A Mobile-Agent-Based Application Model Design of Pervasive Mobile Devices

Xiang Yang ,Yuanyi Zhang',Qinzhou Niu ,Xiaomei Taol, Luo Wu2Department ofElectronic and Computing, Guilin University of Technology, Guilin, P. R. China2School ofElectronics and Information Engineering, Beijing Jiaotong University, P. R. China

yangx@glite. edu. cn, gdzyy9550O] 63. com,djx@,glite. edu. cn, taoxiaomei@glite. edu. cn.

Abstract

On the basis of analyzing the limitations of mobileIP protocol in pervasive computing environment and incombination with the technological advantages ofmobile agent, this paper introduces a mobile-agent-based application model ofpervasive mobile devices.With the introduction ofmobile agent into the pervasivemobile devices, the application model enables allcorresponding IP messages to go through the mobileagent, which can directly detect the status informationofthe IP messages, thus making itpossible to switch themobile devices smoothly. Through handoff delayanalysis and simulation experiments, the paper alsoanalyses and contrasts the performance ofthe pervasivemobile devices before and after the mobile agent isintroduced The results show that with the introductionof mobile agent, the pervasive mobile devices can

switch smoothly and swiftly, decrease handoffdelay andthe status information in the process ofthe transmissionand reduce the loss ratio of handoff of the mobiledevices, thus effectively improving the performance ofthe mobile networks in the pervasive computingenvironment.

Keywords: pervasive computing, mobile agent,pervasive mobile devices, mobile IP protocol

1. Introduction

1.1 Background

As a new computing paradigm advanced in the1990s[1], pervasive computing emphasizes embeddingcomputing devices in almost any kind of objectimaginable in our daily life and work. Withoutcomputers being everywhere in their life, people can

focus on the tasks in their life again. People almostmean the similar thing when they mention pervasivecomputing. However, there hasn't been any precisedefinition of pervasive computing so far. According toXu Guangyou[2], pervasive computing is an integrationof information space and physical space. In theintegrated space people can be provided withtransparent digital service anywhere and anytime.

The definition reflects the two key characteristics ofpervasive computing: ubiquity and transparency.However, the limitations of mobile IP protocol inpervasive computing environment often result in thetriangle routing problems of mobile devices in thecommunicating process, thus causing low bandwidthand high delay and to some extent are restricting thedevelopment and application of pervasive computing.

1.2 Relevant Research

Many researchers have put forward some optimizingprograms to solve the triangle routing problems. Forinstance, the agent-based MIP route optimization[3],dynamic home agent program[4] and temporary agentprogram[5]. Though these programs have to some

extent improved the performance of the network, thereare some deficiencies. First, no smooth handoff hasbeen achieved, so the switching performance of themobile devices hasn't been improved much. Secondly,every correspondent node (CN) participating in themobile node (MN) communication must be capable ofdealing with the mobile status information, whichrequires every host necessary to communicate with MNto be upgraded. What's more, the massive links in thehandoff process of the communicating hosts may

increase the mobile status information transmitted in thenetwork several times, especially when all CN involvedin the MN communication are in a same domain.

To solve the above-mentioned deficiencies, we

introduce mobile agent, which is intelligent, mobile andsecure, in pervasive mobile devices to smoothly switchthe corresponding mobile devices and achievetransparency of the mobile host, thus effectivelyimproving the performance of the network and enablingthe users to get reliable and transparent informationservice anytime and anywhere. Only under thiscircumstance can we achieve complete pervasivecomputing.

2. Definition

2.1 Mobile Agent

Mobile Agent is the computer program which can

automatically move among different network hosts andperform the user's operations[6]. Mobile Agent has the

1-4244-0971-3/07/$25.00 ©)2007 IEEE.

characteristics of being automatic, cooperative, mobile,secure and intelligent. It can move to the appropriatenode, interact with its environment or communicatewith other agents as tasked.

The system structure, as illustrated in figure 1, showsthat mobile agent environment (MAE), which is abovethe host system, is the software environment wheremobile agent exists. It supplies secure and accuraterunning environment for mobile agent (MA), performsthe start-up and move of MA, establishes theperformance status of MA, performs the restrictingmechanism, fault tolerance strategy, security controland communication system of MA and provides basicservice.

A4DMMAE

Agent

IS-Rourc:s Host

ADM: Agent Administer Module MAE: Mobile Agent EnvironmentCM: Communication ModuleS-Rources: System-Rources SM: Security Module

Figure 1: System structure ofMA

MIPv4[8] is composed of correspondent node (CN),home agent (HA), foreign agent (FA) and mobile node(MN). The basic principles are:

(1) Each mobile node has a fixed IP address, whichis called home of address (HoA).

(2) When a mobile node (MN) moves to a newnetwork, it will get a temporary IP address called careof address (CoA) and then inform HA the CoA throughbinding update (BU). Then, HA responds to BUthrough binding acknowledge (BA).

(3) Correspondent node (CN) always sends HoA ofMN before sending data to MN. Then HA decides howto deal with the data according to the status ofMN: X IfMN is in the home network, it will directly accept thedata packets from CN. X If MN is not in the homenetwork, HA will intercept the data packets and forwardthem to the current foreign agent (FA) of the mobilenode. FA then sends the data packets to the mobile node(MN).

The basic principles of MIPv6[9] is similar to thoseof MIPv4. The only difference is that MIPv6 doesn'thave foreign agent (FA). Therefore, before CN sendsdata packets to MN, MN will inform CN the CoA. Afterreceiving the binding update (BU) of MN, CN willdirectly send the data packets to CoA.

2.2 Technological advantages of Mobile Agent

As MA's transmission strategy of transferring codeto data has met the needs of the network technology andthe application and development of the current network,the application of MA, which has excellent potential, ina network, especially the application of mobile devicesin a wireless network, has great technologicaladvantages[7].

Firstly, Mobile Agent can encapsulate networkprotocol and its autonomy is applicable to the dynamicchanges of the terminal equipment of the network.Secondly, the characteristics of dynamic migration ofMA can afford great flexibility for network equipment.Finally, MA can be applied in the massive datatransmission and the real-time control and it has goodrobustness to the network and the devices, thus it cansatisfy the increasing specific needs of the users of thenetwork.

2.3 The basic principles of mobile IP protocol

With the popularization of mobile interconnectednetwork, more and more pervasive computing devices(such as notebook computer, smart phone, PDA, etc)are demanding wireless access to network and smoothhandoff without disconnection in different domains. Asa mobile performance scheme of network layer, mobileIP protocol can carry out the seamless handoff ofmobile terminals in heterogeneous network and enablepeople to acquire information anytime and anywhere.

3. The Mobile-Agent-Based ApplicationModel Design of Pervasive Mobile Devices

The limitations of mobile IP protocol in pervasivecomputing environment result in the triangle routingproblems of mobile devices in the communicatingprocess, thus causing the problems of low bandwidthand high delay.

In view of the above mentioned situations, weintroduce mobile agent technology in pervasive mobiledevices and put forward a mobile-agent-basedapplication model of pervasive mobile devices.

3.1 Model Design

As is illustrated in figure 2, the mobile-agent-basedapplication model of pervasive mobile devices iscomposed of four parts: mobile host (MH), fixed host(FH), address server (ADS) and pervasive computerserver (PCS). The functions of each part are discussedas follows:

iH.MA)_ Sendagent-------0 MHl(MAl)]

FM2(MA2:) T----ae C'tSend agentdevcM2(Me

Move

Figure 2: The mobile-agent-based pervasive mobiledevices

3.1.1 Mobile Host (MH) and Fixed Host (FH)

Mobile host (MH) and fixed host (FH) are twoindispensable basic infrastructures in pervasivecomputing. As major pervasive computing equipment,MH and FH consist of information access equipment,intelligent controller, intelligent home appliances andentertainment system. Mobile host, the portable devicewith wireless network interface and mobile agentsystem, can establish agent in pervasive computingenvironment according to the demands of the user andinteract with other equipment or agent easily throughwireless network to get digital service transparently.Fixed host, which includes the server, desktop computerand intelligent home appliance, is the pervasivecomputing equipment fixed in the network.

3.1.2 Address Server(ADS)

Address server is one of the key parts in theapplication model. As a public server in the Internet,ADS has its IP address or DNS known in advance bymobile agent. ADS is mainly used to record the currentlocation of the mobile host. When the pervasive mobiledevice enters the pervasive computing environment, itwill register the CoA of its current position to addressserver through mobile agent. If a mobile host wants tocommunicate with another mobile host, it will inquirethe current CoA of that mobile host to ADS, i.e. it willsend the HoA of the host to be communicated to theADS, which, then, enquires about the current registeredCoA of the target mobile host according to its HoA andreturn the information to the host that requests theenquiry.

the needed information in the mobile agent environmentofPCS and return the results to mobile host.

3.2 Key technologies

3.2.1 Mobile detection

Mobile detection is also called agent discovery. Themajor functions of mobile detection are: Mobile nodefinds out the link to which it is connected byexchanging agent enquiry and agent advertisementmessages with mobile agent. If it is in a foreign link, themobile node can also get the CoA of the current linkand the available address of the current Router. Thenthe mobile detection is ready for the registration of thenew CoA and the log-out of all the CoAs.

3.2.2 Agent registration

Agent registration plays a very important role in theapplying process of the whole application model sincemobile node must attain the CoA if it is to communicatein a foreign link. The registering process is finished byexchanging registration enquiry and registrationresponse packets among mobile node (MN), foreignagent (FA) and home agent (HA) with the cooperationof the monitoring process of the user mode and thesupport of the core mobile IP. After the mobile nodereceives the agent advertisement, it will judge whetherit's in a foreign link or in a home link. If it is in aforeign link and hasn't registered any CoA, mobilenode will start up the registration process.

3.3 The applying process

3.1.3 Pervasive Computing Server(PCS)

Pervasive Computing Server (PCS) is a specialcomputer fixed in the network. It is a key equipment inthe pervasive computing system. PCS is responsible ofproviding digitized information to various devices inthe pervasive computing environment. Additionally,PCS has a wireless communication interface, which isresponsible of establishing a wireless unit (such as awireless cellular unit or a wireless local area networkunit). The pervasive mobile devices (Mobile Host) inthe wireless network can connect with PCS through thewireless network and then communicate with otherdevices fixed in the network.

In addition, PCS also include mobile agentenvironment (MAE), which can provide secure andaccurate running environment to start up and move MA,build the executing estate of MA, return MA, carry outMA's restriction system, fault tolerance strategy,security control and communication system and providethe basic service for MA. The mobile host can createMA and send it to PCS through the wireless network.Then M\A can deal with all kinds of requests and collect

In this application model, we assume that there aretwo hypothetical mobile devices, MH1 and MH2, in thepervasive computing environment. First of all, they arestarted up, and two mobile agents, MA1 and MA2, arecreated respectively. Then MA1 and MA2 are sent tothe pervasive computing server (PCS) through wirelessnetwork. Figure 3 illustrates the intercourse process ofMAI and MA2 in the pervasive computingenvironment.

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timne-a 9

1213

(A,)

78 _

_4 10

14

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Figure 3: Intercourse process of mobile agent(1) MA1 registers the CoA of MH1 to address server

(ADS) through pervasive computing server (PCS), i.e.MA1 sends the current CoA ofMHI to ADS.

-0 j.

(2) After receiving the CoA, address server (ADS)answers the registration information of M\A1 andinforms PCS of its acceptance of the registration ofMA1.

(3) MA2 registers the CoA ofMH2 to address server(ADS) through pervasive computing server (PCS), i.e.MA2 sends the current CoA ofMH2 to ADS.

(4) After receiving the CoA, address server (ADS)answers the registration information of MA2 andinforms PCS of its acceptance of the registration ofMA2.

(5) MA1 provides the HoA ofMH2 to ADS throughPCS and asks for the current CoA ofMH2.

(6) After receiving the HoA of MH2, address server(ADS) seeks the current CoA ofMH2 through HoA andsends it to M\A1.

(7) MA2 supplies the HoA of MH1 to ADS throughPCS and asks for the current CoA ofMH1.

(8) After receiving the HoA of MH1, address server(ADS) seeks the current CoA ofMH1 through HoA andsends it to MA2.

(9) MH1 communicates with MH2 through theinteraction ofMA1 and MA2.

(10) MH2 moves to a new place according to theuser's demand. The CoA of MH2 changes and MA2'(We use MA2' to distinguish it from MA2 since thelocation of MA2 has changed. As a matter of fact,MA2' is same with MIA2) registers a new CoA to theaddress server.

(11) After receiving the new CoA, address serveraccepts and answers the updated registration ofMA2.

(12) Address server sends the new CoA of MH2 toMA1.

(13) After receiving the new CoA, M\4A1 answers theaddress server.

(14) The communication between MH1 and MH2continues through the interaction ofMA1 and MA2.

The above-mentioned application process sufficientlytestifies that mobile agent plays an important role inimproving the connection performance of pervasivemobile devices. Of course, context awareness is alsovery important for people to acquire information serviceanywhere and anytime transparently in pervasivecomputing environment. Context awareness is definedas "any feature information used to describe theenvironment of an entity, which can be a person, a placeor a related object"[10]. Therefore, context awareness isoften achieved through awareness entity, which, in thisapplication model, refers to the mobile agent (MA).

4. Performance analysis

Since handoff is an important feature of the mobilenetwork in pervasive environment, the performance ofthe handover mechanism can directly affect theperformance of the whole application model. Therefore,in the next part we analyze and contrast the handoffdelay, the loss ratio and the relationship between the

before and after the introduction of mobile agent in thepervasive mobile devices through handoff delayanalysis and simulation experiments and further discussthe important role the application model plays in themobile network in improved pervasive environment.

4.1 Handoff delay analysis

To simplify the process, the delay of communicationprocess of pervasive mobile devices MH1 and MH2 isdiscussed and the time delays of pervasive mobiledevices before and after the introduction of mobileagent are analyzed and their performances are alsocontrasted.

According to the key technologies introduced in theprevious part, the delay of the handoff process of thepervasive mobile devices is:

Thd=Tmd+Treg (1)

In formula (1), Tmd stands for the time delay ofmobile detection, and Treg is the time delay of agentregistration. First of all, let's have a look at theinteraction process before the introduction of mobileagent. As is illustrated in figure 4, TI and T5respectively stand for the link delay of the process inwhich MH1 and MH2 sends their registration request toFA; T2 and T6 respectively stand for the link delay ofthe process in which FA forwards the registrationrequests of MH1 and MH2 to HA after finishingprocessing the requests; T3 and T7 respectively standfor the time delay of the process in which HA answers

FA; T9 stands for the time delay of the process in whichMH2 communicates with HA after establishing thecommunication; T1O stands for the time delay of thecommunication between HA and FA and T 11 stands forthe time delay of the process in which FAcommunicates with MH1. In addition, Tfp and Thp,which stand for the time when FA and HA process thepackets, are also two important parts in the handoffdelay. So the delay of the handoff process is:Thdl=Tmdl+Tregl=Tmdl+ T1+ T2+ T3+ T4+ T5+

T6+ T7+ T8 +Tfp+Thp (2)The transmission time delay of the normal

communication between MH1 and MH2 after thehandoff is:

Tdelayl= T9+ T1O+ T1 1 (3)~~FA 5

~10 2 3 6 2

HAk

Figure 4: The interaction process before theintroduction of mobile agent

Figure 5 illustrates the interaction process betweenMH1 and MH2 after the introduction of the mobileagent. TI represents the link delay of the process inwhich MA1 sends its registration request to ADS. TI iscomposed of two parts: one is the link delay of theprocess in which MAt registers the CoA of MHI, and

CN numbers and the status information flow capacity another is the link delay of the process in which NlAl

provides the HoA of MH2 to ADS. T2 stands for thetime delay of the process in which ADS answers M\A1after dealing with its registration request. T2 is alsocomposed of two parts: the answer to CoA and theanswer to HoA. T3 and T4 are the registration timedelay and the response time delay of MA2. T3 and T4are also made up of two parts, and they arecorresponding to TI and T2 respectively. Tap stands forthe time delay of the process in which ADS processesthe packets. T5 is the time delay of the communicationbetween M\4A1 and MA2. According to Formula (1), thetime delay of the handoff process after the introductionofmobile agent is:

Thd2=Tmd2+Treg2=Tmd2+ T1+ T1+ T2+ T2+ T3+T3+ T4+ T4+Tap (4)

The transmission delay of the normalcommunication between MH1 and MH2 after thehandoff is:

Tdelay2= T5 (5)Mill ~~~MH2

Figure 5: The interaction process after theintroduction of mobile agent

Tmdl and Tmd2 are same because the mobiledetection algorithm adopted before and after theintroduction of mobile agent is same. Therefore, weonly discuss about the registration delay Tregl andTreg2. As we know, when the communication range isfixed, the more the link nodes there are in the handoffprocess, the longer the handoff delay will be. And thismay easily cause network congestion and decrease thenetwork performance to a great extent. As thecommunication range between MH1 and MH2 beforeand after the introduction of mobile agent is same, let'sassume that in an ideal environment the time delays ofall links are same, put as T and the time for the middlenodes to deal with the packets is same, put as Tp, then

Tregl=8T+ 2Tp, Treg2=8T+ Tp,So Tregl>Treg2 or Thdl>Thd2;Therefore, the handoff delay is obviously decreased

after the introduction of mobile agent and theperformance of the network is improved effectively.

In addition, the contrast of formula (3) and (5)indicates that before the introduction of mobile agent,MH1 communicates with MH2 through the exchange ofstatus information between FA and HA. The result ofthis is that MH1 and MH2 can not obtain the latestnetwork access of each other in the process of thecommunication, and all the communication messagescan be sent to each other only after being re-encapsulated by HA and FA. Then triangle routingproblems occur, i.e. the delay of the communicationbetween MH1 and MH2 is not only affected by thetransmission delay in the regular network, but also is

affected by the time delays of the communicationbetween MH2 and HA, between HA and FA andbetween FA and MH1. Therefore, a nice service qualitycan not be achieved since the pervasive computingdemands a very low time delay. Since MA1 and MA2have completed binding update operation after themobile agent is introduced, the transmission delay ofMH1 and MH2 is the transmission delay of the normalnetwork. So the introduction of mobile agent cangreatly cut down the network cost of the data exchangeprocess and effectively improve the performance of thenetwork.

4.2 Simulation experiment

To analyze the performance of the mobile-agent-based application model of pervasive mobile devices,we experiment on the loss ratio of the handoff and therelationship between the communication CN numbersand the status information getting in and out the CNdomain before and after the introduction of mobileagent by extending the functions of simulation networksoftware NS2.28[1 1]. The experiment assumes that thewireless link is an ideal error free link and the wirelesscell has no overlaps and is ideal seamless coverage. Thebasic parameters of the experiment are listed in chart 1.

Chart 1: The basic parametersParameter Value

Wireless data link layer protocol 802.11Routing protocol DSDV

Wireless transmission rate 500KbWire transmission rate 1Mb

CN number 50Delay IOms

In the experiment, we use A and B to represent whatthe situation is before and after the introduction ofmobile agent. The experiments results are illustrated infigure 6 and figure 7. Figure 6 illustrates that in thewhole simulation process, the loss ratio of the handoffbetween MH1 and MH2 after the introduction ofmobileagent is much lower than that of the handoff before theintroduction. The major reason of this is the IPmessages encapsulated by MA through binding themessages are sent directly out through the output bywhich MA is linked with the network and then sent tothe current access agent ofMN. In this way, the trianglerouting problems can be avoided and the loss ratio isreduced greatly.

0. 20

.2 -" O. IS _El]

0. 00A

Figure 6: Loss ratio of the handoffFigure 7 illustrates that the mobile status information

that is necessary to be sent to CN before theintroduction of the mobile agent increases as the CNnumber of the communicating MN increases. After the

introduction of mobile agent, the mobile informationrelated to CN basically remains the same and is close tothe mobile information when there is only one CN.Even in the worst situation, i.e. the N CNs of the MNcommunication are located in different domains, CurveB is only gradually approaching Curve A and it willnever rise above it.

O 20

15

!" 1 1 0 20 30 40 50CN number

Figure 7: The relationship between CN numberand the status information

The experiment results show that after theintroduction of mobile agent (MA), M\A encapsulatesthe IP messages through binding them. Smooth handoffis achieved without affecting the normal running of themobile IP protocols, thus the handoff delay is reduced,so is the loss ratio in the process of the handoff of themobile devices and the network resources occupied bymobile status information in the transmission process isalso decreased. Therefore, the mobility of the host istransparent to the current network and the computer andthe performance of the mobile network in pervasiveenvironment is effectively improved.

5. Conclusion

As a new computing paradigm, pervasive computinghas gradually come into our life. In pervasivecomputing environment, people can get dependableinformation service anywhere and anytimetransparently. In view of the limitations of mobile IPprotocol and pervasive mobile devices and incombination of the technological advantages of mobileagent, this paper introduces mobile agent in pervasivemobile devices, puts forward a mobile-agent-basedapplication model of pervasive mobile devices andanalyses and contrasts the performance before and afterthe introduction of mobile agent through handoff delayanalysis and simulation experiments. The analysisresults indicate that the introduction of mobile agentinto pervasive mobile devices can help achieve thesmooth handoff between the mobile devices, hencereducing handoff delay and the status information in theprocess of transmission, lowering the loss ratio in theprocess of the handoff of the mobile devices and

effectively improving the performance of the mobilenetwork in pervasive environment.

Of course, since mobile agent is a new technologyintroduced in pervasive computing, security is also avery important issue to it. Several effective strategieshave been presented by Mu Hong[12] in view of thecharacteristics and mobile devices. Although thesestrategies can somewhat ensure the success of mobileagent applications, they are obviously insufficient forpervasive mobile devices, which require very highsecrecy and security performance. Therefore, the futureresearches should focus on the security of mobile agentused in pervasive mobile devices and the securitysystem of pervasive computing.

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