A Stability-Oriented Overlay Multicast for Mobile Ad Hoc Networks

Fucai Yu, Younghwan Choi, Soochang Park, Ye Tian and Sang-Ha Kim Dept. of Computer Engineering

Chungnam National University, Daejeon, Korea {yufc, yhchoi, winter, tianye}@cclab.cnu.ac.kr and shkim@cnu.ac.kr

Abstract—Stable multicast data delivery structure contributes high data delivery ratio and low control overhead in Mobile Ad Hoc Networks (MANETs). Due to the dynamic topology of MANETs, to construct a stable multicast data delivery structure becomes a challenge in ad hoc environment. In this paper, we propose a stability-oriented overlay multicast protocol for MANETs based on a modified Delayed Forwarding mechanism. In this protocol, each member node precalculates its stability according to residual battery and its local mobility. A source rooted overlay multicast tree is created if source has to send data packets. The overlay construction follows the stability of each member node. By using a modified Delayed Forwarding mechanism, a member node with relatively high stability will have highly populated subtrees, SOOMP improves multicast routing stability by trying to push the unstable member nodes to the edge of the multicast tree. Data packets are transmitted via IP-in-IP tunnels between member nodes in the overlay tree.

Keywords-mobile ad hoc network; overlay multicast; delayed forwarding mechanism

I. INTRODUCTION

There have been a number of multicast protocols proposed for Mobile Ad Hoc Networks. They can be classified into two categories: Proactive and Reactive (on-demand). In terms of the proactive category, data delivery structure is maintained and updated disregarding the data traffic. When source begins to forward data packets, the data packets travel through a predetermined data delivery structure to destinations. However, Proactive protocols require periodical exchange of control packets for data delivery structure maintenance, even though there is no multicast data sent by source. Proactive protocols consume lots of network resources (e.g., bandwidth and power) and generate heavy control overhead. In contrast to proactive protocols, reactive protocols e.g., [2]–[4] avoid these drawbacks by update and maintaining the data delivery structure only if source has data to be sent. By doing so, periodical control messages exchange are not always necessary, hence saves network resources and reduces control overhead.

There also have been a number of overlay multicasting protocols for MANETs e.g., [5][6]. These overlay multicast protocols aim at group topology, network topology is tracked by the arbitrary underlying unicast protocol. This meachanism

allows overlay multicast to be more stable and to have lower control overhead even in a highly dynamic environment.

A reactive overlay multicast protocol ODOMP [1] has been proposed recently. In ODOMP, overlay tree is created only if source has to send data packets. This reactive manner significantly reduces overlay maintenance overhead. ODOMP overlay creation (or recreation) is initiated by the source broadcasting a JREQ message throughout the network. Once created an overlay multicast tree, ODOMP utilizes periodic overlay recreation to update the overlay. However, too frequent overlay recreation results in high network overhead, this is the main drawback of ODOMP. ODOMP requires frequent overlay reconstruction to optimize the overlay for a number of reasons including: first, ODOMP aims at creating a shortest path overlay tree. It does not consider the overlay stability. Due to network mobility, a created overlay tree can’t keep in optimized state for long time. Second, new joining group members directly connect to source and hence become the downstream members of source. Also, if one member node fails (leave of power-off), all its downstream members also directly connect to source to rejoin the overlay. It is obvious that these new overlay links are un-optimized. All these reasons require frequent overlay recreation to optimize overlay tree, which highly increases the network overhead.

In this paper we propose a reactive multicast approach for MANETS named Stability-Oriented Overlay Multicast Protocol (SOOMP). The goal of SOOMP is to create a stable overlay tree for multicast communication in ad hoc network. In SOOMP, each member node precalculates its stability according local mobility and its residual battery. The source node initiates overlay multicast tree creation only if source has to send data packets. SOOMP exploits a modified Delayed Forwarding mechanism to create overlay. The delay time of SOOMP is based on the stability of upstream member node. In other words, a more stable member node will have highly populated subtrees than an unstable one.

The rest of the paper is organized as follows. In section we describe member node stability calculation mechanism. In section we present SOOMP overlay creation, overlay maintenance and data forwarding process in detail. The performance evaluation of the proposed mechanism is

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followed in Section . Finally, Section covers the concluding remarks and future works.

II. MEMBER NODES STABILITY

One of the biggest drawbacks of tree-based multicast protocols is their weak stability, if a branch point fails or leave the group, all its subtrees can not receive data packets until they reconnect to the tree. This affects the performance of multicast protocol. As we know, a node with much residual battery will have long lifetime, a node with low local mobility means a low link break probability, so a node with more residual battery and low local mobility is certainly stable. The principle of SOOMP overlay multicast tree creation is that a stable member node should have highly populated subtrees. By this way we can enhance multicast tree stability.

SOOMP overlay tree creation is based on the Member Node Stability (MNS) of each member node. Member node MNS is calculated according to the member node local mobility and residual battery. We define MNS as:

localM%BatteryMNS = ,(1)

Where Mlocal is local mobility of one member node and %Battery is percent of residual battery of the member node. The Mlocal can be calculated by the algorithm proposed in MOBIC [7].

III. SOOMP OVERLAY CREATION AND MAINTENANCE

A. SOOMP Overlay Creation As described earlier, SOOMP aims at creating a source

rooted stable overlay multicast tree. For this goal, SOOMP exploits a modified Delayed Forwarding mechanism for overlay creation. Comparing with the Delayed Forwarding mechanism described in ODOMP, the difference is that, in ODOMP, the delay time that a JREQ packer is to be delayed by a non-member node is determined by the hop distance to the member node which has sent the JREQ packet. While in SOOMP, the delay time that a JREQ packet is to be delayed by a non member node rests with the stability of the member node which has sent the JREQ packet. SOOMP overlay creation is described as follows, see Fig.2, only the parts those needed to be expatiated on are drawn out. All transmission delays of wireless media are ignored.

First, if source node A has to send a multicast data packet and does not have a valid overlay tree for this packet, it buffers the packet and initiates overlay creation by broadcasting a Join REQuest (JREQ) packet. This JREQ contains the fields ScrAddr, GrpAddr, SeqNumber, LastMember and T_DELAY. The SrcAddr is set to source address and the GrpAddr is set to the multicast group address. By using the source address and a sequence number a JREQ packet can be uniquely identified. The LastMember field contains the address of the last member node which has

forwarded this JREQ. Initially the source node sets the LastMember to its own address. T_DELAY is used to inform the non member node how long this JREQ should be delayed. T_DELAY is calculated as follow:

)( %BatteryMTMNSTT_DELAY localunitunit ⋅== , (4)

The Tunit in (4) is a small constant time unit. Here we can see a member node with high stability will generate low T_DELAY. Source node calculates T_DELAY according to its stability and sets it to the T_DELAY field of the JREQ.

On receiving a JREQ, a node first checks whether the packet is a duplicate. Duplicates are silently discarded. If the node is non member node e.g., node B and C in Fig.2, it waits T_DELAY long time after received the JREQ and then rebroadcasts the JREQ to its neighbors, the T_DELAY value is got from the received JREQ. Here both node B and C will wait T_DELAYA which was calculated by source A and stored in the JREQ. This is our modified Delayed Forwarding mechanism.

If a member node e.g., node D and F in Fig.2, receives a JREQ packet, it stores the lastMember of the JREQ as its upstream member node for this group. Then it sets the lastMember field of the JREQ to its own address and sets T_DELAY of the JREQ to a new value which was calculated by (4) according to its stability. Then it broadcast the JREQ to its neighbors. In this example, member node D and F each receives JREQ from non member node B and C at the same time, and both store source A as upstream member node. Then both D and F set T_DELAY of the JREQs to the value calculated according to their stability. And then both D and Frebroadcast the modified JREQ to their neighbors. Here we assume the stability of member node D is less than that of member node F, i.e., MNSD < MNSF, then according to (4), the delay time generated by member node D is bigger than that of member node F, i.e., T_DELAYD > T_DELAYF. So though non member nodes G and H each receives a JREQ from D and F at the same time, node H will rebroadcast the JREQ received from F earlier than G rebroadcast the JREQ received from D. So member node I will receive the JREQ from Hearlier than the JREQ from G, hence stores F as its upstream member node. The later received JREQ from G will be silently discarded as a duplicate. Here we can see that though member

Figure 1. SOOMP overlay creation

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node I has the same hop distance to D and F, because member node F has high stability and generated short T_DELAY, member node I will connect to member node F which has higher stability. This process continues until all member nodes join the overlay.

Each member node except the source node has exactly one upstream member node. After receiving a JREQ a member node generates a JOIN REPLY (JREP) and forwards it to its upstream member node by unicast. The upstream member node stores the sender as one of its downstream member nodes. During this process a source rooted overlay tree is created step by step.

IV. PERFORMANCE EVALUATIONS

SOOMP was implemented in the network simulator QualNet 3.8. Its performance is compared with the performance of ODOMP. Since among all proposed multicast approaches, ODOMP is the most similar one to our approach.

A. Simulation Environments Our simulation modeled a network of 100 mobile hosts

placed randomly within a 1500 x 1500 m2 area. Each node has a radio transmission range of 250m and channel capacity of 2Mbit/s. The mobility model follows a random waypoint model with varying pause time. A two-way ground model is used as radio propagation model. Traffic is generated as constant bit rate (1 packets/second). Finally, we use IEEE 802.11 as MAC layer protocol. The speed range was set to from 0 to 2M/s. Simulation time is 600s.

B. Evaluation Metrics 1) Link Break Effect Ratio: Link Break Effect Ratio

(LBAR) is defined as the ratio of average number of affected member nodes by link break to the total number of member nodes.

( ) mNALBAR m −=m

01 , (5)

Where Am is the number of member nodes affected by one link break, i.e., the number of the member nodes locating in the subtree of the broken link. N is the group size. m is the number of link breaks during simulation time. As we know, all member nodes in the subtree of a broken link can not receive data packet temporarily. This problem becomes even more serious if there are a number of member nodes locate in the subtree of the broken link. SOOMP focuses on creating a stable overlay tree. This goal is achieved by making stable member nodes have highly populated subtrees. In other words, SOOMP tries to let unstable member nodes locate in the edge of the tree. The breaks of the links between these fringe and unstable member nodes have low effect on protocol efficiency. So by this metric, we can evaluate protocol stability.

2) Packet Delivery Ratio: Packet Delivery Ratio (PDR) is defined as the ratio of the average number of data packets received by all members to the number of the data packets transmitted by source.

sm TN-RPDR 1= , (6)

Where N is the group size, Rm is the total number of data packets received by all members, and Ts is the number of data packets transmitted by the source. This metric reflects the efficiency of the multicast protocol.

3) Control Overhead: Control overhead is defined as the ratio of the number of control packets transmitted and the number of delivered multicast data packets.

C. Simulation Results 1) Mobility: We varied the maximum speed from 2m/s to

20m/s to evaluate the effects of mobility on performance of SOOMP and ODOMP. The simulation results are presented in Fig.3. Fig.3-1 shows that the packet delivery ratio of both protocols slightly decrease with increasing speed. This is due to increasing probability of link breaks which lead to packet losses in both protocols. Comparing with ODOMP, SOOMP creates a stable overlay tree by making stable member nodes have highly populated subtrees. This means unstable member nodes usually locate on the edge of the tree, and relatively have a small number of member nodes in their subtrees, so the number of lost packets due to these link breaks of these unstable members in SOOMP will be less than the lost data packets in ODOMP. Fig.3-2 shows that the link break effect ratio of both protocols slightly increase with increasing speed. This is due to increasing probability of link breaks. However, in SOOMP, unstable member nodes usually locate on the edge of the tree, this means these unstable member nodes relatively have a small number of downstream members. So the link breaks of these unstable member nodes have lower effect on link break effect ratio than ODOMP. Control overhead is composed of routing discovery overhead and routing maintenance overhead. Both SOOMP and ODOMP exploit periodic flooding message for routing discovery, so SOOMP and ODOMP have the same routing discovery overhead if used with the same OVERLAY_TIMEOUT. The routing maintenance overhead of two protocols increases slightly with increasing speed. This is due to increasing probability of link breaks. ODOMP exploit unicast protocol e.g., AODV, to handle link breaks, while SOOMP exploit ERS to handle link breaks. As we know, ERS is an efficient and range-limited routing discovery mechanism, especially for short distance routing discovery. Fig.3-3 shows the control overhead comparison of two protocols with different mobility metric.

2) OVEYLAY_TIMEOUT:We varied the OVERLAY_ TIMEOUT from 5s to 30s to evaluate the effects of OVERLAY_TIMEOUT on performance of SOOMP and ODOMP. The simulation results are presented in Fig.4. It is

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obviously that OVERLAY_TIMEOUT has bigger effect on performance of ODOMP than SOOMP. As shown in Fig.4-1, if the OVERLAY_TIMEOUT is set to more than 15s, the packets delivery ratio of ODOMP significantly decreases. Since ODOMP does not focus on creating stable overlay for data delivery. So the created overlay tree might not keep optimized state for long time, so, with an increasing OVERLAY_TIMEOUT, the number of link breaks also increases. While in SOOMP, firstly, a stable overlay is created for data delivery. Second, link breaks usually occurs on the edge of the tree. So OVERLAY_TIMEOUT has low effect on SOOMP packet delivery ratio. The effects of OVERLAY_TIMEOUT on link break effect ratio of two protocols are shown in Fig.4-2. For control overhead, as we describe above, the link breaks ratio of ODOMP is bigger than SOOMP with OVERLAY_TIMEOUT increasing. So ODOMP pay more control overhead on link recreation. Simulation results about control overhead are shown in Fig.4-3.

V. CONCLUSIONS

In this paper we propose a reactive multicast approach for MANETS named Stable On-demand Overlay Multicast Protocol (SOOMP). The goal of SOOMP is to create a stable overlay tree for multicast in ad hoc network. In SOOMP, each member node precalculates its stability according local mobility and its residual battery. Similar to ODOMP, SOOMP

also uses Delayed Forwarding mechanism to create overlay, but the difference is that the delay time of ODOMP bases on distLastMember, the hop distance to upstream member node, while that of SOOMP is based on the stability of upstream member node. In other words, in SOOMP, a more stable member node will have highly populated subtrees than an unstable one, by this way to improve multicast routing stability. Simulation result indicates than SOOMP achieves high performance than ODOMP.

REFERENCES

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(1) Packet delivery ratio (2) Link break effect ratio (3) Control overhead

Figure 2. Effects of mobility on performance of SOOMP and ODOMP

(1) Packet delivery ratio (2) Link break effect ratio (3) Control overhead

Figure 3. Effects of OVERLAY_TIMEOUT on performance of SOOMP and ODOMP

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