## 3 Overview of ICN Architectures
#### Named Data Networking (NDN)  
In NDN, Named Data Objects (NDOs) are published at designated nodes, facilitated by routing protocols to disseminate information on NDO locations. Routing within NDN can capitalize on aggregation through a hierarchical naming convention. Requests for NDOs--referred to as \"interest\" packets--are forwarded hop-by-hop through the network by content routers (CRs). Each CR relies on three core data structures to manage this process: the content store (CS), the Pending Interest Table (PIT), and the Forwarding Information Base (FIB), as illustrated in Figure 1.  
The CS serves as a local cache, temporarily holding data objects that have passed through the router. When a CR receives an interest packet (steps 1-3 in Figure 1), it first checks the CS for a matching content object. If found, the data are sent along the incoming interface, and the interest is dropped.  
If the requested content is not in the CS, the router consults the FIB, which maps name prefixes to one or more outgoing interfaces. Each FIB entry consists of a name prefix and an ordered list of next-hop interfaces, enabling multi-path forwarding and supporting path-performance evaluation [26]. The router uses the longest prefix match to select the appropriate forwarding path and sends the interest packet toward the next CR.  
Meanwhile, the router records the interest in the PIT. Each PIT entry is indexed by the content name and includes a list of interfaces from which the interest has arrived. This  
Figure 1: Overview of content-centric networking (CCN) or Named Data Networking (NDN) architecture.  
\begin{table}
\begin{tabular}{l l} \hline \hline
**Abbreviation** & **Description** \\ \hline CR & Content router \\ CS & Content store \\ FIB & Forwarding Information Base \\ LAN & Local Area Network \\ LRU & Least Recently Used \\ MANET & Mobile Ad Hoc NETwork \\ NDO & Named Data Object \\ PIT & Pending Interest Table \\ VANET & Vehicular Ad Hoc Network \\ WAN & Wide Area Network \\ WLAN & Wireless Local Area Network \\ WSN & Wireless Sensor Network \\ \hline \hline \end{tabular}
\end{table}
Table 1: List of abbreviations used frequently in this article.  
ensures that when the corresponding data object is retrieved, it can be forwarded back to all requesting interfaces. Once the requested content is located--either at a publisher or cached in a downstream router--it is returned in a \"data\" message. The interest is then discarded, and the data travel back to the requester(s) along the reverse path, guided by the PIT entries at each hop (steps 4-6).  
CCN natively supports on-path caching; any data message passing through the router is cached by default, allowing future requests for the same content to be served directly from the local CS without forwarding the interest further. From the perspective of a CCN node, a balance exists between requests and responses, with each request generating either a single response or none at all. Depending on local configurations or observed network performance, CCN nodes can adopt various strategies for request transmission pacing and interface selection.  
Security for NDOs is ensured through public key cryptography, while trust in cryptographic keys can be achieved through multiple approaches. These include deploying certificate chains modeled after a hierarchical naming system, akin to public key infrastructure, or utilizing authenticated data from a trusted authority.