2284 IEEE COMMUNICATIONS LETTERS, VOL. 20, NO. 11, NOVEMBER 2016

Use of Proxy Mobile IPv6 for Mobility Managementin CoAP-Based Internet-of-Things Networks

Sang-Il Choi and Seok-Joo Koh

Abstract— Recently, the constrained application protocol(CoAP) has been standardized for remote control of varioussensor devices in Internet of Things networks. In CoAP, tosupport the handover of mobile devices, service discovery shouldbe performed again. So, the handover delay may be increasedsignificantly. To address this limitation of CoAP, in this letter,we propose the two mobility management schemes based on theproxy mobile IPv6 (PMIPv6): CoAP-PMIP and CoAP-DPMIP.In CoAP-PMIP, local mobility anchor (LMA) and mobile accessgateways (MAGs) are used to provide network-based mobilitysupport for sensor devices. Each device has to register its IPv6address with LMA, and all messages are transmitted throughLMA. In CoAP-DPMIP, the role of LMA is distributed to eachMAG. By using distributed MAGs, this scheme can provideoptimized transmission path and also reduce the handover delay.From ns-3 simulations, we can see that the CoAP-DPMIP schemeprovides better performance than the CoAP and CoAP-PMIPschemes, in terms of total delay associated with binding update,data transmission, and handover.

Index Terms— Mobility management, PMIPv6, CoAP, mobilesensor devices, handover, internet of things.

I. INTRODUCTION

IN INTERNET of Things (IoT) environment, a variety ofsensor devices are connected to Internet [1]. To address

the battery power problem of constrained sensor devices, theConstrained Application Protocol (CoAP) has recently beenstandardized in the Internet Engineering Task Force [2].

In the meantime, the mobility management in IoT serviceshas been considered as one of the critical issues [3]–[7],in which the seamless data transmission should be providedfor mobile sensor devices. Such typical applications includethe IoT-based healthcare services [3], [4], in which a doctor(client) needs to monitor the status of moving patients (mobilesensors) by using the CoAP-based IoT communication, andin an emergency case, the doctor should be able to treat thepatient appropriately.

Some works on IoT mobility management have been madeso far. In [5], a resource mobility scheme was proposed forservice continuity in IoT networks. However, by using thetunneling between old and new gateways, it is still subjectto a non-optimized path problem. In [6], a new protocolfor IP-based wireless sensor network was proposed, but itdid not consider the characteristics of CoAP. In [7], a cen-tralized mobility management scheme for CoAP was alsoproposed. However, the scheme is a host-based mobility

Manuscript received June 30, 2016; revised August 1, 2016; acceptedAugust 14, 2016. Date of publication August 18, 2016; date of current versionNovember 9, 2016. The associate editor coordinating the review of this letterand approving it for publication was T. Han.

The authors are with the School of Computer Science and Engineer-ing, Kyungpook National University, Daegu 41566, South Korea (e-mail:sjkoh@knu.ac.kr).

Digital Object Identifier 10.1109/LCOMM.2016.2601318

Fig. 1. Existing CoAP procedures.

scheme, and thus it may give potential overhead to sensordevices by frequent message transmission and increased powerconsumption.

In this Letter, we propose the two network-based mobilitymanagement schemes using Proxy Mobile IPv6 (PMIPv6) [8]with CoAP: CoAP-PMIP and CoAP-DPMIP. It is noted thatthe network-based mobility scheme is preferred to the host-based scheme for energy saving of sensor devices in wirelessnetwork. In the proposed schemes, we define an IPv6 addressof mobile sensor device as a combination of the prefix valueof its MAG and the hashed value of Uniform ResourceIdentifier (URI) of a mobile sensor device. In CoAP-PMIP,a Local Mobility Anchor (LMA) manages mobile sen-sor devices by storing the address information of sensordevices. In addition, all messages are passed via LMA.In CoAP-DPMIP, the role of LMA is distributed to DistributedMAG (DMAG). By using DMAG, CoAP-DPMIP can providea more optimized data transmission path and also reduce thehandover delay.

The rest of this Letter is organized as follows. Section IIbriefly reviews the mobility management operations of CoAP.Section III presents the proposed PMIP-based mobility man-agement schemes. Section IV discusses the ns-3 simulationanalysis and comparisons. Section V concludes this Letter.

II. EXISTING COAP MOBILITY SCHEME

In CoAP, when a sensor device turns on, it is attached toan Access Router (AR), and it will register its resource listwith the Resource Directory (RD) that is equipped with AR.If a handover occurs during communication, the client willrealize the handover of the sensor device by receiving anICMP destination unreachable message. Then, the client justperforms the device discovery procedure again so as to obtainthe changed location information of the mobile sensor device.

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CHOI AND KOH: USE OF PROXY MOBILE IPv6 FOR MOBILITY MANAGEMENT IN CoAP-BASED IoT NETWORKS 2285

Figure 1 shows the mobility operations of CoAP. If a sensoris attached to an AR, it registers its resource list to the AR withRD by exchanging the Router Advertisement and Solicitationmessages (Step 1). When a client sends some request/responsemessages to a mobile sensor, it performs the multicast-basedservice discovery operations so as to get the IPv6 addressof the sensor device (Step 2-5). On reception of a discoverymessage, the sensor responds with its IPv6 address to client.Then, client and sensor device can exchange the data messages(Step 6). Let us assume that the handover of the sensordevice occurs (Step 7). CoAP does not provide the handoversupport operation. So, the client cannot know the handoverevent until it receives a destination unreachable message as anICMP error (Step 8, 9). On reception of ICMP error message,the client realizes the movement of target device. Then, itperforms device discovery procedure again to find the movedsensor device (Step 10-12). After that, the client and thesensor device can continue their communications (Step 13-15).Due to this delayed handover detection and repeated devicediscovery operations, the CoAP scheme tends to generatemany additional messages and to induce large handover delay.

III. PMIPv6-BASED CoAP MOBILITY SCHEMES

To reduce the handover delay of CoAP, we proposethe two PMIP-based mobility schemes: CoAP-PMIP andCoAP-DPMIP. For efficient mobility management, we definean IPv6 address of mobile sensor device as follows: the prefixof MAG::hashed URI of sensor device. By this, the locationand movement of sensor device can be managed.

A. PMIPv6 With CoAP (CoAP-PMIP)

In this scheme, we add a mobility anchor as LMA andextend the functionality of AR by including the roles of MAG.

If a sensor is attached to MAG, the MAG sends a bindingupdate message to LMA to register the URI and IPv6 addressof mobile sensor device. Then, LMA stores the informationof sensor device and the associated MAG. For data delivery, aclient get the contact information (including IPv6 address) ofmobile sensor by sending a binding query message to LMA.If a handover occurs, the mobile sensor device sends a bindingupdate message to LMA through the newly attached MAG.Then, LMA updates the associated IPv6 address and forwardsthe data message to the new MAG.

Figure 2 shows the procedures of CoAP-PMIP. If a sensoris attached to the network, MAGA sends Proxy BindingUpdate message to LMA (Step 1, 2). Then, LMA updatesits table and responds with Proxy Binding ACK messageto MAGA (Step 3). For data delivery, client sends querymessage to LMA through MAGC to get an IPv6 addressof sensor device (Step 4, 5). Then, LMA finds the mappinginformation of sensor from its table and sends Binding QueryACK message to client (Step 6). Now, client can exchangedata messages with sensor (Step 7). If a handover occurs,a newly attached MAG performs the proxy binding updateoperation with LMA (Step 8, 9). Then, LMA responds withProxy Binding ACK message (Step 10). Now, the client canexchange messages with the sensor device via LMA (Step 11).

Fig. 2. CoAP-PMIP procedures.

Fig. 3. CoAP-DPMIP procedures.

B. Distributed PMIPv6 With CoAP (CoAP-DPMIP)

In CoAP-DPMIP, the role of LMA is distributed to eachMAG, named distributed MAG (DMAG). DMAG maintainsits table and stores mapping information of URI and IPv6address of sensor devices. It is assumed that DMAG alreadyknows the prefix and IPv6 address of other DMAGs. To findthe DMAG that the target sensor device is initially connectedto, each DMAG has only to know the IPv6 address of targetsensor device, since the address is a combination of prefixof the initially attached DMAG and a hashed URI value ofsensor device. If a handover occurs, a new DMAG can performhandover operation with the old DMAG by checking the prefixfield in IPv6 address of sensor device.

Figure 3 shows the procedure of CoAP-DPMIP. If asensor is attached to DMAGA, the DMAGA stores theIPv6 address and URI of sensor device, prefix, and theIPv6 address of DMAG (Step 1). The client sends BindingQuery messages to DMAGC to get an IPv6 address ofsensor (Step 2). Then, DMAGC confirms the URI oftarget sensor device and forwards Binding Query messageto other DMAGs. Based on the Binding Query ACKmessage, DMAGC realizes that the target sensor deviceis managed by DMAGA in this example (Step 3, 4).Now, the client can send data message to the sensor devicevia DMAG (Step 5). If a handover occurs, a new DMAG sendsHandover message to DMAGA using the prefix contained in

2286 IEEE COMMUNICATIONS LETTERS, VOL. 20, NO. 11, NOVEMBER 2016

Fig. 4. Network model for NS-3 simulation.

TABLE I

PARAMETER VALUES USED FOR ANALYSIS

the IPv6 address of sensor device (Step 6, 7). Then, DMAGA

performs handover operations with DMAGC to inform the newDMAG of sensor device (Step 8, 9). When the handover proce-dures between old DMAG and DMAG of client are completed,DMAGA sends Handover ACK message to DMAGB and thehandover procedures are completed (Step 10). Now, the clientcan exchange messages directly with the sensor (Step 11).

The additional messages of proposed schemes are used onlyin the wired network. Thus, the energy consumption of mobilesensor will be almost the same for both existing and proposedschemes in handover operation. Rather, the proposed schemescan reduce multicast discovery messages to be processed atmobile sensor, compared to the CoAP scheme.

IV. EXPERIMENTAL ANALYSIS

A. Test Environments

For performance comparison, we used the ns-3 networksimulator [9], based on the network model for simulation, asshown in Figure 4.

Table 1 shows the parameter values used for simulation.In simulation, the hop count between AR and AR is set to 5,and the default hop count between AR and LMA is set to 8.

B. Simulation Results and Discussion

Figure 5 shows the trace of all data packets that the targetsensor receives from the client during simulation, in whichclient sends totally 40 data messages to the mobile sensor,and the handover occurs at the 20th data message.

In the figure, we can see that the packet arrival timesfor initial data messages are almost the same for all ofcandidate schemes. However, as the number of data messagestransmitted increases, the packet arrival time of CoAP-PMIPsignificantly increases, compared to the other two schemes,since it uses non-optimized transmission path via LMA.Such gaps of performances tend to get larger, as the number of

Fig. 5. Traces of all data packets during simulation.

Fig. 6. Impact of wired link delays on total delay.

data messages increases. In the meantime, the CoAP-DPMIPscheme provides steadily shorter arrival time, compared to theother two schemes. This is because CoAP-DPMIP can reducethe handover delay and optimize data transmission path bydistributing the role of LMA to each DMAG. Consequently,the CoAP-DPMIP scheme gives the best performance amongthree candidate schemes.

In the remaining experimentations, we will focus on theperformance analysis by handover event in various networkenvironments, in which the client sends two data messagesto the target sensor: one is transmitted before handover, andanother is done just after handover.

Figure 6 shows impact of wired link delays on total delay.In the figure, it is shown that CoAP-PMIP severely dependson the wired link delay. This is because this scheme usesthe non-optimized path between MAG and LMA, and trans-mits the binding update and data messages over the wiredlinks between MAG and LMA. Thus, in CoAP-PMIP, thetotal delay increases, as the wired link delay gets larger.The existing CoAP scheme also gives large handover delays,since the discovery operations should be performed again byhandover. Overall, the CoAP-DPMIP scheme provides thebest performance. This is because the CoAP-DPMIP schemedistributes the role of LMA to each AR and optimizes the datatransmission path.

Figure 7 shows the impact of Wi-Fi (wireless) link delayson total delay. The existing CoAP scheme shows the worstperformance, since it frequently uses the wireless link between

CHOI AND KOH: USE OF PROXY MOBILE IPv6 FOR MOBILITY MANAGEMENT IN CoAP-BASED IoT NETWORKS 2287

Fig. 7. Impact of Wi-Fi link delays on total delay.

Fig. 8. Impact of the number of handover events.

mobile sensor device and AR during the discovery procedureby handover. However, the two proposed PMIP-based schemesprovide the fast handover mechanisms by using the bindingupdate procedure, and thus the two proposed schemes showbetter performance than the existing CoAP scheme. It is notedthat the impact of Wi-Fi link delay on total delay is alsomuch larger for CoAP than for the proposed schemes. This isbecause CoAP depends on the multicast discovery messagesover Wi-Fi link, whereas the PMIP-based schemes performeffectively the network-based handover operations by usingthe handover update messages.

Figure 8 shows the impact of the number of handoverevents. In the figure, we can see that the existing CoAPshows the worst performance owing to the repeated multicast-based discovery procedures. However, the two proposedPMIP-based schemes provide better performance, since theseschemes can reduce the handover delay by using LMA orDMAG. In addition, the CoAP-DPMIP scheme optimizes thedata transmission path. Overall, the CoAP-DPMIP schemeshows the best performance among the three candidateschemes.

Figure 9 shows the impact of hop count between MAG andLMA for candidate schemes. In the figure, we can see that theCoAP-PMIP scheme tends to give good performance when the

Fig. 9. Impact of hop count between MAG and LMA.

hop count is small. However, it provides the worst performancefor large hop count (4 or more in this simulation). This isbecause CoAP-PMIP depends on the hop count between MAGand LMA. Overall, the CoAP-DPMIP scheme shows the bestperformance among the three candidate schemes.

V. CONCLUSION

In this Letter, we proposed the two PMIP-based CoAPmobility schemes to reduce the handover delays in IoTnetworks: CoAP-PMIP and CoAP-DPMIP. The proposedschemes use LMA or DMAG to manage the mobile sensordevices and to perform the handover operations.

From the performance comparison, we can see that theproposed CoAP-DPMIP scheme provides the best performanceamong the three candidate schemes by using distributed MAGsand optimized data path. In the meantime, CoAP-PMIP givesbetter performance than CoAP for small number of datamessages. However, CoAP tends to provide better performancethan CoAP-PMIP for large number of data messages.

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