1. NGMAST 2008 - Mobile DHT Energy1 Optimizing Energy Consumption of Mobile Nodes in Heterogeneous Kademlia-based Distributed Hash Tables Imre Kelényi Budapest University of Technology and Economics Department of Automation and Applied Informatics Jukka K. Nurminen Nokia Research Center

2. NGMAST 2008 - Mobile DHT Energy2 Agenda  Distributed Hash Tables (DHT)  DHT on mobile phones  Probabilistic message dropping  Measurements and results  Analytical model  Conclusion

3. NGMAST 2008 - Mobile DHT Energy3 Distributed Hash Tables (DHT)  Peer-to-peer systems that enable the storage and retrieval of pairs in a completely distributed and scalable way  Distributed storage systems, file sharing, service discovery, P2P SIP, etc.  DHT nodes communicate with each other via messages (e.g. over UDP datagramms)  There has not been any mobile implementation

4. NGMAST 2008 - Mobile DHT Energy4 DHT on mobile phones  Mobile Kademlia for Symbian OS Implements the Kademlia DHT protocol It can connect to the global Mainline BitTorrent DHT network (usually has more than 1 million concurrent users) Open source project More information: http://www.aut.bme.hu/MobileDHT http://www.aut.bme.hu/MobileDHT

5. Problem with the mobile DHT  When connected to a large DHT, the energy consumption is so high that the battery is depleted in a couple of hours  This is mostly the result of the large incoming traffic: requests coming from other DHT nodes NGMAST 2008 - Mobile DHT Energy5

6. First solution: block all requests  It is possible to block all incoming requests and thus operate in client-only mode  Significant energy conservation but also decreases DHT performance NGMAST 2008 - Mobile DHT Energy6

7. Probabilistic message dropping  Concept: do not block all incoming requests but a fraction of them Energy is conserved The node still participates in the DHT to some degree  Implementation: if an incoming request is received, it is dropped with a certain probability P drop The larger P drop is, the more messages are blocked and more energy is conserved Full peer: P drop =0, client-only: P drop =1  It does not require any protocol modifications in the other nodes (backward compatible)! NGMAST 2008 - Mobile DHT Energy7

8. Probabilistic message dropping  Protocol requirement The DHT must store multiple replicas of the stored values  Energy is conserved Responses are not sent out Less requests are received NGMAST 2008 - Mobile DHT Energy8

9. Measurement results  Performed with a Nokia N95, connected to the Mainline BitTorrent DHT  Average of 3-hour-long sessions NGMAST 2008 - Mobile DHT Energy9

10. Message traffic statistics NGMAST 2008 - Mobile DHT Energy10

11. Analytical Model on DHT Performance  Increasing P drop decreases the performance of the DHT, but to what extent?  Our analytical model characterizes the lookup latency of the DHT as a function of P drop and the ratio of mobile nodes in the network m NGMAST 2008 - Mobile DHT Energy11

12. Analytical Model on DHT Performance  Expected lookup latency in Mainline BitTorrent DHT as a function of m (ratio of mobiles) and P drop NGMAST 2008 - Mobile DHT Energy12

13. Conclusion  Probabilistic message dropping enables heterogeneity in DHTs without protocol modification  It enables mobile nodes with scarce resources, most importantly battery capacity, to join DHTs  Finding the optimal value for P drop is still an open question Dynamical adjusting NGMAST 2008 - Mobile DHT Energy13

14. Thank You!  More information: http://www.aut.bme.hu/mobiledht http://amorg.aut.bme.hu  Questions? NGMAST 2008 - Mobile DHT Energy14