1. Rarest First and Choke Algorithms Are Enough
speaker: Yongming Chen

2. Outline
Introduction
Methodology
Experimental Results
Conclusion

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

4. 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

5. 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

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

7. 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

8. Torrents

9. 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)

10. 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

11. Results

12. 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

13. Results

14. Results

15. 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

16. Results

17. Results

18. 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

19. 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

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

21. 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