multi-torrent: a performance study yan yang, alix l.h. chow, leana golubchik internet multimedia lab...

MULTI-TORRENT: A PERFORMANCE STUDYYan Yang, Alix L.H. Chow, Leana Golubchik

Internet Multimedia Lab

University of Southern California


OUTLINE

Background and Introduction Proposed Approach Simulation Study Related Work and Conclusion

2


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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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


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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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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OUTLINE

Background and Introduction Proposed Approach Simulation Study Related Work and Conclusion

8


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.


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

⎧ ⎨ ⎩


OUTLINE

Background and Introduction Motivation and Proposed Approach Simulation Study Related Work and Conclusion

11


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.


STAY IMPROVEMENT: # OF TORRENT

13

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.


STAY IMPROVEMENT: INTER-ARRIVAL TIME

14

More improvement for

longer inter-arrival time.

More improvement for


More obvious # nodes change for


More obvious # nodes change for


Setting Value

Node class Fast nodes (5000kbps / 512kbps)

File selection Fixed selection 3 torrents out of 10.


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


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%


OUTLINE

Background and Introduction Motivation and Proposed Approach Simulation Study Related Work and Conclusion

17


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


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


Q & A

20

Thank You!


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


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


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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multi-torrent: a performance study yan yang, alix l.h. chow, leana golubchik internet multimedia lab...

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