Simplified fast handover in mobile IPv6 networks

Computer Communications 31 (2008) 3594–3603

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Simplified fast handover in mobile IPv6 networks Nguyen Van Hanh *, Soonghwan Ro, Jungkwan Ryu Kongju National University, 182 Sinkwan-dong, Gongju-si, Chungcheongnam 314-701, Republic of Korea

a r t i c l e

i n f o

Article history: Available online 24 June 2008 Keywords: Mobile IPv6 Fast handover Simplified fast handover

a b s t r a c t The Fast Handovers for Mobile IPv6 (FMIPv6) protocol provides seamless handover; it uses anticipation based on layer 2 trigger information of the mobile node (MN) to obtain a new care-of address at the new link while still connected to the previous link, thus reducing handover delay. A bidirectional tunnel is then established between access routers to minimize packet loss during the handover. However, this method incurs higher signaling costs compared with the standard Mobile IPv6 protocol. In many cases, the mobile node cannot complete the fast handover in predictive mode due to lack of time, especially with high-speed movement of the mobile node. This paper proposes an enhancement to the FMIPv6, the Simplified Fast Handover in Mobile IPv6 Networks (SFMIPv6), which significantly reduces the anticipation time of the fast handover and thereby increases the probability that the protocol can perform the fast handover in predictive mode. In this paper, we also present performance evaluations in terms of the influence of two factors, the break-down point and the velocity of the MN, using evaluation models. The numerical results prove that the network performance of the proposed protocol is effectively improved compared to the original protocols, the FMIPv6 and Fast Handover Support in Hierarchical Mobile IPv6 (F-HMIPv6). Moreover, the results show that the proposed protocol could appropriately operate with high-speed mobile node movement. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Mobile IPv6 (MIPv6) [1] is a global mobility management protocol in mobile IPv6 networks that specifies the operations through which a mobile node (MN) maintains its connectivity to the internet while being handed over from one access router to another. These operations cause considerable and unacceptable packet losses and latencies in certain circumstances such as real-time applications. Several extensions to Mobile IPv6 have been proposed in order to reduce these handover latencies and packet losses. The FMIPv6 [2] is an enhancement to Mobile IPv6, proposed by the Internet Engineering Task Force (IETF) that provides seamless handover in Mobile IPv6 Networks. Layer 2 (L2) trigger information from the mobile node (MN) is used to obtain a valid new care-of address (NCoA) while it is still connected to the previous link, and then a bidirectional tunnel is established between the previous access router (PAR) and the new access router (NAR) in order to reduce packet loss during the handover. The MN can use the NCoA immediately after establishing a connection with the new link; thus, the handover latency and packet loss can be considerably reduced. Furthermore, if the MN has enough time to perform fast handover in the predictive mode, there may be no packet loss. * Corresponding author. Tel.: +82 10 2891 7299. E-mail addresses: [email protected] (N. Van Hanh), [email protected] (S. Ro), [email protected] (J. Ryu). 0140-3664/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.comcom.2008.06.009

However, a certain cost is also incurred in terms of additional overhead; for example, new signaling messages are required for the anticipated handover procedure. Therefore, in many cases the MN cannot successfully perform the fast handover procedure in predictive mode due to lack of time. In these cases, the MN must revert to the normal MIPv6 or switch to the reactive mode of the FMIPv6, depending on the link-break time; both handover latency and packet loss are thereby increased. Several extensions [7–12] have been proposed to improve the performance of the FMIPv6, but these studies did not consider reducing the anticipated handover delay that limits the time for the MN to perform the fast handover procedure in predictive mode. Moreover, all of these enhancements issue more signaling messages during this critical period; therefore these proposals are inappropriate for high-speed MN movement. This paper proposes a modification to the FMIPv6 that considerably reduces the overhead associated with fast handover, including the signaling cost and the packet delivery cost. This method may significantly increase the probability that the MN can successfully perform the fast handover procedure in predictive mode, thus supporting high-speed MN movement. The key idea is to eliminate the useless time during which the MN waits for the PAR’s response after sending a Router Solicitation for Proxy (RoSolPr) message, before completely sending the Fast Binding Update (F-BU) message to the PAR. Instead, the MN uses the F-BU option within the RtSolPr message, such that the PAR can initiate fast handover immediately after receiving the RtSolPr.

N. Van Hanh et al. / Computer Communications 31 (2008) 3594–3603

Accordingly, the anticipation time of the fast handover can be significantly reduced about a roundtrip time (RTT) on the wireless link between the MN and the PAR. For the sake of simplicity, we describe the proposed scheme based on the FMIPv6 [2], but it can also be applied to other schemes that support fast handover, such as the Fast Handover for Hierarchical Mobile IPv6 (F-HMIPv6) [4]. However, in numerical results, we show the comparison between the original schemes (FMIPv6 and F-HMIPv6) and the proposed schemes (SFMIPv6 and SF-HMIPv6 that apply to the respective original schemes, FMIPv6 and F-HMIPv6) in order to illustrate the improvement of the new proposal. The rest of the paper is organized as follows. Section 2 provides the background and related work. Then, in Section 3, we describe our proposed protocol, the Simplified Fast Handover in Mobile IPv6 Networks (SFMIPv6) Scheme. In Section 4, the performance of the proposed protocol is estimated through evaluation models. Numerical results are given in Section 5. Next, the security issues of the proposal protocol are discussed in Section 6. Finally, we conclude this paper in Section 7. 2. Background and related work 2.1. Fast Handovers for Mobile IPv6 In this section, we briefly introduce the FMIPv6. Detailed information can be found in [2]. The basic operation of the FMIPv6 is illustrated in Fig. 1(a). The FMIPv6 introduces seven additional message types: Router Solicitation for Proxy Advertisement (RtSolPr), Proxy Router Advertisement (PrRrAdv), Handover Initiate (HI), Handover Acknowledgement (HAck), Fast Binding Update (F-BU), Fast Binding Acknowledgement (F-BAck), and Fast Neighbor Advertisement (FNA). A fast handover procedure starts with the MN sending an RtSolPr message, and ends with the MN receiving an F-BAck message on the previous link. In the FMIPv6 protocol, when an MN is aware of its movement towards an NAR through an L2 trigger, the MN must perform a fast handover procedure. Then, after connecting to the NAR, the MN immediately sends an F-NA message without the need for route discovery in order to inform its presence, so that arriving and buffered packets can be forwarded to the MN.

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Finally, as in the MIPv6 protocol, in order to complete the handover, the MN must perform home registration with the Home Agent (HA) and correspondent registration, including a return routability procedure and binding update with the CN. 2.2. Predictive and reactive modes of the FMIPv6 If the MN receives the F-BAck message on the previous link before connecting to the new link, it is in the predictive mode of fast handover. However, since the FMIPv6 depends on the layer 2 trigger, there is no assurance that the MN has enough time to initiate and complete the fast handover procedure while it is still connected to the previous link. If connectivity to the previous link is lost unexpectedly, the MN has to operate in reactive mode and one of the following cases can occur: 1. RtSolPr message cannot be sent or PrRtAdv message cannot be received over the previous link: In this case, the MN has no information about the new link, anticipation fails, and the MN reverts to the standard Mobile IPv6 handover. 2. F-BU message cannot be sent over the previous link: In this case, the PAR cannot initiate fast handover with the NAR; therefore the NCoA cannot be validated, and the PAR cannot buffer and forward packets to the new link. Accordingly, packets that are en route to the PCoA of the MN may be lost. 3. F-BAck cannot be received over the previous link: In this case, the MN sent the F-BU but did not receive F-BAck before the link break. The PAR redirects packets to the NAR, which buffers the packets until the MN announces its presence. The MN may possibly receive F-BAck on the new link. From [5,6], the probability, Ploss, of an MN losing its connection with the previous link during the fast handover procedure, is given by

Ploss ¼ 1  extL2Trigger

ð1Þ

where – tL2Trigger is the L2 trigger time taken from the occurrence of an L2 trigger event to the link break-down point; – x is a decreasing factor which is introduced to account for a variety of decreasing patterns.

Fig. 1. The basic operation of protocols in predictive mode.

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The link break-down point usually depends on the size of overlap area and also the velocity of the MN. 2.3. Extensions to the FMIPv6 protocol Several extensions [7–12] have been proposed to improve the performance of the FMIPv6, but these studies did not consider reducing the anticipated handover delay that limits the time for the MN to perform the fast handover procedure in predictive mode. In addition, all of these enhancements issue more signaling messages during this critical period; therefore these proposals are inappropriate for high-speed MN movement. Malki and Soliman [7] introduced the Fast Handover protocol with a simultaneous binding function to minimize packet loss at the MN. To reduce packet losses, a mobile node’s traffic is bi-casted or n-casted to all locations where the MN could roam next in the near future, as well as to its current location. This procedure eliminates any ambiguity regarding the moment at which the traffic should be rerouted toward the mobile node’s new location after a fast handover, and it enables the protocol to decouple L2 and L3 handovers. However, Simultaneous Binding still retains all the steps of the fast handover procedure; thus the probability that the MN cannot perform the fast handover in predictive mode is still high, as in normal FMIPv6. Leoleis et al. [8] proposed an integrated unicast and multicast handover support solution to the FMIPv6, Seamless Multicast Handover. It is approached in a twofold manner: first by enabling the new access router to become a recipient of the multicast traffic of interest via tunneling, and second, by buffering the tunneled traffic for the period during which the mobile is unable to communicate due to link layer communication unavailability. Tunneled multicast packets are de-capsulated before being natively forwarded on the wireless link, eliminating the overhead caused from packet encapsulation over the air interface. However, signaling messages are increased even more, and the probability of the MN performing the fast handover in predictive mode cannot be improved. Chen and Zhang [9] presented a modification to the FMIPv6 protocol using extra binding updates to reduce tunneling time between the PAR and NAR. Pre-Binding Update and Pre-Binding Acknowledgement messages are exchanged between the MN and the Home Agent (HA)/Correspondent Node (CN) before the Fast Binding Update message is sent to the PAR. Thus, the reverse tunnel between the PAR and the NAR need not be established. However, other issues could arise. First, the Pre-Binding Update message is sent with the unverified NCoA to the CN. Thus, if the NCoA does not then pass the Duplicate Address Detection (DAD) test, these binding update messages would be useless. Second, since two more new signaling messages are issued during the critical time of the fast handover procedure, this method could prolong the fast handover delay, easily leading to fast handover failure. Zhang and Pearce [10] proposed that the care-of address test, a part of the Return Routability test for Mobile IPv6 route optimization, be run proactively in the context of the FMIPv6 protocol so

Type=150 Subtype=2

M

Code=0 Reserved

Checksum Identifier LLA Options...

that the latency caused by the care-of address test after movements can be reduced. The key idea is to deliver the Care-of Test Init (CoTI) message as soon as possible from the MN to the NAR. Once the NCoA is determined by the NAR, the NAR modifies (if necessary) and forwards the CoTI message to the CN to launch the care-of address test. There is no improvement for the fast handover procedure. Furthermore, the CoTI message is encapsulated in the fast binding update message, so it can incur even higher signaling costs than the original protocol. Kim and Kim [11] introduced an early binding fast handover (EBFH), in which an MN performs an early fast binding update with its current access router before a trigger that signals that an MN is closed to handover. The FMIPv6 initiates movement detection through a link-going-down trigger, whereas EBFH completes its binding update for the NCoA before the link-going-down trigger. The purpose of EBFH is to provide a fast handover for fast-moving nodes. If the MN moves at high speed, it reverts to the FMIPv6. This requires that the MN detect the speed and direction of movement; the implementation of such a detection mechanism is outside the scope of this proposal. Furthermore, EBFH issues many signaling messages before the link-going-down trigger, so it consumes a large amount of network performance and creates significant useless overhead. Hsieh et al. [12] presented an architecture which enhances the integrated hierarchical and fast handover scheme in conjunction with a handover algorithm based on a pure software-based movement tracking technique. This scheme could minimize handover latency and virtually eliminate packet loss at the layer 3 IP layer. However, this scheme introduces a new agent, the Decision Engine (DE), and six more new messages. Thus, it consumes a large amount of network performance and creates significant useless overhead. Moreover, too many messages are exchanged during the critical time of the fast handover procedure, prolonging the anticipation time of the fast handover and easily leading to failure of the fast handover in predictive mode. The supposed advantages of this scheme therefore cannot be achieved.

3. The proposed protocol The proposed scheme, SFMIPv6, is an optional and fully backward-compatible enhancement to the FMIPv6 protocol. In the FMIPv6, after sending the RtSolPr message, the MN must wait for a PrRtAdv message from the PAR before obtaining the NAR’s information to generate a new CoA. The PAR must also wait for an F-BU message from the MN in order to initiate fast handover. Such delays are unnecessary and could be eliminated. Fig. 1(b) shows the basic operation of the SFMIPv6 scheme (in comparison with the FMIPv6 scheme in Fig. 1(a)). The SFMIPv6 scheme does not define any new messages. A bit, M, and a new option, F-BU Option, are introduced as shown in Fig. 2(b). Bit M is used within the RtSolPr and PrRtAdv messages to instruct the receiving node to operate in the SFMIPv6 scheme. Upon receiving the F-BU Option within the RtSolPr message, the PAR forms a new CoA on behalf of the MN. Immediately afterward,

Type

Length Lifetime Sub-options...

F-BU Option...

Fig. 2. The modification of the RtSolPr message.

A H L K

Sequence # Reserved

N. Van Hanh et al. / Computer Communications 31 (2008) 3594–3603

the PAR initiates the fast handover; to the PAR does not wait for the F-BU message from the MN after it sending the PrRtAdv message. The basic operation of the protocol in predictive mode (Fig. 1(b)) is as follows: – Upon receiving an indication from a wireless link-layer trigger, the MN initiates the fast handover procedure by sending an RtSolPr message containing the F-BU option and the M bit to the PAR. – After receiving the RtSolPr, the PAR checks whether the M bit is set in order to follow the SFMIPv6 scheme. If the M bit is set, the PAR forms a prospective new CoA on behalf of the MN, then simultaneously sends an HI message to the NAR, and the PrRtAdv with M bit set to the MN. – According to the M bit within the PrRtAdv received by the MN, it will decide to follow the FMIPv6 scheme or the SFMIPv6. If the M bit is set, it indicates that the PAR supports the SFMIPv6 protocol, so the MN does not need to send the F-BU to the PAR. It just needs to wait for an F-BAck. – Upon receiving the HI message, as specified in the FMIPv6, the NAR validates the uniqueness of the NCoA that was already formed by the PAR and is included in the HI message. The NAR then responds to the PAR with a HAck message. – After receiving the HAck message from the NAR, the PAR sends an FBAck message to the MN on both links, the previous link and the new link, and starts the tunneling of buffered and arriving data toward the NCoA. – As soon as it is connected to the new link, the MN sends an FNA message to inform the NAR of its presence. Packets are delivered to the MN from this point on. – In order to complete the handover, as in the MIPv6 and the FMIPv6 protocols, the MN must perform a home registration with the Home Agent (HA) and correspondent registration including return routability procedure and binding update with the CN. Then new packets can be sent directly from the CN to the NCoA of the MN. If the PAR does not support SMIPv6, the M bit in the RtSolPr and PrRtAdv messages and the F-BU option within the RtSolPr message

a

would be ignored and have no effect. All normal FMIPv6 messages [2] remain unchanged and retain their original meaning. The proposed scheme can also be applied to other schemes that support fast handover, such as the Fast Handover for Hierarchical Mobile IPv6 (F-HMIPv6) and called SF-HMIPv6. F-HMIPv6is the combination of Fast Handovers for Mobile IPv6 (FMIPv6) [2] and Hierarchical Mobile IPv6 Mobility Management (HMIPv6) [3]. The basic operation of SF-HMIPv6 is similar to the SFMIPv6, but the MN exchanges the signaling messages for the handover with MAP, rather than with PAR as the F-HMIPv6. 4. Performance evaluation Since the fast handover procedure depends on the layer 2 trigger, in some cases the MN may not have enough time to initiate and complete the fast handover while still connected to the previous link. In this paper, we evaluate the performance in terms of the impacts of two factors, the break-down point and the velocity of the MN. To evaluate the impact of the break-down point and the velocity of the MN on the network performance, the evaluation model is as follows. 4.1. Evaluation model 4.1.1. Coverage model For the sake of simplicity, we consider a wireless network system in which access points (AP) are used with omni-direction antennas organized in a hexagonal pattern as shown in Fig. 3, and placed at the center of cells with uniform topographical and local signal propagation conditions. Fig. 3(a) shows the layout of overlapping coverage areas in the system. The coverage area can be defined in terms of Signal Strength; the effective coverage is the area in which MNs can establish a link with acceptable signal quality with the AP. This area can be modeled by a circle centered at the corresponding AP. The coverage radius, r, is defined as the distance from an AP to its coverage boundary. The cell radius, c, is the distance from an AP to its cell boundary. [14] shows four different coverage models based on the ratio of the coverage radius to the cell radius (c/r). In this paper, we use a model as shown in Fig. 3. There are three overlapping coverage areas shown in

Effective Coverage Cell Boundary

B r c G

C

d

b A

F

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D

E

Fig. 3. Overlapping coverage areas.

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Fig. 3(b), denoted by I, II, and III. Area I is a non-overlapping region in which the MN can receive signals from only one AP, while both areas II and III are overlapping regions in which the MN is able to receive from different APs: two APs in area II, and three APs in area III. 4.1.2. The minimum overlapping distance In order to evaluate the influence of the velocity of the MN on the network performance, we consider the worst case, in which the traveling path of the MN through the overlapping area is the minimum overlapping distance. Fig. 4 shows the minimum overlapping distance and related notations that are defined above. From [12], A0 B0 is a side of an internal equilateral triangle that is formed by overlapping area III (in Fig. 4). The distance between APs, d, is given by

pffiffiffi d¼c 3

dn

AP1 y x

MN

z

AP2

Signal Strength

Optimal Handover Point

h

Handover Triggered

ð2Þ 0

0

To find the minimum overlapping distance, x (A B ), as a function of r and c, consider DBPA. Using trigonometric relationships we obtain

cosðbÞ ¼

BP2 þ BA2  ðPAÞ2 2BP  BA

sinðbÞ ¼

PH BP

or

cosðbÞ ¼

r2 þ 3c2  x2 pffiffiffi 2rc 3

or x ¼ 2r  sinðbÞ

2

ð5Þ

dh ¼ ð6Þ

Applying (4)–(6), the minimum overlapping distance, x, is given by

!! pffiffiffi  pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 2 2 c þ 4r  3c 4r

1

x ¼ 2r  sin cos

ð7Þ

4.1.3. Handover scenario We assume that there is no change in direction while the MN moves inside the overlapping area. Fig. 5 shows the basic handover scenario. The optimal handover point occurs at position A. However, the relative signal strength with hysteresis margin is a commonly used approach to prevent

Distance

the ping-ponging phenomenon [15]. In this approach, a handover is only triggered if the signal level of the AP to which the MN is currently attached differs from one of new APs by at least the hysteresis margin h, as at position B. The associated delay dh of the hysteresis margin h is introduced in [13], and is given by

From (3)–(5), we obtain

pffiffiffi  pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 3 cosðbÞ ¼ c þ 4r 2  3c2 4r

B

Fig. 5. Basic handover scenario.

ð3Þ

ð4Þ

sin ðbÞ ¼ 1  cos2 ðbÞ

A

d 1 þ 10h=K 2 2v 1 þ 10h=K 2

ð8Þ

Applying (8), the distance dh from the optimal handover point (position A) to position B given by

dh ¼ vdh ¼

d 1 þ 10h=K 2 2 1 þ 10h=K 2

ð9Þ

4.1.4. Network topology model In order to evaluate the network performance, we use the system model shown in Fig. 6. There are six nodes participating in the system model, and each node is connected to the others by a link. Each link is assigned a notation associated with the latency or cost of a packet delivery between two nodes of the link. The specific notations are described in Table 1. 4.2. Influence of break-down point

Effective Coverage Cell Boundary

r

If connectivity to the previous link is lost unexpectedly, network performance could degrade depending on the link-break time. Fig. 7 shows the timeline for the fast handover procedure,

B

c

Q β

P

B' O H

x

A'

C

A Minimum Overlapping Distance

Fig. 4. Minimum overlapping distance.

Fig. 6. Network topology model.

N. Van Hanh et al. / Computer Communications 31 (2008) 3594–3603 Table 1 List of evaluation parameters Symbol Description m n a b X D k D0

DL2 DL3 DIP DDAD DRA DMD DMN-HA DMN-CN DHA-CN DBU Dnew Dtotal Cbuffer Closs r c d x v h K2 dh dh z

Latency or cost of a packet delivery between MN and Access Router (PAR or NAR) Latency or cost of a packet delivery between Intermediate Router (IR)/ Mobility Anchor Point (MAP) and Access Router (PAR or NAR) Latency or cost of a packet delivery between HA and IR/MAP Latency or cost of a packet delivery between CN and IR/MAP The exact time point at which the MN loses its connectivity with the PAR The additional time taken for the MN to recognize the link-break with the PAR after the link has actually been broken Packet arrival rate The time span between the moment the MN sends the RtSolPr message to the PAR/MAP and the moment the F-BAck arrives at the MN (in theory). The value of D0 depends on protocols. It is (4m + 4n), (2m + 4n), (4m + 6n), or (2m + 4n) in FMIPv6, SFMIPv6, F-HMIPv6, or SF-HMIPv6 protocols, respectively Layer 2 handover delay Layer 3 handover delay: the time span between the moment the MN completes layer 2 handover and receives the first packet from CN IP connectivity delay, which depends on each scheme and the failure or success of the fast handover Duplicated Address Detection Delay The mean value of the Router Advertisement intervals The Movement Detection Delay in standard Mobile IPv6 One way transmission delay between MN and HA One way transmission delay between MN and CN One way transmission delay between HA and CN Binding update delay with HA and CN in Mobile IPv6 Dnew = DMN-CN Overall handover latency Buffering Cost: total number of buffering packets at the PAR during the fast handover Loss Cost: total number of lost packets during the fast handover The coverage radius in coverage model The cell radius in coverage model The distance between APs in coverage model The minimum overlapping distance The velocity of the MN Hysteresis margin, representing the signal level about which the new AP is stronger than the old one. K2 represents environment-specific attenuation characteristics [13] The associated delay dh of the hysteresis margin h The distance from the optimal handover point (Fig. 5) to the hysteresis margin point (position B), which is associated with dh The remaining distance in the overlapping area of the MN from the hysteresis margin point

FMIPv6

Fig. 8 shows the timeline for failure cases, and Fig. 9 shows the comparison of handover latency among FMIPv6/F-HMIPv6 with/ without the proposed protocol. In this section, we evaluate performance while varying the link break-down point, which is related to the failure cases of the fast handover. We then compare the original FMIPv6 protocol with the proposed protocol.

PAR receives PAR receives PAR sends F-BU PrRtAdv HAck MN sends MN receives NAR receives MN receives RtSolPr PrRtAdv HI F-BAck

•

SFMIPv6

0

•

m

•

2m

•

3m

•

•

3m+2n 3m+4n

•

4m+4n

Time

PAR sends PrRtAdv/HI MN sends RtSolPr

• 0

MN receives PrRtAdv

•

m

•

2m

MN receives F-BAck

•

2m+4n

Fig. 7. Timeline for fast handover procedure.

Time

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T specified in Tables 2 and 3 and the following is given by T = X, or T = min{z/v, D0} depending on the context of evaluation, the break-down point, or the velocity of the MN. [6] classified failure cases in the FMIPv6 into three cases as described in Section 2.2. In the SFMIPv6, we also classify failure cases into three cases and evaluate network performance for each case, different from those in the FMIPv6, as follows: – Failure Case 1 (X < m): In this case, the MN recognizes the linkbreak with the PAR before the PAR completely receives the RtSolPr message. The MN has no information about the new link, and the NAR has no information about the MN. Thus, the MN must switch to the standard MIPv6 protocol.Having no information about the new link, the MN has to perform movement detection. As the discussion in [18], the movement detection delay (DMD) of the MIPv6 is given by DMD = DRA/2. Then, upon receiving the new network prefix advertised from the NAR, the MN configures an NCoA and performs DAD process for this NCoA, i.e. DIP = DDAD + DMD. Since the fast handover was not inititated, no packets are bufferred, i.e. Cbuffer = 0. Accordingly, all of packets destined to the MN during the disruption time are lost, i.e. Closs = k(D + DL2 + DIP + DBU + Dnew). – Failure Case 2 (m 6 X < 2m): In this case, the MN recognizes the link-break with the PAR before receiving the PrRtAdv message. Although the MN has no information about the new link, the PAR can still initiate fast handover.Having no information about the new link, the MN has to perform movement detection. As the discussion in [18], the movement detection delay (DMD) is given by DMD = DRA/2. However, the MN performs the fast handover in reactive mode, and thus DIP = DMD + 2(m + 2n). Packets arrived at PAR after the reception of the HAck message (i.e. at the time of m + 4n) are buffered until the the MN compeletes the conrespondent registration and receives the first packet directly from the CN, i.e. Cbuffer = k{T + D + DL2 + DIP + DBU + Dnew  (m + 4n)}. If the link break-down point before the buffering time (i.e. m + 4n), packets destined to the MN during in the interval [X, m + 4n] are lost. In addition, packets still in the wireless link (from the PAR to the MN) while the link is broken are also lost, i.e. Closs = max{0, k(m + 4n  T)} + km. – Failure Case 3 (2m 6 X < 2m + 4n): In this case, the MN recognizes the link-break with the PAR before the MN receives FBAck message. However, the MN has information about the new link before the link break-down point according to the reception of a PrRtAdv message (with M bit set). Subsequently, immediately after connecting to the new link, the MN needs not to perform movement detection process, i.e. DMD = 0. The MN performs fast handover in reactive mode, thus DIP = 2(m + 2n). As described in failure case 2, Cbuffer = k{T + D + DL2 + DIP + DBU + Dnew  (m + 4n)}, and Closs = max {0, k(m + 4n  T)} + km. In successful case (X P 2m + 4n), no packets are buffered and lost due to the handover. Immedidately after connecting to the new link, the MN sends a F-NA massage to the NAR to inform its presence and receive packets destined to the MN via previous path. Then, after the completion of the correspondent registration, the MN can receive packets directly from the CN. Thus, DMD = 0, DIP = 2m, Cbuffer = 0, Closs = 0. The summary of the above evaluations is shown in Table 3. In this paper, we compare network performance between the FMIPv6, F-HMIPv6 schemes and the proposed enhancements to those original schemes, SFMIPv6 and SF-HMIPv6, respectively. For the sake of simplicity, in this section we evaluate the performance of only the FMIPv6 and SMIPv6. The others can easily be derived from the evaluations in Tables 2 and 3.

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MN sends Link breakRtSolPr down point Link break recognition delay

•

•

•

X+

Binding Updates Completed

CoA Configured

L2 handover IP connectivity delay (DI) delay (D L2)

•

X

0

Link Up

D new

DBU

•

•

•

X+ +DL2+DL3

Time

L3 handover delay(DL3 ) Receive Packet from CN

Link break recognized Fig. 8. Timeline for failure cases in fast handover.

NAR

HI F-BU

lPr RtSo

F-HMIPv6

HI F-B U

lPr

C total ðtÞ ¼ Ploss ðtÞ  C packet ðtÞ

MN 4m+2n

4m+4n

NAR

ð11Þ

where Ploss(t) is the probability that an MN can lose its connection with the previous link at time t as shown in (1). In this case, we consider the link break-down time as a variable and it is a unique variable in the equation.

ck F-BA

RtSo

MAP

v tAd PrR

ck F-BA

tAdv

MN

4.3. Influence of velocity

NAR

HI

HI

olPr

SF-HMIPv6

olPr

MRtS

v

MRtS

ck

RtAd

F-BA

MPr

dv

ck

F-BA

RtA

MN

MAP

ck

ck

MPr

PAR

In this section, we evaluate the performance while varying the velocity of the MN. To evaluate the influence of the velocity of the MN on network performance, we consider the worst case, in which the traveling path of the MN through the overlapping area is the minimum overlapping distance, x. Let

HA

HA

SFMIPv6

ck HA

ck HA

PAR PrR

FMIPv6

The total packet delivery cost Ctotal(t) for a fast handover can be measured by

NAR

y¼

MN 2m+2n

2m+4n

Fig. 9. Comparison of handover latency among the FMIPv6/F-HMIPv6 with/without the proposed protocol.

C packet ¼ sC buffer þ ð1  sÞC loss

ð12Þ

From (9) and (12), we obtain the position of the MN in the overlapping area, y, which is given by

y¼ From [6], the packet delivery cost for failure, Cpacket, can be measured by

x þ dh 2

x d 1 þ 10h=K 2 þ 2 2 1 þ 10h=K 2

Thus, the remaining distance in the overlapping area of the MN for handing off, z, is given by

ð10Þ z¼xy¼

where s is the weighting factor.

ð13Þ

x d 1 þ 10h=K 2  2 2 1 þ 10h=K 2

ð14Þ

Table 2 Performance evaluation of FMIPv6 scheme Break-down point X

X < 2m

2m 6 X < 3m

3m 6 X < 4m + 4n

X P 4m + 4n

Velocity of MN Failure case DMD DIP Dtotal Cbuffer Closs

v > z/2m 1 DRA/2 DDAD + DMD T + D + DL2 + DIP + DBU + Dnew 0 k(D + DL2 + DIP + DBU + Dnew)

z/3m < v 6 z/2m 2 0 2(m + 2n) T + D + DL2 + DIP + DBU + Dnew 0 k(D + DL2 + DIP + DBU + Dnew)

z/(4m + 4n) < v 6 z/3m 3 0 2(m + 2n) T + D + DL2 + DIP + DBU + Dnew k{T + D + DL2 + DIP + DBU + Dnew  (3m + 4n)} max{0, k(3m + 4n  T)} + km

v 6 z/(4m + 4n) Successful 0 2m T + DL2 + DIP + DBU + Dnew 0 0

Table 3 Performance evaluation of SFMIPv6 scheme Break-down point X

X
m 6 X < 2m

2m 6 X < 2m + 4n

X P 2m + 4n

Velocity of MN Failure case DMD DIP Dtotal Cbuffer Closs

v > z/m 1 DRA/2 DDAD + DMD T + D + DL2 + DIP + DBU + Dnew 0 k(D + DL2 + DIP + DBU + Dnew)

z/2m < v 6 z/m 2 DRA/2 DMD + 2(m + 2n) T + D + DL2 + DIP + DBU + Dnew k{T + D + DL2 + DIP + DBU + Dnew  (m + 4n)} max{0, k(m + 4n  T)} + km

z/(2m + 4n) < v 6 z/2m 3 0 2(m + 2n) T + D + DL2 + DIP + DBU + Dnew k{T + D + DL2 + DIP + DBU + Dnew  (m + 4n)} max{0, k(m + 4n  T)} + km

v 6 z/(2m + 4n) Successful 0 2m T + DL2 + DIP + DBU + Dnew 0 0

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N. Van Hanh et al. / Computer Communications 31 (2008) 3594–3603

Fig. 5 shows the representation of x, y, and z in the context of the handover scenario. Let V denote the velocity of an MN in a cell, which is a continuous random variable in the interval [0, Vmax], where Vmax > 0 is given. Let us assume that the velocity is distributed uniformly in the interval [0, Vmax]. From [17], the CDF (Cumulative Distribution Function) of velocity is given by

F V ðvÞ ¼

8 > < 0;

v

V max > : 1;

if ; if if

v<0 0 6 v 6 V max

ð15Þ

v > V max

The probability, PS(v), of an MN successfully performing the fast handover procedure in predictive mode is given by

  Dz 0 v z z ¼ PS ðvÞ ¼ P v < ¼ D0 D0 V max 0 V max

ð16Þ

Thus, the probability, PF(v), of an MN failing to successfully perform the fast handover procedure in predictive mode is given by

  z z ¼1 PF ðvÞ ¼ P v > D0 D0 V max

ð17Þ

Therefore, the overall packet delivery cost for failure can be given by

C total ðvÞ ¼ P F ðvÞ  C packet ðvÞ

ð18Þ

The evaluation analysis of the influence of velocity on the network performance of the FMIPv6 and the proposed protocol, SFMIPv6, based on z, are evaluated and shown in Tables 2 and 3.

trated in Fig. 8. When the MN loses its connectivity with the previous link before receiving the PrRtAdv massage, it has no information about the NAR and must switch to the standard MIPv6. Thus, the total handover latencies of the protocols are high. In this case, the handover latencies of the original and proposed protocols are the same. However, if the link is broken after the MN has completed sending the PrRtAdv message (at 6 ms for SFMIPv6; at 8 ms for SF-HMIPv6), the handover latencies of the proposed protocols are greatly reduced compared to the original protocols. Fig. 11 shows the comparison of the packet loss cost of the protocols while varying the link break-down point. The packet loss costs of the proposed protocols, SFMIPv6 and SF-HMIPv6, are much lower than those of the original protocols. Indeed, in the SFMIPv6/ SF-HMIPv6, the PAR can initiate the fast handover with the NAR immediately after receiving the RtSolPr message (at 6 ms and 8 ms in the SFMIPv6 and SF-HMIPv6, respectively), therefore the packet loss cost is very small or zero from 6 ms/8 ms. However, in the FMIPv6/F-HMIPv6, the PAR can initiate the fast handover with the PAR only after receiving the F-BU message, three times later than in the SFMIPv6/SF-HMIPv6 (at 18 ms/24 ms, respectively). Consequently, the packet loss cost of the SFMIPv6/SF-HMIPv6 is greatly reduced as shown. Fig. 12 shows the comparison of the total packet delivery cost of the protocols while varying the L2 trigger time. The packet delivery cost of the proposed protocols is much lower than that of the original protocols.

5. Numerical results The parameter values used in the numerical analysis are shown in Table 4 based on discussions in [6,13,16,18]. Among these parameters, the packet arrival rate has been chosen for a high traffic volume with k = 2 packets/ms (i.e. 2000 packets/s). 5.1. Influence of link break-down point In this section, we compare the network performance of the FMIP, SFMIPv6, F-HMIPv6, and SF-HMIPv6 protocols in terms of the impact of the velocity of the MN. Fig. 10 shows and compares the handover latencies of the protocols while varying the link break-down point X, which is illus-

Fig. 11. Packet loss cost in terms of break-down point.

Table 4 Parameters used in numerical analysis

s

m

n

D

k

DL2

a

b

x

DDAD

DRA

K2

h

Vmax

0.2

6 ms

2 ms

20 ms

2 packets/ms

20 ms

5 ms

8 ms

0.05

500 ms

50 ms

40 dB

5 dB

180 km/h

Fig. 10. Handover latency in terms of break-down point.

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N. Van Hanh et al. / Computer Communications 31 (2008) 3594–3603

Fig. 12. Total packet delivery cost in terms of L2 triggering time.

5.2. Influence of velocity of MN Based on discussions in [13,14], we consider two scenarios depending on the coverage radius (r) and cell radius (c) in order to evaluate the network performance in terms of the influence of the velocity of the MN as follows: – Scenario 1: r = 60 m and c = 40 m. – Scenario 2: r = 30 m and c = 20 m. Fig. 13 presents the effect of the velocity of the MN on the handover latencies of the protocols and compares the performance between these protocols. In Fig. 13(a), r and c are fixed at 60 m and 40 m, respectively; in Fig. 13(b), r and c are set to 30 m and 20 m, respectively. In the context of Fig. 13(a), the proposed protocols, SFMIPv6 and SF-HMIPv6, have enough time to initiate fast handover for MN velocities up to 180 km/h, whereas the F-HMIPv6 and FMIPv6 have to switch to the standard Mobile IPv6 when the

speed reaches 125 km/h (F-HMIPv6) and 170 km/h (FMIPv6). In the context of Fig. 13(b), the overlapping area is decreased by half, and the traveling distance is also decreased by half. Thus, the impact of the velocity on the handover latency of the protocols is significantly increased due to lack of time to complete the fast handover, but it is much lower for the proposed protocols. Indeed, the protocols must revert to the standard Mobile IPv6 when the speed of the F-HMIPv6, FMIPv6, SF-HMIPv6, and SMIPv6 reach 60 km/h, 85 km/h, 125 km/h, and 170 km/h, respectively. Thus, in this context, the limits of the velocity of the MN in the proposed protocols, such that the handover latency can be acceptable, are twice as high as those in the original protocols. Fig. 14 shows the packet loss cost of the protocols as a function of the velocity of the MN. In the context of Fig. 14(a) (r = 60 m and c = 40 m) the proposed protocols have enough time to perform the fast handover procedure, so the packet loss cost is zero or very small. However, the original protocols must revert to the standard Mobile IPv6 when the speed of the MN reaches 125 km/h (FMIPv6) and 170 km/h (F-HMIPv6). Thus, the packet loss costs of these protocols are very high. In the context of Fig. 14(b), the coverage and cell radii are decreased by half, so the traveling distance of the MN in the overlapping area is also decreased by half. In this case, the MN does not have sufficient time to perform the fast handover when the MN moves at high speed. However, in the proposed protocols, the effect of the speed of the MN on the packet loss cost is much lower than that in the original protocols. Fig. 15 shows the comparison of the total packet delivery cost between the protocols while varying the velocity of the MN. In the context of Fig. 15(a) (r = 60 m and c = 40 m) the speed of the MN is not so influential in the total packet delivery cost of the proposed protocols, whereas it significantly affects the total packet delivery cost of the original protocols. Moreover, in the context of Fig. 15(b), when r and c are decreased by half, the impact of

Fig. 13. Handover latency in terms of velocity.

Fig. 14. Packet loss cost in terms of velocity.

N. Van Hanh et al. / Computer Communications 31 (2008) 3594–3603

3603

Fig. 15. Total packet delivery cost in terms of velocity.

the speed of the MN is much higher on the overall packet delivery cost of the original protocols, whereas the proposed protocols could effectively operate at much higher MN speeds, up to 125 km (SF-HMIPv6) and 170 km/h (SFMIPv6). In summary, the performances of the proposed protocols are effectively improved compared to the original protocols. It has been shown that our proposal could appropriately operate with high-speed MN movement. 6. Security discussion The proposal does not introduce new security vulnerabilities than those already described in the FMIPv6 [1]. However, the proposal does not use the F-BU message; instead the F-BU option is included in the RtSolPr message. Therefore, the RtSolPr message in the proposed scheme must be secured and authenticated as the F-BU message in the FMIPv6. 7. Conclusion This paper proposes an enhancement to the FMIPv6 which significantly reduces the overhead associated with fast handover, including the signaling cost and packet delivery cost, by optimizing the fast handover procedure. The key point is to eliminate the useless time during which the MN waits for the response of the PAR after sending a RoSolPr message, before completely sending an F-BU message to the PAR. Instead, the MN uses the F-BU option within the RtSolPr message, such that the PAR can initiate fast handover immediately after receiving the RtSolPr. Accordingly, the anticipation time can be remarkably reduced about a RTT on the wireless link between the MN and the PAR. Therefore, this method can significantly increase the probability that the MN successfully performs the fast handover procedure in predictive mode. In this paper, we considered evaluation models in order to evaluate and compare network performance between the original protocols, the FMIPv6 and F-HMIPv6 protocols, and the proposed protocols (that apply to the respective original protocols), SFMIPv6 and SF-HMIPv6, while varying the link break-down point and the velocity of the mobile node. The numerical results show that the network performances of the proposed protocols are effectively improved compared to the original protocols. Moreover, the influence of the velocity of the MN on the network performances of the protocols was evaluated, and the evaluation results showed that

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