REDMAN Replica Distribution

The RD facility operates to transparently distribute resource replicas in the dense MANET. REDMAN associates each resource of common interest with a metadata-based description, which includes the expected replication degree depending on the estimated resource criticality decided by service developers. When a delegate enters a dense MANET, it communicates the RDF description of its resources to the replica manager, which decides the replication degree (deg) to enforce by considering the hint about resource criticality and the number of current participants in the dense MANET. Future work will improve the replica manager decision capability, by taking into account additional indicators such as the current and average network load. Afterwards, the manager creates new SRT (Shared Resource Table) entries for the newly arrived resources; any entry contains the enforced replication degree and weakly consistent information about the nodes currently hosting replicas for that resource. Finally, the manager commands the delegates to start the replication operations according to the strategy exposed below. When a node decides to host a replica, it notifies the replica manager; in this way, the manager can update the information in its SRT.
Since the replica manager node has the duty of coordinating replica operations, it tends to consume more resources than other nodes. For this reason, the proposed replica distribution strategy aims at reducing the manager communication load, by giving it the only charge of sending short description messages, while delegates have the burden of actually enforcing resource replication. In addition, REDMAN periodically reallocates the replica manager role by forcing a new manager election (as detailed in the next section). This also contributes to improve the suitability of nodes acting as replica managers because, during long time intervals, an elected node could lose the properties that motivated its election, e.g., its central position in the dense MANET topology.

The main guideline of the RD protocol is to disseminate resource replicas on nodes at r-hop distance along an approximately constant direction. When a delegate is commanded to replicate one of its resources, it sends a replication packet specifying the number of replicas still required and the desired r-hop distance between replicas. The replication packet is propagated on nodes placed along an approximately straight line with a fixed direction. Let us observe that REDMAN does not require dense MANET participants to be GPS-equipped and exploits lightweight heuristic-based estimations, specific for dense MANETs, to determine constant directions. Roughly speaking, the solution guideline is that a node determines its successor by choosing, among its neighbors (the nodes at single-hop distance from it), the one sharing fewer neighbors with its predecessor. To this purpose, RD locally broadcasts the neighbor list of its predecessor to all neighbors; only the neighbors sharing with the predecessor a number of neighbors lower than a threshold reply. This determines a roughly constant direction if the node density is almost uniform in the dense MANET. When a replication packet reaches a node at r-hop distance from a replica, that node becomes a delegate; the new delegate commands the manager to update the information about replica placement and reiterates the process by decreasing the number of requested replicas.

Figure 1: Node A disseminates replicas and information about replica placement along approximately rectilinear directions toward E.