multi-torrent: a performance study yan yang, alix l.h. chow, leana golubchik internet multimedia lab...
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MULTI-TORRENT: A PERFORMANCE STUDYYan Yang, Alix L.H. Chow, Leana Golubchik
Internet Multimedia Lab
University of Southern California
Internet Multimedia Lab
OUTLINE
Background and Introduction Proposed Approach Simulation Study Related Work and Conclusion
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Internet Multimedia Lab
BIT-TORRENT (BT) OVERVIEW
An extremely popular P2P file sharing application.
Nodes downloading same file (torrent) form an overlay network.
File is divided into pieces (chunks) with fixed size.Default size is 256KB.
Upload while downloading Nodes download chunks from their neighbors. Each node upload file chunks to a number of
neighbors. Local scheduling file exchange.
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Internet Multimedia Lab
HOW BT WORKS?
NodeSeed Leecher
UnchokingTit-For-Tat (TFT) Leecher: 4TFT +
1 optimisticSeeds: 5 random
4
seed
leecher
leecher
leecher leeche
r
leecher
4TFT + 1 optimistic4TFT + 1 optimistic
5 random5 randomseed
Internet Multimedia Lab
TORRENT IN REAL LIFE
Downloading multiple torrents simultaneously is commonPrevious study shows ~85% peers participate
in multiple torrents. [Guo et al. IMC 2005] Overlap in downloading torrents: common
interests do exist in real life! Many torrents are related to each other.
Different episodes of a TV show. Movies/music form the same genre, etc.
Start-up information are obtained from a similar source. Similar web site, online forum, etc.
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MULTI-TORRENT APPROACH Current BT: single-torrent approach
Torrents are independent Doesn’t take advantages of comment interest
How about multi-torrent approach?Relating torrents together. A natural idea but is significantly overlooked. Encourages seeding. (Key to system
performance) Advantages of multi-torrent
Help newly joined nodes ramp up faster.Help nodes nearing end of their downloads find
the last few file chunks faster. (end-game behavior)
Keep a torrent “alive”.
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Internet Multimedia Lab
CONTRIBUTIONS
A multi-torrent BT system Small modification to the current BT protocol.Completely decentralized, all local decisions.
A “cross-torrent” TFT (CTFT)Simple modification to current BT TFT.
Extensive simulation-based study shows:Multi-torrent approach improves system
performance.CTFT provides incentives for nodes to stay as
seeds.
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Internet Multimedia Lab
OUTLINE
Background and Introduction Proposed Approach Simulation Study Related Work and Conclusion
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Internet Multimedia Lab
MOTIVATIONAL EXAMPLE
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Slow: +21% Fast:
+17%
Slow: +21% Fast:
+17%
Staying nodes are
better!
Staying nodes are
better!
Setting Value
Node class Fast nodes (5000/512kbps) (0.6), Slow nodes (1500/128 kbps) (0.4).
File selection Fixed selection 2 torrents out of 10 (200MB each).
No Stay Node leaves torrent as soon as it finishes.
Stay Node leaves torrent until all participating torrents finishes.
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PROPOSED APPROACH Current BT uses download rate for nodes to decide unchoke.
Such unchoking for each torrent is independent. Cross-Torrent TFT (CTFT) Aggregate download rate on each torrent in peer’s all participating
torrents. Aggregation is done in a weighted manner.
Higher weight (> 1) given to the downloading rate from seeding torrents. Normal weight (=1) given to the downloading rate from leecher torrents.
Total contribution of a peer Ny to node Nx is:
€
wi(y) ×Di(x,y)i=1
#Torrents
∑
10
Download rate on torrent i of node Nx from
node Ny
Download rate on torrent i of node Nx from
node Ny
€
€
wi(y) >1 Ny is seed on torrent i
wi(y) =1 otherwise
⎧ ⎨ ⎩
Internet Multimedia Lab
OUTLINE
Background and Introduction Motivation and Proposed Approach Simulation Study Related Work and Conclusion
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Internet Multimedia Lab
SIMULATION SETUP
Parameters Parameters Value
File size 200MB
Simulation time 12 hours (+3 hours warm-up)
Average node inter-arrival time
45 sec
Peer set size 40
#Leccher unchoking 4 TFT + 1 optimistic
#Seed unchoking 5
Original seed 1 per torrent
CTFT weight 4
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Simulator • Event based in [Bharambe et al. INFOCOM 2006]
• Extension for multi-torrent and latest BT protocol.
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STAY IMPROVEMENT: # OF TORRENT
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Have nodes staying
helps both metrics.
Have nodes staying
helps both metrics.
Setting Value
Node Class Fast nodes (5000kbps / 512kbps)
File selection 1. Fixed selection out of 10. 2. Random selection out of 10.
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STAY IMPROVEMENT: INTER-ARRIVAL TIME
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More improvement for
longer inter-arrival time.
More improvement for
longer inter-arrival time.
More obvious # nodes change for
longer inter-arrival time.
More obvious # nodes change for
longer inter-arrival time.
Setting Value
Node class Fast nodes (5000kbps / 512kbps)
File selection Fixed selection 3 torrents out of 10.
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APPLICATION: GAME PATCH SYSTEM
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492MB: -77%
256MB: +84%
80MB:+92%
492MB: -77%
256MB: +84%
80MB:+92%
Setting Value
Node class Fast nodes (5000kbps / 512kbps)
File size 492MB, 256MB, 80MB (WoW major patch 2.0, 2.1, 2.2)
File selection Fixed selection of all 3 torrents
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APPLICATION: GAME PATCH SYSTEM
Dynamic seeding (DS) Each node does a local estimation of the ratio of seeds to leechers. Only participate as seed in those torrents where seed to leecher
ratio is below R. This local estimation re-evaluated every S time units.
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492MB: +2%256MB: +47%
80MB: +69%
492MB: +2%256MB: +47%
80MB: +69%
Internet Multimedia Lab
OUTLINE
Background and Introduction Motivation and Proposed Approach Simulation Study Related Work and Conclusion
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Internet Multimedia Lab
RELATED WORK Explore multi-torrent
Multi-torrent system using tracker overlay. [Guo et al. IMC 2005]
Our focus is on performance. Our CTFT provides incentives to stay as seeds. Our design requires only local client modification.
Explore BT incentives in single torrent A market based incentives. [Freedman et al. IPTPS
2008] Reputation based incentives. [Lian et al. IPTPS
2006] Our CTFT-DS matches seeding capacity demand
and supply with local knowledge. Investigating other incentive approaches are part of
our future work. 18
Internet Multimedia Lab
CONCLUSION Performance gains are possible through
multi-torrent.Nodes staying as seeds improve performance.CTFT provides incentive for nodes to stay as
seed.CTFT-DS makes seeding efficient.
Multi-torrent is a promising research area with remaining future directions.Malicious users.More exploration on system parameters.
R, S, CTFT weight, etc. Real word implementations. 19
Internet Multimedia Lab
STAY IMPROVEMENT: CTFT WEIGHT
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Staying nodes perform better in
both class.
Staying nodes perform better in
both class.
Setting Value
Node class Fast nodes (0.6), Slow nodes (0.4)
File selection Random selection of 3 out of 10 torrents
Stay probability 0.5
Internet Multimedia Lab
STAY IMPROVEMENT: FRACTION OF STAY
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No IncentivesNo Incentives Incentives!Incentives!
Setting Value
Node class Fast nodes (0.6), Slow nodes (0.4)
File selection Random selection of 3 out of 10 torrents
Internet Multimedia Lab
DISCUSSION (BACKUP)
Performance in the real worldWe expect even bigger improvement.
System parametersExploration of various system and proposed
schemes’ parameters is our on-going effort. Malicious behavior
A malicious node can take advantage of the weight of CTFT, how to detect malicious node is one of our future works.
How malicious behavior affects the system performance is one of our future works.
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