Effective Replica Allocation in Ad Hoc Networks for Improving Data Accessibility

1. Effective Replica Allocation in Ad Hoc Networks for Improving Data Accessibility Takahiro Hara (Proc. IEEE Infocom 2001,pp1568-1576) Presented by Mingsheng Peng CS401 presentation

2. Contents • Why Data Replication • Related Work • System Model • Replica Allocation Methods • Simulation Model • Conclusion CS401 presentation

3. Why data Replication? • Ad hoc networks : temporarily constructed by only mobile hosts. • Mobile host plays the role of a router, even if source and destination are not in range, data packets are forwarded by relaying • Since hosts move freely, disconnections occur frequently, this causes frequent network division. CS401 presentation

4. Why data Replication?(contd...) • Example, if some link goes down and the network is split • Nodes on the right cannot access D2 • Nodes on the left cannot access D1 CS401 presentation

5. Why data Replication?(contd...) • A possible solution is by replicating data items at mobile hosts which are not the owners of the original data. CS401 presentation

6. Related Work • Ad hoc network routing protocol: Such as DSDV,AODV,DSR,ZRP,CBRP • can only improve the connectivity among MHs which are connected to each other, • but cannot do anything when the network is divided as in the case in Figure 1. • Distributed database systems • data Replication in database helps in reducing response time • since failures occur infrequently, a small number of replicas is sufficient • Mobile computing • mobile hosts access databases at sites in a fixed network, create replicas on mobile hosts • address issues of maintaining consistency with low communication costs • assume only one-hop wireless communication CS401 presentation

7. System Model • The system environment is assumed to be an Ad-hoc network where: • mobile hosts access data items held by other mobile hosts (single or multiple hops) • each mobile host creates replicas of the data and maintains the replicas in its memory • data item available if it is present locally or if it is available at one of the neighbors • Assumptions: • unique host identifier: Mj (set of all mobile hosts M = {M1, M2,…, Mm}) • unique data identifier: Dj (set of all data items D = {D1, D2,…, Dm}) • Assume all data items are of the same size • each host has a memory space of C data items for replicas (excluding the space for holding originals) • data remains the same and does not change (simplifying assumption) • access frequencies of data items from each mobile host is known and does not change CS401 presentation

8. Replica Allocation Methods • Approach: • replicas are relocated in a specific period (relocation period) • replica allocation is determined based on the access frequency and network topology CS401 presentation

9. Three replica allocation methods • Three replica allocation methods: differ in emphasis put on access frequency and network topology. • SAF : (Static Access Frequency) only the access frequency to each data item is taken into account. • DAFN : (Dynamic Access Frequency and Neighborhood) The access frequency to each data item and the neighborhood among mobile hosts are taken into account. • DCG : (Dynamic Connectivity based Grouping) The access frequency to each data item and the whole network topology are taken into account. CS401 presentation

10. SAF(static Access Frequency) • Each host creates replicas in descending order of access frequencies • Advantages: • No control information regarding replicas need to be exchanged • Once each host has its all necessary replicas, there is no more replica relocation. • Low overhead and low traffic • Disadvantages: • hosts with similar access characteristics have the same replicas, but a MH can access data items held by other connected MHs,and it is more effective to share many kinds of replicas among them.Thus it gives low data accessibility when many hosts have the same or similar access characteristics. CS401 presentation

11. SAF example CS401 presentation

12. DAFN(Dynamic Access Frequency and Neighborhood) • The algorithm of this method is as follows: • 1)  At a relocation period, each mobile host broadcasts its host identifier and information on access frequencies to data items. After all mobile hosts complete the broadcasts, from the received host identifiers, every host shall know its connected mobile hosts. • 2)  Each mobile host preliminary determines the allocation of replicas based on the SAF method. • 3) In each set of mobile hosts which are connected to each other, the following procedure is repeated in the order of the breadth first search from the mobile host with the lowest suffix (i) of host identifier (Mi). When there is duplication of a data item (original/replica) between two neighboring mobile hosts, and if one of them is the original, the host which holds the replica changes it to another replica. If both of them are replicas, the host whose access frequency value to the data item is lower than the other one changes the replica to another replica. When changing the replica, among data items whose replicas are not allocated at either of the two hosts, a new data item replicated is selected where the access frequency value to this item is the highest among the possible items. CS401 presentation

13. DAFN(Dynamic Access Frequency and Neighborhood) • Eliminates replica duplication among neighboring hosts • The above procedure is executed every relocation period • Overhead and traffic is much higher than SAF • Does not completely eliminate replica duplication • If network topology changes during the execution of this method, replica relocation cannot completed CS401 presentation

14. DAFN example CS401 presentation

15. DCG(Dynamic Connectivity based Grouping) • Biconnected component: A maximum partial subgraph which is still connected if one of the vertices is removed (high stability!) • The algorithm is as follows: 1) At a relocation period, each mobile host broadcasts its host identifier and information on access frequencies to data items. After all mobile hosts complete the broadcasts, from the received host identifiers, every host knows the connected mobile hosts. 2) In each set of mobile hosts which are connected to each other, from the mobile host with the lowest suffix (i) of host identifier (Mi), an algorithm to find biconnected components is executed. Then, each biconnected component is put to a group. If a mobile host belongs to more than one biconnected component, i.e., the host is an articulation point, it belongs to only one group in which the corresponding biconnected component is first found in executing the algorithm. 3) In each group, an access frequency of the group to each data item is calculated as a summation of access frequencies of mobile hosts in the group to the item. The calculation is done by the mobile host with the lowest suffix (i) of host identifier (Mi) in the group. CS401 presentation

16. DCG(Dynamic Connectivity based Grouping) 4)  In the order of the access frequencies of the group, replicas of data items are allocated until memory space of all mobile hosts in the group becomes full. Here, replicas of data items which are held as originals by mobile hosts in the group are not allocated. Each replica is allocated at a mobile host whose access frequency to the data item is the highest among hosts that have free memory space to create it. 5) After allocating replicas of all kinds of data items, if there is still free memory space at mobile hosts in the group, replicas are allocated in the order of access frequency until the memory space is full. Each replica is allocated at a mobile host whose frequency to the data item is the highest among hosts that have free memory space to create it and do not hold the replica or its original. If there is no such mobile host, the replica is not allocated. CS401 presentation

17. DCG(Dynamic Connectivity based Grouping) • Data accessibility is expected to be higher since replicas are shared among a group of hosts • Overhead and traffic higher than the other two methods since it consists more steps and needs to take the largest time among the three methods to relocate replicas in a wide range. • the probability is higher that the network topology changes during executing this method, and in this case, the replica relocation cannot be done at mobile hosts over disconnected links CS401 presentation

18. DCG example CS401 presentation

19. Simulation Model • 50  50 flatland • Each host randomly moves in all directions • Movement speed is randomly determined between 0 to d • Radio communication range is a circle of radius R (1-19) fixed to 7 • Number of hosts = Number of data items = 40 • Each host has creates up to C replicas (1-39) fixed to 10 • Access frequency of each host to Di is pi given by one of the three cases: • Case 1: pi = 0.5(1 + 0.01i) • Each host has same access characteristics, access frequencies vary in a Small range • Case 2: pi = 0.025i • Each host has same access characteristics, access frequencies vary in a Wide range • Case 3: pi is determined as a positive value based on N( 0.5(1+0.01i) , σ ) • larger the value of σ, higher the difference in the access characteristics of the hosts • Relocation period = T (1-8192) fixed to 256 • Simulated for 59,000 time units and traffic measured (traffic = number of hops used for relocating data) CS401 presentation

20. Conclusion • Introduced replica allocation in ad-hoc networks as a mechanism of improving data accessibility • Proposed 3 replica allocation methods that use access patterns and the network topology • Simulation results show that DCG gives the highest accessibility at the cost of increased traffic and SAF has the least traffic with low data accessibility • The replica allocation method depends on the system configuration and access patterns CS401 presentation

21. Any Questions? Thank you! CS401 presentation