Adapting Distributed Hash Tables for Mobile Ad Hoc Networks

Transcription

1 University of Tübingen Chair for Computer Networks and Internet Adapting Distributed Hash Tables for Mobile Ad Hoc Networks Tobias Heer, Stefan Götz, Simon Rieche, Klaus Wehrle Protocol Engineering and Distributed Systems Group Wilhelm-Schickard-Institute for Computer Science University of Tübingen Mobile P2P 06, Pisa 1

2 Mobile Ad Hoc Networks and DHTs? Scenarios for large MANETs Disaster scenario / emergency response teams Campus scenario / loosely organized groups Mass events / police squads Military use / troops No central components available Full decentralization Basic demand for managing and exchanging data Mobile P2P 06, Pisa 2

3 Distributed Hash Tables for Ad Hoc Networks? DHTs are know to be Fully decentralized Scalable Fault tolerant More rigidly constrained than unstructured P2P relationships between Relationship between Nodes (constrains connections) Relationship between Nodes and data items (constrains data location) Higher maintenance cost MANETs are challenging environments for DHTs Mobile P2P 06, Pisa 3

4 Overview Differences MANET vs. Internet Overlay-level adaptations Node-level adaptations Routing-level adaptations Conclusions Mobile P2P 06, Pisa 4

5 MANETs vs. Internet from a DHTs point of view Mobile P2P 06, Pisa 5

6 Assumptions made by conventional DHTs Assumptions made by conventional DHT approaches Stable network Long lasting connections Stationary peers Relatively high bandwidth Hierarchical structure efficient underlay routing Efficient connection establishment Dedicated routers These assumptions can not be made in mobile ad hoc networks! Mobile P2P 06, Pisa 6

7 Structural Differences: Ad-Hoc Networks vs. Internet No global connectivity but local domains Network splits, mergers, no central services Heterogeneous networks Unreliable nodes and unreliable links Changing network characteristics Unpredictable router behavior Underlay characteristics Expensive connections, high initial cost & delay, scarce resources Mobile P2P 06, Pisa 7

8 Overlay-level Adaptations Mobile P2P 06, Pisa 8

9 How can you find a DHT in a MANET? Problems No global connectivity No rendezvous points or IP caches DHT discovery necessary Possible solution: restricted flooding Efficient Identify close DHT nodes Multiple DHTS can be discovered Decide which DHT to join (size, connectivity) Mobile P2P 06, Pisa 9

10 How robust are DHTs in MANETs? MANET splits up DHT view: many nodes fail simultaneously How robust must a DHT be? Robustness via redundancy 300 nodes, 2 equally sized parts 6 successors: 90% failure prob. 12 successors: 4% failure prob. Redundancy is expensive Maintenance Path finding Copying of item backups Redundancy limits the scalability of the system Mobile P2P 06, Pisa 10

11 How robust are DHTs in MANETs? MANET splits up DHT view: many nodes fail simultaneously How robust must a DHT be? Robustness via redundancy 300 nodes, 2 equally sized parts 6 successors: 90% failure prob. 12 successors: 4% failure prob. Redundancy is expensive Maintenance Path finding Copying of item backups Redundancy limits the scalability of the system Mobile P2P 06, Pisa 11

12 Underlying networks can merge Multiple DHTs in one MANET All data items should be accessible over the DHT Data items are stored in one of the DHTs Merge DHTs Many data items have to be moved Topological changes Costly Communication between two DHTs Decreased performance Further splits lead to fragmented DHTs Mobile P2P 06, Pisa 12

13 Underlying networks can merge Multiple DHTs in one MANET All data items should be accessible over the DHT Data items are stored in one of the DHTs Communication between two DHTs Decreased performance Further splits lead to fragmented DHTs Merge DHTs Many data items have to be moved Topological changes Costly Mobile P2P 06, Pisa 13

14 Underlying networks can merge Multiple DHTs in one MANET All data items should be accessible over the DHT Data items are stored in one of the DHTs Communication between two DHTs Decreased performance Further splits lead to fragmented DHTs Merge DHTs Many data items have to be moved Topological changes Costly Mobile P2P 06, Pisa 14

15 Simple merging scheme Goals: Move as few items as possible Establish as few new connections as possible! Different DHT topologies require different merging schemes Solution: Keep the identifiers of each node Keep flexible links (e.g. fingers in Chord) Adjust constrained links (e.g. ring connections in Chord) Move nodes from the smaller to the bigger DHT Mobile P2P 06, Pisa 15

16 Merging scheme performance 7,5 7, ,5 6,5 6 6 Hops 5,5 5 4,5 50% 40% 20% One ring Hops 5,5 5 4,5 50% 40% 20% One ring 4 4 3,5 3, Nodes Nodes One successor per node Four successors per node Mobile P2P 06, Pisa 16

17 Node-level Adaptations Mobile P2P 06, Pisa 17

18 Weak or unreliable nodes Nodes can be disconnected from the network The node is still operational Temporary disconnections Recovery mechanisms cause overhead Recovery is unnecessary if node reenters the MANET Possible solutions: Delay maintenance operations Backup nodes (neighbors) perform the tasks for absent nodes Build DHT of reliable subset of nodes (node ranking) Increased robustness and fewer maintenance operations Mobile P2P 06, Pisa 18

19 Weak links Unreliable underlay links Weak, short lived, only temporary available Using these links for a DHT is costly (recovery) Solution: Force network splits Identify and tag weak and unreliable links Indicators: link lifetime, signal strength, node properties Don t use these links for DHT traffic Requires: traffic classification on routing level (DHT control traffic) Requires: classification of route finding requests (DHT induced requests) Virtual partitioning into reliable sub networks! But: not all data items are accessible Mobile P2P 06, Pisa 19

20 Routing-layer Adaptations Mobile P2P 06, Pisa 20

21 Routing topologies MANETs and DHTs are routing topologies Similar or duplicate functionality ID propagation Neighbor discovery Failure detection Routing Both routing instances are unaware of each other Cross layer modifications Mobile P2P 06, Pisa 21

22 Cross-layer optimizations Use the underlay to propagate DHT IDs Information about close DHT nodes Additional DHT routing information for free Use multicast for maintenance messages Efficient message delivery! BUT: numerous multicast groups Applicability depends on the choice of the underlying MANET protocol DHT aware underlay routing DHT nodes are MANET routers Each forwarding node has DHT routing information Use DHT information to relay packets Shorter overlay paths Mobile P2P 06, Pisa 22

23 Conclusions Problematic combination of two dynamic routing layers Affects overlay, node, and routing levels Solutions must address all levels Mobile P2P 06, Pisa 23

24 Thank you for your attention. Questions? Mobile P2P 06, Pisa 24