retrieving content (e.g., road congestion information, weather conditions) regardless of the identity of the node(s) producing it and (_ii_) the requested contents have a spatial and/or temporal scope. NDN implements a naive caching strategy that lets nodes cache _all_ the received contents. However, indiscriminate caching may waste network resources and reduce the cache efficiency and it is poorly suited for contents that exhibit a limited validity, which are frequently exchanged in VANET. Examples of transients contents requested by vehicular applications are, for instance, those related to parking lots availability, road congestion, maps of the surroundings [2; 3]. If the content lifetime is not properly conveyed in packets and accounted for in the caching decision, stale contents risk to be propagated by affecting the behaviour of applications relying on them. Such an effect is particularly exacerbated in VANETs, due to the broadcast nature of the wireless medium that facilitates sharing of data, also of useless data, if they expired. However, literature on caching transient contents in NDN is almost unexplored in VANETs and still in its early stage for Internet of Things (IoT) contents [4; 5; 6] and wired networks [7; 8]. To fill this gap, the following contributions are provided in this paper:  
* We showcase the benefits of tracking the content lifetime in NDN packets to prevent stale information from becoming disseminated in the vehicular network. * We propose a simple but effective NDN-compliant caching strategy that accounts for the content lifetime, not only for replacement purposes but also for the caching decision. * We perform a comprehensive simulation campaign in ndnSIM [9], the official ns-3-based simulator of the NDN community, to study the impact of the content lifetime in the caching decision when considering two distinct vehicular scenarios, urban and highway, and varying traffic load and content popularity settings. The remainder of this paper is organized as follows: Section 2 introduces the NDN paradigm. An overview of vehicular applications and connectivity options is provided in Section 3. Section 4 discusses the status quo on Vehicular NDN (V-NDN), with special focus on caching strategies in the literature. Section 5 motivates our study, by also providing early results, and presents the proposal.