Modeling Fault Tolerant and Secure Mobile Agent Execution in Distributed Systems - Distributed Artificial Intelligence, Agent Technology, and Collaborative Applications - page 133

regarding reliable agent execution, as well as compatibility between two different agent systems or secure
agent migration, have been considered. Some of the proposed schemes are either replicating the agents
(Hamidi & Mohammadi, 2005) or check-pointing the agents (Park, Byun, Kim, & Yeom, 2002; Pleisch
& Schiper, 2001;) For a single agent environment without considering inter-agent communication, the
performance of the replication scheme and the check-pointing scheme is compared in Park et al. (2002)
and Silva, Batista, and Silva (2000). In the area of mobile agents, only few works can be found relating
to fault tolerance. Most of them refer to special agent systems or cover only some special aspects relating
to mobile agents, such as the communication subsystem. Nevertheless, most people working with mobile
agents consider fault tolerance to be an important issue (Izatt, Chan, & Brecht, 1999; Shiraishi, Enokido,
& Takzawa, 2003). Mobile agents are becoming a major trend for designing distributed systems and
applications in the last few years and foreseeable future. It can bring benefits such as reduced network
load and overcoming of network latency (Chan, Won, & Lyu, 1993). Nevertheless, security is one of
the limiting factors of the development of these systems. The main unsolved security problem lies in
the possible existence of malicious hosts that can manipulate the execution and data of agents (Defago,
Schiper, & Sergent, 1998). Most distributed applications we see today are deploying the client/server
paradigm. There are certain problems with the client/server paradigm, such as the requirement of a
high network bandwidth, and continuous user-computer interactivity.
In view of the deficiencies of the client/server paradigm, the mobile code paradigm has been developed
as an alternative approach for distributed application design. In the client/server paradigm, programs
cannot move across different machines and must run on the machines they reside on. The mobile code
paradigm, on the other hand, allows programs to be transferred among and executed on different com-
puters. By allowing code to move between hosts, programs can interact on the same computer instead
of over the network. Therefore, communication cost can be reduced. Besides, mobile agent (Fischer,
Lynch, & Paterson, 1983) programs can be designed to work on behalf of users autonomously. This
autonomy allows users to delegate their tasks to the mobile agents, and not to stay continuously in front
of the computer terminal. The promises of the mobile code paradigm bring about active research in its
realization. Most researchers, however, agree that security concerns are a hurdle (Greenberg, Byington,
& Harper, 1998).
In this chapter, we investigate these concerns. First, we review some of the foundation materials of
the mobile code paradigm. We elaborate Ghezzi and Vigna's classification of mobile code paradigms
(Ghezzi & Vigna, 1997), which is a collection of the remote evaluation, code on demand, and mobile
agent approaches. In the next section, we address the current status of mobile code security. The fol-
lowing section presents the model for fault-tolerant mobile agent. In the next section, security issues of
the mobile agent are discussed, and we discuss security modeling and evaluation for the mobile agent
in the section after. In the following section, simulation results and influence of the size of agent are
discussed. We then conclude the chapter.
t he MoBile code Paradig M
The mobile code paradigm is essentially a collective term, applicable wherever there is mobility of code.
While different classes of code mobility have been identified, Ghezzi and Vigna proposed three of them,
namely remote evaluation, code on demand, and mobile agent (1997). This classification, together with
the client/server paradigm, is summarized in Table 1.