# Reputation-Based Trust Approaches in Named Data Networking
## 4 Design Options  
Depending on the implementation domain of a reputation-based trust management scheme, there are various design options which need to be examined. In this section, we explore the design space by laying out all options and assessing the trade-offs of each choice. While analyzing the use cases in which the reputation-based scheme will be deployed, it is vital to examine three basic dimensions, namely formulation, calculation and dissemination, so that a comprehensive framework is tailored [36]. The formulation dimension describes the mathematical model and input for the assessment of reputation values. Formulation includes two main aspects: the reputation measure and the mathematical model used to aggregate ratings. Reputation can be measured using binary, discrete, continuous, string or vector type of values. The aggregation model can be a simple summation or average of ratings, discrete, fuzzy logic, flow-based or probabilistic [37]. **Content rating versus Network entity rating (Figure 1):** A basic dilemma in an NDN trust management scheme is whether the feedback (i.e., ratings or reputation value) should be tied to the disseminated content itself or to the entity which forwards the content (e.g., initial producer or intermediate router). The policy-based approach in force, relates content objects to certificates. Each time a pending Interest is satisfied, the requesting consumer has to verify the signature of the providing party, and thus accept or reject the incoming Data Packet. Managing trust that way, ensures resistance to attacks at consumers and can be utilized to exclude malicious content from intermediate router's caches [35]. Nonetheless, using a credential-based approach, consumers and intermediate routers cannot make any proactive assumptions regarding the trustworthiness of providers, or their future content quality. Introducing a reputation-based approach can have two benefits for NDN operation. On the one hand, when reputation values are assigned to providers, they can function as a criterion to (proactively) avoid transactions with untrustworthy providers, or perform selective forwarding and/or caching. On the other hand, assessing content quality by itself can be useful for consumers or for detecting unexpected or malevolent behaviors (e.g., malfunctioning sensors sending corrupted/false data). Even though rating content has been investigated as an option for either ranking already cached content [22] or to determine the probability of content to be cached [32], the majority of previous works assume that ratings are assigned to network entities (e.g., end-users [34; 35; 32] or routers [33]). Figure 1: (**a**) Content Rating vs (**b**) & (**c**) Network Entity Rating. Regarding reputation calculation and dissemination, the calculation dimension addresses the practical design and implementation of the reputation algorithm, whereas the dissemination dimension focuses on the mechanisms used for distribution and storage of ratings and reputation values among entities within the system. Based on classifications [38] and measures [37] for reputation-based schemes, we highlight below the ones that are most relevant for NDN. **Centralized versus Decentralized (Figure 2):** A primary design decision regards the process through which reputation is being calculated and/or distributed in the network. Based on this, trust models are divided into centralized or decentralized ones. In the centralized case, a trusted central authority undertakes the responsibility to collect the ratings, compute the reputation values, store and announce them to network entities. Although this method limits computational complexity, since there is no need for every node to evaluate reputation, there is a trade-off with the introduced overhead for ratings' and reputation values' exchange. Another important drawback of this approach is that the single trusted entity represents a single point of failure. Once this is compromised, the entire network operation can fail.