Leveraging Social Networks for Increased BitTorrent Robustness

1. Leveraging Social Networks for Increased BitTorrent Robustness Wojciech Galuba, Karl Aberer EPFL, Switzerland Zoran Despotovic, Wolfgang Kellerer Docomo Euro-Labs, Munich, Germany

2. Motivation 1 Freerider • BitTorrent relies on cooperative behavior • But freeriding possible • 15-80% of peers are freeriders

3. Current solution: Tit-for-tat • Tit-for-tat (TFT) strategy: • Reward good uploaders by giving them higher priority • TFT defense ineffective: • Large-view exploit • Seeders don’t download  all peers look equal to seeders

4. Friend-to-Friend (F2F) systems P2P network Social network • Source of the problem • In P2P nets everyone is a stranger • A solution: connect to friends’ computers only • Reliable, stable identities • Take advantage of existing social trust • Cooperation more likely

5. Motivation 2 BitTorrent How can we leverage the social network to solve freeriding? Is the social network a good content distribution medium?

6. The social BitTorrent • Approach • BitTorrent on top of a social network • Friends’ computers added to the peer list • Protocol unchanged • Communication only along the social links • Mutually authenticated • Can be encrypted for privacy • Friend discovery • We use a centralized friend server • Possible decentralized implementation: • PGP • Jabber/XMPP

7. Freeriding disincentive Alice Upload/download: 1.2 Bob UL/DL: 2.5 Carol UL/DL: 0.9 Daniel UL/DL: 1.2 Hmmm... I need to talk to Eve Eve UL/DL: 0.02 • Users unlikely to freeride on friends User interface:

8. Evaluated systems • BT – BitTorrent implementation • Using trackers to discover peers • SBT – the social BitTorrent • Only friend connections • HBT – hybrid implementation • Both on peer list: • friend peers • peers obtained from the trackers • Friend uploads have absolute priority

9. Experimental setup • Simulated in protopeer.net • Tit-for-tat (TFT), downloaders ordered by: • TFT on: how much they upload • TFT off: how much they download • 1024 peers, 25 MB file, single initial seeder • Mixture of peers with 0.5, 1 and 2Mbit/s upload bandwidth • Social net generated based on MSN Messenger data

10. Social network alone is an efficient content distribution medium • Though: long-tail problem

11. Swarm density Source No access to source Def: Fraction of peers in the social network interested in downloading a file Low swarm density  distribution graph becomes disconnected

12. Low swarm density  many failed downloads Trackers (in BT and HBT) solve the problem

13. Social links give HBT advantage only if swarm density > 2%

14. Linear scaling with content size

15. O(logN) scaling with the number of peers

16. Open problems • We assumed uniform distribution of interested peers • In reality interest clustered in social nets • Even better performance? • Content discovery • Can friends-of-friends be trusted? • Social BT  geo locality  less inter-AS traffic?

17. Conclusions • Social network alone an efficient and scalable content distribution medium • Social BitTorrent • Increased robustness to freeriding • As long as swarm density >2% • F2F approach to content distribution: • Authenticated connections, trust • Increased privacy