A survey of survivability in mobile ad hoc networks <s> II. SURVIVABILITY CONCEPTS <s> In this paper we survey issues and challenges in enhancing the survivability of mobile wireless networks, with particular emphasis on military requirements*. Research focus on three key aspects can significantly enhance network survivability: (i) establishing and maintaining survivable topologies that strive to keep the network connected even under attack, (ii) design for end-to-end communication in challenging environments in which the path from source to destination is not wholly available at any given instant of time, (iii) the use of technology to enhance survivability such as adaptive networks and satellites. <s> BIB001 </s> A survey of survivability in mobile ad hoc networks <s> II. SURVIVABILITY CONCEPTS <s> This paper gives the main definitions relating to dependability, a generic concept including as special case such attributes as reliability, availability, safety, confidentiality, integrity, maintainability, etc. Basic definitions are given first. They are then commented upon, and supplemented by additional definitions, which address the threats to dependability (faults, errors, failures), and the attributes of dependability. The discussion on the attributes encompasses the relationship of dependability with security, survivability and trustworthiness. <s> BIB002 </s> A survey of survivability in mobile ad hoc networks <s> II. SURVIVABILITY CONCEPTS <s> The Internet has become essential to most enterprises and many private individuals. However, both the network and computer systems connected to it are still too vulnerable and attacks are becoming evermore frequent. To face this situation, traditional security techniques are insufficient and fault-tolerance techniques are becoming increasingly cost-effective. Nevertheless,intrusions are very special faults, and this has to be taken into account when selecting the fault-tolerance techniques. <s> BIB003
In general, security mechanisms follow two defense lines: one preventive and another reactive  . The former provides mechanisms to avoid any type of attack, as firewalls and cryptographic systems. The latter consists in taking action on demand to mitigate intrusions, as intrusion detection systems (IDS). Nevertheless, preventive and reactive solutions are not efficient to put all attacks and intrusions off  , BIB003 . Thus, research groups have built security mechanisms toward one third defense line, called intrusion tolerance (IT) BIB003 , as illustrated in Figure 1 . Tolerance approaches complement the other ones and its goal is to develop mechanisms to make systems (networks, means of communication, services and others) tolerant to attacks and intruders, that is, to afford some essential network services in the presence of malicious actions BIB003 - BIB001 . Systems using techniques for tolerating intrusions and attacks are called intrusion tolerance systems. In a broad sense, these techniques can provide certain survivability key properties, supporting the development of survivable systems. Survivability refers to a system capability of completing its goals and requirements in a timely manner in face of attacks, intrusions, failures or accident  . Laprie et. al  consider survivability similar to dependability in terms of goals and addressed threats. Dependability goals consist in the system capability of delivering trusted services and avoiding the most frequent or severe failures. This work addresses survivability as a special case of dependability, where the network is able to complete its goals in the presence of malicious faults. These faults present different conditions and specific necessities that can only be efficiently treated when analyzed individually BIB001 . Hence, survivability aims to increase security effectiveness, and assist dependability and security integration. Making a parallel with dependability, intrusion tolerance applies fault tolerance mechanisms into the security domain. While survivability is a system capability, intrusion tolerance consists of techniques and mechanisms to offer correct services in the presence of intrusions  . Intrusion tolerance emerged with Fraga and Powell's initiative  , however, the development of such systems only had more attention in the last decade with MAFTIA (Malicious-and AccidentalFault Tolerance will be Internet Applications)  and OASIS (Organically Assured and Survivable Information System) [15] projects. The MAFTIA project has designed wide scale distributed systems to tolerate many ordinary faults and malicious attacks in fixed networks. The OASIS project was developed by American Department of Defense (DARPA) to build a tolerant system for high-speed networks. Survivability attributes are reliability, availability, maintainability, confidentiality, integrity and safety  . Survivable systems address a subset of faults, called malicious or intentional faults, comprising of malicious logics and DoS attacks or intrusion  , BIB002 . In general, these faults abuse of existent system vulnerabilities, introduced accidentally or deliberately during the development of the system. An attack can successfully exploit system vulnerabilities resulting in an intrusion. This work suggests that survivability should be reached by the use of preventive, reactive and tolerant approaches operating together. Figure 2 illustrates this behavior where preventive defenses will be the first obstacle for attacks, blocking certain ones and incapable of preventing others. Some attacks can succeed in intruding into system (or network) and reactive defenses will begin to work, trying to detect and stop them. However, reactive defenses have also limitations and intruders can be successful in compromising the system. In order to guarantee the system operation even in the presence of intrusions, intrusion tolerance techniques need to be applied, until preventive or reactive defenses can adapt themselves and take actions against the attack or intrusion. The proposal of combining the three defense lines should be better explored taking into account survivability key properties and requirements for MANETs. Survivability requirements are identified in Section IV and the survivability properties are resistance, recognition, recovery and adaptability BIB001 . Resistance is the capability of a system to repel attacks. User authentication, firewalls and cryptography are examples of mechanisms used to reach it. Recognition is the system capacity to detect attacks and evaluate the extent of damage. Examples of recognition mechanisms are intrusion detection by patterns and internal system integrity verification. Recovery is the capability of restoring disrupted information or functionality within time constraints, limiting the damage and maintaining essential services. Conventional strategies applied for achieving recovery are replication and redundancy. Finally, adaptability is the system capacity of quickly incorporating lessons learned from failures and adapting to emerging threats  , BIB001 . Examples of adaptation techniques are the topology control by the radio power management and active networking technology. The application of active networking technology intends to allow the dynamic selection of MAC or network layer parameters, and the dynamic negotiation of algorithms and entire protocols based on application requirements or the communication environment BIB001 . Figure 3 illustrates the interaction among these key properties.