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IADIS International Conference WWW/Internet 2010Like in most P2P networks, P2MAN slices content for delivery. The owner user decides how the objectwill be splitted in pieces. The owner represents a file using a bitmap (where each bit represents a file slice).The owner also decides which multicast group will be used for transmitting the object. The metadata iscreated by the owner, containing the necessary information for guiding the requesting nodes. After receivingthe reply, the requesting node reads the metadata, joins the respective multicast group and sends anauthorization message to the PC. By receiving the authorization message, the content owner can initiate theobject transmission. Considering that we assume an unreliable routing multicast protocol (i.e., with notransport layer multicast protocol), reliability is granted at the application layer by employing aretransmission mechanism provided by the Repair Mode. On Repair Mode, a receiver node can claim forpieces not received yet. The owner then retries the transmission of the remaining pieces. While there aremissing pieces, new claims and retransmissions will take place until the receiver gets all the pieces. In thiswork, security issues are not addressed at any layer. Table I summarizes our choices, confronting them withthe common approaches in the literature and showing the advantages.Table 1. P2MAN design choicesLiterature P2MAN AdvantagesAdapted DHT Public Channel Native, lightweight, well suited to the scaleUnicast with an incentive mechanism Multicast Native Simple, low need for incentive, low handshake overheadTCP UDP Less overhead, low transport performance degradationTCP retransmissions Repair Mode Less overhead, opportunistic reliability repairing2.2 P2P Reputation SchemesReputation scheme for a peer-to-peer system is a way of recording information about past behavior of peers,for use as a guide to other peers. A challenging issue that a retrieval protocol must deal with is guaranteeingthat the information gathered about peers is reliable, as malicious peers may tamper with the information theystore. Aberer and Despodovič (2001) early presented their reputation scheme specifically designed for P2Pnetworks. Eigen algorithm (Kamvar et al., 2003) maintains a global score for each peer i computed of thescore of i given by all other peers weighted by their own global score. Some replicated mother peers computeand store a global reputation value for a peer. The mother peers find the peers they must keep track of, andare found by peers who need information, through a distributed hash table. However, collusion attacks mayoccur, where rogue peers collude in order to reduce the reputation of honest peers. Chun et al. (2003)proposed architecture for secure resource peering based on ticket exchange. This architecture, however,assumes a shared cryptographic infrastructure and the establishment of relations of trust between peers toallow resource access. In P2PRep (Cornelli et. al., 2002), each peer stores information about their owninteractions with other peers. To assure the reliability of this information, P2PRep relies on voting forgathering opinions about a peer, heuristics to find clusters of potential malicious voters, and on a sharedcryptographic infrastructure to verify the identities of the peers involved in a transaction.2.2.1 The Network of FavorsThe central idea of the Network of Favors is that the users who are greater net contributors of resourcesshould get higher priority access to the spare resources. This principle acts as a guide to the balanceddistribution of the available resources among the users and, thus, as an incentive for collaboration. Itcircumvents the need to provide the reliability of the collected information by not aggregating a globalreputation value for a peer. Instead, peers only use reputation information involving peer-to-peer interactionsin which they themselves have participated. This information is stored locally by the peer. Maliciousstrategies based on lying about the behavior of third parties (e.g., collusion attacks) cannot be applied.In the Network of Favors, allocating a resource to a peer that requests it is a favor, and the value of thatfavor is the value of the work done for the requesting peer. Each peer keeps a local record of the total valueof favors it has given to and received from each known peer in the past. Every time it does a favor or receivesone, it updates the appropriate number. The peer calculates a local reputation for each peer based on thesenumbers, such that a peer who has given many favors and received few will have a high reputation. The peeruses the current reputations to decide to whom to offer a favor when it has to arbitrate between more than onerequester. Thus, whenever there is a resource contention, requesters with higher reputation get priority.221