## 1 Introduction
The algorithm takes explicitly into account communication constraints among the agents, in terms of maximum communication range achievable with a desired level of performance. It allows the vehicles to autonomously position themselves in order to cover the maximum area around the asset by means of the detection sonars, adapting their behaviour to the specific communication performance encountered as the mission proceeds. It is a distributed approach, since each vehicle takes local decisions in order to achieve the final goal, and it is based on the concept of _emergent behaviour_ defining the action and the behaviour of each agent through simple elementary rules [9, 10, 11]. The algorithm allows for reconfigurability in response to oceanographic variations and/or external events. When a vehicle leaves or a new one joins, the vehicles may change their positions to adapt themselves to the new configuration so to achieve again the maximum asset protection guaranteeing the communication connectivity among the currently available agents. * A set of network security solutions has been designed, functional to the cooperative strategy and tailored to the communication limitations of the medium. In contrast to traditional wired networks, an adversary equipped with an acoustic modem can easily eavesdrop as well as modify and insert fake messages [12, 13]. In order to address these threats, we implement the cooperation algorithm according to the group communication paradigm. Vehicles in the group share a group key they use to encrypt and authenticate broadcast messages. Whenever a vehicle leaves the group, either because it has finished its mission or because it is (suspected to be) compromised, the current group-key is revoked and a new one distributed (forward security). Communication confidentiality and authentication is addressed with a cryptographic suite that keeps at minimum message expansion. This is done to cope with bandwidth and range limitation and to limit the amount of message overhead required by other cryptographic approaches. Furthermore, we use a secure rekeying protocol that is efficient from two standpoints: the number of rekeying messages is logarithmic in the number of vehicles; a rekeying message does not carry any additional proof of key authenticity so avoiding message expansion. It is remarked that in this work the communication limitations in nodes cooperation are tackled from the application level point of view, _i.e._, proposing a cooperation strategy that attempts to minimize the information exchange among the nodes. The strategy will be shown to be robust with respect to connectivity loss or communication range degradation. The aspects more related to the physics of underwater acoustic propagation have been discussed elsewhere [8], where it has been shown how to compute communication figures of merit (bandwidth, signal-to-noise ratio, communication range) from oceanographic environmental properties. In this paper, each node uses the maximum communication range as the available information on the communication channel properties. Finally, one more consideration can be done with respect to Denial of Service (DoS), one of the most severe threats against network availability in the underwater scenario. In fact, contrasting DoS in underwater networks is even more complicated than in traditional networks due to the intrinsic limitations of the acoustic channel [6]. The proposed security suite is aimed at communication integrity and confidentiality and thus does not provide any countermeasures against DoS. However, the cooperative algorithm is intrinsically reactive against DoS due to the emergent behaviour approach. Whenever an enemy succeeds in disrupting communication among vehicles, the simple rules that drive vehicle motion make them move closer to the asset in order to ensure protection regardless what the other agents are doing.