Many mobile agent system-related services and applications require interacting with a mobile agent by passing messages. However, an agent’s mobility raises several challenges in delivering messages to a mobile agent accurately. Consisting of tracking and message delivery phases, most mobile agent location management schemes create or receive many update messages and interaction messages to ensure the effectiveness of the schemes. In addition to downgrading the overall performance of a mobile agent location management scheme, excessive transmission of messages increases the network load. The migration locality of a mobile agent and the interaction rate between mobile agents significantly affect the performance of a mobile agent location management scheme with respect to location management cost. This work presents a novel Dual Home based Scheme (DHS) that can lower the location management costs in terms of migration locality and interaction rate. While the DHS scheme uniquely adopts dual home location management architecture, a selective update strategy based on that architecture is also designed for cost-effective location management of mobile agents. Moreover, DHS is compared with available schemes based on formulations and simulation experiments from the perspective of location management costs. Simulation results demonstrate that the proposed DHS scheme performs satisfactorily in terms of migration locality and interaction rate.
As a software program, a mobile agent [
Location management of a mobile agent is essential for a mobile agent system, in which many significant services are accomplished via mobile agent interactions that need precise location information. Notable examples include task communication [
The popularity of mobile agent technology has led to numerous mobile agents to execute tasks on a network simultaneously, thus incurring considerable location management costs. Location management costs of a mobile agent consist of the transmission costs of update messages and interaction messages. In addition to degrading the overall performance of a mobile agent location management scheme, excessive transmission of messages increases the network load.
Information retrieval is one of the important applications in mobile agent technology. Among the many Internet applications in which a mobile agent has been applied to retrieve information are web searching [
Available mobile agent location management schemes can be classified into hierarchy-based, region-based, and home-based location management architectures. Hierarchy-based and region-based architectures can localize location management tasks and are highly promising for developing an economically feasible cost mobile agent location management scheme. However, both architectures have several limitations. For instance, globally deployed cooperative LMSs for providing location services are difficult to achieve, while installing, managing, and maintaining these designated and stationary LMSs incur extra costs. Moreover, previous studies that adopt both location management architectures are concerned with reliability and scalability, yet they do not attempt to reduce location management costs [
Although a home-based location management scheme is simple and easily implemented, reducing location management costs is problematic because a contradiction arises between the transmission cost of update messages and interaction messages. Specifically, increasing the frequency of updates of a mobile agent decreases the transmission cost of interaction messages, while the overall transmission cost could be increased due to the increasing number of update messages. Current mobile agent location management schemes based on this architecture adopt complex update strategies for minimizing location management costs.
To maintain low location management costs in terms of migration locality and interaction rate, this work presents a novel Dual Home based Scheme (DHS) by incorporating the advantages of the above three location management architectures. The proposed DHS scheme uniquely adopts a dual home location management architecture, as well as a selective update strategy based on the architecture to ensure cost-effective mobile agent location management. Additionally, the proposed DHS scheme is compared with available schemes based on formulations and simulation experiments from the perspective of location management costs. Simulation results demonstrate that the proposed DHS scheme performs satisfactorily in terms of migration locality and interaction rate.
The rest of this paper is organized as follows. Section
The channel of a mobile agent MA is a path that can locate MA. It is constituted by location information that is left by MA on hosts that provide location services. The location information is a pair in the form of
A region is a group of agent platforms that are in the same administration scope [
Hierarchy-based location management architecture generally divides hosts in a mobile agent system into groups based on geographic locations. In each group, one or more LMSs are deployed to administrate the status of each mobile agent in the same group. In such an architecture, a
A LMS in a hierarchy-based architecture generally refers to a designated, independent, and stationary host that induces inestimable costs, including the installation, management, and maintenance of LMSs, as well as selection of the best position for LMS. Budhkar et al. [
A region-based location management architecture originates from the feature of transmission latencies in local area network (LAN), which is significantly lower than that in wide area network (WAN). In such architecture, a network contains an independent and stationary host, which administers active mobile agents in the region. Migration of a mobile agent is either intraregion or interregion. The former is attributed to the fact that the source and the destination hosts of a mobile agent’s migration are located in the same region, while the latter is attributed to the fact that the source and the destination hosts are located in different regions. Schemes such as Search-by-Path-Chase (SPC) [
In RAMD, the LMS is called region server (RS) in a region. Location tracking in RAMD is also based on the migration behavior of a mobile agent. In intraregion migration, a mobile agent only updates location information to the current RS as it arrives at the destination host. In interregion migration, a mobile agent updates to the home host, home RS, and current RS when it arrives at the destination host. Hosts on a mobile agent’s
BSPC scheme is a variation of SPC scheme. A LMS in a region is called a region agent tracker (RAT) in BSPC. When a RAT perceives that a queried receiver agent is located in its region, it determines the actual location of the receiver agent by broadcasting. Only the host where the receiver agent resides will reply to the RAT as receiving the broadcast. Importantly, BSPC improves scalability and reliability, as well as reducing migration and interaction overhead.
In the home-based location management approach, the home host that creates mobile agents serves as a LMS for the mobile agents. The location management architecture is simple and easily implemented. Models such as home-proxy scheme (HP) and forwarding-proxy scheme (FP) belonging to this type significantly differ in location update strategies. In HP, a mobile agent arriving at a new host reports to its LMS the new location by using an update message. However, constantly updating location information incurs a large overhead during the tracking phase. In FP, a mobile agent that is ready to depart leaves a forwarding pointer at the current host to indicate its next stop; hence, FP produces no messages during the tracking phase. Nevertheless, with an increasing frequency of the migration of a mobile agent, a
(a) HP scheme, (b) FP scheme.
Current researches tend to use update message and forwarding pointer alternately during the tracking phase to reduce location management costs, by which a mobile agent updates the location information to LMS periodically. Another movement based scheme, the dU scheme, is described elsewhere [
Flocchini and Xie [
Several studies have evaluated mobile agent location management schemes. Deugo [
In information retrieval applications, a mobile agent visits as many hosts as possible to acquire large amounts of required information. However, with the rapid growth of the Internet, an increasing amount of valuable information can be retrieved by the mobile agent in a region, subsequently causing the migration locality to change dynamically with the migration of the mobile agent. Moreover, intraregion and interregion migration of a mobile agent in current region-based location management schemes significantly differ in tracking cost owing to the design of update strategies. Therefore, this work compares the location management costs of each location management scheme by considering the migration locality of a mobile agent. Migration locality (
Additionally, an information retrieval application normally dispatches multiple mobile agents to search for information in parallel for efficiency, in which mobile agents coordinate and negotiate with each other to refine the searched results by exchanging messages. This interaction implies that the interaction rate between mobile agents varies with the migration of mobile agents. As the interaction rate is low, most of the location management cost is the tracking cost because of the low number of interaction messages. Under this circumstance, location management schemes that produce a higher cost during the tracking phase introduce additional performance overhead and network load. The extent to which message delivery cost influences location management cost becomes increasingly significant with an increasing interaction rate. Interaction messages are sent to a mobile agent along a
Since an interaction message is forwarded along a
A naming service includes naming and lookup. Naming refers to assigning a unique name to a newly created agent, while look-up refers to querying the location of a receiver agent’s LMS by submitting a look-up request. Two naming services are available in a mobile agent system. The first service is one in which the location information of the home host or the starting LMS on a channel is embedded into a mobile agent’s name. A sender agent attempting to interact with a receiver agent resolves the receiver agent’s name first. The second service constructs a global look-up service similar to the domain name system (DNS) protocol on the Internet. Location information of the home host and the starting LMS on a channel is obtained by querying one or more look-up servers. Although easily implemented, the first naming service cannot ensure that the location can transparently deliver messages to a mobile agent, thus making it impractical for a mobile agent location management scheme. Moreover, although message delivery in the second naming service is transparent to the mobile agent, transmitting query messages to look-up servers increases location management costs, ultimately increasing the complexity of maintaining the look-up servers.
This section introduces the location management architecture of DHS scheme, as well as its tracking phase and message delivery phase. Moreover, a simple method which is called On-Demand Probe and Priority Cache is devised for providing the selective update strategy in DHS the necessary transmission cost information between a pair of hosts on a network when making an update decision.
The proposed DHS scheme adopts a dual home location management architecture, in which a mobile agent has two homes, that is, primary and temporary. The primary home is a host that creates the mobile agent, while the temporary home is the host where the mobile agent updates location information to the primary home. Restated, the latter is the host next to the primary home on the mobile agent’s
Figure If Else
An example to illustrate tracking operation of DHS. (a) If
The philosophy behind the proposed DHS scheme is to reduce the total location management costs by preventing update and interaction messages from being transported on network links with higher transmission costs. To achieve this every host has to maintain a location table for storing the location information of mobile agents in the proposed DHS scheme. Location information stored in the location table of each host forms a channel that can forward an interaction message to a mobile agent. Either
The formed channel after updates to (a) temporary home, (b) primary home.
A location table consists of two entries, that is, AgentID and NextAddress, in each record. AgentID is the unique identifier of a mobile agent, while NextAddress is the next destination of a mobile agent. As a host receives an update message, the value in the NextAddress field is updated to the address in the update message.
When making update decision, MA requests the transmission costs from
Interaction messages are handled in the message delivery phase, which are sent among mobile agents along
Fields of an interaction message vary in different applications that require mobile agents to communicate, coordinate, or negotiate with each other for completing different tasks. Under such circumstances, interaction messages must include the name of the sender agent for the receiver agent to reply to the message. Obviously, a reply message sent by a receiver agent is another interaction message and, therefore, is delivered in the same way. A sender agent that remains stationary and requests to initiate a session that interacts directly can also add the address of the host, where it currently stays, in the interaction message for a direct delivery to the host.
Reliable message delivery is vital for mobile agent location management. Communication failure and message chasing are of major concerns in this issue. The message chasing problem does not last long in the proposed DHS scheme because there are at most three hosts on a mobile agent’s channel, that is, the primary home, the temporary home, and the current host. As an interaction message reaches the mobile agent’s current location and the mobile agent has left, communication failure occurs. The mailbox approach [ the received message is assigned a serial number based on the incoming sequence; a copy of the message is stored in the mobile agent’s mailbox; the message is forwarded to the next host on the mobile agent’s channel.
A mobile agent always includes in the update message the serial number of the message that it last received and, then, sends it to the home host. The home host that receives the update message not only refreshes the location information of the mobile agent in the location table, but also checks the mobile agent’s mailbox to see if the mobile agent has missed any messages. The home host then pushes the lost messages to the mobile agent directly.
The tracking cost in DHS can be reduced by using forwarding pointers at the right time. While a mobile agent MA updates the location information to the primary home at host
Hop counts and Round Trip Time (RTT) are two commonly used network latency metrics for counting message transmission costs. The former represents the network resources a message consumes, while the latter reflects the efficiency of transmitting a message. Evaluating the cost for transmitting messages among hosts on a network is network distance measurement [
Measuring network distance in real-time helps DHS make more precise decisions, but it is too expensive for a mobile agent system that provides communication services for mobile agents because most network distance measurement methods use exhaustive explorations between a host and its peer hosts. Result of a simulation experiment in this paper shows that probing network distances constitute the major cost for making update decision. DHS requires each host in a system knowing the costs of transmission message between itself and other hosts for visiting mobile agents to decide whether to update. To reduce such probing cost, distance information, once measured, will be cached in a
A host’s
Currently, Ping and Traceroute are two network tools that can be used to measure network distance between two hosts. However, both of them cannot satisfy the requirements of DHS because they are not originally for this purpose. The DHS needs a quick response and low overhead tool to measure both the RTT and hop counts between two hosts. This paper implemented an extended Ping tool that uses network practice for this goal. Currently, most network hosts use platforms such as Linux, Unix, FreeBSD, or Windows. The default time-to-live (TTL) value in Linux-like system, for example, Linux, Unix, or FreeBSD, is set to 64 or 255, while in Windows system, for example, Windows 2000, XP, Vista, Server 2008, or 7, it is set to 128. The disparity between these TTL values is at least 64. Although the scale of the Internet is huge, the distance between a pair of hosts should not exceed 64 hops. Therefore, the hop counts between a pair of hosts are obtained by using the three default TTL values to subtract the remainder of TTL value after pinging the destination host. And finally choose the calculation result that is between 0 and 64. For example, the remainder of TTL value is 246 after pinging an IP address. Using 255, 128, and 64 to subtract 246 separately equals 9, −118, and −182. The distance between both hosts is 9 hops. The extended Ping tool has been verified by Traceroute, and the results are all correct in the randomly selected IP address.
This section compares the proposed DHS scheme with HP, FP, dU, SPC, RAMD, and Ratio schemes by formulating the location management costs in each of them. In doing so, the constituents of location management cost in each scheme are analyzed and the performance of DHS is evaluated. Abbreviations section lists the parameters used in the comparison.
Several assumptions are made for the following comparison. The first is that the probability that a mobile agent migrates to the home region is considerably low, and the second is the probability that the sender agent’s current host and the receiver agent’s home are within the same region is also extremely low.
Location management costs are constituted by the tracking cost and the message delivery cost. The tracking cost is the total cost of the update messages produced in the tracking phase, while the message delivery cost is the total cost of interaction messages created in the message delivery phase. The formulation is based on the region, where the migration of a mobile agent is divided into intraregion migration and interregion migration. Additionally, the transmission cost of a message is also classified into the costs of intraregion transmission and interregion transmission.
The total tracking cost is denoted as
According to the operation of each scheme, the tracking cost in each scheme is explained as follows. In the HP scheme, the total number of update messages is
The tracking cost in DHS considers not only a mobile agent’s migration, but also which home the mobile agent updates to. There are four cases in the following:
The total tracking cost in DHS is expressed as follows:
Additionally, the tracking cost in DHS also includes the cost for calculating the transmission cost between a pair of hosts on the network. Assume that the cache miss rate is denoted as
The definition of
The message delivery cost (
This section compares the proposed DHS scheme with HP, FP, dU, SPC, Ratio, and RAMD schemes by using the above derived formulae. The network latency metric used in the comparison is hop count. Assume the network scale is 30 hops for the longest distance between hosts, and the maximum migration of each region is 5 hops. In such a network, the transmission cost of a message is between 1 and 5 hops in a region and 6 to 30 hops between regions. Accordingly,
Since the probability that the home hosts of a sender agent and a receiver agent has been assumed to be very small,
The total costs in tracking phase.
The total costs in message delivery phase as (a)
Although slightly affecting the tracking cost in DHS, migration locality does not affect home-based schemes, that is, HP, dU, Ratio, and FP. This figure further demonstrates that the tracking costs in SPC and RAMD are especially high as the migration locality is low. This is because SPC and RAMD produce more than one update message in each update process for the reliability.
Figure
Assume that the transmission costs of update messages and interaction messages are the same. The total location management cost is then computed by summing up both the tracking cost and the message delivery cost. Figure
The total location management costs as (a)
In Figure
Evaluation results in the previous section are obtained from the formulae with many postulations. This section verifies the validity of the results from formulae by using the simulation approach which is more realistic to actual circumstances. This work devises a simulation environment in which experiments can be performed precisely. The network topology of the environment used here is established by using the nem [
This section evaluates and compares the tracking costs, message delivery costs, and total location management cost in each scheme by simulation experiments. The cost of determining network distance between hosts (
The total costs in tracking phase.
The total costs in message delivery phase as (a)
The results in Figures
The total location management costs as (a)
According to Figure
This section analyzes why the proposed DHS scheme in the simulation experiments performs better than that in the formulae evaluations. As mentioned earlier, the proposed DHS scheme reduces the total location management cost by preventing update and interaction messages from being transported on high transmission cost network links. Therefore, the values of
The comparison of
According to Figure
Moreover, that proposed DHS scheme can select a network link with a lower transmission cost for message because there are two homes for a mobile agent to update its location information. Theoretically, the possibility that a mobile agent updates to the temporary home increases when the migration locality increases. Nevertheless,
The values of
Obviously, the values of
In this paper, the location tables are used to maintain location information for DHS to track the current position of mobile agent. Location tables use storage space in host servers. In our environment, a location table is created by the MySQL database management system. The space complexity of location tables is evaluated by analyzing the data structure. In this paper, a location table consists of two fields: AgentID and AgentID: this field stores the unique identification of a mobile agent with field type and size of varchar(29). The format of AgentID is the IP address of the host where the mobile agent is created plus a timestamp that is specific to millisecond. The IP address needs 12 bytes and the timestamp needs 17 bytes.
According to the above specification, an entry in the
When a mobile agent finishes the tasks in a host and moves to the next host, it updates the location table according to the tracking mechanism. Suppose that a LMS maintains location information for
This work develops a novel mobile agent location management scheme that can minimize total location management cost. A location management scheme called DHS is proposed for this purpose. The development of DHS involves considering the migration locality of mobile agent and the interaction rate between mobile agents, that is, two important factors that affect the location management cost in mobile agent based information retrieval applications. For reducing the location management cost in all migration localities and interaction rates, the proposed DHS scheme uniquely adopts dual home location management architecture. Additionally, a selective update strategy based on the proposed architecture is designed for cost-effective mobile agent location management. The proposed DHS scheme is initially compared with several conventionally adopted location management schemes by formulation. Simulations are then performed not only to verify the formulae, but also to evaluate the total location management cost of the proposed DHS scheme. Simulation results demonstrate that the total location management cost in the proposed DHS scheme is nearly the lowest in all migration localities and interaction rates.
Number of hosts in a mobile agent’s itinerary
Cost of transmitting an update message between a pair of hosts
Cost of transmitting a query message between a pair of hosts
Cost of transmitting an interaction message between a pair of hosts
Probability of intraregion migration
Probability of interregion migration
Probability that a sender agent and a receiver agent are within the same region
Probability that a mobile agent updates to the primary home
Probability that a mobile agent updates to the temporary home
Number of hosts that forward the th message to a mobile agent, that is, the number of hosts on the mobile agent’s channel
Source host and the destination host are within the same region
Source host and the destination host are located in different regions
The total number of interaction messages received by a mobile agent during an itinerary
Cost of transmitting an interaction message between a pair of hosts, which are locating in the same region
Cost of transmitting an interaction message between a pair of hosts, which are locating in different region.
The authors declare that there is no conflict of interests regarding the publication of this paper.