A Review on the Communication Mechanism of Mobile Agent

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1 International Journal of Video & Image Processing and Network Security IJVIPNS-IJENS Vol: 11 No: 01 5 A Review on the Communication Mechanism of Mobile Agent Arif Hidayat Abstract Mobile agent technology is becoming more popular and has been implemented in many areas. Several researches have been conducted to address its challenges, while one of the most important is agent communication. This paper will reobserve the problem of mobile agent communication. It explains the background concept of mobile agent communication as well. Subsequently, various current agent communication mechanisms are discussed. It finally summarizes how these mechanisms answer the challenges found in mobile agent communication including their advantages and disadvantages. Index Term Mobile agent, agent communication, Message Efficiently Forwarding Scheme, Distributed Sendbox Scheme, Virtual Agent-Based Communication. I. INTRODUCTION Mobile agent can be seen as a program code which is able to migrate from a host to another host within a distributed environment and access the local resources in which it resides. While Remote Procedure Call (RPC) needs an uninterrupted communication link between hosts to access remote resources, the unique mobile characteristic of a mobile software agent takes out this requirement and therefore preserves the connection channel as well as its bandwidth [1]. Typically, the deployment of mobile agent technology involves several agents collaborating to achieve a final objective. Each agent is assigned in a particular task, of which is a breakdown of the whole operation [2]. In order to perform its task, a mobile agent often needs to collaborate with its partner agent. This collaboration requires the agents to logically meet in order to passes or exchanges the information. Because of the nature of mobile agent which travels across the hosts, the communication between agents becomes a complicated issue. A reliable communication is, therefore, essential to guarantee that the agents will not face a problem during their coordination. To realize this purpose, the communication mechanism should fulfil the needs of location-transparency and reliability [3]. This paper will present several current mechanisms for agent communication and discuss their Arif Hidayat is with the University of Brawijaya, Indonesia, Jl. Veteran Malang 65145; arifhidayat@ ub.ac.id advantages and disadvantages. However, it focuses solely on the mechanism of how an agent could deliver a message to the targeted agent. It will not cover the semantic side of agent communication such as the language used by the agent to communicate between each other and so on. II. PROBLEM DEFINITION In mobile agent system, several agent communication problems arise due to its characteristic of mobility, however this paper will only discuss two of them. The first one is how to locate an agent, to which another agent needs to communicate. While an agent is continuously moving between hosts within a network, it is quite difficult to know where it resides at a specified moment. This condition leads to the need to map the location of all agents affiliate in a certain group of agent system. Allowing each agent to be able to send messages to other agents with unknown location indicates a communication mechanism to be location transparent [4]. The next issue is regarding with the delivery of the message itself. Being aware of the targeted agent does not guarantee that an agent will be able to deliver the message to its targeted partner. One reason is because the target agent may have migrated to another host before receive the sent message [5]. This situation is likely happen to a highly mobile agent. Therefore a reliable communication mechanism should be able to make sure that a message is delivered to its targeted recipient. Almost all mobile agent toolkits introduce the communication handling inside their system. This paper will only consider the most current agent communication mechanisms and discuss how they tackle afore-mentioned problems. III. BACKGROUND A. Mobile Agent Mobile agents are defined as software objects equipped with a certain level of intelligence and autonomy which can migrate from one host to another host within a distributed network to complete a particular task given by its owner [6]. Contrasting with the previous remote communication method such as Remote Procedure Call (RPC), which applies conventional Client/Server model and requires continuous

2 International Journal of Video & Image Processing and Network Security IJVIPNS-IJENS Vol: 11 No: 01 6 connection between hosts, mobile agent technology is typically designed to bypass the communication link and utilize the local resources directly from the host where the resources are located [5]. For its capability, mobile agent is considered as a new potential paradigm to fulfil the need of effective and scalable answer for the problems in the area of distributed computing. Being able to move between nodes is a special characteristic of mobile agent which makes it as a favourable solution to address the problems in distributed environment. Firstly, by being mobile, an agent can access remote resources without requiring constant connection with the host, therefore saves network bandwidth and reduces the latency. Moreover, it will be able to deal with the physical problems possibly happen to the host during the execution, such as connection fault or power failure. In addition, while conventional remote access communication obligates the hosts to be always synchronous, the mobile agent implementation allows the communication between hosts to be performed asynchronously[7] Several issues must be considered in designing a mobile agent system. They are including mobility control, naming of the agents and hosts, security of the host and the agent itself, applicability within heterogeneous environment, authentication and so on. However, this paper only concerns to agent communication in the mobile agent system. B. Agent Communication In order to perform a given task effectively, a mobile agent should interacts and cooperates with others. Agents, therefore need to be able to communicate between themselves. It has been realized that communication between mobile agents is not as simple as the common communication method in a computer network. The problem is how to deliver a message to a migrating-recipient with unknown location. Several researches have been done to solve this problem. Mole system [8] introduces the session-oriented communication. In this system, a session must be established by agents want to communicate among themselves. Agents are equipped with badges, by which they can identify each other and therefore are able to set up a session. However, using an assumption that agents will not migrate during a session makes this system can not deal with a highly mobile agent. It can not offer a reliable communication as well, since agents are allowed to terminate the session anytime. Moreover, this system does not put location-transparency forward. Communication mechanism presented in Epidaure [9] uses a special communication agent to be the message dispatcher. Each message containing the address of the targeted agent and should be sent to the dispatcher, which will then deliver it to the recipient. This system suffers from the high network load if there are a lot of agents which frequently move, because the system will need to frequently update the location of all agents. Message Delivery Protocol (MDP) [6] offers a mechanism to track the location of an agent accurately. It maintains all agents addresses in a tree-like hierarchy structure. Agents are categorized into groups, each of which will be put in a domain. Each domain has a gateway and the top of all domain gateways is the root. Each server within a domain updates the address of an agent when it migrates to another domain. This procedure is logically simple, however building the hierarchical tree is a complicated task, especially in a very large distributed network such as internet. Furthermore it can not address the situation when a message is continuously chasing a highly mobile agent. There are numerous other systems have been introduced, not to mention several toolkits that cover basic mechanism for handling mobile agent communication such as Telescript, Aglets, Concordia, Voyager and so on. Regarding with these two problems of mobile agent communication, [10] has mentioned the techniques used to address the problem of either location tracking or message delivery. To deal with location tracking, each proposed system may use one of the following techniques: --Broadcast/Multicast scheme (BM), on which a sender agent broadcasts a request of recipient s location or the message itself to all hosts in the system. --Hierarchical Scheme (HS), which uses a hierarchy to store the location of all agents. Each agent has a path of its location from the root to the leaf, which is a group domain in which it resides. --Central Server Scheme (CS) utilizes a dedicated server to maintain the location path of a mobile agent. --Forwarding Pointer Scheme (FP), on which the migration path of a mobile agent is maintained by a forwarding pointer indicating its next host so that the message can reach it through the path. In order to ensure that a message is delivered to its recipient, each system may use one of these techniques: --Ostrich, on which a message is discarded when the host does not know the recipient s location. --Avoidance which applies a synchronous method, on which a mobile agent is not allowed to migrate until it receives the acknowledgments for all messages. --Detection which requires the system to be able to identify any failure of message delivery and performing an action to handle it accordingly. IV. SOLUTION APPROACH This paper will discuss three different mechanisms used in mobile agent communication, they are Message Efficiently Forwarding Scheme (MEFS), Distributed Sandbox Scheme, and Virtual Agent. These mechanisms are considered as the most recent techniques in mobile agent communication. In the discussion, this paper covers how each mechanism deals with the problems of mobile agent communication. It explores the

3 International Journal of Video & Image Processing and Network Security IJVIPNS-IJENS Vol: 11 No: 01 7 operational work of each method. It will explain the advantages of each mechanism as well as its disadvantages. A. Message Efficiently Forwarding Scheme (MEFS) Jingyang et.al [10] have proposed a reliable mobile agent communication mechanism which referred as MEFS. This mechanism combines two techniques, Central Server and Forwarding Pointer Schemes, to solve the problem of location tracking as well as two other techniques, Avoidance and Detection, for delivering the message. The author illustrates the standpoint of MEFS and compares it with other systems based on the technique implemented in Figure 1. MEFS offers location-transparency to tackle the problem of Fig. 1. The position of MEFS in two dimension design space of agent communication protocol [10] location tracking. In this system, each host has the information about agents created on it and agents residing on it at present. This information is kept to be always updated by forcing a migrating agent to unregister it self and put a forwarding pointer in the current host, informing the destination to its initial host, as well as register it self in the new host. Maintaining this procedure makes each agent can be easily tracked within a network. When sender and recipient agents reside in the same host, the message will be directly delivered. Otherwise, a query asking the recipient s location will be sent by a dedicated module in the host called communicator to the recipient s home host. This home host informs the current or the destination host of its agent, so that the message will be delivered successfully. Each mobile agent always put a forwarding indicator before its migrate from a host, therefore, the communicator frequently finds the recipient s location without necessarily request it from the recipient s home host. MEFS introduces Chasing Message Register, Over-speed Agent Blocking and Regular Synchronization to particularly resolve the problem of message chasing. A message chasing happens when a message is forwarded more than a certain number of times (Co). When it happens, then the message will be stored in recipient s home host. Over-speed Agent Blocking method forces an agent to synchronize itself with its home host whenever its migration velocity is over a certain specified level (Vo). It then waits for chasing messages for an amount of time (Ts), so that it can receive them. This mechanism guarantees that each message will be received by its recipient. It completely solves the problem of location tracking and message delivery as well. However, forcing the agent to perform a regular synchronization with its home host will limit the mobility of the agent itself. The communication overhead caused by the synchronization must be considered as well, especially in an environment with a high number of agents. Moreover, it is quite difficult to configure the suitable level of Co, Vo and Ts, at which the system can optimize the trade off between reliable delivery, limitation of agent mobility, time for forwarding the message and communication overhead caused by synchronization process. B. Distributed Sendbox Scheme Leu, et.al [11] have proposed a two-modes mobile agent communication mechanism termed as distributed sandbox scheme. It manages the message delivery in the mobile agent based on the standard of Session Initiation Protocol (SIP). To deal with location-tracking, this scheme deploys a centralized server dedicated to maintain the path of all agents within a mobile agent system. To be able to migrate, each agent should register its destination host and un-register its current host to the server, so that its location can be updated. Inside the server, each agent is identified by its SIP-standard identity, formatted as agent_id<agent_id@hostname>. This mechanism introduces real time tracking as well as unreal time one. While real time tracking requires the agent to register itself to the server each time it migrates, unreal time tracking allows the agent to perform a single registration for several migration. The latter mode is beneficial for agents with high mobility, because it will not slow up their movement like its real time counterpart does. In this scheme, each agent s state is defined as either active, waiting or moving. When an agent (S) resides in a host (PS) wants to communicate with another agent, it must send a request about recipient (R) agent s location to the home server (H), which will then check the recipient s location and state. If the recipient s state is active in its host (PR), then the server asks it to turn into waiting state and replies the sender s request with the recipient s location. Both agents are now synchronized and the communication between them can be performed. This practice is called as direct mode. When the recipient s state is moving, the scheme deploys another approach called forward mode. In this mode, the sender does not directly send the message (MSR) to the migrating recipient, instead, it sends the message to the sendbox of the host on which it resides. The home server, accordingly, creates an entry (ESR) in its table containing the information about the host saving the message and the recipient s identity. When the recipient finishes its migration and registers itself from its new host (PR ), the server asks it to move to the waiting state. It then notifies the hosts that save

4 International Journal of Video & Image Processing and Network Security IJVIPNS-IJENS Vol: 11 No: 01 8 Direct Mode Forward Mode Fig. 2. The operation of direct mode and forward mode [11] messages destined to this recipient to retrieve and forward them. These two modes of message delivery are depicted in Figure 2. The implementation of this scheme eliminates communication failure and message chasing. It also gives an advantage on the low network load, since each agent does not necessarily carrying un-sent messages while it is migrating. Based on the experiments, it is known that the more frequent the recipient move, the more frequent the forward mode utilized in the system. More over, it is confirmed that the number of sent messages does not affect the utilization of either direct or forward mode. The distributed sandbox scheme offers a reliable communication mechanism, however this scheme is vulnerable to communication failure for two reasons. First, there is a significant reliance of the system to the server. When there is a problem on the server because of security attack or physical damage, then the whole system will be down. Therefore a welldesigned recovery mechanism is important to be implemented. The next issue is regarding with the waiting time for an agent to receive a message. The scheme does not define the time limit for an agent to stay in waiting state in order to receive messages. When the sender agent, or sender host in forward mode, faces a problem while sending a message, then the recipient has to wait for an uncertain time. It makes the agent unable to perform any operation before the message arrives. C. Virtual Agent-Based Communication Mechanism Zhao et.al [12] have proposed a virtual agent-based mechanism for mobile agent communication. It introduces the use of virtual agent to provide a reliable message transmission. In addition, instead of deploying a server, this mechanism uses multicast to track the location of each agent within the multi agent system. Each host in the system keeps the information about all agents in three tables. The HOME_T table consists of five columns: Local_ID for the name of each agent created in this host, Addr for its address, State for its status, either static or migrating, Onway_Count for total on the way messages destined to it and VA for the presence of its virtual agent.the next is AddrLocal_T which records the agent migrating to this host, and the last is AddrRemote_T that keep the information of other agent located in other host. Each agent is identified by P_ID:L_ID, of which P_ID is the host where it resides and L_ID is a logical name given by the owner. Once it is created in home host, it should be multicasted to other hosts in order to inform this agent s presence. However, when it migrates, it registers its location only to its new host as well as its home host without necessarily multicasts other hosts. In this scheme, when the message s sender (A1) and recipient (A2) reside in the same host, then the message can be directly delivered using the information in the AddrLocal_T table. In the other situation, when both of them reside in their original hosts (HI, H2), the information in AddrRemote_T table will be useful to make the message deliverable to its recipient. The only condition in which a virtual agent needs to be created is when the recipient is migrating, identified by its migrating state in HOME_T table, to another host (H3) except the sender s host. In this case, the home host of the recipient will create a virtual agent which is a replicate of the recipient. This virtual agent receives and holds the message. When the original migrating agent arrives in new host and registers itself in both new and home hosts, then the virtual agent knows its location from Home_T table and delivers the message accordingly. This virtual agent can not migrate to other host. It can be destroyed by its host to save the host s resources. This communication mechanism provides locationtransparency. It also saves network bandwidth when it is implemented in a small mobile agent system, since the migrating agent only needs to register itself to its new and home hosts, which then store all information using three tables. However, there is no definition on what situation a virtual agent should be destroyed and when it should be kept alive. Keeping a non-operating virtual agent to stay alive will waste the host resources. Moreover, this mechanism is not efficient to be used in a large network with a lot of agents migrating frequently. One reason is because there will be high communication overhead caused by agents and their virtual counterparts to update their location and send messages. Based on the discussion, these three mechanisms provide location-transparency as well as reliable message delivery. However, each mechanism has its own primacy in a certain situation. While MEFS completely answers the problem of message chasing, Distributed Sendbox scheme is more

5 International Journal of Video & Image Processing and Network Security IJVIPNS-IJENS Vol: 11 No: 01 9 Mechanism T ABLE I SUMMARY OF REVIEWED MOBILE AGENT COMMUNICATION MECHANISM MEFS Distributed Sendbox Virtual Agent-based Location transparency X X X Reliable Delivery X X X Message Chasing Solved Solved Reliance on Server X Hosts Resources Used Low Low High beneficial in term of saving bandwidth whereas Virtual Agentbased mechanism presents the best performance in small network with small number of mobile agents. The comparison over these mechanisms is summarized in Table I. V. CONCLUSION Mobile agent technology has been implemented in many areas of business and industry. This paper reviews the problem of mobile agent communication and discusses several approaches to address it. While there are number of methods have been proposed in mobile agent communication, three most current mechanisms with different approaches are discussed in this chapter. These mechanisms offer locationtransparency as well as reliable message delivery in mobile agent system. The selection of mechanism should be carried out based on the characteristics of the network, the performance of the mechanism on tackling the challenges as well as the problem may arise during the implementation. Several factors to be considered are network size, number of agents and their level of mobility, the use of hosts resources, message chasing, communication overhead and vulnerability to the failure. [8] Baumann J, Hohl F, Radouniklis N, Rothermel K, Straβer M. Communication concepts for mobile agent systems. Mobile Agents 1997: [9] Desbiens J, Lavoie M, Renaud F. Communication and tracking infrastructure of a mobile agent system. System Sciences, 1998, Proceedings of the Thirty-First Hawaii International Conference on; 1998; p vol.7. [10] Zhou J, Ja Z, Chen D. Designing reliable communication protocols for mobile agents. Distributed Computing Systems Workshops, 2003 Proceedings 23rd International Conference on; 2003; p [11] Fang-Yie L, Hsiao-Hong T, Wen-Kui C. Distributed sendbox scheme for mobile agent communication. Mobile Business, 2005 ICMB 2005 International Conference on; 2005; p [12] Jianmin Z, Xinzhong Z, Huiying X, Xinpeng Z. A New Communication Mechanism Based on Virtual Agent for Mobile Agent. Computer Supported Cooperative Work in Design, 2006 CSCWD '06 10th International Conference on; 2006; p Arif Hidayat, received the bachelor degree in agroindustrial technology from Brawijaya University, Malang, Indonesia, in 2004 and MAIT degree in applied information technology from Monash University, Australia in His current research interests include networking, decision support system and information system REFERENCES [1] Dalmeijer M, Hammer DK, Aerts ATM. Mobile software agents. Computers in Industry. 2000;41(3): [2] Pietro Picco G. Mobile agents: an introduction. Microprocessors and Microsystems. 2001;25(2): [3] Jiannong C, Liang Z, Jin Y, Das SK. A reliable mobile agent communication protocol. Distributed Computing Systems, 2004 Proceedings 24th International Conference on; 2004; p [4] Jiannong C, Xinyu F, Jian L, Das SK. Mailbox-based scheme for mobile agent communications. Computer. 2002;35(9): [5] Murphy AL, Picco GP. Reliable communication for highly mobile agents. Agent Systems and Applications, 1999 and Third International Symposium on Mobile Agents Proceedings First International Symposium on; 1999; p [6] Lazar S, Weerakoon I, Sidhu D. A scalable location tracking and message delivery scheme for mobile agents. Enabling Technologies: Infrastructure for Collaborative Enterprises, 1998 (WET ICE '98) Proceedings, Seventh IEEE International Workshops on; 1998; p [7] Tripathi A, Ahmed T, Pathak S, Carney M, Dokas P. Paradigms for mobile agent based active monitoring of network systems. Network Operations and Management Symposium, 2002 NOMS IEEE/IFIP; 2002; p