Trust Solution and Trust Challenges

The trust mediator addresses different trust challenges, as follows:

• Community-based challenge (domain-specific information): The trust mediator is not community-based, because the trust solution utilizes general TMs, which are categorized in different categories and suit different domains. Requestors have many requirements, or trust preferences, and services have many properties, which are published as TMs. For example, if a requestor requires a service based on response time and security, he/she is able to search from different service brokers in different domains and communities for services that meet his/her requirement, because services support different TMs that can satisfy his/her trust preferences.

• Rating providers challenge: The trust solution rates service providers as well as services. The rating of providers helps to impede the problems of whitewashing, cold start, and malicious behaviour as well as reducing the overhead of trust bootstrapping new services in the trust mediator.

• Trust bootstrapping challenge: The trust solution includes an approach for trust boot- strapping services and providers. While current solutions in the trust literature address

reputation bootstrapping, our approach is unique in addressing trust bootstrapping. In addition, the trust mediator bootstraps service providers beside bootstrapping services, and there are no previous works that consider the trust bootstrapping of service providers. Furthermore, the proposed trust bootstrapping approach addresses other limitations on the current bootstrapping approaches in the literature. For instance, our trust bootstrap- ping approach does not assign default values, is not based entirely on feedback, limits

the overhead on the requestor side, is not based on communities, thus it is a universal approach, and addresses cold start and whitewashing challenges.

• All services will be trust bootstrapped at publication time and have initial trust rates. The

TT M,Ts, andTprare stored in the rating registry ready to be returned to the requestors. In

particular, the initialTT M support the bootstrapping of the trust preference rates. There-

fore, trust rates are available and the trust-based search can be conducted immediately at the search time, which results in shorter searching time.

Other approaches in the literature may evaluate services’ rates on the basis of information that is not supported by the services as trust information. For example, a service may have a low response time, but it does not publish the response time as a TM. Our approach will not consider the response time TM in the trust evaluation of the service, because the service does not provide that TM as a trust information; however, other approaches will consider the response time on the service trust evaluation. The other approaches rank all services that meet the non-functional QoS properties of a requestor regardless of whether or not the services publish the selected information as a TM.

• The trust bootstrapping of services and providers is initially conducted, and then the trust mediator continues the evaluation of theTT M,Ts, andTpr. This continuous and dynamic

evaluation, based on different approaches, such as rating service providers, allows for additional evaluations of the trust rates and helps to impede the whitewashing and cold start. In contrast, other approaches are based on a one-time evaluation [7, 52].

• Change identity challenge or whitewashing: The trust mediator addresses the whitewash- ing challenge and establishes trust in a way that discourages providers from changing identities for the following reasons:

1. Trust establishment based on a fair trust bootstrapping approach. The rejected providers cannot enter the service broker with initial default ratings values, such as average or high ratings.

2. Trust establishment occurs through a long-term interactions with many of the provider’s services. Thus, service providers who have changed their identity will undergo

a long process of trust validation to rebuild their trust, competence, and honesty. Moreover, the service broker can develop a more robust trust rate by considering the time and the value of the transactions.

3. Trust establishment is based on the providers’ services. Thus, the trust rate of a service affects the rate of the provider. A provider who attempts to change a service’s identity will not benefit because the provider’s rate has already been affected by that malicious service. As a result, providers will be encouraged to behave in a trustworthy and in consistent manner.

• Cold start: The trust solution addresses the cold start issue by trust bootstrapping and conducting continuous evaluations of trust rates. Because rating is conducted at registra- tion time, the trust mediator may perform a number of evaluations of theTT M, Ts, and Tpr. The number of trust evaluations for Ts andTpr is presented as snum and pnum, re-

spectively, as presented in Section 8.3.3. Thus, services and service providers may have enough initial rates at searching time. Ts and Tpr, along with snum and pnum, enable

requestors to make better selection decisions.

• Services and service providers may accumulate high trust ratings and then attack the consumers or systems. The trust models use exponential averaging, which assigns more weight to the most recent interactions, thus minimizing the importance of past observa- tions and mitigating variation and bias. In addition, exponential averaging reflects the true value of the current rating, and therefore, it is more capable of detecting malicious behaviour.

• The trust bootstrapping technique minimize the overhead on the requestor side. Specif- ically, requestors do not play a significant role in the trust bootstrapping process. Their contribution is relatively low, as they have the option of providing feedback after consum- ing services.

• The trust solution eliminates the need for matchmaking approach and the disclosing of requestors policies. Specifically, the solution identifies the TMs, from which requestors can select a service based on their trust preferences.