Leveraging Social Networks for Increased BitTorrent Robustness

Wojciech Galuba, Karl AbererEPFL, Switzerland

Zoran Despotovic, Wolfgang Kellerer Docomo Euro-Labs, Munich, Germany

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Motivation 1

BitTorrent relies on cooperative behavior But freeriding possible

15-80% of peers are freeriders

Freerider

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Current solution: Tit-for-tat

Tit-for-tat (TFT) strategy:Reward good uploaders by giving them higher

priority TFT defense ineffective:

Large-view exploitSeeders don’t download all peers look

equal to seeders

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Friend-to-Friend (F2F) systems

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

Social network

P2P network

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Motivation 2

How can we leverage the social network to solve freeriding?

Is the social network a good content distribution medium?

BitTorrent

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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

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Freeriding disincentiveUser interface:

AliceUpload/download: 1.2

BobUL/DL: 2.5

CarolUL/DL: 0.9

DanielUL/DL: 1.2

EveUL/DL: 0.02

Users unlikely to freeride on friends

Hmmm... I need to talk to Eve

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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

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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

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

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Swarm density

Def: Fraction of peers in the social network interested in downloading a file

Low swarm density distribution graph becomes disconnected

Source

No access to source

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

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

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Linear scaling with content size

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O(logN) scaling with the number of peers

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Open problems

We assumed uniform distribution of interested peers In reality interest clustered in social nets

Even better performance?

Content discoveryCan friends-of-friends be trusted?

Social BT geo locality less inter-AS traffic?

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Conclusions

Social network alone an efficient and scalable content distribution medium

Social BitTorrent Increased robustness to freeridingAs long as swarm density >2%

F2F approach to content distribution:Authenticated connections, trust Increased privacy