# Behavioural Verification: Preventing Report Fraud in Decentralized Advert Distribution Systems
## 4 Model Overview
### System Architecture  
As previously mentioned, the advert reporting system that we submit with this work is an extension of our Advert Distribution System (ADS) that we briefly explained in Section 3 and therefore shares much of the same components and architecture. Figure 2 illustrates the architecture of the system and provides a high-level overview of the system's operation which can be divided into three stages with each of them performed under a different sub-protocol later examined in Section 5.3. The Users, Publishers, Agents, Ad-Dealers and Advertisers represent the same stakeholders as in our earlier ADS system but at the same time have the added functionality of managing Ad-Reports with the help of the Broker. The Broker assumes the role of a trusted representative of the Advertisers to the Ad-Dealers and Publishers. As the Advertisers are too numerous to operate independently while still remaining coordinated, they employ the services of the Broker whose job is to collect and verify the Ad-Reports that are dispatched by the Ad-Dealers. The user initiates the first stage that is marked in Figure 2 as **(1) Report Form or RF Collection** by composing an RF-Request the purpose of which is to inform the Broker of the user's intention to submit Ad-Reports. The RF-Request is then sent to an Agent who operates as an intermediate node of the opportunistic network. The job of the Agent is to physically transfer the RF-Request to one of the Ad-Dealers, who then forwards to the Broker. Upon receiving the RF-Request, the Broker issues a Request Form (RF) which is sent in the opposite direction: back to the Ad-Dealer, who forwards  
Figure 1: Decentralized advert distribution system over opportunistic network. it to the Agent, so that it may be conveyed back to the requesting user. The RF contains necessary information which the user needs in order to compose and submit his/her Ad-Reports later on. The second stage, which is marked as **(2) AD Collection**, is essentially the same advert delivery operation which we described in Section 3. Through the use of an Agent, the user sends an Ad-Request message to the Ad-Dealers in order to demand specific adverts. The Ad-Dealer who receives the Ad-Request may then obtain the requested adverts from the appropriate Advertisers and send them back to the user via the same Agent. Note that this operation can take place multiple times via different Agents and Ad-Dealers. Furthermore, it may also take place in combination with the first operation (RF Collection) as one Agent can transfer both the adverts and the RF at the same time. The final stage, which is noted as **(3) Ad Report or AR Submission**, takes place after the user has viewed the adverts through a Publisher. To notify the Broker that the user has viewed the adverts, the user composes Ad-Reports and forwards them to a Commissioner with the intent to be delivered to an Ad-Dealer. The receiving Ad-Dealer shares the Ad-Reports with the Broker, who verifies their authenticity and notifies the appropriate Advertisers. Based on the information that is provided within the Ad-Reports, the Advertisers can eventually reward the Publishers who featured the adverts as well as the **two** involved Ad-Dealers (the first that delivered the adverts to the user and the second that submitted the Ad-Reports). It is evident that the Commissioner serves a very similar function as the Agent. The only difference is that the Agent ferries Ad-Requests and adverts, whereas the Commissioner ferries Ad-Reports. At this point we must note that **the terms Agent and Commissioner are designations that are given to a user based on the type of service that they provide**. Consider an example where a user named [PERSON] sends an Ad-Request to [PERSON] who transfers it to an Ad-Dealer, collects the advert and delivers it back to [PERSON]. [PERSON] then views the advert, generates an Ad-Report that she sends to [PERSON] who forwards it to the Ad-Dealer.