Tunnel-Free Distributed Mobility Management (DMM) Support Protocol for Future Mobile Networks

: The number of wireless services and devices have remarkably increased, especially since the introduction of smart phones. The population of mobile nodes (MNs) is now exceeding the traditional non-mobile nodes. Mobility is a key factor in mobile core networks as it is responsible for providing continuous communication when a MN is on the move. Currently, a centralized mobile core network architecture is implemented, which has certain limitations. Distributed mobility management (DMM) is often seen as a solution to the problems associated with centralized mobility management (CMM). Address and tunneling management are big challenges for current DMM-based mobility protocols. the current advancement of mobile network architecture, this paper proposes a novel tunnel-free distributed mobility management support protocol intended for such an evolution. In addition, the of the existing mobility management the of handover latency, handover blocking probability, and data packet loss is analyzed and compared to the proposed framework. The performance analyses show that the proposed tunnel-free method can reduce about 12% of handover latency, 71% of handover blocking probability, and 82% of data packet loss. Conceptualization, M.Z., S.K.; S.M.; Co-Supervision,


Introduction
The advent of a huge plethora of wireless technologies have opened new opportunities besides the typical voice calling services. Wireless services and devices have seen a remarkable increase, especially since the introduction of smart phones. As reported in Sandvine [1], the population of mobile nodes now exceeds that of traditional wired nodes. With such an increase, the IP paradigm is now shifting from IPv4 to IPv6 as the IPv4 addresses are not enough. The current implementation of a mobile core network architecture is in a hierarchical and centralized manner. Therefore, the mobility management protocols, such as mobile IPv6 (MIPv6) and proxy mobile IPv6 (PMIPv6), have also been deployed in this manner [2,3]. More devices need more IP addresses, which results in the need for network scalability. A scalability issue has been identified as one of the limitations of the current centralized and hierarchical network architecture [4,5]. Other limitations include sub-optimal routing, central point of failure, and reliability problems.
Distributed mobility management (DMM) [5] is often seen as a solution to the scalability, reliability, central failure point, and routing issues of the centralized network architecture since the DMM approach distributes the responsibility of providing connectivity and mobility by going toward a flat network approaches, i.e., a host-based DMM approach and a network-based DMM approach, along with their comprehensive comparison. They have pointed out the four main challenges faced by DMM: address and tunneling management, registration delay and signaling overhead, network configuration and resource management, and security considerations. Lee, J.H. et al. [8] have proposed a DMM protocol for host-based IPv6 mobile networks. They have introduced an access mobility anchor at an access network level. Mobility signaling is handled by the MN itself. The MN is attended by its nearest mobility anchor. They have compared their design to MIPv6 in terms of throughput and handover delay. On the other hand, they have kept their analysis limited to only one mode of MIPv6, i.e., bi-directional tunneling, and they have not considered a MIPv6 route optimization mode. Giust, F. et al. [10] have discussed DMM architecture and deployment scenarios. Two types of deployment scenarios were given: network function virtualization and selected internet IP traffic offload/local IP access scenario in 3GPP. The first type suggests the separation of data and control planes. Mobility management is provided in two ways: mobility management functions with separate control and data planes under one controller, or with separate control and data planes with separate control in core and access networks. Ali-Ahmad et al. [11] have proposed a DMM design that offers global and local mobility support in a distributed fashion called distributed dynamic MIPv6. They have performed an analysis considering the mobility costs. Global mobility is ensured by keeping a record of the most recent session entry even after it is terminated.
Chan, H.A. et al. [12] have explored distributed and dynamic mobility management in the context of mobile internet. They have discussed the issues, challenges, and existing DMM approaches. They have highlighted the fact that DMM can address the CMM issues since its features are well-suited to the developing trends of mobile networks. Lee, J.H. et al. [13] have evaluated the performance of current IP mobility management protocols using an analytical cost model. The comparison parameters were the packet delivery cost, signaling cost, tunneling cost, and total cost. They have pointed out the weaknesses and strengths of each mobility protocol for use in a consumer network design. Lee, J.H. et al. [8] have evaluated the handover performance of current IP mobility management protocols. They analyzed the characteristics of handover operations in terms of handover delay, packet loss, and handover blocking probability. These characteristics can be used in the development of new mobility management protocols. Giust, F. et al. [14] have proposed a MIPv6 client-based DMM solution using cryptographic generated addresses. They called it flat access mobility architecture. Chan, H.A. et al. [15] have presented an architecture that unifies various possibilities of DMM. They proposed a method to co-locate the logical functions at the access router (AR) or gateway. Condeixa, T. et al. [16] have introduced a make-without-break concept during horizontal IP handovers in DMM. The handover is performed once the MN is in the overlapping area of the two access points (AP). A double MAC (Media Access Control) association is needed during the handover. DMM solutions are integrated with MAC layer handover approaches. This concept is called dynamic mobile IP anchoring. Lee, J.H. et al. [17] have discussed a host-based DMM approach with mobile data offloading. They have given a qualitative analysis highlighting the strength of their approach using a selective data offloading mechanism. In their proposed approach, an offloading system is built with distributed mobility anchors called offloading points. The MN maintains the binding updates and flow table.
Cominardi, L. et al [18] have provided a DMM framework based on PMIPv6 that makes use of software-defined networking (SDN). The authors have investigated and evaluated the two major DMM protocol groups: the SDN-based group and the IP-mobility group. They have designed and implemented protocols for both groups. They have also worked on breaking down the aspects that should exist in a DMM solution and how it could be implemented with an SDN solution. The major contribution of their research is an insight on the ways to apply DMM concepts in future mobile networks. Carmona-Murillo, J. [19] have proposed a hybrid DMM solution for future mobile networks. They have pointed out that there are cases where the DMM approach has not performed its best, which has compromised real-time and many over the top applications, in addition to increasing the signaling overhead in the network. The authors have analyzed the possibilities of different trade-offs between the centralized mobility architecture and DMM, in this way taking the best of both CMM and DMM and provided a hybrid solution based on it. According to their analysis, the hybrid solution performs well in terms of the packet delivery cost and signaling overhead. Huang, C.-M. et al. [20] have focused on the DMM handover issues when multiple MNs perform a handover rather than a single MN handover. They have proposed a solution to this problem by using partial DMM, proposing a bursty multi-node handover scheme. The technique that they have deployed is that instead of using multiple sets of control messages for a group of MNs, only one set of control messages is used for the handover. According to the authors, for exchanging handover signaling messages, their proposed scheme can decrease bandwidth consumption by 30%. Huang, C.-M et al. [21] have addressed the issue of overloaded AP or a mobility anchor and access router due to an increase in the number of MNs attaching to them. They have proposed mobile edge computing (MEC) as a solution to this issue. Their research introduces a MEC-assisted technique that facilitates early handovers using full DMM. Their proposition is to force a MN into performing an early handover by defining trigger-timing. Furthermore, to sidestep the unbalanced network, a decision is made regarding the MN suitable for early handovers. Fafolahan, E.M.O. et al. [22] have focused on the limited performance and increased cost issues of DMM protocols in the event of a short cell-residence time. They have proposed an approach that is based on fast handover concepts, locator identifier separation protocol, and local mobility anchoring for 5G dense small cells. The proposed technique splits up a local domain into many location service areas, where a local anchor controls each area. They have analyzed their protocol in terms of different handover parameters like handover latency, total signaling cost, packet loss, and data delivery cost, improving the performance of each parameter.
All these proposed DMM approaches have tried in their own way to provide mobility services to a MN in a distributed manner, proving that a distributed approach best suits the current mobile networks' needs rather than the centralized approach. On the other hand, DMM also faces many challenges when it comes to practical deployment. One of the major challenges is managing tunneling when providing session continuity to a MN. With tunneling comes other problems as well, such as the management of multiple IPs, the registration delay and signaling overhead as a result of increasing tunnels, the network configuration and resource management, and some security considerations. All the DMM schemes discussed above use the concept of tunneling, and hence face the issues and problems mentioned above.
The main contributions of this research include the proposition of a framework that provides a tunnel-free DMM support protocol for mobile networks and then comparing it with the existing IPv6 mobility management protocols. The endorsement of a flat network architecture, as opposed to the hierarchical network architecture, is made to overcome the limitations of a centralized core network architecture. There is also no hassle in terms of tunnel management and address management. Endorsement of network scalability is done through a DMM approach. Lastly, communications and transmissions are made more seamless by mitigating the delays involved in the process, e.g., with no tunneling involved, there will not be additional registration delays. With the conducted analysis in this research, it confirms that the proposed tunnel-free DMM support protocol results in a considerably mitigated and reduced handover delay, which produces a reduced handover blocking probability and data packet loss.
The rest of the paper is organized as follows. Section 2 explains the proposed tunnel-free DMM support protocol. Section 3 discusses the analytical modelling and performance evaluation. Section 4 discusses the numerical analysis and results. Section 6 concludes the paper.

Proposed Tunnel-Free DMM Support Protocol
This section introduces a novel tunnel-free DMM support protocol. It removes the MIPv6 limitations by making use of the current advancements in mobile network architectures.

Overview
At an access network level, the proposed DMM approach depends on distributed mobility anchors as follows: • Base Mobility Agent (BMA): The HA of MIPv6 is expanded to a BMA. The BMA performs mobility and routing management for the MN. The network prefix is assigned to the MN by BMA. The MN is supplied by the BMA it is currently attached to. The architecture is distributed, and therefore multiple BMAs are deployed. For a MN on the move, it gets attached to another BMA in its range when it is about to leave the coverage area of the previous BMA. In this design, session continuity of a MN on the move is ensured by using the MBB approach. In traditional break-before-make infrastructures, only one connection can exist between a node and an access point. This requires nodes to continually break and re-establish connectivity as they move between access points. By using the incrementally evolved MBB [23,24] approach, a network can scan for and determine the next best available path prior to handoff. User data packets that go to and from the BMA and MN are transferred within a radio access network where the MN is currently connected. The handover control within the domain is focused on accommodating user mobility, while at the same time avoids interrupting or delaying ongoing user packets transfer, degrading the QoS of such a user packet transfer, or dropping user packets that should not be dropped. Therefore, the MBB approach is adopted, i.e., network resources at the target side are reserved prior to the movement of a MN to the target cell. Mechanisms used to reserve resources at the target side are carefully designed to ensure the success of the handover. During handover, BMAs may be relocated due to old BMAs that could not serve the MN. The handover takes place when the MN is in the overlapping region of the two BMAs. Connections can be established to the two BMAs simultaneously, and in this manner, the MN can communicate with two BMAs. The availability of more than one active connection through the handover process leads to a more seamless and reliable handover performance. The adjacent cells work on similar channels or frequencies that MBB uses for simultaneous connections, thus the MNs can communicate simultaneously with the old and new BMAs during the handover. Once the handover with MBB is complete, the new BMA informs the Corresponding Node (CN) about the MN's updated path and the connection to the previous BMA is dropped. Once the MN is attached to the new BMA, a new address is assigned to it. If the MN has an unfinished, ongoing transmission session with the previous address, the previous address is maintained by the MN until the new address is completely configured. When the new address is completely configured, an IP handover takes place. The previous ongoing session is then continued on the newly configured address. The previous address is then discarded by the MN. The previous session, as well as the new session, will now be anchored through the new BMA.
Signaling Messages: There will be two separate signaling messages. The BMAs communicate with each other with the help of NBU and NBAck messages. The newly configured address is registered by the MN while the previous address is still valid. The address information is extracted by BMA2 upon receiving the NBU message, where the movement of the MN to BMA2 is updated with the help of this extracted information. The BMA2 then sends the NBAck message containing the newly configured address of MN to BMA1, indicating the complete configuration of the new address.
In the architecture, at the access network level, the BMAs are distributed. The address is assigned to the MN based on a network prefix provided by the BMA. Using the BU message, the MN then registers the assigned address with the BMA. When the MN on the move changes its attachment point to another BMA, the MN then configures a new address based on the network prefix provided at the new access network; however, it retains the previous address. Upon registration by a BU message, the MN informs the current BMA about the previous address along with the configuration of the new address. As the current BMA attains the previous address data from the BU message, it sends an NBU message to the previous BMA, which updates the mobility and routing status of the MN. The IP handover is confirmed by an NBAck. Figure 1 shows this scenario.
As discussed above, the proposed approach depends on distributed BMAs at the access network level, preventing a network bottleneck and avoiding a single point of failure. The proposed approach gives network scalability a fair chance, and it improves the performance of end-to-end packet transmission by removing the tunneling tradition and making use of the MBB.
sends an NBU message to the previous BMA, which updates the mobility and routing status of the MN. The IP handover is confirmed by an NBAck. Figure 1 shows this scenario.
As discussed above, the proposed approach depends on distributed BMAs at the access network level, preventing a network bottleneck and avoiding a single point of failure. The proposed approach gives network scalability a fair chance, and it improves the performance of end-to-end packet transmission by removing the tunneling tradition and making use of the MBB.

Protocol Operation
The steps of the protocol operation of proposed approach, as shown in Figure 2

Protocol Operation
The steps of the protocol operation of proposed approach, as shown in Figure 2 are as follows: 1. Connection to BMA1. 2.

Analytical Modeling and Performance Evaluation
A mathematical analysis was performed to evaluate the performance of tunnel-free MIPv6 with MIPv6, PMIPv6, FMIPv6, and HMIPv6 in terms of the handover latency, handover blocking probability, and data packet loss.

Network Topology
The network topology for performance evaluation is presented in Figure 3. The proposed tunnelfree protocol with DMM was assessed with other protocols. Since for MIPv6 and PMIPv6, there exists no benchmark deployment model [25], we assumed that BMA was in the similar point as the AR and MAG in MIPv6 and PMIPv6, respectively. In this situation, distance dHA-AR was considered to be larger than dMAG-LMA.

Analytical Modeling and Performance Evaluation
A mathematical analysis was performed to evaluate the performance of tunnel-free MIPv6 with MIPv6, PMIPv6, FMIPv6, and HMIPv6 in terms of the handover latency, handover blocking probability, and data packet loss.

Network Topology
The network topology for performance evaluation is presented in Figure 3. The proposed tunnel-free protocol with DMM was assessed with other protocols. Since for MIPv6 and PMIPv6, there exists no benchmark deployment model [25], we assumed that BMA was in the similar point as the AR and MAG in MIPv6 and PMIPv6, respectively. In this situation, distance d HA-AR was considered to be larger than d MAG-LMA. The network topology for performance evaluation is presented in Figure 3. The proposed tunnelfree protocol with DMM was assessed with other protocols. Since for MIPv6 and PMIPv6, there exists no benchmark deployment model [25], we assumed that BMA was in the similar point as the AR and MAG in MIPv6 and PMIPv6, respectively. In this situation, distance dHA-AR was considered to be larger than dMAG-LMA.

Handover Latency of MIPv6
IETF developed MIPv6 at the network layer for handling mobility management. It keeps an MN accessible while it is on the move within the IPv6 network. A home address (HoA) identifies its MN regardless of its present point of connection to the network. The MN is linked with a care-of-address (CoA), as well as when it is away from its home domain. The MN uses mobility signaling messages to register the CoA with the HA. The packets received by the HoA for the MN are then routed to the MN's CoA. The handover latency of MIPv6 in this case can be given as [26]: where L HO represents the handover latency, T L2 is the link layer handover latency, T DAD is the duplicate address detection latency, and T R is for registration latency.

Handover Latency of FMIPv6
FMIPv6 arranges for a CoA beforehand in association with the new access router (nAR) with the help of L2 triggers. It then creates a bi-directional tunnel amid the new access router (nAR) and the old access router (oAR) to reduce the handover latency. Upon connection with the new link, the MN informs the nAR that packets can now be received. Once the MN successfully obtains the FBAck (Fast Binding Acknowledgement) message that has been sent by the oAR before moving to the nAR, predictive FMIPv6 is executed; otherwise, reactive FMIPv6 is performed. The handover latency for predictive FMIPv6 can be given as [27]: where T PRE represents the time the time at which the nAR receives the Unsolicited Neighbor Advertisement (UNA) message sent from the MN and the time at which the MN receives the first data packet sent from the nAR. T L2 is the link layer handover latency. The handover latency for reactive FMIPv6 can be given as [8]: where T R is for registration latency, T DAD is the duplicate address detection latency and T RE represents the time to send FBU (Fast Binding Update) message, exchange required information between the relevant ARs and receives the first data packet sent from the nAR.

Handover Latency of HMIPv6
MIPv6 makes the MN exchange mobility messages with the HA/CN and configure a new CoA whenever the MN is on the move. In the case where the distance between the MN and HA/CN is large, it becomes very time consuming and also results in unnecessary signaling overhead. HMIPv6 [28] addresses this problem by introducing a MAP (mobility anchor point), a new functional entity for local mobility management. The handover latency for HMIPv6 can be given as: where T L2 is the link layer handover latency, T MD is the movement detection process delay, T DAD is the duplicate address detection latency and T MAP represents the time required to send the binding update message, receive the binding acknowledgement message and also receive the data packet sent from the MAP.

Handover Latency of PMIPv6
The MN on the move is managed in a localized manner in PMIPv6. The entities providing a mobility service supports the mobility for the MN. The mobile access gateway (MAG) senses and registers the movement of the MN when it gets attached to a new access network. For a handover in the domain of PMIPv6, the Duplicate Address Detection (DAD) process and address configuration is not compulsory since the same home network prefix in the RA message sent from the MAG at the new access network is obtained by the MN. The handover latency for PMIPv6 can be given as [29]: where T LMA includes the time needed to receive the LMA's first data packet, time needed to send the RS message, and the time needed to exchange the update and acknowledgement messages between the LMA and MAG.

Handover Latency of Proposed Approach
The handover latency for MIPv6 is found using Equation (1). For our proposed approach, as shown in the handover timing diagram in Figure 4, the handover latency is comprised of the handover delay of link layer depending on the chipset implementation for a wireless interface (d L2 ), the link layer authentication latency (d AU ), the movement detection process delay (d MD ), the duplicate address detection delay (d DAD ), the new binding update and acknowledgement process delay (d N ), the delay of the proposed protocol's handover registration (d R ), and the arrival delay of the first packet of the previous session from the current BMA to the MN (d S ). The delay in the movement detection process (d MD ) is comprised of the time required for sending the RS notification and receiving the RA notification. If t RS denotes the time required for the RS notification, and t RA denotes the time required for the RA notification, then movement detection process delay d MD can be calculated using: where T RS is the time required for sending the RS notification and receiving the RA notification and T R is the handover registration delay. where TRS is the time required for sending the RS notification and receiving the RA notification and TR is the handover registration delay. The factors involved in the delay in the RS/RA notification are the message size of RS/RA, the bandwidth and propagation time of the wireless link, and the link failure probability of the wireless link. The RS/RA delay has been calculated according to the AR involved [9,30]. We derived tRS and tRA for our proposed approach focusing on the factors involved as mentioned above. tRS and tRA can then be calculated using: In Figure 4, dR specifies the proposed protocol's handover registration delay. It begins with the BU notification. For the proposed approach we derived dR as follows: The proposed approach eliminates the need for tunneling by anchoring the sessions via a new BMA using the MBB approach. Let dN represent the new binding update and acknowledgement latency caused while sending a signaling message. Then, dN consists of a new binding update represented by dNBU and a new binding acknowledgement represented by dNBAck. We can calculate dN using: The proposed approach also provides a mobility option for an address bound to the MN. It is represented by mM. Adding this factor to dN, dNBU and dNBAck can be calculated as follows: The arrival delay of the first packet of the previous session from the current BMA to the MN is represented by dS. It involves a BA and the data packet represented by mD. We calculated dS as follows: The factors involved in the delay in the RS/RA notification are the message size of RS/RA, the bandwidth and propagation time of the wireless link, and the link failure probability of the wireless link. The RS/RA delay has been calculated according to the AR involved [9,30]. We derived t RS and t RA for our proposed approach focusing on the factors involved as mentioned above. t RS and t RA can then be calculated using: In Figure 4, d R specifies the proposed protocol's handover registration delay. It begins with the BU notification. For the proposed approach we derived d R as follows: The proposed approach also provides a mobility option for an address bound to the MN. It is represented by m M . Adding this factor to d N , d NBU and d NBAck can be calculated as follows: The arrival delay of the first packet of the previous session from the current BMA to the MN is represented by d S . It involves a BA and the data packet represented by m D . We calculated d S as follows: As can be seen in Figure 4, the overall registration delay is represented by d REG. Hence, it can be given as: Meanwhile, the combined latencies of link layer handover and authentication can be given as: Let T HO denote the handover latency combining all the factors mentioned above. We can now calculate the handover latency for the proposed approach by summing up the involved timing factors as follows: where the default values used for d L2 , d A , d WRS , and d DAD are the same as in References [8,30].

Handover Latency Analysis
If the different handover latencies are compared, the order of latencies can be given as [31]: When comparing HMIPv6 and PMIPv6, the order of latencies can be given as [32]: When comparing PMIPv6 and FMIPv6, FMIPv6 outperforms PMIPv6 [33]. Therefore, we can say: FMIPv6 > PMIPv6.
Our evaluation above shows that TFMIPv6 outperforms other mobility management protocols. Therefore, we can give the order of latencies as follows: For comparison purposes, the relative gain is defined here for the handover latency to the MIPv6 process as: where L HO is the handover latency for MIPv6 and T HO is the handover latency for the proposed approach. If G H is greater than 0.1, signifies that the proposed handover technique outperforms the basic one.

Handover Blocking Probability of the Proposed Approach
For the proposed approach, the handover blocking probability presented in References [8,[30][31][32] is used to examine the failures in terms of the handover blocking probability in this study. Many factors can cause a handover failure for the MN, such as the non-availability of a wireless network resource, signal-to-noise deterioration, and handover latency. If T HO represents the handover latency for the proposed approach, let E[T HO ] denote the mean value of T HO . Let T E be the time when MN exists in the network along with its probability density function ƒ E (t). We assumed T HO to be exponentially distributed with the cumulative function F T (t) for the sake of simplicity. If T HO is the only factor blocking the handover, then the handover blocking probability p b is given as: where µ c is the rate of boundary crossing for the MN, i.e., its mobility rate. Supposing that the BMA's coverage area is circular, then µ c is calculated as follows [8,[32][33][34]: where v represents the average velocity of the MN and R represents the radius of the BMA's coverage area.

Handover Blocking Probability Analysis with Other Approaches
Suppose the handover latency of a particular mobility management protocol is denoted by L HO (.) , where (.) specifies a specific protocol. If the mean value of L HO (.) is given by E[L HO (.) ] and the residence time in the network is denoted by T R , along with its probability density function ƒ R (t). Supposing the handover latency is exponentially distributed along with the cumulative function F T (.) (t), for all other protocols with their respective handover latencies, the handover blocking probability can be calculated using: where µ c is the rate of boundary crossing of the MN, i.e., its mobility rate. Supposing that the BMA's coverage area is circular, then µ c is calculated with Equation (19) [8,[32][33][34].

Data Packet Loss with the Proposed Approach
During the handover process, upon the absence of a buffer management technique deployed at the network, the data packets sent to the MN might get lost. The total data packet lost during a handover process can be defined as the sum of all the lost data packets sent from a CN to the MN. Let δ P represent the total data packet loss, which can be calculated using: where λ S represents the average session arrival rate at the MN's wireless interface, S(L) represents the average session length in packets, and T HO is the handover latency for the proposed approach.

Data Packet Loss Analysis with Other Approaches
Let ϕ p (.) represent the total data packet loss for any protocol, which can be calculated as follows [8,32]: ϕ where λ S represents the average session arrival rate at the MN's wireless interface, E[S] represents the average session length in packets, and L HO is the handover latency for a specific approach.

Results and Discussion
This section provides the results of the performance evaluation of the mobility management protocols in comparison with the proposed tunnel-free distributed mobility management approach. NS2 is chosen as the simulation platform for this research. Network Simulator 2.33 (NS2.33) [35] was used to investigate the proposed framework, along with Mobiwan, which is an extension tool for MIPv6. Mobiwan running on NS2.33 was installed on CentOs Linux 7 (a linux based operating system). Network Animator NAM 1.15 [36] was used for the NS2.33 simulation. Xgraph was used to generate the simulated results. For comparison purposes, system parameter values for the numerical analysis were used from References [30,[37][38][39].

Handover Latency
Handover latency versus ρ ƒ is given in Figure 5, where ρ ƒ varied from 0 to 0.7 with a step value of 0.05. The likelihood of an erroneous packet transmission increased with a greater value of ρ ƒ on the wireless link. This in turn raised the number of retransmissions of mobility signaling, resulting in a larger handover delay. Consequently, as shown in Figure 5, the handover latency was relative to ρ ƒ for each mobility management protocol.
In this analysis the proposed tunnel-free MIPv6 outperformed the other mobility management protocols in terms of handover latency due to the reason that the MN in the protocol utilized a tunnel-free approach and prepared its handover beforehand at the previous BMA before it actually moved to the new BMA. This analysis placed PMIPv6 second in terms of handover latency. PMIPv6 managed its MN locally. The LMA and MAG exchanged the mobility signaling, hence there was no need to perform the mobility signaling upon a wireless link leading to the minimized effect of ρ ƒ in the performance of PMIPv6. handover blocking probability for each mobility management protocol. For this analysis, the handover latency was considered as a blocking factor for the handover blocking probability. Synonymous with the previous result in Figure 5, an increase in the value of ρƒ consequently increased the handover blocking probability. The handover blocking probability of the proposed TFMIPv6 was lower than all the other protocols, while that of MIPv6 was higher than all the other protocols. Again, this analysis placed PMIPv6 second in terms of the handover blocking probability.

Handover Blocking Probability
For comparison purposes, the values of velocity v and radius R were set to 20 m/s and 500 m, respectively, whereas the value of ρ ƒ varied from 0 to 0.7 with a step value of 0.05. Figure 6 shows the handover blocking probability for each mobility management protocol. For this analysis, the handover latency was considered as a blocking factor for the handover blocking probability. Synonymous with the previous result in Figure 5, an increase in the value of ρ ƒ consequently increased the handover blocking probability. The handover blocking probability of the proposed TFMIPv6 was lower than all the other protocols, while that of MIPv6 was higher than all the other protocols. Again, this analysis placed PMIPv6 second in terms of the handover blocking probability.  For comparison purposes, the values of ρƒ and R were set as 0.2 and 500 m, respectively, whereas the value of v varied from 0 to 30 m/s. Figure 7 shows the handover blocking probability versus v. With an increase in v, there were frequent changes in the MN's point of attachment, which implied that it was necessary for a MN with a higher velocity to complete its handover in less time than the MN with a lower velocity. Therefore, the increase in the velocity consequently increased the handover blocking probability for each mobility management protocol. In the simulation, the proposed TFMIPv6 outperformed the other mobility management protocols in terms of the handover blocking probability, which was below 0.05 even with an increase in velocity up until 30 m/s. PMIPv6 again placed second in terms of the handover blocking probability with a value below 0.1 when the velocity reached 30 m/s. Synonymous with the earlier results, MIPv6 performed poorly yet again in terms of the handover blocking probability.
Furthermore, for comparison purposes, the values of ρƒ and v were set to 0.2 and 20 m/s, respectively, whereas the value of R varied from 400 to 800 m. The handover blocking probability For comparison purposes, the values of ρ ƒ and R were set as 0.2 and 500 m, respectively, whereas the value of v varied from 0 to 30 m/s. Figure 7 shows the handover blocking probability versus v. With an increase in v, there were frequent changes in the MN's point of attachment, which implied that it was necessary for a MN with a higher velocity to complete its handover in less time than the MN with a lower velocity. Therefore, the increase in the velocity consequently increased the handover blocking probability for each mobility management protocol. In the simulation, the proposed TFMIPv6 outperformed the other mobility management protocols in terms of the handover blocking probability, which was below 0.05 even with an increase in velocity up until 30 m/s. PMIPv6 again placed second in terms of the handover blocking probability with a value below 0.1 when the velocity reached 30 m/s. Synonymous with the earlier results, MIPv6 performed poorly yet again in terms of the handover blocking probability.

Data Packet Loss
During a handover process, data packets sent from the CN to MN tend to get missing. Figure 9 depicts the packet loss in the course of the handover process. For comparison purposes, the values of λS and E(S) were set to 1 and 10, respectively, whereas the value of ρƒ varied from 0 to 0.7. As can be seen in Figure 9, the proposed TFMIPv6 had the lowest amount of data packet loss when compared to other protocols. It was observed from the previous section that the handover latency was directly proportional to the packet loss during the handover process. To illustrate, MIPv6 took more time to complete its handover process, which resulted in more packet loss in contrast with the other protocols. The proposed TFMIPv6 outperformed the rest of the protocols in terms of data packet loss because it had the lowest handover latency. Furthermore, for comparison purposes, the values of ρ ƒ and v were set to 0.2 and 20 m/s, respectively, whereas the value of R varied from 400 to 800 m. The handover blocking probability versus R is shown in Figure 8. An increase in the value of R meant a bigger access network for the MN, which consequently increased the residence time for a MN to stay in that access network, giving the MN more time to complete its handover process, hence decreasing the handover blocking probability. As can be seen in Figure 8, R influenced most of the mobility management protocols but not the proposed TFMIPv6.

Data Packet Loss
During a handover process, data packets sent from the CN to MN tend to get missing. Figure 9 depicts the packet loss in the course of the handover process. For comparison purposes, the values of λS and E(S) were set to 1 and 10, respectively, whereas the value of ρƒ varied from 0 to 0.7. As can be seen in Figure 9, the proposed TFMIPv6 had the lowest amount of data packet loss when compared to other protocols. It was observed from the previous section that the handover latency was directly proportional to the packet loss during the handover process. To illustrate, MIPv6 took more time to complete its handover process, which resulted in more packet loss in contrast with the other protocols. The proposed TFMIPv6 outperformed the rest of the protocols in terms of data packet loss because it had the lowest handover latency. All the results presented in Figures 6-8 show that the handover latency of the proposed TFMIPv6 was minor enough to not to be influenced and affected by the handover blocking problems caused by ρ ƒ , v, and R. The reason for the extraordinary performance of the proposed TFMIPv6 in comparison with the other protocols was the tunnel-free approach introduced, along with the MBB approach discussed earlier. The MBB approach allowed the MN to prepare its handover at the previous BMA before the MN performed the actual handover to the new BMA, meanwhile continuing the previous ongoing session anchoring through the new BMA instead of the previous BMA, hence eliminating the need for tunneling.

Data Packet Loss
During a handover process, data packets sent from the CN to MN tend to get missing. Figure 9 depicts the packet loss in the course of the handover process. For comparison purposes, the values of λ S and E(S) were set to 1 and 10, respectively, whereas the value of ρ ƒ varied from 0 to 0.7. As can be seen in Figure 9, the proposed TFMIPv6 had the lowest amount of data packet loss when compared to other protocols. It was observed from the previous section that the handover latency was directly proportional to the packet loss during the handover process. To illustrate, MIPv6 took more time to complete its handover process, which resulted in more packet loss in contrast with the other protocols. The proposed TFMIPv6 outperformed the rest of the protocols in terms of data packet loss because it had the lowest handover latency.

Conclusions
This paper proposed a tunnel-free mobile IPv6 protocol using a DMM approach. In addition, it analyzed and compared the existing IPv6 mobility management protocols in terms of the handover latency, handover blocking probability, and packet loss. From the results of the analysis conducted, it was confirmed the proposed tunnel-free approach with MBB concept allowed an MN to prepare for the handover beforehand, and anchored the transmission from the current BMA instead of tunneling it from the previous BMA, avoiding a reduced handover delay, consequently reducing the handover blocking probability and data packet loss.

Future Work
Like most of the DMM-based protocols proposed, our work also focused on a single MN's handover. In future work, to further improve the dimensions of the proposed research, we will be focusing on the DMM handover issues when multiple MNs perform a handover from the same previous network to the same destination network simultaneously, rather than a single MN handover. Moreover, we are also currently analyzing the cost of the proposed approach. In future work, we intend to compare the proposed approach in terms of cost with the benchmark protocols. In addition, we also intend to undertake a performance evaluation with other mobile IPv6-based schemes using the MBB approach. We have considered this aspect in the future work for this research. For now, we have performed performance evaluations with current existing deployed protocols for mobility management.

Conclusions
This paper proposed a tunnel-free mobile IPv6 protocol using a DMM approach. In addition, it analyzed and compared the existing IPv6 mobility management protocols in terms of the handover latency, handover blocking probability, and packet loss. From the results of the analysis conducted, it was confirmed the proposed tunnel-free approach with MBB concept allowed an MN to prepare for the handover beforehand, and anchored the transmission from the current BMA instead of tunneling it from the previous BMA, avoiding a reduced handover delay, consequently reducing the handover blocking probability and data packet loss.

Future Work
Like most of the DMM-based protocols proposed, our work also focused on a single MN's handover. In future work, to further improve the dimensions of the proposed research, we will be focusing on the DMM handover issues when multiple MNs perform a handover from the same previous network to the same destination network simultaneously, rather than a single MN handover. Moreover, we are also currently analyzing the cost of the proposed approach. In future work, we intend to compare the proposed approach in terms of cost with the benchmark protocols. In addition, we also intend to undertake a performance evaluation with other mobile IPv6-based schemes using the MBB approach. We have considered this aspect in the future work for this research. For now, we have performed performance evaluations with current existing deployed protocols for mobility management.

Conflicts of Interest:
The authors have no conflict of interest to declare.