# Rarest First and Choke Algorithms Are Enough

## Presentation on theme: "Rarest First and Choke Algorithms Are Enough"— Presentation transcript:

Rarest First and Choke Algorithms Are Enough
speaker: Yongming Chen

Outline Introduction Methodology Experimental Results Conclusion

Introduction recent studies have proposed replacement of rarest first and choke algorithm to improve BT Didn’t Cohen think of efficiency and fairness?

Terminology pieces and blocks interested and choked peer set
leecher and seed peer state rare and available pieces # of copies rarest pieces least # of copies

BitTorrent Rarest First Algorithm Choke Algorithm
piece selection strategy download at random in rarest pieces set random first, strict priority, end game mode Choke Algorithm peer selection strategy regular unchoked peers and optimistic unchoked peer in leecher state seed kept unchoked peers and seed random unchoked peer in seed state

BitTorrent rarest first may lead to the scarcity of some pieces
choke algorithm is not fair and favors free riders

Experiment instrument a mainline client default parameters:
add some logs default parameters: 20kB/s max. upload rate, no download limit max. peer set size is 80 4 unchoked peers limitations: single client instrumentation limited torrent set

Torrents

Methodology use entropy instead of system capacity to evaluate efficiency of a piece selection strategy characterized with peer availability peer availability of x according to y = (the time peer y is interested in x) ∕ (the time x is in the peer set of y)

Methodology bit level tit-for-tat fairness is not appropriate in the context of p2p file replication excess capacity will be lost a seed cannot evaluate the reciprocation of a leecher alternative: for any leecher i, j, Di < Dj if Ui < Uj a seed should give the same service time to each leecher

Results

Rarest First Algorithm
almost all torrents have the ideal entropy 1,2,4,5,6,7,8,9 in top graph are in startup phase 2,18,19,21,26 in bottom graph have a small # of leechers rarest first does not perform optimally in some extreme cases peer join with only a few missing pieces super seeding

Results

Results

Rarest First Algorithm
transient state: only one seed steady state: no rare pieces the duration of transient phase is constrained by the initial upload capacity rarest first attempts to minimize it rarest first algorithm is efficient at keeping a torrent in steady state

Results

Results

Choke Algorithm strong correlation between the amount of bytes uploaded and downloaded choke algorithm leads to an equilibrium in the peer selection new version of choke algorithm in seed state is a significant improvement over the old one

Conclusion The rarest first algorithm guarantees a high diversity of the pieces. first block problem rather than last pieces problem To achieve fairness with a bit level tit-for-tat strategy is not appropriate The choke algorithm is fair, fosters reciprocation, and is robust to free riders

Conclusion piece and peer selection are key ideas for designing a scalable p2p system choke is simple but not easy to explain

Reference A. Legout, G. Urvoy-Keller, and P.Michiardi. Rarest first and choke algorithms are enough. In Proc. ACM SIGCOMM’06 S. Jun and M. Ahamad. Incentives in bittorrent induce free riding. In Proc. SIGCOMM’05 Workshops