# Reliability Model Based Dynamic Multi-Level Trust Analysis
## 2 Related Work  
Many literatures have established different trust models. Each model models trust from different angles and describes the trust in different ways. Refs. [4; 5; 6; 7] describes the trust information as binary discrete values, that is, trust and distrust. This way is simple to describe and easy to calculate the combined trust, but the value is rough. Another form of discrete trust representation is multileveldiscrete values [9]. This way describes the trust just like \"very trust, trust, distrust, very distrust\". It can describe the trust more accurately and is easy for users to choose, but this method is not convenient for combined calculation of trust [26], so there is less research on it. Continuous trust information more intuitively and accurately represents the user's beliefs, probabilities, etc., but it is not good for users to accurately define their own trust. Therefore, in [16], a multi-level trust model based on probability values is proposed, which combines the characteristics of continuous model and discrete model.  
Although many literatures indicate that using distrust information when calculating trust information can improve the accuracy of prediction, in fact, not many literatures deal with distrust alone. The two important issues in distrust research are distrust propagation and calculation [4; 26; 31]. Ref. [10] uses subjective logic to express trust information, which includes trust, distrust and uncertainty. Here distrust must be propagated by trust. Uncertainties in subjective logic are distinguished into the posteriori uncertainties and priori uncertainties in [32]. Ref. [33] regards distrust as a separate concept from trust. However, this model adopts the same calculation model for trust and distrust. An important difference in the calculation of distrust and trust is the difference in the way of propagation. In [4], the author discusses the transitivity of distrust and adopts the propagation rule of \"the enemy of the enemy is a friend\". \"Don't take into account your enemies' opinions\" is adopted in [34]. That is if A distrust B, then we can't adopt B's opinions. Ref. [34] introduced that not only trust and distrust are important, but lack of confidence also distinguishes distrust from low-level trust. Therefore, distrust needs to be considered separately and the dissemination mode of distrust is different from trust. In addition to trust and distrust, uncertainty is also an indispensable part of trust information.  
[PERSON]. [27] introduces six dynamic trust models, including blindly positive, blindly negative, slow positive, fast negative, balanced slow, balanced fast and slow negative, fast positive. In the real world, a good reputation is not easy to construct, but trust is easier to lose than to gain [27]. [PERSON]. [28] designs a Bayesian based trust model, but it penalizes too strong. [PERSON]. [27] proposes a SinAlpha trust aggregation engine which adopts trust increase slowly but decrease fast. [PERSON] [35] considers the dynamics of trust from changes in time, and considers the changes in trust weights during the propagation process. However, the current research is aimed at binary trust and does not support the dynamic changes of multi-level trust.  
In this paper, the direct trust relationship is modeled by multi-level trust model. The distrust propagation mechanism is discussed specially. A corresponding trust evaluation method, which is based on MDDs is proposed. This method combines the information on all paths between two parties within OSN, especially considering the propagation rules on one path and the integration rules on all the paths. The dynamic changes of different levels, especially on distrust and uncertainty are addressed.