*2.9.6. CORE - Collaborative reputation mechanism to enforce node cooperation in mobile ad hoc networks*

Another famous reputation mechanism in literature is CORE protocol (Collaborative Reputation Mechanism to Enforce Node Cooperation in Mobile Ad Hoc Networks) [5]. It is a complete reputation mechanism that defines three different types of reputation: (i) Subjective Reputation - reputation observed locally by a node with regards to other nodes (direct observations), (ii) Indirect Reputation - reputation provided by nodes to other nodes which includes only the positive reports by others and (iii) Functional Reputation - also referred as task-specific behavior, which are weighted according to a combined reputation value that is used to make decisions about cooperation or gradual isolation of a node. That is, Subjective Reputation and Indirect Reputation are merged by means of a weighted combining formula in order to compute a final value of reputation concerning a specific evaluation criterion (e.g. packet forwarding) forming Functional Reputation, the last type of reputation considered. By combining different functional reputation values concerning different evaluation criteria, a global reputation value may be estimated. The subjective reputation is computed by giving more relevance to past observations than to recent ones. Subjective Reputation values are updated on the basis of a Watchdog mechanism, if misbehaviour is identified. Indirect Reputation values are updated by means of a reply message that contains a list of all entries that correctly behaved in the context of each function.

In this work, distribution of positive ratings is allowed so as to avoid potential denial of service attacks. In case reputation of an entity is negative, the execution of any requested operation will be denied by all other entities in the system. The system assumes a DSR routing in which nodes can be requesters or providers. The rating is done by comparing the expected result with the actually obtained result of a request. Here, nodes exchange only positive reputation information. The authors argue that this prevents a false-negative (badmouthing) attack, but do not address the issue of collusion to create false praise. In CORE, members have to contribute on a continuing basis (thereby enforcing node cooperation) to remain trusted or they will find their reputation deteriorating until they are excluded. CORE does not provide for a second-chance mechanism.

## *2.9.7. SORI - Secure and objective reputation-based incentive scheme for ad hoc networks*

SORI Scheme for Ad Hoc Networks) [4] targets only the non forwarding attack. SORI monitors the number of forwarded packets from neighborhood and the number of forwarded packets to neighborhood. Reputation rating is then acquired by computing the ratio between the two numbers with a consideration for the confidence in the rating proportional to the number of packets that are initially requested for forwarding. Second hand information is delivered only to the immediate neighbors. This rating source; however, is weighted by what is called credibility, which is derived from the rating ratio. The delivery of the second hand information is achieved by hash-chain based authentication. SORI consists of three components, namely, neighbour monitoring (used to collect information about packet forwarding behaviour of neighbours), reputation propagation (employed so as to share information of other nodes with neighbours) and punishment (involved in the decision process of dropping packet action, taking into account the overall evaluation record of a node and a threshold so as to consider collision events). Reputation rating formation considers first-hand information weighted by a confidence value used to describe how confident a node is for its judgement on the reputation of another node and second-hand information weighted by the credibility of nodes which contribute to the calculation of reputation. Credibility of a node is defined on the basis of a node's behaviour as forwarder and not as a witness. Reputation rating itself is based on packet forwarding ratio of a node. SORI does not discriminate between selfish and misbehaving node. SORI does not comprise a second-chance / redemption mechanism. Finally, SORI, in order to tackle with impersonation threats, constructs an authentication mechanism based on a one-way-hash chain.
