Mitigating Packet Loss in Mobile IPv6 Using Two-Tier Buffer Scheme

Transcription

1 Vol. 3 (2) June Mitigating Packet Loss in Mobile IPv6 Using Two-Tier Buffer Scheme Salim M. Zaki 1c and Shukor Abd Razak 1 1 Department of Computer Systems and Communications, Faculty of Computer Science and Information Systems, Universiti Teknologi Malaysia, MALAYSIA c Corresponding Author: Salim M. Zaki salimzki@gmail.com Abstract Mobile IPv6 was designed to allow nodes to be reachable and maintain ongoing connections while they are within the network topology. A packet loss problem still occur when a node handover from one base station to another in default Mobile IPv6, which may affect real-time traffic. This study proposed a new scheme to reduce the real-time traffic packet loss in MIPv6 environment. Network Simulator (ns-2) and its extension MobiWan that supports IPv6 have been used to simulate the proposed scheme and tested its efficiency. The proposed scheme on the Correspondent Node showed that it could reduced packet loss occurred during Mobile Node handover as the Correspondent Node sending data to Mobile Node. Apart from the number of lost packets, some other metrics such as throughput and delay have also been analyzed to investigate the performance of the proposed twotier buffer compared to the normal MIPv6 scheme. Keywords MIPv6; packet loss; two-tier buffer; packet buffering; real-time traffic 1. INTRODUCTION Mobile IPv6 [1] adds mobility feature to IPv6 enhanced features. MIPv6 considered as an evolution of Mobile IPv4 [2], and MIPv6 promises a good characteristics to mobility management in IP-based networks. In current implementation, Mobile IPv6 allows nodes to remain reachable while moving around in the network [1]. While moved away from its home and attached to another network, Mobile Node (MN) will inform the Home Agent (HA) about its new address through exchanging Binding Update (BU) and Binding Acknowledgement (BA) messages. After the handover process, the incoming packets from Correspondent Node (CN) will be either forwarded to MN using its new

2 IPv6 address (care-of-address) or directly send to the MN from CN if the route optimization is supported. In such situation, the handover for a mobile node in MIPv6 environment causes packet loss [3]. This problem suffered the quality of real-time sessions between MN and CN due to the loss of packets [4]. However, the handover latency is considered as the main reason behind packet loss [11, 12, and 15] due to the time MN takes to reach new access point. Moreover, it is not known when the MN moves out of the first access point coverage area which causes many problems where one of them is packet loss. The lost link by mobile nodes is causing packet loss in most of the cases [19]. Even in hierarchical MIPv6 packet loss, the problem is still occurring [18]. Packet loss effect is an active area of research and many factors can contribute to the effects [20]. Even though numerous mechanisms were proposed to solve this problem, the problem still exists. Thus, reducing packet loss is a big challenge in this area. By proposing a buffer on the CN side to retain the forwarded packets, the buffered packets will be retransmitted to the mobile nodes once the binding updates have been received by the corresponding node. This procedure reduced the packet loss. The rest of the paper is organized as follows. Section 2 gives an overview of some related work on MIPv6 improvements. Section 3 describes the proposed scheme followed by the simulation experimental setup in section 4. Analysis of the experiments are presented and discussed in Section 5. Finally, Section 6 concludes the paper. 2. RELATED WORK A number of solutions have been proposed to overcome packet loss problem in MIPv6 during handover. Some of the studies proposed to reduce the number of packet loss through reducing latency of handover procedure which causes the packet loss. Whilst other researches proposed to reduce loss of packets using buffer by duplicating buffers on the access routers. Fast handover of mobile IPv6 uses anticipation to obtain new address for the mobile node from the new access router even it is still connected to the previous link. This procedure reduces handover latency time which then reduces packet loss. Even though it reduces latency and packet loss, some authors proposed an enhancement for this protocol to make it faster and low packet loss. Van et. al., [14] proposed two enhanced protocols called SFMIPv6 and SF-HMIPv6. In both protocols, previous access router can initiate fast handover with new access router instantly after receiving Router Solicitation for Proxy (RoSolPr) message. This enhancement reduced the delay and packet loss also reduces consequently. In the original protocols, the latency is higher due to waiting for fast binding update messages. Authors in [5] proposed a scheme called Tunnel Buffering (TB), which is an interoperable enhancement to Mobile IPv6 to reduce packet loss during movement. TB does that by 'holding' the packets (in a buffer) which are to be send over MobileIPv6 or Hierarchical Mobile IPv6 (HMIPv6) tunnel until MN s movement is complete. The buffer in this scheme is proposed to be on the Home Agent if the MIPv6 is used. The function of this buffer is to hold the incoming packets to a MN while the packets were being handover from network to another, and send these buffered packets to MN when BU was received from the MN. This scheme also proposed to be working on HMIPv6 [6] by duplicating the buffer on the Mobility Anchor Point (MAP) which is a new node introduced by HMIPv6. In Fast Handovers for Mobile IPv6 [7], the author proposed to use a buffer on the New Access Router (NAR) to buffer the forwarded packets from Previous Access Router (PAR) and keep them from losing. The authors in [8], proposed an enhanced buffer management scheme for MIPv6 Fast handover. Such proposed scheme has two parts. First, while the original Fast Handover protocol only buffers packets in the New Access Router (NAR), they use buffer in the Previous 1

3 Access Router (PAR) as well during the handover process. This helps improved the total buffer utilization in the network. Second, they define three types of services in the handover process so that packets can be treated differently based on their traffic characteristics. The evaluation of the proposed method was carried out using ns-2 simulator and it showed that this method reduced the number of lost packets in the handover. Leoleis et al. [13] proposed an enhancement for FMIPv6; they achieved the enhancement in a dual way: first, through tunneling the traffic to the new access router (NAR) which becomes a receiver of this traffic. Second, buffering the tunneled traffic for the period during link layer handover where mobile node is incapable to communicate. The utilization of buffers reduced packet loss better as compared to the original FMIPv6. However, the cost of this enhancement is that the signaling became higher [14] which could cause some unwanted problems. Proxy Mobile IPv6 [16] is a management protocol proposed by IETF. The main idea behind it is that mobility entities in the network are managed by the network itself. Thus, mobile node plays no role in mobility management and all required signaling for mobility is managed by the network. As standard MIPv6, PMIPv6 suffered from packet loss. Choi, et. al., [17] proposed a buffering scheme called the Smart Buffering scheme. It anticipates an MN s movement through utilizing the network information, and buffers the packets that were expected to be lost. The anticipation is based on the receiving signal strength indication (RSSI). When the value reached beyond a threshold, then the mobile access gateway will start buffering the packets and forward the packet simultaneously. Those buffered packets are time stamped and when packets are expired, they will be discarded. However, the new mobile access gateway will communicate with the old one when MN finishes the attachment to it and the old mobile access gateway will forward the buffered packets. 3. THE PROPOSED SCHEME In the proposed scheme, the CN has two tier buffers to retain the forwarded real-time traffic packet as shown in Figure 1. While MN is moving to a new location, incoming packets may be lost or may arrive out of order. Therefore, this study proposed to use the first tier of the buffer on the CN to retain the packets that was already sent before MN handover procedure starts. When the MN attached to a new location and sending binding updates (BU) to the correspondent node, the CN will send the first buffer contents to the Mobile node. The contents of this buffer will be cleared after MN receives them and substituted with new packets that the CN wants to send to the MN and so on until the real-time session finished between the two nodes. The size of the buffer depended on the amount of packets that need to be sent. 2

4 Figure 1: the proposed scheme It is possible that the MN moves again while the CN sending the buffered packets. This could make the forwarded packet (that were buffered in the first buffer) unable to reach the MN. To avoid this problem, duplication for the first buffer contents into the second tier of the buffer has been proposed. The second tier buffer was aimed to retain the packets for the first buffer until the MN gets the packets that were retained in the first buffer. After that, the second buffer will be cleared. Pseudo code in Figure 2 shows the steps of using the two-tier buffer scheme during handover time. This approach reduces the number of lost packets during the MN handover procedure by retaining packets in double buffer scheme. Furthermore, it reduces overhead on the routers by using corresponding node to manage retaining packets during handover time. The existing work that used buffer to reduce packet loss was proposed for fast, hierarchical MIPv6 and PMIPv6; not on the standard MIPv6. They used buffer on routers; however in this paper, it is used on corresponding node. The aim of the two-tier buffer here is not to overload router with redundant packets from mobile nodes, but to distribute the load of mobile nodes communications in the networks. 3

5 Figure 2: steps of using the two-tier buffer scheme during handover time pseudo code. 4. EXPERIMENTS Performance Evaluation of the proposed scheme was done by using the discrete event simulator (ns-2) [9] and MobiWan tool [10]. Enhancement was done on the standard MIPv6, and simulation was compared to standard MIPv6 as well. Figure 3 illustrates the simulation topology and Table 1 lists all simulation parameters. The wireless coverage area of each Access Router (AR) in this topology is 250m and both coverage areas were overlapped within a few meters. The MN moved between these two ARs along a straight line at the speed of 10 meter/second (36km/Hr). CN was used to send data to MN when MN was still attached to HA during the handover process. Links between nodes were set to a transmission rate of 10Mbit/s with a delay of 2ms on each link. UDP protocol was used with Constant Bit Rate (CBR) traffic generator to generate packets with 160 bytes size with 20ms interval (64-kb/s audio). Total simulation run time, t=100s. TABLE 1: PARAMETERS USED FOR THE EXPERIMENTS Parameter Simulator Simulation time Topology size Transmission range Traffic type Maximum speed Value Ns-2 with MobiWan 100 second 800*800 meter 250 meter Constant bit rate 10 m/s 4

6 5. RESULTS DISCUSSION Figure 3: the network topology This section presents the performance of the analysis of the proposed scheme as compared to the existing normal MIPv6 scheme. Five different performance scenarios were gathered for both normal MIPv6 and the proposed scheme. Differences between these scenarios were the time period of sending data from source (CN) to destination (MN). As depicted in Figure 4, the average delay in the normal MIPv6 schemas (i.e. without buffer) is increasing when the MN starts moving and going outside its Home Agent coverage area. With buffer, the average delay is also increased, but, with a very low rate compared to the standard scenarios. However, there is no significant increased in delay measured in two-tier buffer scheme compared to normal scheme (i.e. without buffer) as the delay did not go beyond 7.5ms. This indicates that the proposed buffering packets reduced the delay in packet transmission between MN and CN, especially in high mobility rate environment. 5

7 Figure 4: Delay comparison The average throughput decreased significantly when MN moves out of its home network. The decreasing can be seen in Figure 5. The throughput of the proposed scheme shows an improvement as opposed to the decreasing patterns measured in existing scheme. The fall in the throughput in the second scenario is normal and expected as this is happened when the MN starts to move to a new location. However, unlike the existing scheme (i.e. without buffer), as the downfall pattern continue in scenario 3, 4, and 5, in the proposed scheme (i.e. with buffer), the throughput rate is bouncing back from downfall pattern starting from scenario 2 onwards. Thus, the damage can be minimized by having two tier buffers as proposed in this paper. Figure 5: Throughput comparison 6

8 The ratio of lost packets in both schemes (i.e. with and without buffer) starts with zero because MN does not start its handover yet, as shown in Figure 6. In the second scenario (without buffer scheme), it can be seen that the percentage of packet loss is the highest. This is because, in this stage, the MN started to move away from its home location to attach to new AR. After a while, (i.e. scenario 3 and 4), the percentage of packet loss is lesser than in the second scenario. This is because, in these stages, the MN has informed the CN about its new CoA, thus recovering from the packet loss state. The packet loss percentage keep decreasing in scenario 5 as the links between CN and MN becomes more stable. As expected, the percentage of packet loss is increasing in scenario 2 (with buffer scheme), when the MN starts to move. However, compared to 16.5 percent measured in existing scheme (without buffer), the performance of the proposed scheme is still better at only 4%. The improvement in packet loss percentage is similar to one without buffer as the links between MN and CN became more stable in scenario 3, 4, and 5. Thus, it is proven that the proposed scheme reduced the packet loss in MIPv6 environment. 6. CONCLUSION Figure 6: Packet loss comparisons In this paper, a new packet loss reducing scheme was introduced for real-time traffic in MIPv6 networks. The buffering concept was proposed to retain forwarded packets sent by the CN to the MN. The results of the existing and the proposed scheme have been discussed and the comparison between them was shown as well. In the proposed scheme, it is shown that packet loss could be reduced by buffering the forwarded packets on the correspondent node. Overall, the results showed that the proposed two-tier buffer scheme could minimize the impact at handover procedure to real-time transmission in MIPv6 networks. A future direction will be testing the proposed scheme with many active mobile nodes moving between many access points in order to evaluate the efficiency of the scheme. 7

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